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I want to put some solar cells on my EV (120V VW) to charge the accessory battery. However, the Acc Batt doesn’t need much charging since I don’t often drive at night, so it’s just running the contactor and blinker/brake lights.

I would like to create a switching charger that will use a bunch of strings of solar cells in parallel to charge the Acc Batt, then when it is full will switch the strings to series and charge the traction pack. Does anyone know if anything similar has been done before?
 

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wire the solar in series to get enough voltage to directly (and totally unregulated except for protection diodes and fuses) charge the traction pack, and leave the DC/DC running all the time to charge 12V.

With the surface area available on the car for solar you won't be able to charge at more than an amp or two at 120V under the best of conditions, but that should still be enough to keep the DC/DC going and charging the 12v system. You might want to verify the DC/DC's power draw under various conditions to convince yourself this will be a net positive electrically speaking though. If it draws too much power "idling" all the time you may make things less efficient, not more.

good luck.
 
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