With “RV Doctor” Gary Bunzer
January 2012
Dear RV Doctor:
I am interested in adding solar panels to my motorhome to help power it when I’m dry camping. My motorhome uses four (U2200) golf cart batteries. I typically run them down to approximately 50 percent charge overnight. What size solar panels will I need to completely recharge the batteries each day, assuming it’s a sunny day?
John Zumwalt, F404761, Hanford, California
At approximately 230 amps per battery, your total amp-hour capacity is slightly more than 460 amps. Before we get into your solar panel requirements, let’s talk about what size battery charger to use when you are plugged into shore power, since that will remain your primary method of charging when not dry camping.
The recommended battery charger for any battery bank is one with an output of, approximately, the total amp-hour capacity divided by 5. At 460 amp-hours, that equates to a battery charger approaching 90 amps of output. (Realistically, you could use two 40-amp stackable chargers.) But since you do one of the best things possible for your batteries by limiting the discharge to only 50 percent, a three-stage 40-amp to 60-amp charger will suffice. If the depth of discharge (DOD) does happen to dip below the 50 percent mark, it will just take a little longer to recharge the batteries.
Now I’ll address the solar panels. Because of voltage loss due to air quality, sun exposure time, high cell temperatures on the panel, etc., solar panels do not operate at 100 percent efficiency. In fact, even the better panels rarely exceed 15 percent efficiency, so you’ll want a little overkill on the voltage produced by the solar panels. There are 100-watt panels available that contain 40 cells with an output voltage of around 20 volts DC. The minimum output for your system would be panels that produce at least 17 volts DC via 36 cells. With your battery bank, I also would recommend a very sophisticated charge controller. Well-designed controllers take into consideration what is best for the batteries.
A common rule of thumb is to have a little more wattage available from the solar array than you have battery capacity. For instance, a 300-amp battery bank should have 300 to 400 watts of solar output. At a little more than 460 amp-hours in your bank, mathematically you’d need five 100-watt panels in order to effectively charge a fully discharged battery bank. But since you’re only discharging to 50 percent, which equates to roughly 230 amp-hours, I would recommend you start with three 100-watt panels and keep an accurate log of current usage over a period of a month or so. You may find you’re not really using that much battery power each day.
The good thing about today’s photovoltaic systems is that you can always add panels if necessary. If you desire enough solar power to fully charge a depleted battery bank, then go with the five 100-watt panels. If you go with the lower number but think you may want to add panels at some point, make sure the charge controller you select is large enough to allow for this.
Down The Drain
Dear RV Doctor:
My motorhome has a combination tub and shower. The shower diverter mechanism fell apart when the plunger was raised, and some small parts went down into the drain where I can’t get to them or gain access to the P-trap. Is there a kit available to repair this, or do I have to replace the entire faucet assembly?
Second, I will soon need to replace my two 12-volt house batteries. I have heard that going to two 6-volt golf cart batteries or AGM batteries wired in series would be a better solution. What is your opinion? Do you have an archived article on this subject to which you could refer me?
George Vlases, Seattle, Washington
All P-traps must be accessible in an RV, so there has to be a way to access, remove, and check the trap for those parts. If you can find the pieces, there’s a chance it can be reassembled. If not, then you’re probably better off just getting a whole new diverter assembly, unless you can identify the brand of the existing one and find replacement parts. Residential faucet assemblies will fit in a motorhome since they use the same cutout spacing, so you’ll have plenty to choose from at any of the home repair stores.
Typically, two flooded commercial-grade 6-volt batteries wired in series will provide a little more capacity than two like-sized flooded 12-volt batteries wired in parallel (nominally speaking). As timing would have it, an article I recently wrote about configuring battery banks appears on page 46 of this issue.
Into The Electrical
Dear RV Doctor:
I have a 2004 National RV Tropi-Cal with an electrical problem. It appears that shore electric is getting to the breaker box, but it’s not available through the motorhome’s electrical outlets. When I run the inverter or the generator, the motorhome has power, but not when I plug in.
Bill Cook, Harrison, Michigan
It appears that the transfer switch is not operating properly. It’s probably best to take the motorhome to a certified RV technician for a complete diagnostic check. But with two out of the three AC power sources providing voltage as they should, the problem is most likely in the transfer switch. It would at least be the logical starting point for diagnosis.
Most transfer switches are automatically triggered when the shore cord is plugged in. Full transfer of voltage occurs after a short delay. Are you sure you’re getting voltage to the breaker panel while on shore power? In most cases, the transfer switch is wired between the shore cord and the distribution panelboard, so be sure you are measuring pedestal power at the breaker box. There’s an outside chance a problem exists with the panelboard, but without performing an inspection and taking some measurements, I’d suspect the transfer switch first.
Up To The Ceiling
Dear RV Doctor:
Is there a way to flatten down or repair the small pimples that have formed (from water damage) on the ceiling of my 2-year old motorhome? They have appeared in an area that measures 8 inches by 12 inches on an otherwise pristine ceiling.
Carl Camillo, Philadelphia, Pennsylvania
It may be possible to repair the area, depending on the extent of the water damage. I’m assuming it has been confirmed that the pimples were caused by water damage and are not a symptom of off-gassing. If the substrate under the material (plywood or lauan) remains intact, it may be possible to inject glue or liquid cement into each bubble and then press or roll them out by hand. Using an applicator with a hypodermic-like needle attachment should allow you to penetrate the bubbles, squirt some glue underneath, and then flatten them out without having the pinpricks noticeable afterward. Much depends on the type of covering used on the ceiling. Be sure to apply glue or liquid cement that is compatible with the surface material that’s used.
{loadpositionEndBlurbHouseCalls}