Off grid in the RV

RobertR · September 2013

Hello I am new here. I live in an RV on a few acres in East Texas. I have 700watts of solar cells, three 75amp hour Interstate batteries, 30 amp charge controller, inverter that came with the RV.

Typically I use the inverter about 3 hours a night watching TV and using the computer. Read out panel says I'm using 225 to 250 watts which I compute to be 750 watt hours, or divide by 12 = 63 amp hours, which is ~ 30% of the battery amp hours. Typically the voltage after use will have dropped down to ~ 11v.

I then shut down, and in the morning solar panels start recharging the batteries.

Problem is I am going through a set of batteries ~ once a year. Last week inverter started shutting off due to low voltage after ~ 30 minutes of use. Batteries at least some cells had dark brown fluid, which I take to be sulfated.

I have done some research and have ordered a MPPT 60 amp charge controller. I am still researching batteries to figure out which ones I need. At the least golf cart deep cell seem to be in order, which would if my math is right increase amp hours to 420. A 6 v golf cart battery says it has 210 amp hours, I assume two in series would have same amp hours, and then double it for two sets in parallel.

What can I do to improve my system and avoid having to buy new batteries every year? Thanks Robert

Cariboocoot · September 2013

Re: Off grid in the RV

Welcome to the forum.

Let's examine everything carefully, as 700 Watts of panel in your area out to be a good producer. I have the same size array up here and 9 months out of the year it is a winner.

We don't know the specs of the panels or how they are arranged. This is possibly an issue as there could be a Voltage mismatch between the array and the battery. The PWM controller would deliver the current just the same, but you wouldn't get full power potential.

If you could, it would be about 45 Amps, which is more than a 30 Amp controller of any type is going to pass. It certainly should be enough to recharge 325 Amp hours of battery.

Now about those batteries. Three connected in parallel, if not done right, can have current sharing problems which cause one to do most of the work and the third may be doing nothing or very little. Can you give a diagram or picture of the batteries connected together?

You've got your Watt hours right at 250 * 3 = 750. There will be additional power needed for the conversion between DC and AC and for running the inverter. Possibly around 900 Watt hours from the batteries, depending on the inverter used. In Amp hours that's 75 or about 1/3 of the capacity.

So the batteries should work for this if they are wired properly and of the right type. Most likely the problem lies in utilizing the array fully. 700 Watts ought to be capable of 1kW hour AC per day at least.

mike95490 · September 2013

Re: Off grid in the RV

In the afternoon, what is the voltage reading on the batteries, does the PV ever get them up over 14V ? I suspect chronic deficit charging is killing the batteries.

niel · September 2013

Re: Off grid in the RV

do you also shut off the inverter when not being used? just because there's no load turned on doesn't mean there's no draw on the batteries from the inverter.

zoneblue · September 2013

Re: Off grid in the RV

Better batterys will last longer for sure. Care and feeding: you do need to keep somewhat of an eye on them to make sure they are living in the happy zone. I guess yove figured out that 11v is not the happy zone :-(

Things that you should get a handle on are things like, days since last floated. Time of day that they float. Absorb voltage. Rest voltage in the morning. Those 4 things will be a good start.

A happy battery will on a super sunny day reach float before noon; Not go without floating for more then a few days. Be about 14.4V in absorb for a little while each sunny day, come down to 12.6V or so on dusk, and drop no lower than 12.4V by dawn.

How big is the inverter anyway, those batterys will only support a little one.

braak · September 2013

Re: Off grid in the RV

Typically I use the inverter about 3 hours a night watching TV and using the computer. Read out panel says I'm using 225 to 250 watts which I compute to be 750 watt hours, or divide by 12 = 63 amp hours, which is ~ 30% of the battery amp hours. Typically the voltage after use will have dropped down to ~ 11v.

I think your battery are never fully charged. Have a look here:

https://handybobsolar.wordpress.com/the-rv-battery-charging-puzzle-2/

Good luck!

Bruce

niel · September 2013

Re: Off grid in the RV

good point and possibility braak and welcome to the forum. another along those lines may be that the battery itself has lost capacity through occasionally reoccurring incidents that the batteries never recovered from. this may be where normally the setup would work, but due to extended bad weather or seasonal occurrences as well as loads made to be temporarily heavier or extended that the normally good charging can't make up for.

Off grid in the RV

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