Solar Battery Charger


Imagine backing down the boat ramp, only to find the boat battery is dead.  Every happen to you?  Not me, and now never will.


My father and I own a small power boat.  Neither of us have room, nor the ability, to store it at our homes.  So I keep it at a nearby storage facility.  Good security there, but no power hookup.  When we are using the boat frequently, the battery remains charged between uses.  But it's Winter, and the weather isn't ideal to go fishing.  Consequently, the battery isn't getting charged.

A few weeks ago I hauled my generator to the storage facility and recharged the boat's battery.  This could get old quickly.  The generator is heavy, and this job is time consuming.

I've been installing a small solar array on my home.  It now keeps my house backup batteries charged, and runs my outdoor lighting every night.  Works very well.  So I've decided to do the same for the boat.

Harbor Freight has a 5 Watt panel that sells for $39 when on sale.  A 5 Watt panel can't recharge a dead battery in a day, but it'll certainly maintain a battery.  To calculate the current such a panel can supply, use the formula P (Power) = I (Current) x E (Voltage), or I=P/E.  Current = 5 Watts/12 Volts, or 416 mA.  Tad less than 1/2 Amp.  Over the course of a day, it'll keep the battery topped up.


This is how I recharged the battery in the past.

The boat has been sitting unused for a few weeks.  The battery had dropped to 12.59 Volts.  Still very close to full.  But with the unpredictable weather and busy schedules, it could be a couple more weeks before we take it out again.

The panel must be easily removed for when we do take it out.  The swim platform on the transom has 1" handrails.

I found some "pole clips" at West Marine that were designed to clip onto such rails.  They simply screwed onto the supplied panel mounting clips.  The power cable clips onto the battery terminals.  I'll add a quick disconnect later.

The panel snaps to the rails, holding it firmly in place.  I routed the boat cover so it fully exposes the panel.  I will add a small security cable to discourage theft of the panel.  It is behind the boat, and out of sight.

The weather is very overcast today.

This is how much Voltage the panel supplies with NO load.  Since this is such a small panel, a solar controller isn't necessary.  Even when in full sunlight, it can only produce 416 mA, which isn't enough energy to overcharge the battery.  The Voltage drops as soon as a load is connected.

Moments after connecting the battery to the solar panel, the battery Voltage is already increasing.  I'll check it again tomorrow, but easy to see that it's working. 


13.47 Volts the next day, fully charged.


$39, 1/2 hour worth of installation, and it'll keep my battery ready to go all year, with no grid connection.  This would work on a camper as well.  I keep our travel trailer at the same storage facility, and the site where it is parked does have power.  But I'm paying an extra $10/month just to keep the battery charged up.  I've been storing it there for over a year, so such a panel would have already paid for itself several times over.




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Last updated 02/10/08    All rights reserved.