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Old 05-19-2014, 11:10 AM   #1
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Mixing RV and Starting batteries in MH?

I've done several searches on Forest River forums and several times come across someone saying "Don't mix the two types". Hopefully I'm not beating a dead horse here, but:

By hooking two types up in parallel, wouldn't you get the best of both types? I'm not convinced yet it wouldn't work fine.

I've got to figure out what to do with my '07 Coachman 2600SO. I boondocked all last summer no problem, staying out 2-3 days with occasional generator runs to charge things up and make coffee. I've got LED lights everywhere, and the only big draw is the furnace. First time out this year the batteries died in the middle of the second night. They are both Exide batteries; without pulling them I'd say they are starter batteries (can't read the sides). They were down to 7.0 volts DC. I couldn't start the generator or the diesel engine, and when I punched the emergency start, nothing either. I got out the jumper cables, and hooked up the dead batteries to the emergency battery (it was at 12.6 VDC) to get things going again.

I've had this rig only a year or so, and the old coach batteries are labeled "shipped in Aug 2011" so they aren't too old. They seem to do both coach and engine starting duties together. Maybe go dual purpose batteries? I'd really like to have deep cycle capacity, since I'm always in the sticks.
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Old 05-19-2014, 11:58 AM   #2
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Simple answer... NO.
Everything WILL work just fine...but you will be shortening the life of the starting battery by 75% or more by using it in a deep cycle application.
Taking your deep cycle battery down below 12.2 will kill it early too.
7V DC is murder. You got their hearts pumping again...but the damage is done and you'll be replacing them sooner than later.
Provided you don't camp in the cold too much...when you replace your batteries you COULD go with 2 TRUE deep cycle batteries and use them for starting duty as well as full cycle duty but they are not built individually to deliver the current surge of a starting battery. Of course the amphours used up in starting will detract from the time you have available to run things without a genny.
If you are always in the sticks...based on the results you describe above, I'd find a way to cram in TWO deep cycle batts AND a separate start batt. You'll be paying for it in shorter life and more battery purchases if you don't anyway.
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Old 05-19-2014, 12:22 PM   #3
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Since I have to turn over a 6.0L diesel, I'm concerned that I deliver adequate CCA's to the starter.

I had a previous motorhome that had a separate starter battery and coach battery, which made replacement easy. This newer rig seems a step backwards by pulling double duty on a set of two batteries, while stowing one other battery separately for emergency only. The first week I had this rig I fried the emergency battery by hitting it with 100A off my battery charger, figuring it was there only to start the motor like my last RV. I managed to boil off some H2SO4 before figuring out the electrical system was quite different.
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Old 05-19-2014, 01:01 PM   #4
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It will work hooked up in parallel but they will recharge differently.
The small one will charge faster then the larger one.
Also the smaller will discharge faster then the larger one.
So the smart charger will think it needs to stay on higher charger longer so this may be the problem of the smaller one gassing for being over charged as the larger one still need more charge.

My question is why would you want to do this?
After so much money for the MH you want to change the system for not replacing the bad battery for some the same size.
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Old 05-19-2014, 01:06 PM   #5
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I'm getting rid of both old batteries, so it's not a question of trying to save $$ there. It's trying to get the most deep cycle capacity for dry camping while still being able to start the engine.

Which battery are you assuming to be the 'big one'? I think I can get both types in roughly the same size.

It seemed as though the existing batteries were being charged at a higher voltage by the engine (14.1 VDC) than the generator (13.6 VDC).
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Old 05-19-2014, 04:45 PM   #6
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So I pull out one of the batteries, and they are both deep cycles. Exide Stowaway ST24DC140, which have 400 CCA and 140 reserve capacity minutes each. So I'm pushing ~800 CCA with 2 batteries in parallel. Guess I'll replace both of them with deep cycle batteries.
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Old 05-19-2014, 04:53 PM   #7
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Spike...didn't catch the big diesel part which complicates things! You really need a LOT of cranking power for a 450 diesel which is probably why you ended up with a pair of dual purpose batteries installed in the first place.
Do you know the size, group # or model number off the batts that are there now?
That will allow us to determine the cranking amps you need to reliably start. It may also provide that info in the owners manual.
From there... you might find similar amps in one BIG starting battery allowing you to then get a pair of deep cycles for house duty. (A pair of Costco/Sams DEEP CYCLE 6V batteries would be a good choice wired to provide 12V + -/+ -)
If you don't have any way to carry more batteries...then stick with the compromise dual purpose and get the biggest ones you can fit in the space available.

WHEN the voltage on your batteries is low...i.e. 12.2 or less....the alternator should put out 14.1-14.5 volts. Your generator powers your battery charger/converter which should also put out 14+ Volts. As the batteries near fully charged you would see voltage in the 13.2-13.6 range and very little current flow off a smart charger. Don't know if yours has a smart charger...but in any case...with NO OTHER LOADS on it...the battery charger should be giving you readings of 14+ volts on run down batteries. Of course, it could be a Dumb charger putting out 13.6V forever and costing you battery life and genny run time. If you can find and provide the make model of the charger, we can determine what you should be seeing.
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Old 05-19-2014, 04:58 PM   #8
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Group 24 size battery so they are fairly small.

I guess since they are deep cycle the drop to 7 volts wasn't a mortal wound.

The owner's manual is for a Ford E-450 cutaway cab, and the battery info is not correct since Coachman redid the mounting point along with the electrical system. The power converter is a WFCO - all I know so far.
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Old 05-19-2014, 05:01 PM   #9
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The manual for my truck's Duramax says both batteries must be replaced at the same time even if only one fails. Having mismatched batteries will cause one to work harder and one will loaf until they are both ruined in short order.
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Old 05-19-2014, 06:13 PM   #10
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Ooops...your last post snuck in while I was typing mine Spike...yes indeed...you have TRUE deep cycle group 24 batteries which each have 75 amp hours of deep cycle use or 800cca combined.
This is NOT very much deep cycle capacity since you only get to use half of it or 75 amphours before recharging. You should add bigger or more or both when you replace your existing pair. Four golf cart batts in series parallel would give you plenty of starting powe AND 215 amp hours before you need to recharge. (430 total) ...but if you can't squeeze them in...squeeze in what you can! Herk is absolutely right...don't mix & match...replace all at once.
FWIW...WFCO makes smart chargers today...but who knows what they were installing in 2007...try to get a model number. BTW...once you know how big a battery bank you will be making....your charger/converter should not only be "smart" but also sized to provide around 20% of amp hour capacity... i.e a 200amp hour bank needs a 40 amp converter for optimum charging. Bigger wastes money since the battery can't take in more than that at one time (except for AGM's) ...smaller wastes generator time.
Finally...you probably lost some measureable cycle life going to 7V but the Deep cycles ARE more tolerant than starters of such abuse.
Once you get it all sorted out...YOU should not be without a REAL battery monitor for your boondocking. It will rapidly become the instrument you look at most and you'll wonder how you lived without it! (See Victron or Trimatic for models)
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