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DawgLady View Drop Down
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Posts: 94 		Direct Link To This Post Topic: Dual batteries
    Posted: 31 Mar 2019 at 10:17am
When installing dual batteries is it best to have the two connected to each other or have one connected and use the second one as a backup?

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john in idaho View Drop Down
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Posts: 611 		Direct Link To This Post Posted: 31 Mar 2019 at 10:27am
If you are talking 6v batteries you will need them hooked up in series.  12 volt doesn't really matter that much but if you do hook two up make sure it is a parallel hookup.
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Posts: 99 		Direct Link To This Post Posted: 31 Mar 2019 at 2:32pm
https://www.batterystuff.com/kb/articles/battery-articles/battery-bank-tutorial.html
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Posts: 5290 		Direct Link To This Post Posted: 31 Mar 2019 at 8:41pm
If you're getting two new batteries then its best practive to get 2 6V batteries and connect them in series. That way both batteries will always be getting or providing the same current so they will stay balanced. 

If you already have a new-ish 12V battery then the best bet is to buy another identical one and connect them in parallel (+ to + and - to -) and then take the + output to the trailer from one battery and the - output to the trailer from the other battery. That will balance the current flows in and out of the two batteries as much as possible. 

But if the batteries are different in capacity or significantly different in age (say more than about a year or so) then its probably best not to connect them in parallel, and instead use them one at a time. They just won't ever get balanced.  Be sure to rotate between them though fairly often (say once every few weeks), otherwise the one you're not using won't get charged properly. 
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GlueGuy View Drop Down
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Posts: 2654 		Direct Link To This Post Posted: 01 Apr 2019 at 9:23am
Originally posted by offgrid

If you're getting two new batteries then its best practice to get 2 6V batteries and connect them in series. That way both batteries will always be getting or providing the same current so they will stay balanced.
Be advised that connecting batteries in series is does not guarantee that they will stay balanced. Just that the load they carry is more likely to be balanced. In our business, we have numerous remote communications sites that typically run at 48 volts. Depending on the power requirements, we use 4 12V batteries, or 8 6V batteries or 24 2V batteries. Sometimes, we have used series/parallel combinations due to space and/or power requirements. One of the worst problems we have seen is when one (or more) cells goes bad in one of the series-connected batteries. When this happens, ALL of the other cells in all of the other batteries will get overcharged, and you end up pretty much "toasting" all of your batteries. We call that a bad day in Black Rock.
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Posts: 5290 		Direct Link To This Post Posted: 01 Apr 2019 at 10:15am
Sure, I agree, cells do go bad in series strings, and for sure you can't balance a dead cell. End of life failures are inevitable and will always happen in one cell first, and cell shorting is probably the most likely failure event in any PbA battery. But you're still better off having all the cells in series than having a parallel configuration where one string of cells can be cycling a lot more current than another from day one. 

I've done lots of large battery banks too and at many voltages as high as 240Vdc.  You have to have strings of cells in parallel at some point in a large bank. At that point careful design of counductor lengths and connection points to keep the banks at the same voltage and capacity monitoring of each string can help identify problems. In the rpod world its not a problem, we don't ever have banks big enough to require paralleling batteries. 
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Happy Tripping View Drop Down
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Posts: 473 		Direct Link To This Post Posted: 01 Apr 2019 at 11:36am
Newbie question - Since we're talking dual batteries here, what is the 'down-side' to using one deep cycle battery until it gets too low for optimal use and then switching it out for a new, fresh, battery?

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Posts: 5290 		Direct Link To This Post Posted: 01 Apr 2019 at 11:47am
if say you started with two 12V batteries fully charged, then discharged one to 50% state of charge, then recharged it while you discharged the second battery to 50% SOC, and then repeated that cycle, there wouldn't be much difference at the fairly low discharge rates we have in our trailers. 

The downside would be that it might be hard in practice to get the first battery recharged right away which would mean that you'd be leaving it at a lower state of charge for longer than if you just connected the two batteries in parallel. Lead acid batteries don't like being left at low SOC for long periods.  Then there's the hassle factor in connecting and disconnecting the two batteries to the loads and to whatever you're charging them with. 

But you could certainly do it the way you're suggesting as long as you kept track of it and got the batteries recharged right away, some folks on the forum do it that way successfully I think. 
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GlueGuy View Drop Down
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Posts: 2654 		Direct Link To This Post Posted: 01 Apr 2019 at 7:45pm
One option might be to put a separate disconnect on each of two 12V batteries. You could then "swap" batteries by turning one off and the other on. This would still run the risk of having them both connected when they're badly balanced. Probably not a wise plan.
bp
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offgrid View Drop Down
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Posts: 5290 		Direct Link To This Post Posted: 01 Apr 2019 at 10:54pm
There are nice RV/marine battery 1, battery2, both, and off high current rotary switches. I had one my old class a and used it to isolate the engine start battery (1) from the house batteries (2). One of those and some heavy gauge cables would work well as long as there was a way to get the disconnected battery recharged. The problem would be if you forgot and left one battery discharged for a long time.
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1995 RV6A Experimental Aircraft
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