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CaptainBoing

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Joined: 07/09/2016
Location: United Kingdom
Posts: 2168

Posted: 04:54pm 17 Jun 2025

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Evening forum, hope you are all well.

I have recently taken my first ever foray into solar charging - very small scale, small 6V panel charging 4xAA NiMh cells.

I originally had a 1N5819 schottky diode as reverse-drain barrier and the delta-V between VSol and VBat was quite high - expectedly the same as the Vf of the diode. At 300-400mV, that is quite a proportion of the available 6-5.4 ish volts optimal charge on the cells. So I changed it for an "Ideal Diode" circuit using a balanced pair of PNP transistors to drive a PFET when the two voltages differed, effetively shorting VSol to VBat. Bench tests showed I might get 50mV Vf, tests of the ideal diode showed 10mV, real-world gives 60mV-70mV and sometimes as high as 100mV - all very acceptable. So I am making a lot more of the panel's output available to charge the cells and it shows.

But I have noticed something unexpected that was not there with the previous schottky setup... it's not a problem but has really got me wondering. See the following graphic. This is a little over two days of charging (Y-axis is 10's of mV). Everytime the solar panel output comes off charge and VSol dips below VBat, I see a "rebound" in VBat.

There is another in VSol but as we are moving into indeterminate functionality, I am less curious.

I know there are brains here that have been productive with solar for decades and you must have seen something like this before(?)

I asked google AI but I can't really square it's answer (like who am I to question it when I literally had never done anything with solar charging until a month ago?

):"When a solar panel stops charging a battery, you might see a slight voltage rise because the battery is no longer being actively discharged by the panel's current. The battery voltage will rebound to its open-circuit voltage, which is the voltage it would have when not connected to any load or charging source."

So charging a battery represents a load to it? I get that charging is effectively a negative current flow from the cells, but I am connecting a lower voltage (VBat) to a higher (VSol) so I would expect a relaxation if anything.

h

phil99

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Joined: 11/02/2018
Location: Australia
Posts: 2557

Posted: 09:57pm 17 Jun 2025

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Only a guess, perhaps the ideal diode doesn't turn off until there is some reverse current flowing. The rebound would then be when that reverse current stops.

Or it could be due to a change in temperature. The terminal voltage of the cells is affected by temperature, falling as the temperature rises.
When the cells are almost full much of the charging current can no longer be absorbed so goes into breaking down water in the electrolyte, making hydrogen and oxygen. As they recombine the energy is dissipated as heat.
Note how the battery voltage is falling faster than the solar voltage toward the end. Perhaps that indicates the cells are full and heating up. When charging stops the cells cool down and the voltage rises.

Some high rate chargers use the drop in cell voltage to switch to trickle charge.
Edited 2025-06-18 09:00 by phil99

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