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I have a number of AA- and AAA-sized NiMH rechargeable batteries ('accumulators'). All the chargers I encountered allow to charge 2 (4, 6, 8,..) such batteries at a time, but never a single battery.

Is there a good engineering reason for this?

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2  
Do you mean "battery" when you say "accumulators"? – W5VO♦ May 23 '12 at 1:43
4  
In many Eastern European languages the word accumulator is used for rechargeable batteries. Link to Russian Wikipedia: ru.wikipedia.org/wiki/… I can't think of a reason why a charger for a single NiMH cell can't be built. – Nick Alexeev May 23 '12 at 2:14
Most likely. In a lot of languages (including my native one) "battery" means non-rechargeable (may also be limited to devices with more than one cell), while "accumulator" means rechargeable and since "accumulator" looks like an international word it is easy to make this mistake. – Pentium100 May 23 '12 at 2:15
Thanks everyone; I've changed the wording. – 9000 May 23 '12 at 4:23
And of course, technically a "battery" is a series of cells, while the objects we're discussing are generally single cells. And yet somehow we all understand each other! – Ernest Friedman-Hill May 23 '12 at 13:37

2 Answers

up vote 9 down vote accepted

There is no overwhelmingly good reason to charge cells in pairs, and while many cheaper multi-cell chargers do charge in pairs, not all chargers do this.
They are probably doing it to save cost.
It means they can use higher voltage and lower current and they may be able to simplify the charge circuitry somewhat, as they have halved the number of "batteries" being charged by making each battery consist of two cells.

The very capable (and 'nice' :-)) "Powerex Maha MH-C9000" charger charges 4 x AA or AAA cells completely independently - cells can be charging or discharging or resting and the charger can be programmed to perform test cycles or "reconditioning" cycles and more. This is an excellent example of what a charger can achieve when absolute minimum cost is not the main driver.

http://www.mahaenergy.com/store/catalog/mhc9000.jpg

At about $70 here (price varies with seller) this is a very expensive charger compared to most - and also very good value for money if you use AA rechargeable cells extensively.

Glossary:

The term "battery" is often taken to mean a single item that is handled and installed as a single unit. So people may refer to an AA battery or AAA battery OR a 9 volt PP3 transistor battery. More properly the word cell means a single chemical device for producing electrical energy and a battery is a collection of these.

Cell - a single "electrochemical cell" - a single 'battery'. One 'indivisible unit'.

Battery - 1 or more cells connected together (usually in series) to provide an energy source.

Accumulator - as used in question = cell. As used for cars and similar, usually means a number of cells in a common housing.

Added

Multi-cell-string charging:

I've not seen it done (but it may be) but a relatively simple circuit per cell would allow you to charge NimH cells in a series string with little or no ill effects.

Across each NimH cell place a 1.45 Volt shunt (or clamp) regulator. For extra points temperature compensate it, but the difference is small enough to be bearable in most cases. Circuit below - Vout goes nowhere. Battery connects across two rails. Circuit bypasses current when Vbattery reaches present level. Cost at one off levels is $0.50 - $1.00 worst case per cell. Component cost in manufacturing volumes is 5 to 10 cents per cell.

Two diodes would be cheaper but have a far less sharp "knee" and would not be programmable. TL431, R1, R2 could be replaced by a zener but cutoff knee would be very soft.

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I use a charger just like that one, it can not only charge individual batteries, but it can charge them with different currents. It is truly awesome! Of course, I always choose the smallest current, as it creates the least heat. And I've been using the same NIMH rechargeable batteries for five or six years now. – ajs410 May 24 '12 at 18:47
up vote 5 down vote

The reason that battery chargers take 2, 4 etc batteries at once is cost. Good (expensive) battery chargers charge each battery individually, while the cheaper ones save on the charging circuits by charging two or more batteries in series, a 4x1 battery charger needs 4 charging circuits, a cheap charger can get away with one circuit that can be reconfigured to give 2.4V (2 batteries) or 4.8V (4 batteries).

Charging batteries in series is not good for the batteries, because even identical batteries age a little bit differently and charging them in series means you may be always overcharging one battery and undercharging another.

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If one has e.g. four rechargeable batteries that are likely to be in different charge states, is there any good way to charge the batteries with a double-string charger without damage? – supercat May 23 '12 at 15:22
@supercat, I don't think so without monitoring the voltage manually or building a circuit that does it for you (but then you almost made a new charger). Just by the better charger, it is more expensive but you will use it a long time. – Pentium100 May 23 '12 at 23:07
You can make a simple discharger that works in parallel that leaves cells at a safe level and then charge them from there. NimH or NiCd Cells discharged to say 1V will be reasonably empty. NB NOT just a resistor - rather a sink to a known voltage - one per cell with regulator would be good BUT using some diode arrangement would do. Something like 1 x Schottky + 1 x silicon diode in series per cell - but will need tailoring. – Russell McMahon May 24 '12 at 5:05
@RussellMcMahon, that assumes the batteries have the same capacity. New ones do, but as they age, the capacity does not change equally for all cells (differences in manufacturing, maybe one cell has more cycles or was in higher temperature etc), which would still leave one cell overcharged and one undercharged. Also, a diode still passes current even though the voltage is lower than the stated forward drop voltage, so the battery can still be discharged too deeply (I tried to use a few diodes to drop the voltage of a power supply to charge batteries, ended up slowly overcharging them). – Pentium100 May 24 '12 at 5:21
@Pentium100 - I'm not sure which part of my response you are referring to - you seem to be agreeing with me :-). In my clamp/shunt regulator add-on I specify a TL431 IC driven circuit - don't be fooled by the symbol - it's effectively a programmable zener and that circuit will change from under 1 mA to say 2000 mA clamping within say 10 mV battery Voltage increase, if desired. The 1.45V terminal voltage is about right for most NimH cells so you can use modestly age-wearied cells without much worry. I mentioned diodes and zeners only to say that they were not really good enough. ... – Russell McMahon May 24 '12 at 6:51
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