Help: Connecting a 2nd Battery

[JebTN]

I don't see a problem but wanted other opinions. I roughed out what I think I have currently. A previous owner started to install a 2nd battery and stopped. He was going for an under seat install and I plan to hang it underneath like the primary coach battery, if possible. The sketch shows what I have, the orange box with question marks is all wrapped in plastic tape with one wire in and out and I assume it is a fuse but I haven't unwrapped it yet, if it is a fuse it is a long ways from the isolator switch, I will plan to add one at the isolator.

The question is how to connect the 2nd battery into the current coach charger and electrical. I was thinking of adding a A, B, A+B battery switch. If I was in the A+B position I would have two paths from the alternator while going down the road. I don't see a problem with this, I figure one path or the other would carry a little more current because of resistance in the wiring but shouldn't be enough to overload anything. When I install the second battery I plan to install two new AGM batteries so they can be run in parallel without a load imbalance.

Am I missing something or is there a better way?

[booster]

Originally Posted by JebTN

I don't see a problem but wanted other opinions. I roughed out what I think I have currently. A previous owner started to install a 2nd battery and stopped. He was going for an under seat install and I plan to hang it underneath like the primary coach battery, if possible. The sketch shows what I have, the orange box with question marks is all wrapped in plastic tape with one wire in and out and I assume it is a fuse but I haven't unwrapped it yet, if it is a fuse it is a long ways from the isolator switch, I will plan to add one at the isolator.

The question is how to connect the 2nd battery into the current coach charger and electrical. I was thinking of adding a A, B, A+B battery switch. If I was in the A+B position I would have two paths from the alternator while going down the road. I don't see a problem with this, I figure one path or the other would carry a little more current because of resistance in the wiring but shouldn't be enough to overload anything. When I install the second battery I plan to install two new AGM batteries so they can be run in parallel without a load imbalance.

Am I missing something or is there a better way?

What you are looking for, two 12v battery system, is pretty common in a lot of vans so would likely work fine. I don't think it needs to be as complicated as you have it drawn, though, unless you need to have the battery separation for some reason.



The "normal", if anything in the RV world could be considered normal , would be to come off the starting battery and alternator, though a separator/charger relay/manual control charge relay to the parallel connected 12v batteries that connect to the shore charger. You might need to increase the size or run a parallel second cable from the alternator to get capacity for what two AGMs will accept depending on what you already have in place. Cable from alternator and shore charger to the plus on one battery, ground to chassis from the the other battery to keep balance, and then parallel the batteries. If the batteries are far apart, you could chassis ground them separately and use two equal length plus cables to them from a binding post with the alternator and shore connection on it, in a central location.


Good luck with the project, it can be "interesting" to go into one that has been started and abandoned by someone else.

[JebTN]

There is not room to put a 2nd batteries where the current one hangs underneath the van. The existing coach battery is on the driver side about half way back and the cabling that has already been installed would put the second battery in roughly the same position on the passenger side. Since what should be heavy enough cables for a single battery is already installed on each side I would just as soon use it if possible. If I started from scratch I would do several things different.

[booster]

Originally Posted by JebTN

There is not room to put a 2nd batteries where the current one hangs underneath the van. The existing coach battery is on the driver side about half way back and the cabling that has already been installed would put the second battery in roughly the same position on the passenger side. Since what should be heavy enough cables for a single battery is already installed on each side I would just as soon use it if possible. If I started from scratch I would do several things different.

If the two cables coming to the rear are of the same size and length, that would be no problem at at, but if they vary in length much you will start to knock the alternator charging out of balance. How much would be determined by the length difference. Bringing them from a single relay or separator point is good idea, though, again to help with charging balance and simplicity. I think I would still connect the shore charger, and the DC discharging loads, to a central binding post to keep that charging balanced and the discharge from each battery the same.

[markopolo]

One thing leads to another when you start getting into this ...........

You might end up wanting to put a new converter/charger if you end up putting in AGM batteries. Some/most of the older chargers were single stage with output around 13.8V. That's too low to fully charge AGMs and too high as a float voltage. The batteries likely won't get fully charged via the alternator either particularly with an isolator in the mix as there is an approx 0.5V drop due to the isolator.

I saw 8AWG on your diagram and that was typical back then. 30A is typically max current for that gauge wire. If you upgrade the charger to 50A output for example then you'll need to upgrade the wiring ..........

It seems never ending but it is fun.

[JebTN]

I took my B into an RV shop today and told him what I wanted to do and he didn't like having the batteries so far apart either. He thinks he can replace the existing battery box (battery slides in the end) to one that drops down that will hold two batteries. He will upgrade cable size if needed for the extra current. He said what I saved in labor would be more than I saved trying to reuse what I had. I may still up grade the converter/charger.

Thanks for the advise.

[VocalVirgo]

Would upgrading the wiring to 2 gauge solve the problem of having the batteries far apart from one another? I myself have a bank of 7 batteries- 4 on the passenger side rear, under the bed and 3 on the driver’s side directly across from one another. They are as far apart as the van is wide. We are using 7’ of 6 gauge wire to connect the 3 on one side to the other 4 on the other side of the van. The batteries aren’t charging quite right, and we think the smaller gauge wiring and the distance between the 3 & the 4 batteries may be the problem. We are going to swap out the 6 gauge to 2 gauge tomorrow. I will keep you posted. If that solves my issue, then it may be the answer to yours too.

[booster]

Originally Posted by VocalVirgo

Would upgrading the wiring to 2 gauge solve the problem of having the batteries far apart from one another? I myself have a bank of 7 batteries- 4 on the passenger side rear, under the bed and 3 on the driver’s side directly across from one another. They are as far apart as the van is wide. We are using 7’ of 6 gauge wire to connect the 3 on one side to the other 4 on the other side of the van. The batteries aren’t charging quite right, and we think the smaller gauge wiring and the distance between the 3 & the 4 batteries may be the problem. We are going to swap out the 6 gauge to 2 gauge tomorrow. I will keep you posted. If that solves my issue, then it may be the answer to yours too.

It would be interesting to draw the wiring with wire sizes and lengths to be able to check the resistance to each of the batteries for charging and discharging. If these are normal lead acid batteries you likely have something like 600ah of 12v batteries? If so, you are into 4/0 cable territory if you are using a big inverter or charge sources. #2 is in the 100 amp range for heat, but for voltage drop can still be an issue in balancing situations at that 100 amps, IIRC.

[avanti]

Note, however, that the cross-connect cables only carry half the current of the whole battery.

That said, #2 does seem too small. You need to do the math.