Solar/Battery power needs calculation

Ken & Peggy

Ken & Peggy

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Please correct my calculation errors as needed. We recently acquired a solar panel and are trying to figure out how to be sure we use the setup in a way that doesn't damage or shorten the life of our trailer battery.

Our 2008 550 Ultra came with a smaller 34ah or so battery. CampInn recently installed the larger battery, which I think is an 80ah...? Also added the port to connect the solar panel.

Assuming the new battery is actually 80ah, I've read and seen here that you should use no more than 50% of that capacity to avoid shortening the battery life. So, 40ah is available for use.

We also just bought the Dometic, which is rated at 45W, or 3.75A. If the Dometic runs 15 minutes per hour that would = .9375A. I'd probably assume a 1.5A rate to be conservative. 1.5A X 24 hours = 36ah

Lights @ 1A X 3 hours = 3ah
Phone chargers @ 2A X 4 hours = 8ah
Water pump @ 2.5A X .2 hours = .5ah

That totals 47.5ah, which exceeds the available 40ah.

The solar panel, a Zamp 120W portable kit, is rated at 6.84A. At 8 hours of input that = 54.72ah. Assuming 70-75% efficiency, that adds about 40ah back to the battery.

Am I correct in assuming that I will net out with a 7.5ah deficit per day? Which would give me just over 5 days (40ah of usable battery power / 7.5ah deficit per day = 5.33 days) of battery use before needing to either run the tow vehicle or connect to shore power?
 
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Ken & Peggy said:
Please correct my calculation errors as needed. We recently acquired a solar panel and are trying to figure out how to be sure we use the setup in a way that doesn't damage or shorten the life of our trailer battery.

Our 2008 550 Ultra came with a smaller 34ah or so battery. CampInn recently installed the larger battery, which I think is an 80ah...? Also added the port to connect the solar panel.

Assuming the new battery is actually 80ah, I've read and seen here that you should use no more than 50% of that capacity to avoid shortening the battery life. So, 40ah is available for use.

We also just bought the Dometic, which is rated at 45W, or 3.75A. If the Dometic runs 15 minutes per hour that would = .9375A. I'd probably assume a 1.5A rate to be conservative. 1.5A X 24 hours = 36ah

Lights @ 1A X 3 hours = 3ah
Phone chargers @ 2A X 4 hours = 8ah
Water pump @ 2.5A X .2 hours = .5ah

That totals 47.5ah, which exceeds the available 40ah.

The solar panel, a Zamp 120W portable kit, is rated at 6.84A. At 8 hours of input that = 54.72ah. Assuming 70-75% efficiency, that adds about 40ah back to the battery.

Am I correct in assuming that I will net out with a 7.5ah deficit per day? Which would give me just over 5 days (40ah of usable battery power / 7.5ah deficit per day = 5.33 days) of battery use before needing to either run the tow vehicle or connect to shore power?
Click to expand...

Your calculations and success depend entirely on just how much sun you are getting. In my experience, if you have a good day of sun (and not in winter), the solar will charge the battery more than the Dometic will eat battery. But if you are in a campground with reasonable tree cover, each day is going to be a challenge. One day you may be OK and the next you may not.

One minor suggestion which seems "major" in your circumstance: Don't charge your phones from the TD battery. Buy a small battery (or two - one for each person) that is dedicated to recharging the mobile phones. You should have one in your backpack, anyway, when you take a hike, so you have emergency battery backup for your phone. And you can recharge that extra battery when you have dinner in town or in the campground's bathroom. Your phone is more easily charged from your car, so I think it makes less sense to charge the phone from the TD. (Still, I do it when necessary.) If you have to fully recharge a mobile phone every day, and you are using the fridge, I don't think the 120 Zamp will keep up, except under really good solar conditions (ie, NO trees, or some trees but in the summer).

Second suggestion, use your lights, less. I often use a solar or battery powered lantern in the galley, and sometimes in the cabin, as the lanterns are easier to recharge than the TD. And you are going to have and use a lantern, regardless, so this isn't "extra" equipment.

Third suggestion, use a Yeti instead of the fridge. I am still using the fridge, but it's a more expensive accessory than I'd like.

-Al
 
Thanks Al, all good suggestions. Except the Yeti... we're already too spoiled by the Dometic.
 
Jenny n Rob #943 said:
Ken and Peggy,
I have no answers to your questions or calculations, but I see it has been about 9 months. How has this worked out for you?
Click to expand...

We still have limited experience with the solar panel. On this trip we've only used it 2 day, and they weren't back to backs. It worked well, but that really isn't enough use to determine if it can handle our needs on an extended basis.
 
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