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Boondocking Basics – Batteries are Crucial


My mission this week while I was at the stick house in Dallas was two-fold. I was going to buy replacement house batteries for the Old Girl and I was going to research more about the house battery set up. It had become obvious to me over the past few weeks dealing with the failing 8D absorbed glass mat (AGM) batteries that I was probably dealing with a pre-mature failure due to the way they were set up or the way I was using them.  Or both..

Let me start with the new batteries.  I had just about made up my mind to go with some cheap Duralast Deep Cycle batteries from AutoZone at about $95 all in per battery.  I would have preferred the Wal-Mart batteries because they were a bit cheaper. Every Wal Mart I checked had very few batteries in stock and I could never find an ‘associate’  that had the slightest tendency to even remotely, by chance,  in a blue moon, maybe when hell freezes over, actually assist a customer.   I have a real adversity to giving my money to folks that don’t appreciate it.   I was dragging my feet on the purchase because I knew the batteries I was pricing were pretty puny for what I wanted to do but I just could not justify additional bucks right now.

On Thursday, I ran across an ad on Craigslist  for 4 new Interstate 100ah AGM batteries — for $90 each!  Now that was more like it!

Interstate DCM 0100L Deep Cycle AGM battery

Interstate DCM 0100L Deep Cycle AGM battery

These were sure ‘nuf true heavy duty deep cycle batteries that normally sell for about $300 each. I worked on the guy 2 days trying to get him to come down on the price but gave up Saturday morning. He was a tough cookie and I ended up giving him $85 each for two batteries — still a smokin’ deal.

So I had my batteries; now to figure out what was going on with my system that it killed those pricey 8D AGM batteries so quickly.

First, AGM batteries need to be treated a bit differently from traditional wet cell batteries. They are more finicky on their charge rates and that is my first discrepancy. My Progressive Dynamics 9245 3 stage charger/converter is state of the art -supposedly- but it does not meet the requirements of the AGM batteries. They like to be bulk charged at 14.5-14.9 volts. The PD 9245 is set at 14.4 volts. The AGM batteries like a float charge of 13.5 – 13.8 volts; the PD 9245 is set at 13.2 volts. I

Size comparison- 8D 250ah @ 161 lbs, Interstate battery @ 62 lbs

had been sitting at the previous job site for a year hooked to a 50 amp service with the batteries ‘floating’ at 13.2 volts — being undercharged.

Second, there is such a thing as a balanced hookup for batteries.  Alot of my research this past week led me to sailboat and yachting web sites.  Evidently, these folks have been using AGM batteries for years in pretty severe environments and they have learned plenty about how to get the most out of a set of the more expensive AGM batteries.   My battery setup was typical –

Typical battery setup

it looked just like the illustration to the right only with 2 batteries. Positive to positive, negative to negative and the power feeding off the first battery in the hookup. Well,  this web site had done extensive testing and they found out that the batteries would not discharge or charge evenly in this setup.  The first battery, closest to the feed, was always taking the beating.

The  solution is to add a ground cable to feed the power.  That setup would look like the illustration to the left.

Balanced setup

This setup ‘balances’ the discharge and charge cycles.   Before I left Dallas I picked up 12′ of 2ga cable and some battery lugs to make the change to a balanced battery set up on my RV.  It just makes sense.  It also gets alot more complicated if you have more than 2 batteries.  The web site gives those details as well.

Third, the same web site cautions against what they call ‘various take off points’.   All current must flow in and out through the cables labeled  ‘ to installation’.  You cannot just tap a battery to add a power feed for another accessory.  Dammit!  Busted again. I had tapped my last battery in the chain to power the 1000 watt Xantrex Inverter and that is also where I had installed the shunt for the Trimetric 2020 Battery monitor.  No wonder I was getting some screwy readings!

I left Dallas about Noon today on a mission.  I was going to remove both of those back-breaking 8D batteries, rewire for a balanced setup, move the Xantrex inverter and the shunt for the TriMetric.  I am glad I left early because it was ol’ Dark Thirty before I got everything done.

The new Interstate AGM battery installed.

I left things rougher than I wanted but I was tired. I had dirt in my hair and down my neck from laying in the dirt running the new wires. My arms were sore from horsing those humongous 8D batteries around yet again and I was thirsty for a cold beer.

I went inside the RV at dark and turned on the Generac Generator to top off the new batteries. It ran for 45 minutes and I shut it off. I crossed my fingers and turned on the DISH receiver and the TV. 3 hours later, I had my fill of TV and I had run the water pump for a quick shower. The batteries still showed 90% capacity remaining at 12.5 volts. I gotta say I am pleased as punch!

End Note: Fire on the Mountain by The Marshall  Tucker Band from Searchin’ for a Rainbow.   It never gets old

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1 comment to Boondocking Basics – Batteries are Crucial

  • I installed these same interstate batteries in 2010 to power my Moms RV and now it is 2012 and they are still going strong.I think you are are correct about following the specs for charging. AGM batteries are more picky than the flooded FLA type. Good article. Thanks

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