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Old 04-28-2014, 10:13 AM   #1
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How many watts Solar panel do i need?

Hi,
My class C Chassis battery goes completely dead after sitting for a week or so and I have to jump start it when I get it out of storage to go on a weekend trip. I know that is not good for the battery. I tried a 1.8 Watt. solar panel plugged in to my 12 volt aux jack (yes, it was hot) I left it for two weeks, and still had a completly dead battery again.
I suppose I could use the clips and go straight to the battery terminals to rule out the 12 volt jack but i'm thinking I need a higher wattage panel.
Does anyone use one of these to trickle charge their chassis battery? Not sure what is draining my battery but if someone has a suggestion on what level of wattage works, I'd appreciate any input.
I'd really love to hear from someone who successfuly uses this method to keep the battery charged.
Thanks,
Val
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Old 04-28-2014, 11:10 AM   #2
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AMSOLAR has good sizing recommendations in the education section of their website. A very worthwhile read, IMO.
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Old 04-28-2014, 11:27 AM   #3
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How many watts Solar panel do i need?

If your batteries have been drawn down to "dead" several times they may be holding charge properly anymore and you might need to replace them before you add the solar. If your batteries are dying just sitting around for a week (without any real load on them) it certainly sounds like they may be dying and probably and to be replaced. I would take them to get load tested (Pep Boys or O'Reilly's or someone like that will load test for free).
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Old 04-28-2014, 01:24 PM   #4
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The battery is brand new basically and was kept on a trickle charger over winter. Took it off the charger, hooked it up to the RV, fired up perfectly. There's some sort of parasitic drain which i'm going to put an ammeter on this week to see exactly what that drain level is. I was hoping someone would know what wattage solar panel would beefy enough to overcome that drain level. I may go back to the battery disconnect in the meantime.
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Old 04-28-2014, 03:32 PM   #5
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a solar panels that are called trickle charge the really small one dont even put out a amp

i would find on CL some where a RV used sharp 80watt 5 amp panel and a low cost CW charge controller

if you look in the right place you can get all this for 100$

over the years i found 10 of them at 175 to 75$ here and there and its the good way to save money
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Old 04-28-2014, 03:46 PM   #6
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A 25 watt solar panel will work well on an average chassis battery. The output is small enough that no charge controller is needed usually. I have one this size hooked to a car that I rarely drive and it's average output is about 1 amp. Because it's flat, it produces little power except when the sun is high in the sky, so the 1 amp is not constant for 12 hours.....really only about 6 hours of sunlight is "usable". So it's putting in 6ah per day into the battery on a good day....less on cloudy and rainy days obviously.
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Old 04-28-2014, 04:55 PM   #7
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Better to use a battery disconnect to isolate it from the drain. Then after the parasite is found and rectified, connect it back up.

A Battery disconnect is cheaper and easier than a solar charging panel.
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Old 04-28-2014, 05:10 PM   #8
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Try turning the main breaker off on the inverter
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Old 04-29-2014, 10:04 PM   #9
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How many watts Solar panel do i need?
Well...for time travel...you'll need exactly 1.21 gigawats.....for your motorhome....not so much...


Sorry, I couldn't help myself!
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Old 04-29-2014, 10:24 PM   #10
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Your power drain may be something as simple as a clock. 50watt solar & 7amp controller should be plenty & you will be much better off connecting directly to the battery.

Another option is what is called a booster switch which allows the house batteries help the chassis battery to start the engine.
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Old 06-26-2014, 10:45 PM   #11
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Take the battery to get tested

I had a 6 month new battery start discharging for no reason, 1st needed boost or 4 hours on 6amp charge after 2 weeks sitting, then after 1 week and by 9 months old it need a boost after 6 hours sitting. Turned out the battery was defective and replaced under warranty.
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Old 06-27-2014, 12:45 AM   #12
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Quote:
Originally Posted by simtec820 View Post
Hi,
My class C Chassis battery goes completely dead after sitting for a week or so and I have to jump start it when I get it out of storage to go on a weekend trip. I know that is not good for the battery. I tried a 1.8 Watt. solar panel plugged in to my 12 volt aux jack (yes, it was hot) I left it for two weeks, and still had a completly dead battery again.
I suppose I could use the clips and go straight to the battery terminals to rule out the 12 volt jack but i'm thinking I need a higher wattage panel.
Does anyone use one of these to trickle charge their chassis battery? Not sure what is draining my battery but if someone has a suggestion on what level of wattage works, I'd appreciate any input.
I'd really love to hear from someone who successfuly uses this method to keep the battery charged.
Thanks,
Val
Val, I use a solar panel on both my truck and my trailer. I use one on my 2008 GMC Sierra truck during our Vancouver BC winter, because we don't use it during the winter months, That use to lead to a flat battery after a week, something that drove me crazy for over 4 years. There was enough parasitic draw on the truck's systems to flatten it after seven (winter) days, but I finally solved the problem with a 15 watt solar panel that I plug into an accessory port. I just lay the panel on the dashboard.

On my 19 foot trailer, I needed a 160 watt panel to climb over the top of the parasitic draw. The draws were pulling about 20 amp hours a day, something I couldn't track down, even with factory help. I could have a cut-off switch installed to totally eliminate draws, but it's one more thing to go wrong, and my 160 watt panel keeps the battery at about 95%. I may install another panel or two, because that would likely eliminate the need to carry a generator.

Good luck on resolving to your satisfaction,
Nick B
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Old 06-27-2014, 01:16 AM   #13
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First of all you need to know the battery type, specs and age. Let's assume it has a 100 A/h capacity of which is ~50% or 50 A/H usable before the voltage drops so low that it can not power any appliances anymore. The parasitic draw in our 5th wheel is ~1.5 Amp, in your class C it could be between 0.5 and 1.5 Amp. First thing you need is a battery disconnect to stop this parasitic draw because a battery that has been discharged completely a few times will not last anymore and die prematurely.
To overcome the parasitic draw by installing solar you can multiply 1.5 Amp x 24 hrs. = 36 Amp which need to be put back into the battery on any given day. When you have usable daylight for 6 hrs you need a solar panel that puts out 6 A/h plus you have to calculate for rainy/cloudy days. So depending on your location you might need a 100 - 150 Watt 12 Volt panel and a charge controller to be safe (you don't only want to charge the battery but also maintain it).
This is only an example your actual situation is probably different or my math is off
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