Using multiple batteries

[Ranga]

Currently travelling around, towing a caravan. I have the following batteries.

caravan - 120ah AGM deep cycle
canopy - 105ah deep cycle
canopy - 450cca battery for using the electric outboard
550 & 650cca batteries in the Defender, isolated from the rest by a traxide SC80.

These are all just batteries I've accumulated over time. All of these batteries in the canopy and caravan are connected to the SC80 via nice thick cable and Andersen plugs. Hence they all get charged on long drives, or when I connect them up to the Ctek.

I'm concerned about how these batteries are discharged. Given they're all different capacities, is there a danger of the smallest battery being discharged first and to a level that's not good for it?

Comments/suggestions?

[slug_burner]

If connected in series definitely will discharge the lower capacity ones first and once discharged will result in damage if you keep forcing current through them. The overall voltage will drop and should alert you to the fact that a battery in the string has discharged.

In parallel not as likely to damage as all batteries will be forced to stay at the same output voltage. The current will be supplied from the battery that has the lowest output impedance.

[THE BOOGER]

If they are all 12v and I am sure they are they are in parallel the size isnt a problem but if one battery is shot it will draw from the others when not being charged, shortening the time between charges and eventually damaging the other batteries. Without a voltmeter on each battery not much you can do you could check specific gravity reguarly to check condition of each battery:)

[Brian]

As a retired electronic tech, and considering I am not a battery expert, my training says that all these batteries have different types of manufacture, different rates of use (discharge), therefore all need to be considered to need different charging methods.

Also consider that the caravan will not always be connected to the tug.

I am sure that there is some battery experts out there who can advise the proper way to connect everything.

Hope you get it sorted and you have a great trip around the block.

best regards
Brian

[drivesafe]

Hi Ranga, I’ll put this misunderstanding that lead acid batteries are different batteries, they are all 12v lead acid batteries and while they may have been put together in different ways and have different characteristics.

Their different characteristics are only valid if you are using the batteries in situations where these different characteristics are an advantage or a disadvantage.

The reality is that in most RV applications, the different characteristics rarely come into play in a way that would effect any batteries in a parallel set up.

If you were using the vehicle for competition off roading then high performance cranking batteries would be the way to go to get maximum winching potential from your batteries.

For low current use as is the case with most RV operations, mixing different types of lead acid batteries will have no adverse effect on any battery, they will discharge at the same rate, proportional to their size.

The only thing you have to be careful of is when charging using a battery charger, just make sure the battery charger’s maximum voltage is set to that of the battery with the lowest maximum voltage.

This means is you have a flooded wet cell battery and a STANDBY type AGM, like a Fullriver AGM, the flooded wet cell setting is usually 14.7v but a STANDBY type AGM will only tolerate 14.4v, so set the charger to 14.4v and all your batteries will fully charge without overcharging any of them.

[Ranga]

I have a couple more questions.

I have an EMS2 diagnostic unit measuring the volts at the starter (or alternator, can't remember which!).

Even after hours of driving, when I turn the key back to on and after the glow plugs have stopped drawing, I'm only seeing about 11v. Would this indicate either or both cranking batteries are faulty?

[drivesafe]

Hi Ranga, I don’t have any experience with the EMS2 unit but I suspect something is either wrong with the unit or more likely something is wrong with your wiring.

It has been my experience that having a battery voltage of 11v is not high enough to start a motor and as yours is starting, then there is a good chance you have far higher voltage in the battery(s) then is being displayed.

Just a suggestion but try checking all your connections.

[Blknight.aus]

Im backing drive safe as long as your batteries are of the same generic type
flooded cell (including SLA), SLAB or Gel you just hook them up in parallel and feed them some volts at some amps most batteries have a max float voltage on them somewhere, so long as you are somewhere between the nominal no load off charge voltage and the lowest maxfloat voltage all of your batteries will charge to something like 95% given enough time.

as for the charge indication issue, check the output of the alternator with a multi meter then check the battery terminal voltage with the engine off.

odds are its your EMS2...

oh and no...