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Go Back   SailNet Community > On Board > Gear & Maintenance
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Old 10-27-2008
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Solar Panel Recommendation?

After trying to research solar panels for trickle charging my batteries for the winter off season, I've decided to just ask for a recommendation. Anyone have good/bad experiences with their solar trickle chargers?

Thanks
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Old 10-27-2008
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I don't have a trickle charger, I use a 80w panel and a 3 stage regulator specifically for that panel in everyday usage.
I have a 3 battery house bank and 1 starter charged by that, all AGM's, all the same battery (Spiral Blue tops). I set my monitor in the winter to 99% of total capacity (meaning it THINKS that is full) and my batteries just stay there. No chance of over charge that way.

Temperature and battery type matter, if you have any reasonable expectation of your batteries freezing, you are better off taking them home and putting a regular 110v charger on them monthly for a couple of hours.

Wet cell batteries can, IIRC can degrade / discharge far faster than AGM's, and my AGM's are rated at 2% per month. You need to dig into the spec's of your batteries, figure the unloaded discharge rate, add in the expected bilge pump drain, other current drain's (co2 detector whatever) etc - and then get a trickle charger capable of overcoming all of that - and never trust a solar panel's maker of giving your the real average output of his panel - go with 80% because of dust, dirt, leaves, shade etc.

If you are just doing a starter battery, just take it home and trickle it off a 110v charger every couple of weeks, it'll be cheaper.
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Regarding the amp hours per day from a solar panel,

Some time ago, someone who had about an 70 watt panel and link 2000 current measurement (and I beleive accurate) posted a "near best case" measured daily amp hours from the solar panel system. I think it was from a sunny place in the spring time. I dont remember the specifics but the conversion was:

Panel watts rating * .234 = daily amp hours from the panel. For example, if the panel was rated at 20 watts, the daily amp hours from the panel would be 20 * .234 = 4.68 amp hours.

If I run a bunch of numbers to attempt to calculate the overall daily amp hours from a panel, I arrive at about the same number.

There are a bunch of de-ratings. First, the panel gets run at battery voltage rather than its open circuit voltage - derate 12.5/18 (this assumes an MPPT type controller is not used). The panel is typically not aimed directly at the sun - derate sin(45) = .707 Assuming 5 hours per day, and for the same 20 watt panel,

20watts *(12.5/18)*.707* 5 hours = 49.1 watt hours.

Or using a battery voltage of 12.5, 49.1/12.5 = 3.9 amp hours.

Both numbers are "close" (ie, 4.68 vs 3.9) so its probably reasonable to use the simplfied number for sizing solar panels of

Simple rule of thumb:
amp hours per day = manufacture rated watts * .234

You get less amp hours in the winter - maybe a lot less. I might even cut this number in 1/2 during the winter.
If you have an MPPT controller, you get about 15% more amp hours.

Maybe useful in sizing what size panel you need?
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Old 10-27-2008
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Don't have any lengthy experience, but harbor freight has a 5w for about $50.
Just started using it myself.
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Old 10-28-2008
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GH—

A 5 Watt panel generally isn't enough to provide a maintenance charge on wet cell batteries of any size.
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Been some recent discussion on battery self discharge which I havent considered. Here is a calculation of solar panel size needed. Calculations can be wrong and assume a lot but regardless...

Some references:
Can the lead-acid battery compete in modern times?
How Lead Acid Batteries Work

These web sites give a big range on self discharge but Ill use 2% per month for AGM or Gell and 20% per month for flooded wet cell.

For a 200 amp hour battery bank, you need to replace per month/per day:
AGM - 4 amp*hours per month / .129 ah per day
wet cell - 40 amp*hours per month / 1.29 ah per day

In a previous post, I said you get .234 amp*hours per day / watt from a solar panel. Cutting this in 1/2 because of less sun in the winter gives .117 amp*hour per day / watt.

So for the 200 amp hour battery bank,

For AGM or Gel, the minimun would be .129/.117 = 1.1 watt solar panel
For flooded wet cell, the minimun would be 1.29/.117 = 11 watt solar panel.

You can easilly scale these numbers. For example if you had a 100 amp hour wet cell battery, you would need a 5.5 watt panel. Or a 400 amp hour gel cell battery bank would need a 2.2 watt solar panel.
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