Good morning Arduino-fellows,
I am prototyping a project, where we simply plan to power our MCUs outdoor over 4xNiMH batteries that shall be charged using solar energy and a solar charge controller. Could you please help me there? The Batteries are AA with a capacity of 2100mAh and NiMHs standard voltage of around 1.2V. The batteries should be charged using solar cells that provide 6V or 12V:
6V:
12V:
TI is providing a lot of solar charge controllers:
In particular this one looks interesting since it features I2C communication:
What do you think? I would be happy if you can give me a hand.
Thank you!
If the average load current is less than 1/10h of the battery capacity (C, in this case 2100 mAh) or 210 mA, then you don't need a charge controller with NiMH batteries.
"Trickle charging" is fine -- you might aim for at most 200 mA average charge current over a 24 hour cycle.
Tell us about the intended load. Assuming that the solar panels supply all of the load power over time, the long time average panel current has to be higher than the average load current by about 10%, to take into account battery inefficiency.
jremington:
If the average load current is less than 1/10h of the battery capacity (C, in this case 2100 mAh) or 210 mA, then you don't need a charge controller with NiMH batteries.
"Trickle charging" is fine -- you might aim for at most 200 mA average charge current over a 24 hour cycle.
Tell us about the intended load. Assuming that the solar panels supply all of the load power over time, the long time average panel current has to be higher than the average load current by about 10%, to take into account battery inefficiency.
Hi remington,
Thank you for your helpful reply! The load needs on average a current of 500µA (depending on how often I want to measure values it can vary from 300µA to 1mA average current) at 4xNiMH batteries (which have a very flat voltage-capacity profile and have a voltage of around 5.0-5.2V for most of their operating range).
Is it a problem that I am charging 4xNiMH batteries, each rated with a voltage of 1.2V, with a 12V solar panel? Can some overcharging that damages the batteries occur?
Thank you!
The solar panel voltage must be higher than the battery voltage in order to charge them. Typically people choose an open circuit panel voltage to be around 1.5x the battery voltage. You need only a series diode to prevent battery discharge through the panel in the dark.
The important charging parameter is the solar cell current, which is proportional to the area of the individual solar cells. If the load consumes 500 uA on long term average, a very small solar panel will be enough to deliver it.
I have used these 8.2 V panels (open circuit in full sun) for a similar application, and should be fine: https://www.seeedstudio.com/0-5W-Solar-Panel-55x70.html
Alrighty, thank you! That's a valuable input. I will probably go for exactly the solution you recommend. The Solar Panel with 12V is probably oversized when exposed directly to sunlight but I would also like to use it in cloudy places or in the forest.
Do you think the 12V panel I have reported can damage the 4xNiMH battery pack?
Thank you!
Solar panels are best considered as current sources, not voltage sources.