I've purchased a used boat that came with the following:

The solar panel is continuously connected directly to the battery.

That makes me wonder, is there a risk of the solar panel overcharging the battery? Or does the battery have a mechanism that stops the incoming charge when it's fully charged?

Admittedly, I don't know if it's physically possible to overcharge a battery. Maybe it isn't.

  • It is physically possible to overcharge a battery. Lead-acid batteries are fairly robust (not like those lithium batteries that need careful electronic attention); sealed ones a bit less so. Commented Jun 22, 2023 at 22:16

2 Answers 2


It is possible to overcharge a lead acid battery.

With the solar panel permanently connected, you would really want to keep the charge voltage inside the 'Standby Use' voltage regulation as specified on the battery, but this would depend on how often and how much current is being drawn from the battery.

So you should really keep to 13.8v maximum.

enter image description here

The solar panel has a 15v working voltage, which will likely be reached when the battery becomes fully charged. If the battery was in a low charge state, the battery will have no problem with the max 100mA that the solar panel is able to supply.

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There is a question on the solar panel page as below -

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Most averaged sized car batteries will be able to cope with the small amount of current the solar panel will be delivering even when they are fully charged, but the battery you have is relatively small.

I would think there may be a chance that leaving the solar panel connected for a long period of sunny days with the battery fully charged may lead to overcharging and a shortening of the life of the battery.

  • Agreed. Personally, I'd get something to put between the solar panel and the battery which will monitor the battery for when it is full. There's a list of them on Amazon which would work. It wouldn't take much of a solar panel to keep it up, but wonder if 15w is even enough to do this. That is a pretty miniscule amount of power output. Commented Jun 22, 2023 at 14:21
  • 1
    The manual claims "Can I overcharge my battery? No. The 12 Volt Power Sports Charger contains integrated circuitry which prevents discharge and overcharge." - whether that is true or not...
    – Jon Custer
    Commented Jun 22, 2023 at 17:44

No reason to worry:

A lead acid battery left in storage at moderate temperatures has an estimated self-discharge rate of 5% per month. This rate increases as temperatures rise


I guess this panel will have a hard job just keeping up with the self-discharge unless you live in a very sunny place.

The vendor of the panel says clearly you do not need a controller even with panels 10 times larger than your panel:

enter image description here

(I added the red line)

Your battery is about half size of a standard size battery.

If you are still not convinced, then leave the battery not fully charged, and measure the voltage. Then come back perhaps a week later, and do a new measurement. Unless the voltage increases significant, you do not need any kind of controller, which can contribute to discharge, which probably is a much bigger danger to the lifetime of your battery.

  • 1
    you are wrong, it will not have a hard time. The nominal battery capacity is 216 Wh. 5% per month self-discharge is 0.36Wh per day which is about 15 minutes of full sunlight per day. Commented Jun 22, 2023 at 22:14
  • @user253751 Your calculation is only correct assuming no electrical loses and optimal conditions like the orientation to the sun and no shadows from the boat not too far from noon, and that 15 min of optimal conditions should happen every day. Compare less than1.5W to 5W from the most modest phone charger. I wrote "I guess" because I can not know, but I am certainly more worried for the panel will not do any good job for longer periods during the year.
    – Gyrfalcon
    Commented Jun 23, 2023 at 8:43
  • then you are arguing about the event smaller losses. What do you think are the losses on 0.36Wh? How about 0.036Wh losses? Okay so it's 17 minutes of full sunlight instead of 15. Who cares? Commented Jun 23, 2023 at 12:49

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