Adding a 3rd battery bank to the Fleetwood Bounder

[Michael Oro]

So I have 4 truck batteries in decent condition laying around. The make and model is not apparent, but judging by some numbers these are 31P Group Size, 950 CCA, 1140 CA, 175 Minute Reserve Capacity, Heavy Duty batteries.

I want to add them to my RV as the 3rd power bank with its own inverter for additional redundancy and independence. One usage scenario would be to charge them during the day, when the generator is running anyway for the AC and use the inverter to charge the electric bikes during the night without the risk of draining the coach batteries (you guessed it, I take this RV to the Burning Man).

I found the perfect place for this new battery array in the storage compartment to the left of the entry. Since the 2 deep cycle coach batteries live under the entry steps, everything is in close proximity.

What I am going to do is I will connect the 4 batteries in parallel, using the "equal amperage draw" method where any battery is connected to the output with exactly 3 segments of 2/0 AWG cables. Then I will use the Blue Sea Systems 9001e battery disconnect switch with 4 positions, which will allow me to have the 12 volt coach wiring to be connected to the original coach battery, the new battery array or both.

The new inverter will be connected directly to the new battery bank, so that when the switch is in the 1st position where coach battery is connected to the RV alone, the new bank with the new inverter is a completely independent circuit.

When the switch is in the 2nd position, the coach battery will be disconnected completely (saving charge) and the RV will be connected to the new array along with the new inverter. This means that the generator, alternator and shore power converter will charge the new array instead of the coach battery. I want to believe that it will charge it properly despite the fact that these are not deep cycle batteries. I also want to think that the inverter will not be harmed by the charging current on its inputs. The same charging circuits that protect the batteries from voltage spikes on the generator should also keep the inverter healthy if I accidentally leave it turned on.

Now the position 1+2 is most interesting since now I have the new array of 4 acid-lead batteries joined in parallel with the deep cycle lithium coach batteries. On the surface there shouldn't be any harm done, since the Aux Start interconnect switch and the Isolation Relay in BCC does exactly that - connects the coach battery in parallel with the main chassis battery, which is also acid-led. I also believe they are effectively charged by the same circuit and the isolation relay is simply disconnecting one from another at certain voltage point. But since I don't have any BCC logic or relays here, It would be interesting to learn how exactly will this work in terms of charging.

If there is some inherent issue with charging those batteries in parallel, I could charge them separately by switching to position 1 and then position 2 once the first battery is full.

Any advice or words of wisdom will be appreciated.

And one more question - if I want to get really adventurous and connect the shore power connector to the 3500 watt inverter linked to the new battery bank, it will effectively run through the converter and attempt to charge the battery from the other end. Will it be some kind of a "short circuit" in this case? Or it will just slowly drain itself according to the non-100% efficiency of all the components in the loop?

I also have an option of plugging the output of the inverter into one of the power outlets in the storage compartment. This way I don't need to drag the shore cable all the way across the RV. Common sense tells me that the same 120V will be effectively on all other outlets with the small caveat of going through the storage compartment outlet breaker first. But maybe I am missing something here?

[US1]

You are considering back feeding your AC circuits if I’m not mistaken, with a possibility of a live SP end if you forget to throw the main.
A transfer switch can be had for under $75 to avoid all the possible dangers.

If I understand correctly.

[Michael Oro]

What would the transfer switch do in this case? The idea is to connect the 120v output of the inverter to the nearest power outlet. I guess that's what you mean by "backfeeding" and the danger here is that the 120v shore power plug will effectively have 120 volts on it. Definitely not a good idea, considering that it is coiled in the same compartment with the outdoor shower and sewer valves. I did not consider that. Thank you.

I guess it would not be too difficult to throw an extension cord under the chassis if I ever need to do it. Definitely safer.

[SteveJ.]

Assumption being that the batteries are starting batteries I would think that they would have a fairly short lifespan.

[moisheh]

You are turning something simple into something complicated
There are many ways of isolating coach batteries from starting batteries
Some inverters can charge batteries but only if you have an inverter / charger.

In order to do any charging the inverter/charger needs to be connected to shore power
Installing used batteries that are designed to be starting batteries is a bad idea

[153stars]

I would only tie into the the battery side charge and discharge with house batteries.
First I would just charge each battery at say 15 amps untill fully charged to make sure no cells heating up. Let rest and do battery test to make one is not real tired. Better to have 3 good ones than a dud in the mix. Prop the door open while charging or just set up under or off to side of coach.
You might want to test at home if needed at all or worth it to save an hour on genny.
I have two 8D starting batts with the Bluesea MTL-ACR, I can and will steal an hour or two of of light use or boost amps/cut voltage drop to run my microwave in the morning. They were and will be again at max charge during by next charge cycle.
I also chose a pour over coffee maker with thermos carafe instead of elect. drip . Instead of the big hit on overnighted batteries just to make coffee.I love it. Actually did same thing with old Bunn filter basket for 20 plus yrs lol.

[Iceclimber]

You can do this, boats do it all the time but combining batteries of different types can shorten their lifespan. If they are spare batteries doing nothing guess you have not much to lose except time..and as mentioned start batteries do not make good house batteries.

[Mark_K5LXP]

There are enough flaws in the logic of this entire project that I would encourage stepping back and evaluating just what the end goal is. I see no useful outcome and some outright hazards from the project as stated.

Mark B.
Albuquerque, NM

[Iceclimber]

Originally Posted by Iceclimber

You can do this, boats do it all the time but combining batteries of different types can shorten their lifespan. If they are spare batteries doing nothing guess you have not much to lose except time..and as mentioned start batteries do not make good house batteries.

Actually I just re-read and I retract. You should never mix lead acid batteries and lithium the way you are describing.

[Fiesta48]

What about all the weight added? Batteries are 50-60# each. Plus cables and supporting system.