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-   -   Solar charger size - trolling motor battery (http://www.sailnet.com/forums/electrical-systems/87301-solar-charger-size-trolling-motor-battery.html)

 drcoop 05-11-2012 07:14 PM

Solar charger size - trolling motor battery

I have a 12V battery in my 12 foot O'Day to run a 30lb thrust MinnKota trolling motor. My battery is a small one but works great for short to medium runs in and out of the cove to open water. It has the following specs:
VOLTS 12V
AMP HOURS 28AH
BURST AMPS 1000A
HCA 700A
CCA 300A
MCA 350A
RC 40MIN

I would like to be able to recharge the partially depleted battery after a weekend of use and would have 4-5 days to allow it to re-charge between weekends. Will a less expensive solar trickle charger (like a \$49.00 5W Sun Force I found on Amazon) do this for me?

 donradclife 05-11-2012 10:41 PM

Re: Solar charger size - trolling motor battery

Let's say that you have taken all 28 amp-hours at 12 volts out of the battery--that's 28*12= 336 watt-hours. Now you want to replace that with 5 days of solar power, at an average exposure of 8 hours per day or 40 hours. In theory you would need a solar panel of 336/40 = 9 watts. However, the solar panel guys oversell their panels (they rate them at 17 volts, but they only charge at 12 volts), the sun doesn't always shine, and there are some efficiency losses.

In reality a better rule of thumb is the watts in the solar panel divided by 3 will give you the amp-hours of charge at 12v per day. Using this rule, you would need 28 /5 *3 or about 17 watts of solar for a full re-charge each week.

 Stu Jackson 05-13-2012 03:56 PM

Re: Solar charger size - trolling motor battery

Except you'll kill your battery if you discharge it more than halfway.

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