Charging 39 NiMH cell battery pack

[batteng]

All,
Have you ever heard about a charger charging 39 NiMH cell battery pack?

Looks like usual charger charge up to 24 cells max?

If this kind of charger is not available at all, is there a way I can modify an existing charger to do the job? or can I just add a voltage divider to the battery and charge it by using a normal charger?


Cheers,
Batteng

[Ldanielrosa]

Do you already have a 39 cell pack?  I wouldn't advise making a charger for it as the voltage dip for the first cell completing it's charge may be difficult to pick out of 50+ volts.

[ante]

Hi,

What is the available voltage? Why not build a new charger from scratch?

[Guest Alun]

With the normal 1.2V per cell you'd get 46.8V.

What capacity are these cells?

[audioguru]

My AA size Ni-MH cells measure 1.4V after charging slowly all night. Therefore 39 of them would require a charger output voltage of at least 54.6V plus more volts for the current limiter. A charger for a 48V lead-acid battery would probably have enough volts but the current would be excessive. A current-source circuit would need to be added to reduce the current.

[Guest Alun]

Constant current then float at 54.6V would be perfect, but we need to know the capacity to calculate the current.

[batteng]

All,

These cells are rated to 9Ah. I expect to fast charge them at C/4 or C/5 (2 to 2.5 A).
The total battery voltage is 39*1.2=46.8V nominal and 36*1.65=59.4V max (fully charged).

Hope it helps.

Cheers,
Batteng