I know the answer but gotta ask...

[chriscowles]

I have a Deka DP24 (dual-purpose marine battery) that's in good shape and less than a year old. It's my only battery. I'm prepping for dry camping and need more.

I can get a DC27 for $95 plus core. I need 2. Counting a new (cheap) box, core charge and tax, that's $255 for a pair.

I'm already spending hundreds of dollars in prep for my trip. If I can use the DP24 in parallel with 1 DC27, I'll save some much-needed dough.

I've read recommendations that batteries should be the same brand, type, size and age, for maximum efficiency, and don't dispute that. But, if I make this trip with 1 DC27 in parallel with 1 DP24, will that damage either battery?

No stone-throwing, please.

[ependydad]

Before I paralleled the 2 of them, I'd add a battery switch that lets you pick one and then the other and manage my power usage that way.

[chriscowles]

Hmm... That might work. I hadn't thought of that approach. Good ideas like that is exactly why I posed the question.

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[OldCoot]

I'll get a lot of flack about this, but I've mis-matched batteries on campers for years with no ill effect. Just had matching batteries on this 2012 because I insisted the dealer put them in because our trade in had 2 batteries.(unmatched for 4 yrs) with no switch to select on or the other or both either.

[chriscowles]

Originally Posted by OldCoot

I'll get a lot of flack about this ...

Better you than me, but that's why I asked. [emoji4]



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[chriscowles]

These details may require that I buy 2, anyway:

The DP24 is rated at 65AH at the 20-hour rate. The DC27 is rated at 90.

My wife and I both use CPAP. Combined, they draw 8 amps. That's 64AH overnight.
If I used a switch and ran only the DP24, I'd kill it. If I did the same with the DC27, it would use 72% of capacity.

If I wish to avoid exceeding 50% depth of discharge, the only way to do that would be using them in parallel. In that configuration, would the discharge at the same % rate? Might that mean that the DC27 is discharging into the DP24? Even if so, I'm okay with a little inefficiency if it doesn't result in long-term damage.

If it matters if they charge separately, I could use the switch to isolate them while charging and put them in parallel overnight? Would charging separately require running the generator longer?

[OldCoot]

Just me being me, I would hook them together in parallel without a switch and go on about my business. It might not be the ideal situation, but it's been happening on diesel trucks for years and years before rv's used dual batteries. It might shorten the life on one battery a little but I have seen folks do it all the way back in the 50's and 60's with 6v batteries on tractors, big trucks, etc. I think you are worrying about nothing. JMHO.
I also wouldn't worry about charging them separately either.
Let the flack start!

[Flybob]

The biggest concern with using different ( age/size) batteries is with charging as they will charge differently and eventually one will fail early. If you have the switch shown, you can use the both position when drawing power from the battery and then select only one to charge them separately. Not the ideal situation, but unless you plan to do this forever it should be OK.

[hammick]

For the same money or less you can get a couple Interstate GC2 golf cart batteries at Costco. These are true deep cycle batteries and will give you much more capacity than two marine batteries in parallel. I paid $83 each. Here is my cost breakdown:

$83 (I had one core to give)
$83 ($15 core charge)
$6.99 for series cable from Napa auto parts
$12.50 x2 for GC2 battery boxes

[chriscowles]

I don't have a Costco anywhere near me. Also, if they're 6v, that won't work for me. The batteries will serve a dual purpose in which they'll be used separately, when not camping.

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[OldCoot]

Originally Posted by chriscowles

I don't have a Costco anywhere near me. Also, if they're 6v, that won't work for me. The batteries will serve a dual purpose in which they'll be used separately, when not camping.

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Go for it Chris, use what you have and quit worrying. It's your money!

[chriscowles]

Thanks, OC. I think I'll buy one for this trip, then replace the 24DP with a 27DC in the not distant future. That way the difference between the old 27 and the new 27, if any, will be minimal.

Now, if only I could sell somebody a slightly-used 24DP...

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[OldCoot]

Originally Posted by chriscowles

Thanks, OC. I think I'll buy one for this trip, then replace the 24DP with a 27DC in the not distant future. That way the difference between the old 27 and the new 27, if any, will be minimal.

Now, if only I could sell somebody a slightly-used 24DP...

Keep it on a trickle charger and use it for whatever. You might need to jump start your DW's vehicle or your own this next winter.

[chriscowles]

Originally Posted by Flybob

If you have the switch shown, you can use the both position when drawing power from the battery and then select only one to charge them separately.

The initial suggestion of the switch brought that to mind. If charging the batteries separately, does that double the time I have to run the generator? Or is the total amount of amperage and time the same, just split between the two, or perhaps a little longer than if a matched pair were charged together?




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[RPAspey]

If you parallel them temporarily for your trips and then separate and charge them independently between trips. You won't have a problem and they'll probably last a long time. The big thing to watch when parallel charging odd batteries is that one will undercharge and the other will overcharge. So, watch the battery fluid when you're charging them in parallel. Just carry a bottle of distilled water to top them off if needed. Otherwise save some cash and enjoy your trip.

[chriscowles]

I'll use the switch and charge them separately. We'll have distilled water on hand.

Can I assume the smaller battery would be the one that's subject to overcharge? If correct, can I charge them together until the smaller one is topped off, then switch to continue charging only the larger? I would test their charge voltage independently.

[RPAspey]

One way to know for sure which is overcharging is to charge them together and monitor which one starts to off-gas (bubbles a lot more than the other) that's the one overcharging.

[OldCoot]

Originally Posted by chriscowles

I'll use the switch and charge them separately. We'll have distilled water on hand.

Can I assume the smaller battery would be the one that's subject to overcharge? If correct, can I charge them together until the smaller one is topped off, then switch to continue charging only the larger? I would test their charge voltage independently.

If they're sealed batteries, have fun getting the tops off to add water!

[chriscowles]

Originally Posted by OldCoot

If they're sealed batteries, have fun getting the tops off to add water!

I'm too cheap for sealed batteries.

[chriscowles]

Originally Posted by ependydad

Before I paralleled the 2 of them, I'd add a battery switch that lets you pick one and then the other and manage my power usage that way.

So this seems to be the direction I'm taking. Can you suggest how to mount/wire the switch? I know you attach the positive terminals to the switch and ground the batteries independently of the switch. But, wanting to operate the batteries separately when not on the trailer, and there not being space on either battery box for the switch, where do I mount the switch? I have to build my own battery rack so I can make it wide enough to house a 3rd small battery box to house wiring connections, if useful.

Also, since they're both 12v they should be wired in parallel. I understand that the parallel connection wires should be the same length, both negative and positive. What I think I'll need to do in terms of wiring is to have 2 positive cables and 2 negative cables. The positive cables should go from the switch to the respective batteries. The negative cables will be used to split the main negative cable into two, each going to the negative post on a battery.

I've seen wiring diagrams showing the trailer positive going to one battery and the trailer negative going to another battery, with equal length cables bridging between positives and negatives on the two batteries. What's the difference between that, and what I described?