Charging NiMH batteries with 1W solar panel, charge controller needed?

Question

I want to know if a charge controller would still be needed despite the solar panel never outputting more than C/10 of current to the battery.

This is a follow-up about my question here.

The solar panel is rated at 1 W with a max load voltage of 6.4 V and the product page is here.

The battery capacity is 2450 mAH with a nominal 1.2 V per cell and the datasheet is here.

I actually tested the panel myself and found that in the best case with 90,000 lux (full sun), the max voltage was 5.2 V, corresponding to a max current of 157 mA. Thus, the maximum power point is 814 W. At a lower voltage (1 V), the current still never exceeded 165 mA, which is much lower than C/10 but higher than C/40. I've read that despite this, it would still be detrimental to the batteries to constantly charge them at this rate. However, since this is the absolute best case of light intensity, and even then the sun would only be up for around 13 hours at the most and not even at full intensity the entire day, is this a problem?

Answer 1

If your goal is to make the working circuit as cheap as possible then direct connect is OK for your numbers - it won't charge very well but the battery will likely survive for a year or two. You'd still need a diode to prevent draining the battery through a PV at night.

If you're designing for maximum battery life then you don't want to trickle charge at all. Also, for outdoor environment NiMH discharge curve moves up and down with temperature while maintaining shape - you need to check max and min voltages for whatever temp. extremes your circuit is about to encounter.