Battery addition

Hi,
I've been given a near new, unused 200ah agm battery (e-solar kx12-200, http://esolar.com.au/parts/solar-batteries/12v-solar-battery-kx12-200-).
My camper trailer already has a 100ah agm battery, and the charging is handled by the builtin box (photo attached).

My question is will this charger be able to handle the load if I add the 200ah battery to the 100ah battery in parallel ? I assume that it will be fine, but that it will just take longer to charge the effectively 300ah battery.

Thanks,
Paul

the charger looks like it would handle the load
but you shouldn't link 2 batteries of different sizes as they wont charge completely
I would remove the 100 ah and use the 200 ah, looks like a good battery

You could keep the 100ah job but as sparra mentioned you can't put both in parallel. Internal resistance of one to the other is different so you would need some sort of change over switch and it's starting to get messy.

Perhaps keep the 100ah one fully charged and stashed as a spare when on a trip. Just in case.

I would've thought that the battery with the higher charge would just discharge into the other, keeping their levels the same, and that they would distribute the charge amongst themselves ? so even with a decent charger (CTEK etc) this won't work ?

my understanding is the charger will sense when the smaller battery is full and stop charging,
our resident 12 volt guru "old tony" should drop in soon and give us a more technical description

As above, you can't parallel different sized batteries for the reason Sparra gave.

A smaller battery logically takes less time to charge than a full one. What then happens is the smaller battery's voltage rises (as it should) faster than the larger battery, presenting an apparent voltage to the charger indicating a full charge (which it is), in spite of being pulled down by the larger battery which is still trying to haul amps in.

Discharging is not only similarly disproportionate, but they'll discharge each other in a short period. As you place a load on the batteries, the smaller battery's surface voltage drops more than the larger battery, so the larger battery starts dumping into the smaller battery (as well as the load). When the load comes off (eg a fridge that's cycling), the smaller battery jumps to a higher voltage than the larger battery and the larger battery starts drawing from the smaller one.

I'd do as suggested - drop the 200Ah in, leave the 100Ah to run your security system at home, or maybe the new fandangled 12V LED lighting system in your house, or something else. Parallel it to the 100Ah in the camper. Just don't have two dissimilar batteries together, ever.

Hey Paul,

I would be very wary of using this charger to charge your AGM batteries. These chargers are only designed to charge a Lead Acid Battery. I have one of these chargers in my camper and don't use the charge function as I run a 150Amp/Hr AGM. I am using a separate Ctek charger but I still use the SETEC power supply to run the camer when 240V is available.
The folowing is an extract from the owners manual) "The unit is rated to charge a single 12V (up to 6 cells) lead acid battery at 100Ahr Capacity. " If you want your batteries to charge fully and last, I would suggest not using the SETEC to chare them.

Foxy

Or make a split circuit system but you would need to isolate each battery from each other , like a separate charger for the 100ah

I have just a read of the full document since I now have it and it seems I was given 'duff gen' to start with and have passed that 'duff gen' on. Sorry. The owners manual states later on that it is a 4 stage charger is suitable for AGM's. Here is another extract:
This product is suitable for charging 12V-Sealed Lead-Acid (SLA) batteries including Valve-
Regulated Lead-Acid (VRLA) batteries both Absorbed Glass Mat (AGM) and Gel batteries. One or
two batteries with max.100Ah capacity each can be charged. Charging current is limited to 10A
(ST20-II) and 15A(ST35-II).

I am now ducking for cover with a helmet on.

Near as I can tell, the KX12-200 is a lead acid battery (AGM = absorbed glass matrix, it's a lead acid battery but instead of the acid flowing loose, it's all mopped up in a wad of fibreglass between the plates). Any technology designed to charge a lead acid battery can be used to charge an AGM. Gel batteries are different (they don't like voltages over around 14.1V), and LiFePO4 is vastly different, and I've got to do some homework on the calcium jobs.

No need for ducking ... for AGMs you can generally recommend a higher charge rate too.

I wouldn't be charging a non-AGM battery with that charger. 20A is a LOT of power. AGMs can usually handle around C/5 to C/4 (eg a 100 amp hour battery can tolerate being charged at 20 to 25 amps per hour) and spiral wounds like the Optima D31A can tolerate rates up to C/2 (in that case, a 75Ah battery that can take 37.5A).

Crankers can take a whallop - they have to, since a car alternator is capable of putting out 50A or more (in just about every modern vehicle), but many deep cycles can't, and gels definitely can't - C/10 if you're lucky, so a 100Ah battery shouldn't be charged with anything over 10A per hour.

judging by Foxbat's info, my SETEC unit is probably charging at 10A.

would there be a simple way (drawings please if you can be bothered !) of keeping both batteries, but having a 2 way switch that chooses which battery is 'active' for both charging and power usage ?
IE I can manually switch between the batteries (without re-wiring !) and whichever battery I select, will be charged when the van is plugged in to 240v, and used to provide 12v power when not plugged in to 240v ?
That way, worst case would be I forget to manually switchover and charge both batteries before going on a trip.

Thanks for all the info everyone !

try one of these


http://www.biasboating.com.au/BEP_4_Position_Battery_Switch_p/2333.htm

If you can find a heavy enough switch (put a fuse in to protect it all, less than the rating of the switch so 40A switch = 30A fuse) you could do this:

Thanks guys.
Would the switch in the attached picture be OK ?

If anyone can be bothered advising if there's a better (or cheaper but still suitable !) switch from the same company near my work, it's from the URL: http://www.12volt.com.au/General Htmls/webcat2003/batteryswitches.html

Cheers !

That switch you linked to is massive overkill. 300A continuous for an aux system is way beyond the requirements. Even commercial isolators don't go far beyond 100A and in reality, you'll probably whack a 40A thermal breaker up front to deal with problems, so a 40A switch down the back will more than suffice. If you're keen on one of those, grab the smallest one they have, it'll be enough!

actually I forgot to ask you, where abouts in your diagram would you put the breaker/fuse ?

Immediately off the battery. I'd have another just off the main battery at the start of the cable running down from the front.

Hi Tony,
not sure we're talking about the same things here ?
This is a house battery setup, completely contained inside the camper trailer.
Originally I had a 100ah AGM battery, I'm adding an additional 200ah AGM battery. As you and others have mentioned I can't parallel them as they are different capacities, so I'm looking to setup a switch, both for the charging (from the charger builtin to the Jayco - a SETEC unit), and the output to the rest of the camper.
The cabling from the car is all sorted and already previously put in place. It is fused.
I just meant where in your diagram would you add another fuse ?

Thanks,
Paul

Hey Paul,

We're talking the same thing. I maybe wasn't clear enough. Won't matter WHERE the switch is, when I say "immediately off the battery", this is what I mean:

Old.Tony, just to make sure I understand what you're talking about, attached is a diagram of the current (single battery) setup. Following post will have what (I think) you and I are proposing.

Thanks !