The bit thats confusing is the charging time surely a 200mah nimh battery @10% charge current would take 200mah/10 = 20ma charge current ,200mah/20ma charge time = 10hrs why is speced at 14-16 hour charge for these cells ?
If a nimh battery is half discharged and a recharge is desired then how is charge time recalculated in order to not overcharge the battery ?
So to confirm a charge rate of 3-5%C for nimh cell can be used to charge indefinite without overcharging, 10%C would be pushing it if it was placed indefinite lengths of time in the charger ?
Hi there,
There's no such thing as calculating the time required to recharge, except when you just discharge the battery and you know how much charge you just took out of it, but even then it's not really the right way to recharge it.
There is no way to measure the state of charge without charging it up because there is nothing external about the cell that tells how much charge is left. Once you start to charge it however and it reaches full charge, there are a couple ways you can tell it has become fully charged.
One way that is used a lot is called the "Minus Delta V" method, where the 'V' stands for "Voltage". What happens during charge with a decent constant current (like 1 amp for a 2000mAh cell) is the voltage rises as time goes on, but then all of a sudden it starts to decrease even though you havent decreased the current (kept it at 1 amp all that time). This sudden decrease in voltage, which may be as little as 10mv, tells you that the cell is fully charged.
Then there is another method called "Zero Delta V", which looks for the time when the voltage slope decreases to the point where it is zero or almost so.
Another method relies on temperature instead of voltage, but you have to make sure the temperature measurement is done right. When a cell is charging it actually absorbs some heat so the temperature states somewhat cooler, but as it charges the cell temperature rises little by little. As the cell becomes fully charged suddenly the temperature slope increases, and that's the point where the charge is cut off.
So you see NiMH cells are a little more difficult to charge than say Li-ion or Lead Acid. To use the minus delta V method you have to be able to measure small changes in cell voltage, and with the temperature methods you have to ensure that the temperature probe never looses contact with the cell.