Powersupply: NiMh mixed amp ratings, problem ?


i have x6 rechargable NiMh batteries, 1.2volts

x4 of them rated at 2600mah and x2 of them at 2700mah.

i want to use all of these x6 batteries to supply my Rover 5 Robot platform: https://www.sparkfun.com/products/10336

Should all of the batteries have the same amp rating or would this be ok ?

If one NiCd battery goes completely dead before the others, it will suffer "reverse charging" and probably be ruined. However if you are careful with your discharge so that the voltage on none of the cells drops much below about 1 volt you will probably be OK. In any case, the capacity of NiCd cells decreases with age, so the numbers that are printed on the labels may not be very accurate.

"reverse charging"

Would this happen even if i used x6 of the same amp value ?

Also would it be ok to charge the 2.6A along with the 2.7A with the same charger at the same time ?

The same thing goes for NiMH batteries.

You are probably OK as long as you aren't using batteries with widely different ratings, they aren't old used batteries, and you aren't trying to completely discharge them.

BTW, don't fully discharge your batteries, the memory effect is 99.99% myth. Google for "memory effect NASA nicd" for details.

Ok thanks for your answers ill stick with what i've got then.

"Reverse charging" , from the more-charged cell to the less-charged cell, could only happen if they are connected in parallel.

When you put them in the normal type of charger, they are connected in parallel, but in any typical sort of application circuit, they are connected in series.

Reverse charging, in this instance, refers to what happens if a single cell or two run out of charge before the others do, and you continue to draw current. In that case, the voltage of the now-empty cell reverses polarity.

"reverse charging" Would this happen even if i used x6 of the same amp value ?

Yes, it does happen even if the cells are supposed to have the same capacity (usually quoted in mAh or milliampere-hours). The cells do not all have the same capacity and if the pack is allowed to discharge completely, it is very bad for the cell that dies first.

For this reason, people who race electric R/C cars and airplanes tend to buy battery packs with cells that are, by experiment, matched as carefully as possible in capacity. Those packs are quite expensive.

And for everyone else, just don't discharge all the way. It shortens the lifetime of the cells, anyway.