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I'm pretty new to solar panels and basically, I'm trying to charge a 40V 4Ahr lithium ion lawn mower battery using solar panels. 40V solar panels aren't nearly as common of a size as 12V so I'm thinking maybe I could just wire up a few solar panels in series to stack up the voltage but I'm now unsure of how I should go about that.

Say I did four 12V solar panels in series to get 48V, could I safely charge the 40V battery with that? Is it dangerous to supply more voltage than a battery is rated for? I would have a solar charge controller of course but I'm not sure if that would help with supplying too high of a voltage.

Or maybe I should just do three 12V solar panels to get 36V? Would that harm the battery at all supplying too little voltage?

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    \$\begingroup\$ Bad idea. You cannot charge your batteries straight from solar panels. You need a charge controller or you will quickly damage your batteries. Larger 60-72 cell solar panels output the higher voltages you seem to need (30-36V) and are really not that hard to find. \$\endgroup\$
    – StarCat
    Apr 27 at 7:10
  • \$\begingroup\$ @StarCat Yes, I said in the question that I would use a charge controller. Thank you for the input though! \$\endgroup\$
    – LukeyP
    Apr 28 at 7:38
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Lithium Ion batteries require precise current and voltage control during charging. You cannot (should not) connect solar panels directly to a Lithium Ion battery for charging. If you do, you are almost certain to seriously damage the battery and possibly start a fire.

Since 40V is not a standard battery voltage for solar charge controllers, you are going to have some difficulty finding a solar charge controller for that battery.

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  • \$\begingroup\$ Hmmm, so if I were to find a 40V solar charge controller this should work fine? \$\endgroup\$
    – LukeyP
    Apr 28 at 7:43
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    \$\begingroup\$ Assuming the maximum charge current was appropriate and the exact pack voltage was correct, yes. It is still probably not a good solution to whatever problem you are actually trying to solve. If you are just trying to reuse the lawn mower battery because you have it, I would reconsider and just get a 12V or 48V battery pack with matching charge controller/BMS. \$\endgroup\$ Apr 28 at 18:27
  • \$\begingroup\$ Yeah, I did a little more research and it seems like I would need a charge controller specifically designed for lithium ion batteries too. 40V charge controllers are already hard to find and a 40V designed for lithium ion brings my options close to 0. I ended up biting the bullet and got a 48V battery pack instead. But I appreciate all the input you've given! It probably saved me from some massive headaches down the road. \$\endgroup\$
    – LukeyP
    Apr 29 at 21:44

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