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

[MaestroPNW]

Adding more capacity to the camper.
In-camper I've got to size 24's. Thinking of putting two 110AH in front of the wheel well in the truck bed, run the cable through an Anderson connector.

Searched this board and the rest of the web high and low, and see a lot of comments on not putting different capacity batteries in parallel, but also see a lot of write-ups on people doing it with seemingly no issues.

My options, so far, are:
1) Just run everything in parallel
2) Manual battery switch to switch from one bank to another.
3) anything else?

Thanks!

[HMS_Beagle]

MaestroPNW wrote:

HMS Beagle wrote:

The correct way to wire them is to run cables between the terminals of the two banks, then take the positive power bus cable to one bank, and the negative cable to the other. Since the current from either (and both) banks goes through these wires, any voltage drop due to them is seen by both battery banks and they will charge and discharge at the same rate (or at least, at rates not influenced by the wiring).

This makes sense, but likely not going to happen in my setup - the second bank will be plugged in through an Anderson connector and be on the outside of the camper. Modifying the wiring in the way that it would hook up to the "remote" bank and be able to unplug it is probably possible but complicated.

Still easy enough: I would run the power bus ground to the in-camper battery. An Anderson connector runs + and - to the in-bed battery bank. A second Anderson connects the returning + to the power bus. When the in-bed bank is not used, both are disconnected, the second Anderson is plugged into the first, connecting bus + to in-bed +.

[beemerphile1]

In a perfect world, all of the batteries in a bank should be the same size, same brand, built om the same line, by the same person, on the same day.

BUT

If you already have them. run what you brung, and all will be fine. Battery life may be shortened a little.

[time2roll]

With separate battery banks the balance can get wonky even if the sizes match but basically they will work fine. Add 100 or 200 watts of solar and you are golden. I assume you already have LED lighting. I would pass on the switch but it is fine to have if you prefer or find a need.

[MEXICOWANDERER]

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."


Can you pretty-please send me a link to the document? Thank you.

[MaestroPNW]

HMS Beagle wrote:

The correct way to wire them is to run cables between the terminals of the two banks, then take the positive power bus cable to one bank, and the negative cable to the other. Since the current from either (and both) banks goes through these wires, any voltage drop due to them is seen by both battery banks and they will charge and discharge at the same rate (or at least, at rates not influenced by the wiring).

This makes sense, but likely not going to happen in my setup - the second bank will be plugged in through an Anderson connector and be on the outside of the camper. Modifying the wiring in the way that it would hook up to the "remote" bank and be able to unplug it is probably possible but complicated.

[HMS_Beagle]

Two points have been mentioned here that need more discussion: life of the batteries connected in one parallel bank vs. two switched banks; and the length of the cables connected them leading to uneven charge.

Suppose you have two battery banks that could be connected either in parallel, or switched and used on alternated days (a typical scheme). They are 100 AH each, and each evening you use 50AH in your camper, recharging fully the next day. If they are connected in parallel, acting as a single 200 AH bank, the Depth of Discharge is 50/200 or 75% each night. According to manufacturers battery charts they would withstand about 2500 cycles till end of life (I used Lifeline's chart, but others are similar). If instead you switch between two 100 AH banks, each night you will discharge the one in use to 50/100 or 50% DOD. On the chart this gives 1000 cycles to end of life. However you are alternating between two banks, so for the system you get 2000 cycles. Therefore the life when used as two switched banks is 20% less than used as a single larger bank. As the Depth of Discharge increases, the difference will also increase because cycle life decreases non-linearly with DOD. It used to be common in boats (which are far more dependent on batteries than campers) to use two switched banks, but due to this lifetime calculation most boats now are wired as one large bank.

Regarding the wiring for two banks in parallel. It is common (but wrong) to wire the first bank positive and negative to the power bus, then wire the second bank to the first. Any voltage drop due to the charging current in the second bank will result in its charge being lower. In batteries charged for a long time compared to their discharge, the difference will be small, if charged quickly and for the shortest possible time it could be large.

The correct way to wire them is to run cables between the terminals of the two banks, then take the positive power bus cable to one bank, and the negative cable to the other. Since the current from either (and both) banks goes through these wires, any voltage drop due to them is seen by both battery banks and they will charge and discharge at the same rate (or at least, at rates not influenced by the wiring).

[Reddog1]

Moved to the Tech Issues Forum

Wayne
Moderator

[2oldman]

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

[pianotuna]

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.

[bjbear]

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.

[Jfet]

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

[CA_Traveler]

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.

[tony_lee]

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.

[naturist]

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.

[HMS_Beagle]

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....