Thursday, May 4, 2017

Motorhome Battery Job

We bought two more six volt batteries in the US on the way home this year. I have not been happy with the battery system's ability to hold a charge. When boondocking we have been limited to only about three hours of TV at night before running the batteries down to 12.2 volts or 50% charge, the lowest you want to go without starting to damage the batteries. Adding two more would double the capacity.

Getting 12 volts out of two 6 volt batteries requires wiring them in series and that is how the original batteries are wired. What I had to do today was to wire the two new batteries in series as well giving me what amounts to two 12 volt batteries. These would be wired together in parallel to give 12 volts with twice the amp hours I had before. A simple series-parallel circuit. Simple. Right?

Then I cleared out the basement bin next to the side door and cut a piece of scrap 5/8 inch plywood to strengthen the floor. Norma had been using this bin to carry additional canned food so I knew it would bear the weight.

I then drove up to the local car audio shop where I bought a length of #4 power cable from their scrap bin and added a few connectors to my purchase. They charged me a very reasonable $10 for everything. I was happy.

I cut the cable into lengths for the positive and negative leads and soldered on the connectors. Red for + and blue for - and drilled a couple of holes in the side of the bin. I carefully measured voltages with my Fluke meter as I did not want a spark show when I made the final connection. Everything went well with no sparks!

I will let the batteries work together and equalize with the charger overnight and then tomorrow I will unplug the rig and start monitoring things closely making sure the two solar panels keep everything charged. If they can keep up in the current cloudy weather we are having then I have no doubt they will do well when we are down south. If necessary, I am prepared to add another 100 watt solar panel.


  1. That extra storage capacity should sure help out I am sure, good luck.

  2. We totally enjoyed being "Off the Grid" this past winter but because of the need to run medical equipment for the both of us we use a lot of Battery Power. Having experienced long periods of little to no direct sunlight we will be adding additional Batteries as well as panels to gather Morning and Evening sun.
    Great job on the Series, Series Parallel wiring. Now you just have to tie them down.
    Be Safe and Enjoy!

    It's about time.

  3. Now you need a big screen t.v. Think it would fit? :^)

    1. Do I detect a little sarcasm? LOL Part of the problem was exchanging the bedroom TV for the largest one that would fit in the corner (26").

  4. yees we only have two 6 volt batteries as well and I sure would like to add a few more... but where to put them???? I would have to throw something away to make new space nearby!

    1. Yes, something in = something out! In our case it was only part of Norma's "emergency" supply of canned food. 6 volt batteries have a pretty small footprint (although a lot of weight) and take up surprisingly little room in that bin. Go for it John!

  5. Email comment from Al:

    Hi Croft:
    looks like a good install!

    I'm not familiar with those batts, do they need to be vented to the outside to vent the hydrogen gas? You probably have venting on a lid.

    Another comment FWIW, I found that the batts in our RV drink too much water if we are plugged in and the batts are connected to the convertor. If we are parked for more than a day or so in a full service site I isolate the batts, this virtually eliminated the boil off due to the boost charge from the convertor. I had one RV where the batts lasted less than 2 years because of the boost charge issue.

    1. Yes they need venting but the compartment is nowhere near airtight. However after thinking about your question, I just might use a circular saw and cut a vent hole. Thanks for the tip.

      After being plugged in for months on end, I am sure those converters do a lot of damage to batteries. That "smart" module I bought a few months ago constantly changes the charging voltage making it more of a real life condition which I hope is better for the batteries. Depending on the testing I am doing now on the four battery / 2 solar panel configuration, I may leave the rig unplugged most of the summer.

    2. I was a bit worried about your intiial saw choice

    3. Al, I just discovered what you meant by, "isolate the batts". My converter is now turned off at the breaker. A belated thanks.

  6. You're using a standard controller they don't produce a fast charge and they're great for two batteries but if you're going to have 4 one would suggest a MPPT controller
    As for your solar panels if you have a 100 W up there now Why not think about selling that 100w and put a 240 W Single panel up there if you have the room ( that is unless you're in Canada then all bets are off costwise )

    Correct me if I'm wrong but your golf cart batteries equaling 12 V you could bring your voltage down to no less than 10.5 v before you have an issue (plate flaking)
    Golf cart batteries use the larger plate, just for the abuse factor
    If you're using an MPPT solar controller you should be able to regulate it at 13.4 or 13.6 before it goes in to float
    If you're going to add another solar panel stay within 10W of the existing one

  7. I do have a standard 30 amp charge controller and am thinking of upgrading to an MPPT. I have two panels, a 125 watt and a 100 watt which they say is enough for four batteries. And yes, we are in Canada where they have not heard of the decrease in solar costs.

    One friend of mine regularly runs her golf cart batteries down to 11.5 volts but I have always been under the impression they should not go below 12.2. Perhaps I am wrong?