Re: Battery voltage as indicator of charge level

pjdd@xxxxxxxxxxxxxx wrote:
> Is no-load voltage a reliable indicator of charge condition ? Are there
> universal voltage levels for a particular type of cell (e.g. NiCd,
> NiMH, Li-ion, lead-acid, etc.) that can be taken to indicate
> full-charge and full-discharge conditions ?

Lead Acid battery noload terminal voltage can be used as an indicator
of charge level (See other comment above). Other cells are not quite as
easy.

Li+. Fully charged : 4.2V per cell. Discharged 3.0V per cell (nominal).
Nominal cell voltage: 3.7V. There are charts available, and maybe I'll
dig out the links tomorrow at work. Fairly linear, but as with NiMH
somewhat susceptible to ambient conditions.

NiMH. 1.2V per cell. More difficult, but 1.1V is nominally discharged,
1.3V (roughly) is fully charged. Fairly linear betwen these especially
for the 20-8% range. Varies somewhat, see the manufacturers data
sheets.

NiCad. The noload voltage for a particular cell has zero bearing on the
charge state of a NiCad. A discharged battery might read anywhere
between 1.1V and 1.3V. The loaded voltage likewise has little bearing.
I spent many years with these, and the only method I know that actually
works is measuring the recovery time of the cell postload.

Details: Measure the noload voltage. Load the cell at some arbitrary
current (but well below the max load rating) for sufficient time for
the loaded voltage to stabilise (perhaps 100millisec). Release the
load. Measure the recovery curve (which is very close to a RC charge
curve). The shorter the curve, the higher the charge. For the
mathematical details, you'll have to wait until I dig out some books
from 15 years ago (when I did those experiments and filed patent
disclosure).

So the general answer is --No--, there is no universal voltage for most
cells. Indeed, we used to determine the charge of a lead acid battery
by electrolyte specific gravity (that was quite a while ago).

>
> There are gadzillion articles that discuss the general characteristics
> of various types of rechargeable cells, but I have not found any that
> specifically says that an X-type cell reaches Y Volts at full charge,
> and should not be discharged below Z Volts.
>
> Is it at all possible to specify voltage levels this way ? Temperature
> and internal resistance (and therefore charge and discharge currents)
> must play a part, but standard voltage levels under no-load,
> no-charging at standard room temperature would be very helpful.

Not really very helpful - an indicator, nothing more. For anything
approaching reasonable accuracy, noload voltage is not much use for
most solid battery types.

Cheers

PeteS

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