NiMH battery charging problem

Question

I have a battery (8 cells in series, NiMH AA cells (1.2 V to 1.3 V), 1300 mAh per cell).

Cells used-

1. https://www.amazon.in/Rechargeable-NiMH-2300-Peak-Power/dp/B01MF8HUKO

2. https://www.amazon.in/Uniross-Rechargeable-1500-Discharge-Mechanism/dp/B082KZD296

Total low voltage = 1.2*8 = 9.6 V

Total high voltage = 1.3*8 = 10.4 V

I have a circuit that can give a constant 12 V, 1 A output. When I measure the voltage before charging starts it shows 9.6-9.7 V and as soon as I connect the 12 V source the voltage jumps to 10.1 V and very quickly reaches 10.3 V and 10.4 V (within 2-3 minutes). Even after disconnecting the 12 V source the battery voltage remains at 10.4 V.

Current flowing is about 500 mA and reduces as voltage increases.

Q. I can't understand how the battery is charging so quickly?

I have made cut-off circuitry that measures battery voltage and stops overcharging (cut-off point is 10.3 V). Right now as soon as I connect 12 V it cuts off as voltage increases rapidly.

Is it possible that the 12 V is getting added with the battery voltage and that is why a sudden increase in voltage is seen? After I disconnect the 12 V source a drop of about 0.4 V is seen.

What should I do to properly charge a NiMH battery?

Below is the schematic I am using. It is AC/DC input and a 12 V, 1 A DC output.

Here is the datasheet and schematic link.

Answer 1

With all rechargeable cells you’ll see a higher voltage when they are being charged than when they are idle or under load. NiMH are quite forgiving, you can typically charge them at C/3 and trickle them indefinitely at a lower rate - I’d suggest C/20. So you could use a constant voltage source with a fixed resistor, but ideally you’d want a 2-stage constant current charger. A couple of things here; most importantly you have cells of different capacities so it would be wise to go to a trickle charge current that’s acceptable for the lowest capacity cell by the time it’s fully charged, even if the other cells aren’t. Secondly it’s not easy to detect when a cell is fully charged, in the case of NiCd there’s a measurable voltage drop as the cell becomes full but with NiMH it’s quite difficult to detect and when you have a pack of cells that are likely to reach this point at different times it’s virtually impossible. Perhaps the best approach would be to monitor each cell individually, although practically you could do an initial charge to a set voltage or for a set time and then drop to trickle current.

Answer 2

That's no way to charge NiMH, or any batteries. They cannot be directly connected to a power supply, because a power supply with 12V output must push all the current it possibly can into empty battery pack which could be 8V in case of 8 NiMHs and the power supply cannot keep voltage at 12V even if it pushes 1A into batteries, the batteries still limit the voltage to 8V and take all current they can get.

Also an empty NiMH will be betweem 0.8V to 1.0V, and during charging the voltage could go up to 1.6V.

You could maybe use trickle charging at C/10 for 14h, but again, a proper charger needs to limit the current and stop charging after battery is full. A power supply does not do that.

You need a charger between the power supply and batteries.

Also if you simply put 8 cells in series without something that balances the cells, the battery with least capacity can degrade and damage quickly.