Greetings hobbyists and professionals!
Apologies for the long post in advance - if you don't feel like reading the whole thing there is a TL;DR at the bottom.
I am working on a solar powered sensor where I use a 900mAh 3cell NiMH battery charged using solar charger board made for LiPo. Since charge and discharge voltages of 3s NiMH battery are almost identical to lipo, I thought using a lipo solar charger would be ideal.
I chose CN3065 based charger bought from dear friend in China and when testing theory all worked out perfectly. However, when I have hooked it up to a 6v solar panel the output current of the charger varied significantly with brightness of the sun and ranged from 10mA to 200mA on a bright day. Since I read that you should charge a NiMH at a lower current (generally not exceeding 1/10th of rated battery current capacity), I have looked at ways of limiting current of the charge controller and indeed in the datasheet found that max charge current can be limited by soldering appropriate resistor to one of the controller pins.
I did this mod to the controller and it worked out perfectly, however then I faced another problem. The charger used to shut off when the battery voltage would reach 4.2v and turn back on when v would drop to ~4.10v to top it off. After the modification I have noticed that the controller no longer shuts off charging when the battery reaches 4.2v but instead drops current while still constantly pushing around 20mA into the battery at the mentioned voltage. The circuit itself consumes about 6mA, so net current is still exceeding consumption at the mentioned fully charged voltage level.
Is this behavior of the charger dangerous for the battery or as long as voltage does not go beyond specifications I should be fine? Would be great to hear advise from those who had experience with NiMH charging.
Due to nature of the projects, I cannot use any other type of batteries such as LiPo or lead-acid.
TL;DR - is it dangerous to constantly push current into NiMH battery which is already fully charged based on voltage reading (voltage at which current is pushed is the same as fully charged voltage, so battery V does not increase)?