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Old 02-14-2019, 09:35 PM   #1
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Solar advice please

I am struggling with trying to calculate how many watts I need on my roof to keep my batteries (House and Chassis) fully charged.

I only care about one scenario... When the MH is stored for an extended length of time. I do not care about running anything on solar except a trickle charge for my 2 chassis 12V and 8 house 6V batteries.

Sizing sites on the web use amp-hours as a measure. If I am trickle charging, I assume I am using very little amp-hours.. I think!

Thanks for your advice..

Stutch!
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Old 02-14-2019, 09:40 PM   #2
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If you have everything shutdown...100A per battery is a good rule of thumb. So, 800 watts would work. Your 800watts may only produce 400watts during the winter...but that is enough to bump up each day and get you back to 100%. The BIM will connect the chassis when the parameters are met.

If you are running a residential fridge while in storage...1440watts works nicely...but may require bump up with generator if the clouds roll in for a while.
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Old 02-14-2019, 10:09 PM   #3
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Todays...bump up via Solar.
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Old 02-14-2019, 10:23 PM   #4
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Differing ways to go. A 250w or so panel and a small MPPT controller would do it. A other way is a couple three 100w panels wired in parallel to a PWM controller.
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Old 02-14-2019, 10:35 PM   #5
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If all you need is to trickle charge while in storage, even 100w is overkill. I would think that 20 - 30w is more than adequate. There are a number of these on the market:

https://www.google.com/search?q=sola...hrome&ie=UTF-8

Personally, however, I'd install 100w with a controller that can handle more in case I wanted more power down the road.
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Old 02-14-2019, 11:16 PM   #6
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Just based on todays economics of solar panels, I would not install anything smaller than a 100 watt panel, which is overkill for maintaining batteries while in storage. Something like this would work fine for what you want to do https://www.ebay.com/itm/Newpowa-Sol...r/111430641698
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Old 02-14-2019, 11:44 PM   #7
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OP States a total of 10 batteries. Personally I would want a minimum of 125 Watts.
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Old 02-15-2019, 12:17 AM   #8
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OP States a total of 10 batteries. Personally I would want a minimum of 125 Watts.
Yep, there are a number of batteries. And given low winter sun, flat mounting, weather, etc. All why I commented a couple three hundred watts. The incremental cost is not that much to ensure robust storage charging.
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Old 02-15-2019, 05:45 AM   #9
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These can all be factors, and though the OP does not state where they live, I am making the assumption it is not somewhere that the coach will be buried in snow for months. Having said that 10 watts of solar panel per battery with flat mounted panels should be more than enough to maintain a set of deep cycle batteries.


A few years ago I ran across a study done by the US Army on the use of 6 watt solar maintainers, in this study they equipped a number of trucks, generators and other pieces of auxiliary equipment with 6 watt solar maintainer panels and left them to sit for about 6 months over the winter scattered around a military base (I think in Georgia). Being the military all of these had at least 2 hefty 12V batteries wired in series for 24V electrical systems, so we are talking about 3 watts per battery of solar power. The end result was well over 90% of these randomly placed solar panels provided enough power to keep the batteries charged during the test, the few that did not were placed in very shaded areas.
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Old 02-15-2019, 09:14 AM   #10
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Yep, there are a number of batteries. And given low winter sun, flat mounting, weather, etc. All why I commented a couple three hundred watts. The incremental cost is not that much to ensure robust storage charging.
Very true about incremental costs. Heck, you're already up on the roof (or paying someone to be up there). Already are installing mounts and running cables - so, it's really just adding panels, interconnect wires and brackets.
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Old 02-15-2019, 09:19 AM   #11
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Why not just disconnect the battery bank grounds when you put it into storage?

A healthy battery will easily last the winter. The acid lowers the freezing point to, I forget, -70F I think? At least mine lasted fine in Fairbanks over 3 winters. They did need to be recharged in the spring but just plugging into AC worked for that (I parked the RV next to the cabin and just ran an extension cord out to charge it up come spring).

Put a pair of mechanics gloves and a wrench in the battery tray for a cost of $10. Versus several hundred trying to keep them charged with a solar system.

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Old 02-15-2019, 06:16 PM   #12
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Even in Maine 100W should be enough if you wire straight to the 8 house batteries and turn the disconnect switch off. You will need to run jumper cables to the engine batteries to keep them charged too. A portable system would let you angle the panel for better solar gain and HD had a 100W portable system recently for $100.
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Old 02-15-2019, 06:28 PM   #13
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If there is no load on the batteries , you do not have to trickle charge them when in storage . Just be sure the batteries are fully charged and that they are totally disconnected prior to going into storage . They will self discharge about 5% per month so even after 6 months in storage , they will still have 70% capacity.Cold temperatures will not harm a charged battery.
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Old 02-18-2019, 12:16 PM   #14
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I use the Battery Tender Solar Charger on our class A with single start and dual house.
15W for the start and 10W for the house and the batteries are 100% after sitting since November of last year. Put the panels in the windows facing south in Northern Cal.
Solar
Keeping the voltage up is the important thing over the watts. Just the bigger watts for the starter due to the vampire draw. The starter is via the cigarette lighter and the house is directly on the batteries.
Hope that helps you as to sizing yours.
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