It doesn't know, and it doesn't need to. The typical peak-charge algorithm for NiCd and NiMH batteries doesn't care about absolute pack voltage, it applies a current and only cares about seeing the voltage go up and then decrease slightly (for NiCd) or hit a plateau (for NiMHs).
I read in sci.electronics.design that DaveC wrote (in ) about 'How do chargers determine what battery pack is installed?', on Sat, 17 Jul
I expect they apply the lowest voltage first and monitor the charging current. If it drops to near zero in a few minutes, they increase the voltage to the next value, and repeat the process. A maximum current detector and limiter would detect 'a step too far' or a battery with a shorted cell.
Smart battery system specifications (SBS) - which are not the kind of charging situations the poster ia talking about - specify a wake-up charge cycle to handle cells discharged to zero - if only to wake up the discharge battery's communication interface.
When Bosch makes a battery charger, they're primary concern is charging compatible bosch tools and batteries. This cuts down on the possible combinations and permutations considerably.
Supposedly you can hit any nicad or nimh battery with a hefty current below chargr output terminal voltage limits, for a short period, at any time - just to see the resulting terminal voltage. This will tell the charger quite a bit about the battery, before it has to make any decisions about charging parameters or charging safety.
Charging a battery with a shorted cell using simplest constant current dV termination need not result in a safety hazard. A charger doesn't need to detect bad batteries to operate safely. It does not need to fix those batteries either.
There are many smart things that chargers can be made to do, but there has to be a reason for the feature.
I don't know, and I strongly suspect that it depends on who's in 'charge' :) of the of the algorithm. It's on the order of 10-100mV per cell for NiCd and a couple of seconds for NiMH -- and I think you could go cheap and use the same algorithm for both; just detect the top of the peak and call it a plateau.
Gates used to put out a very nice battery book on Pb-acid and NiCd batteries, I don't know if it's still in print.
I have designed a bunch of these while a "Fluke." I used several methods:
1: keyed terminals. different packs used very slightly different terminal arrangements.
2: extra terminal with connection to circuitry, sometimes just a resistor. This circuit could signal the main device or just be a programming resistor to set charging parameters. All packes used the same terminals, but the insides were different.
3: smart battery IC on the terminals, usually containing E^2 for memory.
4: keys on the packaging
5: then the obvious-- put in a constant current and measure the voltage in the main device.
Depended completely >How do multi-voltage chargers (specifically, cordless power tool battery
I believe this method will either leave both under-charged or destroy a NiMH cell. The generally accepted (rapid) charge method for NiMH is a constant current with a temperature cut-off. During charge perhaps 10% of the energy goes into heating the cell. After charge 100% goes into heat so it's rather easy to sense this difference. This method works well for both technologies. V or delta-V sense works for NiCd, but is highly "not-recommended" for NiMH.
Yes the "Rechargeable Batteries Applications Handbook", originally published (and given away[*] to customers) by Gates Energy is/was a very good reference manual. I believe the rights were sold (Stoneham?) and Gates itself bought (?). The book is available on Amazon: