Hi, I recently bought 3 NiMh 2500mAh batteries (https://www.sparkfun.com/search/results?term=NiMh). They are powering an autonomous, and unattended, pro mini system of about 6mA consumption. I want to charge them with one 5V 50mA solar film (https://www.sparkfun.com/products/14799). I thought of simply connecting a protection diode to avoid nocturnal discharge, but my concern is about safety.
50mA is 0.02C for one of those batteries, and 0.007 for the three of them. Is that current (50mA maximum, only on sunlight time) safe for a long term installation?
I’m not an expert in charging NiMh batteries, but I can tell you a LiPo charger won’t work with them. LiPo and NiMh are different chemistries, work at different voltages and need different charge profiles.
The “standard” way to charge NiMh would be C/10 for 15 hours and with a 2500mAh battery that would be 250mA so 50mA would be totally fine especially since you’re only going to get the maximum 50mA from your solar cell for a few hours out of the day.
You will need the diode to prevent the batteries from discharging back into the solar cell at night. You will want a Schottky diode rather than a standard silicon diode since it has a lower voltage drop. You don’t have a ton of energy coming from your solar cell and don’t want to waste much in the diode.
If anyone has more to add, please jump in! I’m not an expert in this aspect and any additional information would be appreciated.
thanks Chris, I understand then that charging with less than 50mA for some hours a day would be safe for a long term deployment? (provided that the sun compensates the 6mA consumption of the system!)
