Simple DIY Peak Detection Circuit

[Shogun]

Need some help. I have this Hobbico charger for NiCd packs. This charger is capable of charging 1 to 10 cells with a rate of 10mA to 200 mA. I bought a 7-cell NiMh pack but since I have a quick charger for this pack, I would like to build the strength by trickle charging it a couple of times using the Hobbico charger. My only concern is that just going purely by timing the charging process, I may overcharge the NiMh pack and ruin it so I would like to use a simple Peak detection circuit to cut-off the charge process once the cells have peaked. Since I plan on just using 7-cell packs, would someone help me with a DIY simple peak-detection circuit?
Thanks. :)

[vintage1]

About teh only thing I can thoibnk of would be a high pass filter - basically an RC filter into an op amp. that would detect a change in voltage from rising to falling, but it really very very dodgy as a way to detect a peak.

I'd buy a proper charger.

[slipstick]

Unfortunately no peak detection circuit works properly at low (trickle) currents for the simple reason that NiMH cells have no real peak at those currents. The voltage rise just gently slows.

Anyway the whole point of charging at trickle rates (below C/10) is that you do no damage however long you leave it on. Just go for it.

Steve

[Shogun]

Thanks for the replies. Just to confirm, trickle charging would not damage the packs if I left them longer. Correct? I have this 7-cell (8.4V) 500mAh pack and I was planning to charge it at 50mAh rate. This would mean that I could leave the pack charging for 10hours at this rate (C/10) and the pack would be fully charged. Is my understanding correct? Thanks in advance for your replies. :)

[adamdb]

There are losses when charging so the conventional wisdom is that you leave it on for 14 hours at the C/10 rate to get a full charge. 10 hours is unlikely to fully charge it. At 50mAh it won't hurt the pack even if it does overcharge it a little.

Adam

[slipstick]

That's right from empty you need about 14 hours for a full charge. You could leave it on charge for 3 or 4 times as long without doing any harm at all.

Steve