dual battery bank system

[hlrosenberger]

I'm looking to set up a system in my RV where one battery bank is connected to the loads, while a second battery bank is connected to the charger. (The charger is either the solar panels, 120 volt to 12 volt converter, or a DC to DC charger powered by my pickup truck and alternator).

My initial thoughts are that I can accomplish this by having two double-pole double-throw switches. One switch connects the charger to either battery, while the other switch connects the load to either battery. Also with this setup, in my mind I could have both batteries connected to the charger, with neither connected to a load, or both batteries connected to the load with neither connected to the charger.

Has anybody ever set up such a system and if so could you share any of your thoughts and ideas with me?

Thank you

 

[Gary RV_Wizard]

That sounds workable, but I'm curious what you expect to accomplish by doing that? What is the advantage vs the simplest design, i.e.all the batteries in parallel and all loads and charging sources connected to the one battery bank? The amps go in and the amps go out, pretty much the same.

 

[hlrosenberger]

First off, I view it analogous to having two propane tanks. You're only ever drawing off the one propane tank, while the other one is always full and in reserve. Then, the one tank goes empty and you switch over to the full one, at which point you have to fill up (so recharge so to speak), the empty propane tank.

Also I intend to have four 300 ampere hour batteries. I currently have two. Once they're discharged to some percentage, it takes a long time to charge them up. If I'm only charging two at once it cuts the charging time in half, and I have two fully charged batteries in reserve. Hope this makes sense.

 

[Gary RV_Wizard]

I view it analogous to having two propane tanks. You're only ever drawing off the one propane tank, while the other one is always full and in reserve. Then, the one tank goes empty and you switch over to the full one, at which point you have to fill up (so recharge so to speak), the empty propane tank.

Those propane tanks aren't continuously being refilled, whereas batteries are connected to a charge source. And if your charging sources exceed consumption, you will actually have more amp-hours stored with a single bank rather than splitting the same batteries into two banks.

Thanks for satisfying my curiosity.

 

[Kevin Means]

You may have a specific reason for wanting to split your battery bank's capacity that I'm unaware of, but barring that, I'd recommend keeping it simple. A single battery bank with a good battery monitor will help you know your battery's status at a glance, so you're never surprised by dead batteries. It will also reduce the additional costs and complexity of adding more wires, switches and the potential for bad connections. JMO

 

[hlrosenberger]

I understand about the cost and the complexity. I intend to travel across the country and do a lot of boondocking. So my goal is to maximize the amount of battery power I have, while keeping to a minimum the charging time. I intend to have four lithium 300 ampere hour batteries. I want to minimize the time that I would have to be at a campsite or somewhere to recharge it at a friend's house or whatever, and if I'm only charging two batteries at a time that's going to be half the time.

Maybe I'm thinking about this all wrong. Thanks

 

[Kirk]

Maybe I'm thinking about this all wrong.

Most of us believe that to be the case. It takes the same amount of time or electricity to charge all batteries whether you charge them all at once or half at a time. With your manual setup you will not get as much advantage from the charge time as you would with a single parallel system, unless you use some high-quality battery monitoring equipment and sit next to it all of the time to manually throw your switches. There are a lot of very experienced RV folks on these forums who know electrical systems, solar systems, and RV converter/chargers. What you have in mind will make the system more complicated and less effective.

 

[8Muddypaws]

Simply put, more trouble than it’s worth.

 

[Gary RV_Wizard]

You get the same amount of amp-hours either way. Two full batteries and two empty is the same as 4 that are each half full. What you need is a battery monitor that measures the amps in and out, so you you can always see what is available to you. No guesswork and no surprises.

 

[John From Detroit]

I do not know how it works with LI batteries but I now with Lead acid.
THe faster you drain them the faster ^2 the go dead

That is why Walmart shows you the capaciy of the battery at the 1 amp rate instead of the C/20 rate (20 hour

A say 70 amp hour battery at C-20 (3.5 amps) lasts 20 hours and 70 amp aours

But at the 1 amp rate may well last over 100 hours (100+ amp hours)

I do not know if this applies to LiFePO4 but assume it does, I also do not know the relative effect is.
That is how much faster a higher discharge rate affects it.

But all the batteries in parallel means a slower discharge rate (1/2)

The flip side: With two banks when Bank A goes dead. there is bank B

 

[Mark_K5LXP]

The only reason to have isolated banks is if there's a gotta have backup requirement like an oxygen machine or critical system. Having 2 half-size banks only offers complexity unless you like managing things manually. There is no Ah delivery or charge efficiency to be gained, and with Li, SoC is largely irrelevant. Take the money you would spend on contactors, cables and controls and buy a bigger battery or solar panel. That will net an actual increase in power.

Mark B.
Albuquerque, NM

 

[Route 66 Rambler]

That sounds workable, but I'm curious what you expect to accomplish by doing that? What is the advantage vs the simplest design, i.e.all the batteries in parallel and all loads and charging sources connected to the one battery bank? The amps go in and the amps go out, pretty much the same.

A major reason to do this is to always have coach power available. One bank of batteries can be used for running coach power like microwave, TV, AC, lights, fridge, CPAP, etc. for boondocking. The other set (200 AH for example) can be used specifically for isolated coach systems-leveling jacks, slides, emergency brake, etc. Keeping these two systems isolated allows for the situation of using AC overnight (for example) or similar that may run those batteries to zero. The isolated system batteries would always be available to allow for raising jacks, bringing in slides and hitching to get on the road. It's actually a very smart way to go.