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Old 10-20-2020, 01:37 PM   #1
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Pairing a deep cycle with a heavy duty starting battery

I know this is not an optimal solution, but has anyone put a deep cycle battery with a starting (chassis battery)
and had long term success. I replaced my 12v house batteries with 6v golf cart batteries and now have a couple of good deep cycle batteries. I have just discovered that my trip back from Utah (and the terrible roads entering and leaving Colorado) has produced a bad chassis battery (11v).
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Old 10-20-2020, 02:01 PM   #2
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Don't know about your battery failure, but I ran a split DC bus on a 2008 Allegro Bus for over 5 years. Because we were running a household refer, a basement freezer and a 17cu ft chest freezer in the bed of the pickup toad (bringing fish home from Alaska), I used 6v AGM's for the rear of the coach and lead-acid for the front. I had it setup so the engine alternator charged both banks, as well as the Onan generator and my solar panels on the roof. I'm sure that setup was not as efficient as it would have been if I ran all AGM's or lead-acids, but it worked just fine. We boondocked a lot and never had a battery fail or go dead even when running all 3 compressor refers/freezers off the grid. I floated a thread of what I wanted to do on some forums, and got responses from gloom and doom to no problem. I'd have to say "no problem" was the answer.
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Old 10-20-2020, 02:10 PM   #3
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A few questions:
1) Is the chassis (starting) battery a single battery or two in parallel?

2) Are both batteries of the same type, flooded cell, AGM, etc?

3) What's the age differences?

4) Same brand?

As you seem to understand you could put two 12 volt batteries in parallel and it will work. Likely not very well for an extended time. There are very real differences in the construction of deep cycle vs starting batteries. A starting battery is built to provide high amps for a short time while deep cycle batteries provide much less amps over a longer time. Differences in the plate design.

The weaker of the two batteries will drawn down the better one. Recharging requirements of these batteries may be quite different, So one will get likely charged while the other will remain undercharged.

I would mix and match batteries ONLY in an emergency case not as a planned solution.
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Old 10-20-2020, 02:43 PM   #4
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If one starting battery went bad, its brother isn't far from doing the same thing.

Since you have a deep cycle or two laying around, use them in place of the bad one.

A deep cycle battery will work in a cranking situation but just not have the high CCA if a starting battery.

They will both charge the same in parallel. You parallel all of your batteries every time you drive down the highway.

Old batteries don't suck down newer ones. Only bad and shorted batteries can pull current from good ones.

Here is some info on charging and combining batteries.

https://www.yandina.com/combInfo.htm#Q2
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Old 10-20-2020, 02:46 PM   #5
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Others will be wringing their hands, but you will be just fine.

I have been running 2 group 31's with 2 group 24's for 4 years now, doing a lot of dry camping.

I replaced the 24's with the 31's when I bought it, and had the 2 24's just sitting around. I measured a compartment next to the steps that the batteries were under. By relocating the water pump, I had room for the 2 24's. SO, in they went!

All 4 are wired parallel, and are working great.

As long as they all stay healthy, they will go up and down in voltage equally. I didn't upgrade the charger, so they can't get charged too fast, and they all float at 13.4 volts all Winter long.

Happy Glamping.
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Old 10-20-2020, 02:56 PM   #6
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Others will be wringing their hands, but you will be just fine.

I have been running 2 group 31's with 2 group 24's for 4 years now, doing a lot of dry camping.

I replaced the 24's with the 31's when I bought it, and had the 2 24's just sitting around. I measured a compartment next to the steps that the batteries were under. By relocating the water pump, I had room for the 2 24's. SO, in they went!

All 4 are wired parallel, and are working great.

As long as they all stay healthy, they will go up and down in voltage equally. I didn't upgrade the charger, so they can't get charged too fast, and they all float at 13.4 volts all Winter long.

Happy Glamping.
I ran one GP31 12 volt battery in parallel with with a pair of GC2s in series, for 4 years and they weren't even next to each other with same length cables.

Worked great.
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Old 10-20-2020, 07:55 PM   #7
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Thanks for all your input. In the near term I am going to place the deep cycle in parallel to my other battery and eventually will replace both!
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Old 10-21-2020, 12:00 PM   #8
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This will explain in detail why it is not a good practice: The 12volt Side of Life (Part 1)
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Old 10-21-2020, 12:39 PM   #9
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This will explain in detail why it is not a good practice: The 12volt Side of Life (Part 1)
Didn't see pairing used deep cycle and used starting batteies mentioned in the article.
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Old 10-21-2020, 04:00 PM   #10
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Quote:
Originally Posted by jabassmaster View Post
I know this is not an optimal solution, but has anyone put a deep cycle battery with a starting (chassis battery)
and had long term success. I replaced my 12v house batteries with 6v golf cart batteries and now have a couple of good deep cycle batteries. I have just discovered that my trip back from Utah (and the terrible roads entering and leaving Colorado) has produced a bad chassis battery (11v).
I'll assume the chassis battery bank is two 12 volt batteries in parallel. You said that one of the two starting batteries went bad, perhaps due to bad roads, but unknown for sure.

I would question if the second chassis battery is top notch shape if it's "twin" failed. As such there is no real problem in using two deep cycle batteries in parallel for the chassis. In this setup you may need about 20% more battery capacity than if using regular starting batteries.

At this point we do not know the real status of the second starting battery. That is where Twinboat and I have different opinions. I'm assuming that both batteries are not equal. If two batteries with different voltages are wired in parallel they will equalize at some intermediate voltage level. That is what I referred to about a bad battery drawing down a good battery. That is simple electronics. Internal resistance of each battery will likely be different and that does impact both discharging and recharging.

Whereas, the proposed mismatched setup you proposed is not likely to result in any big way, I would not personally do it. I'm sure many people have without issue and that's great for them. In this case I would use both matching deep cycle batteries and leave the starting battery out of play. LoL what ever you do.
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Old 10-21-2020, 06:43 PM   #11
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Quote:
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Didn't see pairing used deep cycle and used starting batteies mentioned in the article.
I think he was referring to the statement that using a starting battery in deep cycle service (like the OP proposed) would kill it in about 30 cycles.
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Old 10-21-2020, 08:10 PM   #12
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Quote:
Originally Posted by twinboat View Post
Didn't see pairing used deep cycle and used starting batteies mentioned in the article.
From the OP

"has anyone put a deep cycle battery with a starting (chassis battery)"

That sounded to me like pairing a good deep cycle with an old starting battery of unknown qualities.
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Old 10-21-2020, 08:24 PM   #13
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Originally Posted by gatorb8 View Post
I think he was referring to the statement that using a starting battery in deep cycle service (like the OP proposed) would kill it in about 30 cycles.
That's not what the OP is proposing. He wants to place a deep cycle in a start situation.
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Old 10-21-2020, 08:32 PM   #14
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Originally Posted by rarebear.nm View Post

At this point we do not know the real status of the second starting battery. That is where Twinboat and I have different opinions.

I'm assuming that both batteries are not equal. If two batteries with different voltages are wired in parallel they will equalize at some intermediate voltage level. ????????

That is what I referred to about a bad battery drawing down a good battery. That is simple electronics. Internal resistance of each battery will likely be different and that does impact both discharging and recharging.

U.
Batteries don't loose voltage as they age, they loose AH or CCA capacity.

My 3 year old battery has the same voltage as a new one 12.6 or 12.7 volts.

A battery that can not hold 12.6 volts has failed and should not be used in any situation. Testing for that is simple.
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