Solar Trickle Charging Vehicle Batteries

Need to store an RV, car, boat, SeaDoo, or ATV for the winter?
Here’s a quick look at your solar options for keeping your vehicle battery maintained through our Canadian winter, and avoid the cost of a ruined battery in the spring.

How much power do you need?
A typical flooded lead-acid battery will self-discharge at approx 40% per year. Doing the math, this means that for a typical group 27 or group 31 battery, you will need to offset at least 1 Watt-Hour per day. This means a solar panel in the 2-5W range will be plenty.
Sounds easy enough.
But, before you run out and purchase one of those little dashboard solar panels, or one of the ones listed at the bottom of the page, there are a few things to consider…
1. What sunlight exposure are you likely to get?
2. Are you wanting to charge through a window?
3. Are there any ‘mystery loads’ on the battery?
4. What solar cell chemistry will be used?
5. Do you need a charge controller?
I’ll deal with each of these quickly…

1. Sunlight Exposure
Winter in Canada. Need I say more?
Even if you get a string of sunny days, if you can’t get a sunburn, then neither can the solar panel!
Check for obstructions like trees. Shading will drastically cut your solar charger’s output.
So, considering only our latitude & weather, solar radiation for most Canadian locations can fall from a summer high of 5-6 peak-sun hours per day to less than 2. In the case of Vancouver, it falls to less than 1 in December!
This means that for most Canadian locales, you will need at least a 2W panel to deliver your desired 1 Watt-Hour Per Day of power (for the smaller group 27 & 31 battery formats).
If you have trees shading your property for part of the day (hopefully not the middle of the day), then double this estimate.
2. Charging Through A Window
Window glass absorbs a lot of UV. So much so, that any solar panel charging through a window will lose as much as 50% of its power output because most of its power is generated from UV light.
So, if you plan to use a dashboard-mounted solar panel, you’d better double your original estimate for power needs. That 2W panel is now up to 4-5 Watts with a window in the way.
3. Mystery Loads?
Clocks, meters, and stand-by control circuits all work to drain your battery power without you noticing. If you can disconnect them all for the winter, great!
If you can’t disconnect them all, you’d better make an allowance for the power lost, which can be at least as much as the natural self-discharge rate of the battery.
Poof! We are now up to 8 Watts of solar to keep our single battery maintained.
4. Solar Cell Chemistry
Some solar cells are more sensitive than others. For Canadian latitudes & climate, we will want the most sensitive so that we can collect the most power in any daylight conditions. It just so happens that the most sensitive is NOT crystalline silicone cells like the ones you see on rooftop systems. Crystalline Silicone panels are the most efficient, and therefore produce the most power per square inch “In Direct Sunlight”.
The solar cells most recommended for low-light & partial shade conditions are the amorphous silicone or “Thin-Film” solar panels. These have a very flat appearance, and do not have the little rectangular or hexagonal wafers wired together under the glass.
The other advantage of thin film panels is that they can be produced in foldable or rollable format for easy storage, and are very light weight.
5. Is A Charge Controller Needed?
The usual answer is ‘YES’, however if the panel is only being used for trickle charging, offers less than 1% of the battery’s capacity in Amp-Hours, and operates in the 15-16V range under load, then you can safely forgo the charge controller.
That being said, I advise getting one just to be certain your batteries are being cared for during peak sun seasons.

Products To Check Out…

Here are a few solar panels that can help you in keeping your vehicle batteries charged over the winter months…
… and their charge controller
& a couple of handy traditional crystalline panels designed for all types of vehicles…
… and a simple little 4A regulator for these to protect your battery
Let me know if you need any help determining how to keep your boat, RV, ATV, SeaDoo, or other vehicle battery maintained over the winter.

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