[sdiy] silly question about NIMH batteries

[René Schmitz]

Hi Bert, Richard and all,

>> I was looking at these AAA NiMH rechargeable batteries that are being 
>> sold in stores everywhere and I was wondering if it was possible to 
>> get several simultaneous voltages from them without the need for 
>> regulators? To clarify what I'm talking about : each battery is 1.2V, 
>> so if you put 4 batteries in series, that would give you 4.8V. But 
>> could it also give you 1.2V, 2.4V and 3.6V all at the same time 
>> without having to use a regulator for each of these voltages? Would it 
>> cause problems if I drew current from each of the cells that is in 
>> series with the other ones?

If you can live with a not absolutely constant voltage, and the unequal 
discharging of the cells. There is no reason why not to do that.
However the NiMHs should ideally undergo the same charge-discharge 
cycling. I.e. if you recharge them all once the weakest cell is drained, 
you'll likely overload a cell that is only partly drained. You might 
loose some capacity or lifetime on those.

> You can't use batteries to provide a stable voltage, because:
> 
> 1. The chemical process that drives each cell depends on temperature and 
> remaining charge.

A battery or accumulator will nearly keep a more or less constant 
voltage over a large portion of its discharge cycle. Nowhere near a 
stabilized voltage of course. Initially the voltage is a little larger, 
then it falls off to a slightly tilted plateau, and then rapidly falls 
off once it is nearly empty. Check some datasheets for the batteries in 
question, there should be a voltage vs. charge graph.

> 2. The effective series resistance inside each battery means the voltage 
> varies according to current load.

Accumluators have quite low internal resistance. Hence it is not 
advisible to short them...

Cheers,
  René

-- 
uzs159 at uni-bonn.de
http://www.uni-bonn.de/~uzs159