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Old 06-19-2021, 12:00 AM   #1
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Good/Bad Ideas on Paralleling another 6 Volt Pair to my Current Pair

Hi All,

I currently have two 6V Motive/Trojan T-105s, obviously in series. I was thinking about 2 possible ideas of bumping up my on-battery time.

- Get another pair of T-105s, connect them in series and then parallel them to the original pair to get 2X the time on battery

- Get a deep cycle 12 volt battery with similar capacity and parallel it with the T-105s

Concerns:
- Will the inverter / charger be ok with that set up?
- Is it a no-go if I can't find exactly another pair of T-105s?
- Is it a no-go to do the 12volt battery in parallel?

I know ohms law-wise this works but don't know if the RV inverter/charger system would work in that modified setup.

This more of "what if" / "can I" brain droppings than an actual need (as of now).

Thanks!
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Old 06-19-2021, 05:14 AM   #2
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my late 90,s RV original design had one 12v under step with option for another..
I put in 2 new group 29, 100AH each.. no issues ...
I added one more 29/31 105AH in another compartment with some large cables ...

My only issue came from my 45 Amp chg, convert.. I added smart chg module upgrade to it and it is great.. no more boiling out the water,,,
BUT the load of 3 batteries when I RUN them way down and charger kicks in would trip the OEM breaker for 12V feed that was 30A.. the wiring is #8 guage.. I found a 40A marine breakers and wired in place of the 30A factory.

I holds fine and my wires do not get warm or any issues..

So adding more AH to your system is fine just be mindful of the charger and load to bump them back up.. Draw from batteries should not be higher as you want to add more time I assume..
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Old 06-19-2021, 06:41 AM   #3
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Whatever battery you parallel to your existing set will "work". Ideally you'd have all identical batteries but we don't always get ideal. The further you diverge from ideal, the less optimum performance you will get. How much "less optimum" matters depends on your application and expectation. If you only periodically cycle your batteries, say a few dozen times a year, you will likely age the batteries out before the less optimum part kicks in. If you cycle your batteries hard, like deep discharges every few days for months on end, the less optimum will probably manifest itself in reduced capacity or cycle life below their spec'd rating. Generally speaking the most cost effective solution is to get the setup as close to "optimum" as you can and run it out. You own the batteries you have so you might as well get what Ah you can out of them. When the mismatched set you've created either ages out or gives up, then you can justify replacing them all at one time and get the "optimum".

Mark B.
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Old 06-20-2021, 06:13 AM   #4
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If I had the room under steps I would have 2 6v series and then 2 more in compartment.. that would have me at 400AH+ and in a more balanced set up, closer to that optimum.
Hence I would NOT be discharging / re charging daily.. Mine are already aging out in 3rd year of occasional use.. LOL

To the OP, you be fine..
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Old 06-20-2021, 02:01 PM   #5
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sibe & Mark,

Thanks for the input! Still mulling it over. I have not even taken the rv out on a trip yet, but I'm a "plan for just-in-case" kinda guy, hate the thought of needing to run off batteries and then finding I've got a very short window to do something. Then again, can't decide if spending another $400+ on 2 batteries, plus the connectors and cables plus a battery box to hold them both - is worth it. I'd likely have over $500 into it by the time I was done.

Thanks again.
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Old 06-21-2021, 07:28 AM   #6
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You don't know what you don't know, so unless you have some specific use data to work with you're trying to solve an unknown problem. Instead of 2 more batteries, buy a battery monitor and watch your usage for a few trips. You'll know after a handful of times how far your usage dips into the batteries and you'll get a feel for the timing of discharging and charging. If at that point you know you need more capacity, you're still close enough to "new" that adding batteries won't result in much if any mismatch due to age or cycles. You may find you have plenty capacity to spare and can save yourself the trouble and cost of expanding a system you don't need to.


Mark B.
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Old 06-22-2021, 09:26 AM   #7
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Mark,

Sounds like good advice. I think I'll go that route and see if I really do "need" to increase the capacity. That would definitely be the easier and cheaper way to go, that's for sure.

Thanks.
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