Two banks of two batteries - different size - no-no?

Moved to the Tech Issues Forum

Wayne
Moderator

I know folks who have all sorts of weird battery combinations and they all claim it works fine.

Hi,

I have my banks manually switched. I used perko switches from Napa that cost $7 each. It allows me to charge one bank while using the other.

If you want longevity, consider a modest solar system.

On my fifth wheel, I had 2 6-volts in series with 12-Volts in parralel. Never had a problem. Was it optimal? Who knows, but it worked very well. in my experience, wire size is not as important as many believe. there is an optimum size. As you go bigger, you enter the realm of diminishing returns.

One idea I had been toying with for maintaining two different size battery banks some distance apart is a dual inverter/charger scheme.

The main bank is hooked to your 12V and 120V system and has a combination inverter charger. The output from this inverter also feeds the input to a remote inverter charger on an auxiliary bank. The output from the inverter on the auxiliary bank feeds the input to the inverter/charger on the main bank (with some sort of transfer switch to disconnect it when shore power is available).

Somewhat complicated, but it ensures both battery banks are charged correctly for longest life. More expensive since you need an extra inverter/charger (but you save some on wiring if there is a long distance between banks).

Efficiency? Well, if each inverter/charger runs at 90% efficiency then the auxiliary bank would charge/supply at 81% efficiency. The main bank would supply 12V at 100% efficiency and 120V at 90% efficiency and charge it's own batteries at 90% efficiency.

If the auxiliary bank were supplying 12V at 100 amps through a round trip 40 feet of 2/0 wire (0.003 ohms total) then the power loss in the wire would be 100^2 * 0.003 = 30 watts.

If the auxiliary bank were instead supplying 120V at 11 amps through a round trip 40 feet of 12 gauge wire (0.064 ohms total) then the power loss in the wire would be 11^2 * 0.064 = 7.75 watts.

Of course the auxiliary bank has to boost the 12V to 120 then convert it back to 12V at the main bank, so the power transferred would be 120V x 11 amps x 0.81 - 7.75 = 1060 watts

At this point I realized it really wasn't worth the hassle, but maybe if you had a 80 foot long rig with 160 foot round trip runs...:D

Use option 1 and everything will just work OK. Your battery life will be shorter but you'll have more power and not be concerned about charging and switching. Fuse protection above is a must IMHO.

No technical reason against different capacity batteries in parallel.

I have a document from Exide that gives acceptable chemistries that will work OK in parallel. eg AGM in parallel with standard flooded cell lead acid work well enough, but GEL in parallel with just about anything else is a no-no.

Other factors mentioned, like large differences in age age and slightly different chemistries just mean that the result will not be optimum rather than being a complete disaster - but unless you have bottomless wallet, all new batteries, same age and same brand, will not be the optimum solution for you and like most of the rest of us, you will likely make the best of what you have.

Identical length leads are another "nice to have" feature that is somehow evolved into a "must have" feature and unless you are charging and discharging at high rates, isn't really a practical consideration.

If you plan to charge them off the same charger or to use them together at the same time, they also will need to connect to the charger/house system with the same length/same size cable. If the cable to the new batteries is a different length, the batteries on the longer cable will never charge fully, and will discharge to a different depth than the originals, which will cause problems down the road. From the sounds of it, you will have to do the switchover thing.

They should be the same chemistry (like flooded or AGM or GEL). Preferably they should be the same brand (because even flooded chemistries will differ a little between brands). They should be the same age (because charge/discharge characteristics will change with age). Of those the first one is the only one I would really worry about.

Switching banks used to be the normal, however since the life of the battery bank is non-linear with depth of discharge (deeper discharge = shorter life) the consensus now is to parallel them into one large bank for longest life.

I would consider putting fuses at each end of the connecting cable. Either end can source fire starting currents if the other end gets loose....