finding the capacity of NiCd or NiMH cells?

[Sukhbinder]

Hi guys can any one tell me a way to find the current capacity for NiCd or NiMH cells.
Usually its written on the cell itself but if u need to replace an internal customised cell , how do u do it. i have a black and decker 2.4V cordless screwdriver. the problem is that we donot have a black&decker authorised dealer or service centre in INDIA , i need to change its batteries but the capacity is not written on it any where, the size also is a non standard type like i think its a 1/2D. the cells are enclosed in a paper jacket and fitted in a plastic case. the screw driver is working fine till now but once the battery life goes down it may be difficult to predict its capacity. Does anyone have a generalised type circuit to find the mAh capacity for these batteries. ;)

[audioguru]

Hi Sukhbinder,
You don't need to know the capacity of the original battery cells.
Just use the biggest cells you can find that fit. I think half-D size is common for models so try a hobby shop. Modern cells have a higher capacity than older cells so change both at the same time. ;D

[Sukhbinder]

thanks for the suggestions, if i don't get NiCd cells then i think NiMH cells will replace the NiCd cells well. i'll try to find the best fitting cells for it so that i can get maximum current also from them.

[audioguru]

Some manufacturers don't make Ni-Cads anymore, only Ni-MH now.

[Sukhbinder]

Thanks for the information but could u still provide me with a method to find the capacity of such type of cells. I need it to check out my cells. something that indicates its charge remaining and life remaing and condition.

[audioguru]

It is difficult to measure how much charge remains in Ni0Cad or Ni-MH cells, but you can measure their capacity with a discharge resistor and a timer circuit.

[Sukhbinder]

How do u measure the capacity with a discharge resistor and timer circuit. is it like this that u fix a discharge current value using a resistor and and find the time for which the cell can supply the current. u surely need high wattage resistors for it but what should the approximate range for the resisitor value be?

[tikbalang]

the quick and dirty formula for constant current charging is:

(cell capacity mAH/constant current mA) * 1.4 = charging time H

so, for a 2500mAH cell, charged at 500mA: (2500mAH/500mA) * 1.4 = 7 hours during charging the cell remains cool to barely warm. when it is about to reach the end of the charging time it suddenly becomes very warm to very hot. at this point they are fully charged. you need to get the average CHARGING TIME of the cells. make sure they are discharged, charge them one at a time at a known constant current, then get the average. you have to disregard the bad cells. check the TEMPERATURE every half hour or so to know if they are "fully charged". reversing the formula:

(charging time H / 1.4) * charging current mA = cell capacity mAH

so if you charged for 9 hours at constant current 250mA:

(9H / 1.4) * 250mAH = 1607.1428571428571428571428571429 mAH


*** 1.4 is a constant derived from charging C10 for 12-16 hours (standard charging). i used 14 hours because it is the median of 12 and 16.

[audioguru]

It is not accurate to calculate the discharge capacity of a recharable battery by its charging time because charging is not efficient.
Energizer use a discharge current of 0.2 of its rated capacity and stop the timer when the loaded battery voltage drops to 1V per cell.

The cells are 1.25V over most of the discharge and you have only two cells, so 2.5V/0.5A= 5 ohms for a load and its power rating is only 2.5V x 0.5A= 1.25W.
Make a timer and a 2V voltage detector. The detector stops the timer when the discharging battery voltage drops to 2V.

[Sukhbinder]

Thanks for all your  suggestions guys.