On 9 February 2011 07:50, alan smith wrote: > So in general, given a NiHM cell, the mAh capacity is the amount of curre= nt it can deliver over a given time into a set load, correct? > > I have some unmarked AAA cells, so I charged them with my commercial char= ger and set up a fixed load (power resistor 75ohm 5W) and then proceeded to= log the current it was delivering over time. =A0The curve was not really w= hat I expected, it showed a slow discharge for about an hour and then a ver= y sharp drop to where it then flattened out. > > Am I measuring this incorrectly to find the mAh capacity of the cell? =A0= >From what I can see, it tells me that I have a 275mAh to 310mAh cell (thos= e are the starting points to where it dropped off sharply) > > > Alan, were you discharging them as a single cell or with the 2 cells connected in series? If the latter, then the first drop-off point would represent the end of one cells capacity, with the other providing the second flat area. (At lower current as you have a resistive load). This would also indicate that the lower capacity battery is being reverse polarised - probably not good for it. OTOH, if this represents a single cell discharge and is the same for both cells, then it's a bit more interesting - how about some figures. Initial voltage, time to first drop-off (1hr ?) , second plateau voltage etc. Could your measurement setup be indicating a voltage when open circuit or connected to a high impedance? The figures you give don't quite line up. With a 75ohm load on a 1.5V cell, the current is 20mA. if the drop-off is at one hour, then the capacity ois 20mAhr. Even if series connected to 3V, this only gives you 40mAhr. RP --=20 http://www.piclist.com PIC/SX FAQ & list archive View/change your membership options at http://mailman.mit.edu/mailman/listinfo/piclist .