Ni-MH Charging

[JoeWawaw]

Hi, i would like to make a circuit to charge 2 Ni-MH batteries in series. A trickle charge method would be good for me (charging the batteries at C/10). I was wondering whether a simple comparator cutoff circuit would be sufficient for trickle charing Ni-MH bateries, aka have a comparator to cutoff the charging at a certain threshold (say 2.8V). Thanks

 

[audioguru]

Batteries come in many sizes and capacities. You forgot to say which ones you have.
C10 is not a trickle charge. If it is continuous then it will kill Ni-MH battery cells. A trickle charge is 1/40C or less. The charger is supposed to detect full charge then switch to the low trickle charge or switch off.

A battery charger circuit does not simply detect a certain voltage because the voltage varies with the charging current, the temperature and the manufacturer of the battery. Instead a small voltage rise then fall is detected.
Go to www.batteryuniversity.com to find out the details.

 

[jpanhalt]

It is hard to find a manufacturer that recommends long-term trickle charging for NiMH batteries. The problem is preventing overcharging at the recommend rates of 0.03 to 0.05C.

Here is what Panasonic has to say about trickle charging* (https://www.panasonic.com/industrial/includes/pdf/Panasonic_NIMH_Precautions.pdf ):

Some manufatcurer's products do not self discharge appreciably over a few months. The Sanyo eneloop is one such product. It retains 85% or more of its initial charge for over a year. Thus, it is hard to mate the purpose of trickle charging with a battery of such low self-discharge rate. If 85% capacity is not enough, periodic charging to a fully charged state may be considered, but then you also have to consider that NIMH batteries are considerably more limited in the number of charge/discharge cycles they can undergo compared to some other rechargeable batteries, such as NiCd batteries.

John

*The language is a bit obtuse, but I think the message is that if you want to "top off" a battery, use a rate of 0.03 to 0.05C and monitor state of charge so as not to overcharge. Do not leave the battery on the charger continuously.