Multiple battery performance with solar question

[LandCruiserPhil]

Trailer has 2 identical deep cycle batteries. From a performance stand are you better off using both together until depleted or use one until depleted and switching over to the other fresh battery?

Now add a solar charger in the mix with the ability to wire into both batteries or just one at a time. When and where do you use the solar for maximum performance?

Discuss

 

[UK4X4]

even with deep cycle batteries you should only discharge to 50% of their capacity

I would use both togther and use a voltage shutoff - the deeper discharge the less cycles you'll get out of them

With the batteries wired in paralel with solar, charge both at the same time

the solar can be connected all the time, as long as the panels either have a diode or your using a charge controller

 

[DaveInDenver]

If they are of the same type, age and condition it would be better to use them together rather than try to balance them manually.

 

[wrcsixeight]

The Peukert effect says that the bigger the load, the less overall capacity a battery will deliver. So 2 batteries in parallel should be able to provide more of their energy, than one at a time. But with sub 5 amp loads, the difference might be marginal.

Now if the battery is subject to a cycling load, like a compressor fridge, then a small solar system will never be able to get and hold 14.5 volts for 90+ minutes and get a true full charge. In this case Then charging a single battery at a time would ensure that the batteries do not suffer from too much progressive sulfation do to chronic undercharging.

I added a 68 more watts to my 130 because my cycling fridge would keep knocking my controller out of absorption voltages( 2 group 27 DC batts), and it would get confused and revert to float prematurely.

The 68 watt panel works great in overcast skies and lower angled sunlight. While it only adds ~50% more wattage, I seem to be getting about 70% more A/h returned into my batteries.

 

[Tennmogger]

There's another consideration. No two batteries are exactly the same. When connected in parallel, the one with lower internal resistance will discharge the other one. So, even with no use they will deteriorate faster than if not connected together. My opinion, and personal usage, is to use them separately, monitor voltage, and swap the batteries when one shows first sign of discharge. I look for about 11v on a 12v system, under mild load of a few amps, then swap.

Second advantage of using them separately is that different types, models, even chemistry, batteries can be used. One goes bad, just replace one. One of my house batteries on the camper is a 110 AH deep discharge. The other is a 75AH marine type, not true deep discharge. I have even used wet cell nicad (36AH) as the second battery.

As for charging, they can certainly be charged from the same source at the same time. If that source is solar, even better, as solar power is a current source rather than voltage source. Add a small 'spreader' resistor in series (just half an Ohm works well), diode isolate them, and connect them to the solar charger permanently (and independently from your load switching). BTW, I don't use a charge controller at all. My battery capacity is so high compared to solar panel output (couple of amps) that there is no possible way the solar panels can over charge the batteries between camping trips (a couple times a month minimum). Simple is better.

The camper house batteries also recharge off the truck system when we're on the road (charged through a 24 to 12v converter set to 13.8v).

Bob

 

[McZippie]

There's another consideration. No two batteries are exactly the same. When connected in parallel, the one with lower internal resistance will discharge the other one. So, even with no use they will deteriorate faster than if not connected together.

The same point could be made about the individual cells in one battery.

For "2 identical deep cycle batteries" best to wire in parallel because of Peukert Effect and the lesser the amount a battery is discharged, the longer it last.

 

[LandCruiserPhil]

Full disclosure on my set up to see if opinions change.

2 - 12v 18AH deep cycle batteries available switching A - B - Both

1 - 26 watt solar set up without charge controller

Its a small trailer with light demands. Netbook is the largest single power user with most other loads are under 10watts.

My thought was to use one battery to ~50% then switch battery and hook to 50% battery to the solar and repeat

 

[dwh]

With 18ah per battery and small loads, I don't think Peukert is going to have much applicability in this situation. It mostly needs to be considered for systems large enough were it can make the difference in hours or days of extra run-time, or where it can mean the difference between a small depth of discharge or a large depth of discharge, or in systems where every watt counts.

There's theory, and then there's the real world.

I gotta go with Tennmogger on this one.


But then, I'm a well-known battery abuser. And I don't have solar on my truck at all - I prefer a tiny generator, a battery charger and parking in the shade.


My only nit to pick in Tennmogger's post is the bit about one battery discharging the other.

Well yea - but - all else being equal (make/model/age/type/size AND properly balanced wiring) then the discharge from one to the other is going to be small. Just a little until they are truly equal - but it won't be much since they'll have been charged and discharged together.

So yea, the slightly better battery will do slightly more work than the other battery and wear out slightly sooner. But that process will increase its internal resistance, and then it'll be pretty much the same as the other battery - resistantially speaking (yes, I just made that word up ).

With large banks and with everything else being equal, it's just something that has to be lived with.

Especially if you've got a large bank made up of 2v L-16s: