Difference between revisions of "Electronics notes/Batteries"

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m (Battery sizes)
Line 971: Line 971:
* AAA  (10.5mm x 44.5mm)
* AAA  (10.5mm x 44.5mm)
* 9V
* 9V - also known as 6LR61, PP3, 006P, NEDA 1604 and IEC 6F22 (most of which refer to specific chemistries, but are not always used consistently), E-block (in a few places)
: (which actually contains six to eight 1.2V to 1.5V cells, possibly AAAA, or other shapes)
Apparently, the A, B, C comes from old portable vacuum tube radio schematics, where the letters referred to differently powered parts,
which manufacturers sold basically to refer to their function.
Presumably the D, AA, AAA, AAAA
Line 985: Line 992:
* 18650, 18500, 18350, 16340, 14500 and such are size references and typically imply Lithium cells
* 18650, 18500, 18350, 16340, 14500 and such are size references and typically imply Lithium cells
: These are millimeters. For example, the 18650 has a 18mm diameter and is 65mm long.
: These are millimeters indicating diameter and length. For example, the 18650 has a 18mm diameter, and is 6.5cm long.
:: (I'm not sure whether the 0 means anything{{verify}})
:: (I'm not sure whether the 0 means anything{{verify}})
: Note that:
: Note that:
:: 14500 is basically AA size (...but not in voltage, if lithium)
:: 14500 is basically AA size (...but not in voltage, if lithium)
Small batteries (button, coin, watch) - many, but I mostly see
* CR2032 (and variants - the last two numbers are thickness and usually a range will fit)
* LR44  (and variants)
3LR12 (4.5-volt)
Lantern battery.jpg 6-volt Lantern (Spring)
===Sensing cell/battery state===
===Sensing cell/battery state===

Revision as of 09:56, 27 June 2022

This is for beginners and very much by a beginner / hobbyist.

It's intended to get an intuitive overview for hobbyist needs. It may get you started, but to be able to do anything remotely clever, follow a proper course or read a good book.

Some basics and reference: Volts, amps, energy, power · batteries · resistors · transistors · fuses · diodes · capacitors · inductors · ground

Slightly less basic: amplifier notes · varistors · changing voltage · transformers · baluns · frequency generation · Transmission lines · skin effect

And some more applied stuff:

IO: Input and output pins · wired local IO · wired local-ish IO · ·  Various wireless · 802.11 (WiFi) · cell phone

Sensors: General sensor notes, voltage and current sensing · Knobs and dials · Pressure sensing · Temperature sensing · humidity sensing · Light sensing · Movement sensing · Capacitive sensing · Touch screen notes

Actuators: General actuator notes, circuit protection · Motors and servos · Solenoids

Noise stuff: Stray signals and noise · sound-related noise names · electronic non-coupled noise names · electronic coupled noise · ground loop · strategies to avoid coupled noise · Sampling, reproduction, and transmission distortions

Audio notes: See avnotes

Platform specific

Arduino and AVR notes · (Ethernet)
Microcontroller and computer platforms ··· ESP series notes · STM32 series notes

Less sorted: Ground · device voltage and impedance, audio and otherwise · electricity and humans · power supply considerations · Common terms, useful basics, soldering · PLL · pulse modulation · signal reflection · resource metering · SDR · Project boxes · vacuum tubes · Unsorted stuff

Some stuff I've messed with: Avrusb500v2 · GPS · Hilo GPRS · JY-MCU · DMX · Thermal printer ·

See also Category:Electronics.

This article/section is a stub — probably a pile of half-sorted notes, is not well-checked so may have incorrect bits. (Feel free to ignore, or tell me)

(Knowledgeable help would be much appreciated)

Pedantically, batteries are (parallel and/or serial) combinations of multiple cells, but in practice people use 'batteries' to refer to individual cells as well.

A cell's voltage is fixed, by its chemistry.

The difference can matter to your projects, in that charging batteries can be more involved than charging cells (particularly with lithium).

Batteries in circuits

Internal resistance/impedance

Real-world voltage sources can be modeled as acting like an ideal voltage source in series with a resistor.

This effective resistance is called the equivalent series resistance (ESR) (and some variations like internal resistance, internal impedance, series resistance, and series impedance.)

The ESR is mostly the result of battery chemistry, and also manufacturing and sizing.

The ESR value is typically below 1Ω, down to nearly zero. For example, AAs do on the order of 100 mΩ, Lithium on the order of 10mΩ

ESR is relevant to how much current you can draw, how much the voltage will drop while drawing that much, and how much warmer the battery will get.

High internal resistance means drawing large currents is impossible, causes a lot of heat, and/or a voltage drop that may be unacceptable.

(If you need lower ESR than the battery you want, but only for intermittent use, then capacitors (ESR of ~10 mΩ or ~1 mΩ for some supercapacitors) are good cheats, but they pack much less energy per volume, so their increased current capacity is usually only useful as bypass capacitor or other transient loads)


  • ESR varies with state of (dis)charge
    • highest when discharged. Often lowest when half charged (verify), somewhat higher than the lowest point when fully charged (verify). Details depends on battery chemistry, though.
  • ESR varies with temperature - which is one reason batteries don't work so well in the cold.
  • ESR will rise with age and abuse
  • Putting cells in series (for higher-voltage batteries) effectively adds their ESRs.

See also:

Battery safety in circuits

In-circuit charging

Quick comparison

This article/section is a stub — probably a pile of half-sorted notes, is not well-checked so may have incorrect bits. (Feel free to ignore, or tell me)

There are roughly four generally usable battery types that easily scale to most needs: NiCd, NiMH, Lithium-based and Lead-based. Alkaline exists both in unchargable form and in special rechargable form, but the rechargable one is not very common.

Each have their own characteristics, energy density (staying power per volume or weight), behaviour when charged and stored, and so on. Important to note is that capacity depends (in varying degrees) on use and calendar time. It can also depend on whether it is a more restricted commercial battery or a more-risk-acceptable industrial battery

The following table should serve as general comparison.

General notes:

  • Many figures here are (necessarily) approximate (and most of it needs verification)
    • There is generally variation between cheap, 'extra durable', industrial and specialist batteries.
    • figures involving weight and volume can vary by specific design, and figures here may be based on product sheets, best cases, or whatnot.
    • ...and by shape, e.g. cylindrical may last longer than prismatic (flat) used eg. in thin laptops)
  • Some things are specified per cell (while some things you think of as batteries are multiple cells)
  • self-discharge
Higher self-discharge usually means you should store on a trickle charge or top off every few months, to avoid diminished condition.
some (e.g. nickel-based, lithium-based) have a higher self-discharge in the first day than after that.
non-rechargeable (a.k.a. primary) batteries have much lower self-discharge
  • Overcharge tolerance - overcharging can be avoided for various battery types, with smarter loaders. In some cases (e.g. LiIon) chargers have to be smart to not treat the cells badly. NiCd/NiMH are occasionally overcharged because it was/is the common consumer tech and there are many dumb constant-current chargers out there.
  • On lithium: There are a number of variations of Lithium-based chemistries, most chargable, some non-rechargable, or higher-capacity versions of basic LiIon (that handles a little less safely). There is also the shape-independant, safer, but more expensive Lithium polymer. (often used in ultra-thin laptops)
  • On charge speed: for example, if the capacity of a battery is 2Ah, 0.2C refers to a charge strength of 0.4A. For commercial NiCd/NiMH batteries, overnight charges are usually done at 0.1C, fast charges at 1C, and very fast at 2C or 3C.
  • Memory effect is a misleading name (it's probably better to file it under the term 'voltage depression') referring to the lessening of batteries' capacity, an effect that can be lessened by proper use, and recovered somewhat by conditioners. See also [#On memory effect, voltage depression, and such|the section on it].

Lead acid NiCd (aka NiCad)
(Nickel Cadmium)
(Nickel Metal Hydride)
(Lithium-ion polymer)
Re-usable alkaline Alkaline (non-chargable) NiZn (Nickel Zinc)
('regularly seen in')
Solar, car, wheelchair, emergency lighting, UPS Standby, Consumer electronics Consumer electronics Portable computing Portable computing Consumer electronics Consumer electronics
(Single-)Cell voltage ~2.1V 1.25V 1.25V 3.6V ~3.7V 1.5V 1.5V 1.6V
(cycles) (order of magnitude before capacity noticably less)
300-600 500-2000 300-600(verify) ~1000?
a few years after manufacture?
a few years after manufacture?
a few dozen NA / 1 600(verify)
label cost
(price per Wh)
(varies anyway)
Lowish Average Average High High Low
(relative to others)
Average Low Average Average/High Average/High High NA (High for its single cycle)
Lead acid NiCd (aka NiCad)
(Nickel Cadmium)
(Nickel Metal Hydride)
(Lithium-ion polymer)
Re-usable alkaline Alkaline (non-chargable) NiZn (Nickel Zinc)

Environmental impact (relative to others) Moderate (better if recycled) High Low Low(ish) Low(ish) Lower than basic alkaline?
Storability Good (should be stored full) Long (any state of charge) Average Average (?) Average (store half full?)  ?
Self-discharge ~5%/month
(some variants up to 40%/month(verify))
15%/month(verify) 15%/month(verify) 7%/month(verify) (half from protection circuit) 10%/month(verify) (half from protection circuit) 3%/month <1%/month
Lead acid NiCd (aka NiCad)
(Nickel Cadmium)
(Nickel Metal Hydride)
(Lithium-ion polymer)
Re-usable alkaline Alkaline (non-chargable) NiZn (Nickel Zinc)

Ruggedness Average (avoid full discharge) High Average (avoid full or fast discharge) Average (if not abused; has care circuitry)  ?
Overcharge tolerance High (but too-fast charging causes damage) Average Low Low Low Average NA
Heat sensitivity Average Average/low High(ish) High High(ish) Average
Lead acid NiCd (aka NiCad)
(Nickel Cadmium)
(Nickel Metal Hydride)
(Lithium-ion polymer)
Re-usable alkaline Alkaline (non-chargable) NiZn (Nickel Zinc)
Charge speed (slow,fast,max?)
in fraction of capacity
0.1C - 1C - 3C ? 0.1C - 1C - 2C ? NA

Discharge speed4 (continuous, burst)
Internal resistance
(order of magnitude)
Internal resistance change

Capacity per weightest.
Wh/kg and
kg/Ah12 at 12V
Capacity per volumeest.
80-90 80-150(verify) 60-300?(verify) ~300(verify) 100(verify)
Voltage depression Low High (exercise helps) Average (exercise helps) Low (but wears over time) High NA
(best every x months)
3-6, but little 1-2 2-3, but more details unnecessary unnecessary NA

Lead acid NiCd (aka NiCad)
(Nickel Cadmium)
(Nickel Metal Hydride)
(Lithium-ion polymer)
Re-usable alkaline Alkaline (non-chargable) NiZn (Nickel Zinc)

est. - Necessarily an estimate. The product may tell you more (but advertisement may lie).

2 - Given to decrease over time whether used or not. After about three years after manufacture it may not be very useful for the same application, lasting perhaps half as long. Manufacturers of batteries, gadgets, or laptops do not like to admit this, partly because it means that if you need a new battery and the model is not available, you're screwed.

3 - Measured to when capacity dips below about 80%. Varies a lot because of the differences between battery design, as well as the difference between perfect use (listed) and practical use (may be a third of listed value). 'Over 1000 cycles' tends to be an ideal claim true for large batteries. The three numbers represent energy inefficient design, usual design, and energy efficient design. (eg. prismatic cell phone NiMH, AA-cell NiMH and D-cell NiMH) and are approximate. Note that NiMH 'twice as long as NiCd' claims are based on the larger capacity, not the amount of cycles.

4 - Discharge speed also relates to how the chemistry reacts. For example, for Lead batteries you may quickly get a decent voltage on only the physical surface, which slowly distributes and may mean you won't be able to start your car later. Also keep in mind that even with high ratings, voltage will drop noticeably by a few C (related to ESR)

12 - Quite approximate for a few reasons, amongst which the fact that you would need to approximate 12V with various cell voltages and various practical cell types. Probably needs some verification too(verify).


On memory effect, voltage depression, and such

This article/section is a stub — probably a pile of half-sorted notes, is not well-checked so may have incorrect bits. (Feel free to ignore, or tell me)

Memory effect refers to the idea that it won't easily charge/discharge beyond what it has not recently been charged/discharged to - so that a battery should always be charged and/or discharged fully.

And interestingly, this idea, that charge/discharge patterns lessen battery life, is mostly (but not fully) superstitious bunk.

First, let's call it Voltage depression, a broader and less ambiguous term, basically the observation that capacity lessens over time.

Some real causes, an the more significant, include:

  • cell age. Batteries will only last so long, mostly due to their chemistry.
Different kinds of batteries have noticably different lifespans.
Some age with use more more so than others
Some vary with the storage state it is kept at (some much less so)
  • cell use.
You could try to express this in expectable amount of charge-discharge cycles.
whether you do partial or whole charges is often less relevant than that you use it at all
  • cell damage
due to overcharging, discharging too far (which is frequently overstated, but still real), and other charging faults
Due to exposure to high temperatures (e.g. due to overly fast charging)
due to discharging problems (drawing overly large currents)
  • means of storage, and what that means to the chemistry.
In some cases this is negligible, in some cases it is noticable (though often only starts being a problem when the charge is fairly low)


  • different chemistries seem more susceptible to voltage depression than others
(Apparently NiCD is more susceptible than NiMH to voltage depression)
  • There is something to be said for conditioning
    • ...but the common "fully charge and discharge it multiple times" suggestion often does more bad than good. It's largely nonsense, often has negligible effect, and in multi-cell batteries and packs, complete discharges are likely to damage some of the cells due to reverse biasing some.
  • Most devices's battery charge state detection is fairly dumb, being just voltage-based. They'll decide the battery is nearly empty when the battery has less voltage even though its capacity is still decent.
(For a similar reason, some devices dislike rechargable AA batterires - their voltage is 1.2V where the non-rechanrgeables are 1.5V)

See also:

Charging, discharging

Battery charging

This article/section is a stub — probably a pile of half-sorted notes, is not well-checked so may have incorrect bits. (Feel free to ignore, or tell me)

At its core, battery charging putting voltage on the battery.

It's not entirely unlike charging a capacitor, with some consideration the battery's chemistry: it's a good idea to choose an appropriate voltage, limit the current, slow down/stop when it's full, and more.

If you know what you are doing, you can do it yourself with very little circuitry, but if you want safety, long life, and/or automatic charging, it takes more care and circuitry.

Simple chargers are constant chargers - they usually have a low, fixed current and a table showing after how much time you need to pull out a battery of a particular capacity. These can damage batteries when you allow them to overcharge (for a considerable time).

The cheap sort are often fixed-current, which means the charge time depends on the battery capacity.

Somewhat nicer chargers are slowish-and-timed, often combining the charger's known current with an assumption about capacity, cutting out some time later when the battery's likely full enough. If the battery has larger capacity than expected, they'll just stop short.

More advanced versions actually measure things, and are often sold as being microprocessor-based. They assume only charge characteristics of a battery, and use measurements to stop charging when it seems to detect the battery doesn't like any more charge. If you (want to) pay no attention to battery charging, these are good for your batteries' life, they may charge your batteries more fully without overcharging or stopping early. (Note: These occasionally to misjudge batteries that are not broken in)

Aspects to charging include:

  • avoiding more current than the cell can comfortably charge with (varies with its chemistry)
    • Charge speed is often chosen proportional to cell capacity
    • There is no standard to 'slow charging' or 'fast charging', and the terms seem biased to NiMH/NiCd, but:
      • slow charge tends to refer to no more than 0.2C, which for NiCd/NiMH means more than half a day from flat to full
      • moderately fast charge is something like 0.3 to 1.0C, perhaps 1 to 4 hours
      • fast charge may be up to 2C or 3C, meaning charge times well under an hour -- given that the battery can take it and there is a good cutoff method.
  • You should stop charging at some point or you'll damage the cells.
    • Just applying a constant current without a cutoff is a bad idea, mostly because once a battery is full, significant applied current will cause the cells to heat up, which eventually builds pressure and releases its electrolyte, or rupture.
    • Simple chargers are often either timed for a reasonably modern average battery capacity and a moderately fast charge - or just slow-charge without cutoff, which will heat the batteries once full, but not unreasonably so.
  • a method of telling when/what full actually is. Options include:
    • In theory, the delivered cell voltage can be used, but this varies per chemistry, and also on cell health, needs to settle a bit after charging, you need to be conservative for batteries with a fairly constant voltage though most of their charge/discharge curve (e.g. NiCd/NiMH) which may lead to terminating a charge before the battery is full. Not the handiest option.
    • Negative delta-V detection refers to the fact that battery voltage of NiCd and NiMH falls a little when it can take no more charge, and zero delta v detection refers to the change levelling off. A charger that can detect such a peak can use it to stop. However, NiMH has a less pronounced negative delta-V than NiCd, and this is not so useful for some other chemistries.
      • Fast chargers with negative-delta-V detection tend to start with a slow charge, because there may be peaks when starting to fast-charge an almost empty battery.
    • a temperature sensor in the battery pack. Good as protection, not as an early signal, as serious temperature increase is not good for the battery, and temperature rises a little in normal charging, and to do this thoroughly you arguably need a a sensor on each cell, making this less of an option on household batteries, though interesting on e.g. laptop batteries.
  • possible wish for cell-specific behaviour, such as
    • start charging slowly when battery was near empty (often better chemically)
    • fast charge for much of the time
    • slowish charge when nearing full
    • trickle/float charge, to top off completely - also useful in storage

Further charger features:

  • switch to a trickle charge (on the order of 0.04C. Varies - NiCd can often take C/10, some NiMH is suggested at at C/40)
Batteries slowly discharge because of their internal resistance, and a trickle charge keeps the battery full. In Lithium Ion batteries the trickle charge is so low that it is almost impractical to do, so not commonly seen (verify), and whether this is a good idea (varying per chemistry) is debated
  • balancers
try to charge individual cells equally
  • occasional cycling: discharge-and-charge.
  • do a top-off charge, particularly after a relatively conservatively terminated but fast charge, to make sure that we didn't stop because of something heat related. Regularly interval-based to avoid heat. May add a few percents of power.
  • in-circuit charging
That is: you do not have to disconnect the battery from the device to charge it, and you may be the device can be used while charging (possibly with uninterrupted power)
not hard in theory, as the charging supply can just power the circuit while it charges. May have a few basic implications to the circuit (e.g. a regulator if it is sensitive to having a few volts more on it)

Charger chips and charger circuits often allow some basic cleverness. They

  • often include a regulator to manage/limit charging voltage:
    • linear regulator is simple but a little wasteful
    • pulsed/switching voltage control - control voltage through pulse width modulator, in general or near the end of a charge. More efficient with power than linear regulator, but needs to be smoothed, can cause interference/noise
    • buck regulator - switching regulator with step-down
    • shunt regulator - effectively shorts out the power source (or at least leads it past the battery) at a certain voltage level. Can help avoid reverse biasing too (so handy for solar)
    • Note that you preferably want a regulator with low drop-out, so that you don't need
  • my do a constant charge and cut off after some fixed time, or don't cut off at all
    • Preferably slow chargers, as this makes damage from heating less likely) More general-purpose (e.g. NiCd+NiMH) chargers are often slow chargers for this reason
  • react to voltage and/or negative delta V characteristics (basic intelligence is becoming more common in chargers)
  • can be a smart element between charge source, battery, and (possibly regulated) voltage going to the load/circuit

Some of the more common ways of applying the power while charging:

  • constant voltage, such as attaching an adapter (preferably regulated?)
    • LiIon is constant voltage -- plus a bunch of protection circuitry
    • works well enough for lead acid(verify)
  • constant current - vary the voltage to maintain a constant current flow
    • common for NiCd, NiMH
  • pulsed current - duty cycle of mostly charge, and a pause with a little discharge
    • arguably can be chemically good
    • pauses also allow for open-circuit voltage measurement
  • Trickle charge - to balance the battery self-discharge current
  • Float charge - battery, load, and charger are all in parallel, at a constant voltage below the battery's limit. Mostly used with lead acid batteries, commonly seen in emergency backup lighting and such.
  • Random charging - describes non-constant, unpredictable power application, such as in solar charging, regenerative braking, car alternators. Generally needs current and/or voltage limiting to stay within battery allowance.

Full discharge is not recommended because of a few reasons. One of them is that most batteries are made of individual cells, the behaviour of which varies. If one cell is empty before another, you start charging it in reverse while discharging the others, contributing to voltage depression. This means that discharging NiCd/NiMH should stop at about 0.8V to be safe.

Battery chemistries are made for moderate temperatures; batteries don't deal well with either high or low temperatures.

The charge should be switched to a trickle charge at this point or the battery will begin to vent and release its electrolyte. My old battery was rated at C=1300mAH and my old charger was rated 400mA (30% of C) so the charger should have been switched off after about 4 hours, provided that they were almost flat to begin with. However there is no way of knowing if C was actually 1300maH or if it had decreased a bit, and once the a battery starts to deteriorate, I suspect this becomes a vicious cycle and the battery deteriorates rapidly due to more and more overcharging. The manufacturer suggests these cells should be good for 500 to 1000 cycles if properly treated!

It is not easy for circuitry to detect the charge left in a battery, or the overall usable charge of a battery. Perhaps the most reliable method is to measure the power delivery over a practical discharge, something that laptops may do.

See also / unsorted / to read:

Discharge speed

A C rating basically says the battery will deal with that discharge rate well enough (means its ESR is low enough).

Chemistry-specific notes

NiCd and NiMH notes

This article/section is a stub — probably a pile of half-sorted notes, is not well-checked so may have incorrect bits. (Feel free to ignore, or tell me)

Usual NiCd charge time is 10-16 hours per Amp-hour (a slow, often 0.1C - 0.2C charge), possibly followed by trickle charge to counter self discharge. Fast charge, 1-3hr, is possible but is likely to reduce lifetime somewhat faster.

NiMH deals better with faster charges, probably 4-8 hours per Ah. Slower is still somewhat better, partly because excessive heat (>40 Celcius) will more easily cause damage (NiCd can stand heat somwehat better than NiMH).

Note that batteries heating up a little near the end of charging is normal.

Note that while NiMH/NiCD batteries charge in chargers not directly intended for them, different care and guards apply to each, so you can avoid a few types of mild life-reducing effects (mild damage) by using a charger specifically made for the battery type.

Avoid storing batteries entirely empty, and if you suspect a stored battery may be empty, charge it before use (avoids possible reverse charging because of a complete discharge).

For long-term storage, consider cells exhibit self-discharge. This is lessened when stored at somewhat colder temperatures, but apparently increases at particularly cold temperatures(verify).

NiCd is more resistant to empty storage than NiMH(verify), but in both cases, avoid it if possible.

You may observe that in first charges the capacity isn't large - that's the reason to break batteries in. Ideally, new batteries (usually empty) should broken in: they should be fully charged and discharged three to five times, and ideally slowly charged at that (slower for NiMH).

Lithium notes

Lead acid notes

  • flooded lead-acid, a.k.a. vented
    • should be used upright, charged upright
  • VRLA (valve-regulated lead-acid) - informaly SLA, Sealed Lead Acid
    • allows faster charging than flooded type
    • do not require occasional addition of water to the cells.
    • have a safety pressure relief valve (do not spill their electrolyte if inverted)
    • Gel battery
      • safer for indoor use than flooded (negligible emissions)
      • gel instead of liquid - orientation does not matter; little spillage concern
    • Absorbent glass mat (AGM) battery
      • better specific power, worse specific energy (?)

Charge state estimation

Battery voltage is influenced primarily by:

  • state of charge
  • current currently drawn
  • temperature (on an order of magnitude that you'll notice the difference between freezing winter and scorching summer, but you can ignore if things stay room temperature-ish)

The discharge curve is mostly straight - down to ~20% is straight enough that open-terminal voltage alone gives a decent estimation of how charged the battery is.

Measure this some time after the last noticable charge/discharge, when the chemistry has settled. That, or watch the voltage slowly settle.

With a little more modelling you can do it known currents too.

  • Open-terminal (note this is relatively meaningless)
    • Full is usually below something like 2.13V per cell (times six cells is ~12.8V)
  • under ~C/100 load
    • Full: perhaps 2.11V per cell (times six cells is ~12.7V)
    • 20% charge left: perhaps 2.02V (times six cells is ~12.1V)
  • under ~C/10 load
    • Full: perhaps 2.06V per cell (times six cells is ~12.4V)
    • 20% charge left: perhaps 1.92V (times six cells is ~11.5V)
  • under ~C/5 load
    • Full: perhaps 2.01V per cell (times six cells is ~12.1V)
    • 20% charge left: perhaps 1.81V (times six cells is ~10.9V)
  • under ~C/3 load
    • Full: perhaps 1.90V per cell (times six cells is ~11.4V)
    • 20% charge left: perhaps 1.74V (times six cells is ~10.4V)

There are various charts out there - these figures are based on one or two of them.

Under charge, things are a little more complex - above ~70%, the voltage starts rising considerably, rather deviating from a straight line.

See also:

Discharge protection; sulfation

Sulfation refers to crystallization of lead sulfate on the negative plates.

Most of the lead sulfate will easily convert back when charged, but some will not, and over time will coat the battery's plate, which isolates them, and isn't reversible.

So for battery longevity, this is best avoided. Which mostly means "don't let it get too empty":

  • Below ~1.8V (10.8V for six cells) damage is pretty certain
  • Charge controllers asked to discharge will often cut out well above that (perhaps ~25% charge, ~2.0V, 12.0V for six cells)

Sulfation at all actually starts happening below ~2.06V (12.4V for six cells), which is at ~70% of capacity, so for the longest lifespan, try to keep it above 50% and preferably even above 75% if you can. This is the main reason behind advice like 'half-discharge cycles are better.'

Self discharge is approx 2% per month(verify), so when storing batteries, a quick recharge once every year helps lifespan.

Charging (and corrosion)
This article/section is a stub — probably a pile of half-sorted notes, is not well-checked so may have incorrect bits. (Feel free to ignore, or tell me)

Basic chemistry means that applying approx 2.15V (per cell) will charge a lead acid battery.

Lead batteries do not like overvoltage, charging too fast may heat them more than they like, and overcharging will start the generation of oxygen, which corrodes the battery.

As such charging lead-acid involves some voltage regulation.

How high is too high, voltage-wise, depends noticeably on the type of battery. Gelled, AGM or flooded differ somewhat in this and other details, such as what they should be trickled at. When in doubt, follow the battery or its datasheet. For example

  • some variants can be fast-charged at 2.4V per cell (15V for 6 cells) for most of the cycle.
  • Still, chargers often have to assume the lowest common denominator, so most stick to 2.3V per cell (13.8V for 6 cells)
  • ...or even just a 2.23V per cell (13.38V for 6 cells) trickle-like rate and to be nicer to the gelled variants

If a charger tries to be smarter, it may

start with a slow charge if the battery was connected entirely flat
faster for the main charge
eventually start tapering off for top-off/trickle charge

You could charge the battery at at its trickle-charge voltage. Since this takes forever, this only really makes sense for things like emergency lighting.

See also:

Battery sizes

This article/section is a stub — probably a pile of half-sorted notes, is not well-checked so may have incorrect bits. (Feel free to ignore, or tell me)

Some of the most commonly sold sizes:

  • AA (13.5–14.5mm x 50.5mm)
  • AAA (10.5mm x 44.5mm)
  • 9V - also known as 6LR61, PP3, 006P, NEDA 1604 and IEC 6F22 (most of which refer to specific chemistries, but are not always used consistently), E-block (in a few places)
(which actually contains six to eight 1.2V to 1.5V cells, possibly AAAA, or other shapes)

Apparently, the A, B, C comes from old portable vacuum tube radio schematics, where the letters referred to differently powered parts, which manufacturers sold basically to refer to their function. Presumably the D, AA, AAA, AAAA

  • Button cells - many.
    • letter-number style are according to IEC 60086-3
      • The first letter indicates chemistry, commonly L (Alkaline, ~1.5V), C (Lithium, 3V), S (Silver, ~1.5V)
      • If there's an R, it means round
      • The numbers relate to size.
        • The longer codes more directly mention diameter and thickness, e.g. CR2032 (lithium, round, 20mm diameter, 3.2mm thick), SR516 (silver, round, 5.8 mm diameter, 1.6 mm thick),
        • The shorter ones are a little less direct, e.g. LR44/SR44 are 11.6mm diameter and 5.4mm thick. See also this table
        • And a number have multiple names, like LR44 is basically the same as LR357, and AG13. This runs into the confusing area of same versus local names versus compatible replacements.

Sensing cell/battery state

This article/section is a stub — probably a pile of half-sorted notes, is not well-checked so may have incorrect bits. (Feel free to ignore, or tell me)

Battery indicators rely on general and/or load characteristics of a specific battery type. Many devices, like MP3 players and such, count on one type and easily effectively lie about the charge left when you use another, such as a rechargeable battery instead a non-rechargeable one. This partly because the cell voltage is different.

Doing battery indication accurately needs a bunch of information - what chemistry it is (how the battery responds to charge and discharge, because the relation between voltage and state of charge varies per chemistry, and simply because the cell voltage differs), how much voltage depression it has suffered, what it's currently loaded with and what its capacity is, and more. Things like laptops can do this because such variables are better known and more controlled, but in many other situations the estimation is a lot rougher.


Measure when drawing current. Open-terminal voltage alone doesn't tell you much. For most batteries, you need to be drawing some current to get a decent indication. Doesn't need to be much (and if it is, you probably need a good model of the battery), and you don't need to draw the current with the divider itself. (you may want relatively high resistor values so you draw negligible current).

You may be interested in a relatively small voltage range. For example, for a 12V lead-acid battery, most of the interesting stuff happens in the 10V-13V range.

Voltage sensing tends to mean doing some voltage shifting/dividing/crafting, plus a comparator if you want an indicator, or an ADC if you want a number on a display somewhere.

Voltage crafting is often some combination of resistors, a Zener or two, though can be more involved if you want to get fancy.

For an ADC, the simplest solution is a voltage divider (possibly using a potmeter for one of the two resistors so you can tweak the ratio better than two standard-size resistors can). <--If the ADC doesn't have a suitable reference voltage, you may need to create your own (e.g. with a Zener)-->

With a voltage divider, you are likely using less resolution than you could. Take the 12V lead-acid example - The original ~3V of interesting variation will is ~20% of the voltage range and therefore uses only ~20% of the ADC's range.

If you need more resolution, it's handy to make a Zener-in-series-with-a-resistor construction, the Zener to drop the voltage by a known amount, the resistor to measure the over.

TODO: read and sort

Battery myths and not-so-myths

"Leaking is caused by too low a charge"

"You shouldn't leave your phone on the charger"

Store batteries in the fridge

See also