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I'm running a small Arduino project out of an old 12V car battery (that was on my car since 2008 till 2015). I also have a 18V/5W solar panel.

Can I connect them together so the solar panel charges the car battery? The Arduino project is really really low on consumption so even a trickle charge will be enough to keep it running "forever" with the help of 1-2h of sun per day.

Thanks!

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  • \$\begingroup\$ No. The peak voltage will boil you battery on a sunny day unless you have a regulator. \$\endgroup\$
    – winny
    Jul 3, 2017 at 16:54
  • \$\begingroup\$ Actually it will probably work fine with a car battery. The battery will prevent the panel from ever reaching 18V. Just don't forget to check the water level in the battery. Also, go out there on a really long sunny day and measure the peak voltage to make sure it is not ridiculous. I would say if it hits 15V on the sunniest day of the year, that is OK. The battery will probably still last for a long time. \$\endgroup\$
    – user57037
    Jul 3, 2017 at 17:31
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    \$\begingroup\$ For the benefit of others who may come along, this only works if the solar panel has a fairly low power output. A 5W solar panel with an open circuit voltage of 18V in bright sun is probably about right for a battery maintainer charger. You shouldn't do this with a large solar panel. \$\endgroup\$
    – user57037
    Jul 3, 2017 at 17:37
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    \$\begingroup\$ The solar panel is likely to produce a maximum of about 300 mA. It's quit impossible for it to 'boil' a car battery. I'd suggest the best possible solution would be an LM317 with a series diode on the output, and set to produce no more then 13.8 Volts (after the diode). If you do connect the solar panel to the battery without a regulator ...make sure you have a series diode. \$\endgroup\$
    – Jack Creasey
    Jul 3, 2017 at 18:46
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    \$\begingroup\$ @mkeith I agree, but without one you have built a device which in a sunny country and light load, will make junk batteries out of good ones, albeit slowly at 5 W. I'm thinking a 14 V zener rated at 10 W. \$\endgroup\$
    – winny
    Jul 4, 2017 at 6:32

2 Answers 2

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With a conventional solar setup, solar panels charge deep-cycle batteries. A charge controller is used to prevent overcharging and correct voltage and current. Car batteries are for a surge of current to start the vehicle. I'm not sure if a conventional charge controller could charge your battery correctly because deep cycle and car batteries have different chemistry. But from what I found Here, you should be able to.

I would just try it with a charge controller, or try to make one. Just experiment with some different setups and find something that works for you. Also, I don't know if 5w could power your battery because it depends on the capacity and the current draw from the Arduino.

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    \$\begingroup\$ Could he not just connect the solar panel directly to the battery? At 5 W max the battery would easily dissipate any excess heat even if fully charged. It would probably equate to about 400 mA trickle charge for < 12 h/day. \$\endgroup\$
    – Transistor
    Jul 3, 2017 at 16:48
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    \$\begingroup\$ 9 year old old car battery? At that age it probably needs to be 'boiled' to remove sulfation. 400mA for a few hours per day (at best) should be no problem. \$\endgroup\$
    – Bruce Abbott
    Jul 3, 2017 at 17:06
  • \$\begingroup\$ Self discharge on a battery that old, especially if it is a larger one, averaged over 24 hours, might be higher than that solar panel can produce, even in fair weather. \$\endgroup\$
    – mcu
    Jul 3, 2017 at 17:11
  • \$\begingroup\$ @Transistor- I was just saying how it could be done properly, but personally, I would buy a $5 buck converter and set it to correct voltage. But I'm sure a direct connection would suffice. \$\endgroup\$
    – Isaac
    Jul 3, 2017 at 18:56
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Place 5 rectifier (minimum 3amp fo better handling of heat dissipation) diodes in series to drod rhe voltage down to 14.5v. thats a safe enough level to charge your battery.

each diode drops 0.7v x5 gives you a 3.5v voltage drop. 18v-3.5v= 14.5v chargung voltage.

if you want it at 13.8v make it 6 diodes in series instead of 5.

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