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Old 06-30-2014, 09:53 PM   #1
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Can I run a 2nd battery only when needed?

I've got dual batteries now, except the dealer installed different guage and length of wire to both...so I'm going to make it equal.

if I go on a shorter trip, can I run with only one battery, then bring along a second only when needed? I know that technically this will work, but I'm asking if it will get one or both batteries out of balance to use them apart in some situations and together in others.

Thanks!
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Old 07-01-2014, 07:42 AM   #2
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Originally Posted by steamboatscott View Post
I've got dual batteries now, except the dealer installed different guage and length of wire to both...so I'm going to make it equal.

if I go on a shorter trip, can I run with only one battery, then bring along a second only when needed? I know that technically this will work, but I'm asking if it will get one or both batteries out of balance to use them apart in some situations and together in others.

Thanks!
Are you talking about having two batteries for engine starting or for powering the lights and other electrical components in your RV?

I am thinking that if you have two batteries to power your RV, it is better to have them both hooked up all the time. I think that if you would use one most of the time then only use the second one on longer trips, the more used battery would pull down the stronger battery, making both not last as long. I think they would both last longer if you put both in and used them as a set.

Why would you want to leave one at home on short trips? To save weight?
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Old 07-01-2014, 08:16 AM   #3
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Yes it would be for powering lights/furnace fan/etc on my popup. I'm trying to save weight, leaving the 2nd battery behind for shorter trips and bringing more toys instead.

I would trickle charge the second battery when not in use, but it would still have less discharge cycles. I could even switch batteries each trip.
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Old 07-01-2014, 09:02 AM   #4
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Set it up according to this photo, then make one cable long enough that when the extra battery is out you can just attach it to the single battery.
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Old 07-01-2014, 12:36 PM   #5
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Thanks @SKnight. I'm assuming I should have a fuse between the positive lead of the two batteries (in addition to the poistive lead comign into the battery bank) to disconnect when not in use/not charging so they don't constantly charge/balance each other out?
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Old 07-01-2014, 02:03 PM   #6
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Another vote for both all the time.

Properly paired and connected, your batteries will work half as hard and last twice as long as a single one.

When connected properly they will charge and discharge equally and never be unbalanced. Only if you try what you suggested, will your batteries get out of balance and one of your batteries (closest to the charge/discharge connection) will fail well before the other one.
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Old 07-01-2014, 02:26 PM   #7
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Don't mean to hijack but how would you work it if you have one brand new battery and one used battery (one battery had to go with the old camper and now I have a new one). Was thinking about putting in a switch to use independently but how I had them before is similar to the drawing above.
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Old 07-01-2014, 02:31 PM   #8
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Don't mean to hijack but how would you work it if you have one brand new battery and one used battery (one battery had to go with the old camper and now I have a new one). Was thinking about putting in a switch to use independently but how I had them before is similar to the drawing above.
I would buy a new battery that matched the one in the new camper and connect it as shown.

The old battery would be dedicated to a back up sump pump in the basement or some other use.

If you can NOT do that; buy a 2 position battery switch and NEVER use the "BOTH" position. Run the positive from each battery to its own terminal and charge/discharge each one separately.
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Old 07-01-2014, 02:52 PM   #9
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OK. That's essentially what I was planning on doing. I even have a switch similar to the one you show. Thanks. Then again, a new battery is only $90 and the last setup worked well so maybe I'll splurge and sell the old battery.
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Old 07-01-2014, 03:04 PM   #10
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OK. That's essentially what I was planning on doing. I even have a switch similar to the one you show. Thanks. Then again, a new battery is only $90 and the last setup worked well so maybe I'll splurge and sell the old battery.
Toby, I would do it before I put too many charge/discharge cycles on the one you have. The longer you wait, the more "out of sync" the two batteries will be.
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Old 07-01-2014, 08:01 PM   #11
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Originally Posted by steamboatscott View Post
Thanks @SKnight. I'm assuming I should have a fuse between the positive lead of the two batteries (in addition to the poistive lead comign into the battery bank) to disconnect when not in use/not charging so they don't constantly charge/balance each other out?
Actually if the second battery is up there you want it to balance out and act as one big battery. You could add a fuse between the batteries but I've never seen it done.

The disconnect goes between the camper and the battery pack.
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Old 07-02-2014, 06:20 PM   #12
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Three final questions:

(1) what guage wire should I use? 10 guage is used from the tow vehicle all the way back to the camper's AC/DC converter, so I'm thinkingi that matching that will be sufficient.

(2) How do I get exact equal lengths of the leads coming to the battery without knowing/measuring the exact lengths of the positive/negative wires that go into the camper? I can get them pretty darn close to the point I tie into the main wiring, and maybe I'm overcomplicating this, but couldn't there be wire hidden inside between either the charging lead or a device drawing power that makes the negative longer than the posititve (or vice versa)

(3) are you saying that I should never disconnect the two batteries (like with a fuse) but let them constantly balance each other out / remain connected? And instead only disconnect the positive lead going into the first battery when not in use (ie. to keep CO detector from draining batteries)?
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Old 07-02-2014, 06:56 PM   #13
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1. If you're referring to the battery line that feeds the unit, 10G will be fine in general, it really depends on the length of your run. There's a chart below, 10G is good up to 10A at 28', so unless your converter/distribution center is on the rear bumper, you're fine.

2. While truly equal leads are best, the entire system doesn't need it. You want equal length leads between the batteries so that they stay balanced. The ground is going to tie into the frame fairly close to the batteries, the positive is going to run to wherever the distribution center is. After the batteries equal lengths aren't as critical.

3. Unless you're going to A; run off one battery, then the other or B; leave one battery at home to save weight, yes. Just tie them together and leave them. That way they're always ready to go, you won't have one that got low dragging on one that was in good shape.

You can disconnect either lead to relieve parasitic draw, most folks put the switch on the positive side but if it's easier to cut the negative side then you can place it there. Just place it between the cable actually going to the camper. One click, or remove one nut, and you're golden.

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Old 07-02-2014, 06:57 PM   #14
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I have a Dual Battery setup and I use them both. they are good for about 2 1/2 to 3 days on a full charge. the weight saving does not equal the confidence of the extra power.
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Old 07-02-2014, 07:05 PM   #15
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Here is how mine is wired. It has a battery monitor system installed, an inverter, and is wired to take a charge from a portable solar panel.
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Old 07-20-2014, 06:21 PM   #16
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Herk, I have been thinking about going with the two battery setup. I currently am switching from one to the other (not matched batteries). When I get matched batteries, I am wondering about the converter handling the charging with the dual battery load. I have a 80A converter, so am thinking it will be fine. Your thoughts?


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Old 07-20-2014, 08:21 PM   #17
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Herk, I have been thinking about going with the two battery setup. I currently am switching from one to the other (not matched batteries). When I get matched batteries, I am wondering about the converter handling the charging with the dual battery load. I have a 80A converter, so am thinking it will be fine. Your thoughts?


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My 55 amp converter handles my 150 AH bank just fine.
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Old 07-20-2014, 09:41 PM   #18
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Thanks
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