What's going on here?

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CaptainBoing Guru Joined: 07/09/2016 Location: United Kingdom Posts: 2171	Posted: 11:37am 20 Jun 2025       cheers for your input guys, It is appreciated.   Revlac said     Quote   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. ... Cant say I have seen it No, and it sounds a bit mad to me.   Revlac said   The larger solar shed lights have 2 battery cells and not sure what else they have inside, but on a long hot sunny day I have on occasion seen the lights turn on with a dull glow, I assumed this was excess power, cover the panel a little and its off again, uncover it and shortly after the lights are glowing again, the rest of the year this doesn't happen. Perhaps it is an excellent charge circuit dumping excess energy when the cells are fully charged - I would design that in if were a thinking man   I have had a day or two more to watch these traces and I agree the odd drop/rebound is almost certainly down to the switching of the ideal diode. There is another, less pronounced, wrinkle at the begining of the day where VSol approaches VBat and starts to level out before getting a little kick back into the voltage climb. I think this might be down to the old schottky diode starting to conduct right before the FET gets switched on.   phil99 said   ... 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. There is a fall in VSol which could be Temperature related or b/c the sun has moved round and is no longer perpendicular (but it is still very strong). I have another theory on the clearly related fall in VBat; Panel open circuit voltage (VOC) is inversly affected by temperature (T). Short circuit current (ISC) increases with T but not by as much ratio-wise, Pmax drops (manifests as a percieved increase in panel resistance) and dV increases. A whole charge cycle starts with a dV of around 50mV and ends with as much as 110mV because the panel is roasting in the June sunshine - If this is the cause, I expect this to reduce/disappear in the cooler months. The sudden drop in VSol is due to the shadow of a 300-yr old Oak tree moving round and hitting the panel around 16:30 ish. The rebound in VSol is likely because it is no-longer under load as the ideal diode has switched it out(?) This has been a very enjoyable canter working out why I see what is happening and the good thing is it is repeatable - I like repeatable. All the above is largely guess work based on a little research - what I have is a theory that fits the observations. I am really open to other explanations if anyone has them. cheers h Edited 2025-06-20 22:22 by CaptainBoing