Ni-MH battery pack charger

[overture]

This is a battery pack for old Zenith Transoceanic radio. It has two different output voltage. (9V and 90V). I am making a pack using 75 AA Ni-MH cells (each 2300 mAH) for the 90V supply and 8 D cells (each 9000 mAH) for the 9V supply (can live with a little higher voltage). Now, charging this pack would be a mess if I have to charge the individual cells using 4 in a bank (using the usually available chargers). If I could make a charger to charge the pack as it is (without taking out the individual cells) it could have saved a lot of hastle. Can anyone lead me to some information for circuits suitable for this type of charger. Thanks

 

[Goldorak_couillon]

Hello !

I'd try MAX712 and put the right voltage divider at BATT+ input to make it believe it drives a 16 elements pack.

Another way may be to use an MCU with an ADC and measure delta peak end of charge.

Considering the cost of your huge pack, a thermal sensor may be a good idea.

Enjoy !

 

[batdin]

Wow!!!
75 NiMH 2300mAh cells...It must cost a small fortune!
If you have not bought them yet I would suggest buying a 12V car battery and make two pulsed power supplies - one for 90V and one for 9V.Both simple and cheap.As well as much more reliable.
But since I think you already have these cells,I advise you to use a very simple circuit to charge all the 75 cells simultaneously.Imagine the following circuit: a capacitor,a resistor and a bridge rectifier.The battery pack is connected to the +/- outputs of the rectifier that is connected to AC mains through the current-limiting capacitor and the resistor whose function is to limit the initial charge current.Very simple and very dangerous,too,because the circuit is hot.But if You take some safety measures it will not be a problem.
And,of course,a timer to limit the charge to the necessary value.