Shouldn’t my solar panel keep RV battery charged during storage?

Dear Dave, 
The factory-installed solar panel charges the RV battery when switched on. When parked for a few weeks in the driveway, shouldn’t the panel keep the battery charged, which, in turn, keeps the refrigerator (which is battery operated) running? —Ted, Grand Design Transcend 200MK

Dear Ted,
There are several variables that come into play with your question and how it will affect the operation and efficiency of your system. According to the Grand Design website, the 2024 Transcend 200MK has a solar package that includes a 600w solar prep, 30-amp solar controller prep, 2000w inverter prep, roof-mounted quick-connect plugs, 12V 8-cubic-foot refrigerator, and a solar disconnect. There is no listing of any size solar panel available from the factory or battery size and type. This typically means they pre-wire all these and have a 3-port connection on the top so the dealer can add three panels totaling 600W.

Grand Design also does not ship trailers with house batteries, but rather leaves that to the dealer, as well. If an owner is not going to boondock or dry camp much, they would install a cheap flooded lead acid (FLA) battery. That most likely would be a Group 24 and only have about 80 amp hour capacity. A FLA battery should only be drained to 50%, so that means 40Ah.

More variables regarding power consumption

Another variable is the 12-volt refrigerator, which can consume anywhere from 40-80 watts per hour, and that can increase on hot days. With the unit in storage and not connected to shoreline power, the roof air conditioners will not be running. So a 90-degree day would mean well over 100 degrees inside the rig—which means the refrigerator will run a lot!

I went to the General RV website as they had several Transcend 200MKs listed. They all had 180 watt panels listed; however, not the brand or the size of battery. So I contacted my “go to” solar expert at Zamp Solar and asked him to take a look at a unit that had a 180 watt panel. Here is what I got:

There are unfortunately a lot of variables in that question, and it doesn’t sound like any of this is Zamp stuff, so I’ve got to assume the performance of the panel could be significantly below what we would expect to see from one of ours. That said, if it WAS one of our panels…

180 Watts should generate about 9-10 amps in full sun every hour.

Figure you get 5 hours total of full sun a day (sometimes we’ll call it 6 hours, but for sake of being conservative…)

The fridge, if it’s super efficient, is consuming between 3.5 and 6.5 amps an hour.  Split the difference, call it 5.

If the fridge is the ONLY parasitic draw (and it’s likely not), it’s then eating a total of 120aH in a 24-hour period (5×24=120).

Panel’s producing 50.

Fridge is consuming 120.

Doesn’t really matter what kind of battery bank you’ve got – there’s an excess draw of at least 70ah A DAY when that thing’s sitting in a parking lot.

So basically if the thing was Fully Charged with 2 standard 120aH AGMs, it’d die in less than 48 hours.

If you can provide additional information on the solar panels and batteries, we could tweak it a little more. However, in this situation, it won’t last very long.

 You might also enjoy this from Dave 

Can solar panels charge batteries in storage with disconnect applied?

Dear Dave,
My Riverstone has four six-volt batteries with one 100-watt solar panel on the roof. For the past three years, I had access to store our rig indoors with access to 110 v electricity to keep the batteries charged with a trickle charger. When doing this, I switched the main battery switch to “off” to avoid any battery drain. Now, unfortunately, I no longer have access to inside storage with electricity, so I must store my rig outside and depend on the 100-watt solar panel for keeping the four batteries charged….

Read the rest of Greg’s question and Dave’s answer.

Dave Solberg is a leading expert in the RV industry and the author of the “RV Handbook.”

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