Charging AGM Batteries in parralel

Submitted: Friday, Jun 08, 2007 at 10:16

ThreadID: 46424 Views:4729 Replies:5 FollowUps:7

This Thread has been Archived

TentEnKaMan

Gday guys just picked up 2x 100ah agm batteries recently, in the hope of inflicting our gennie less on fellow campers lol. I was planning to keep these in a van or enclosed trailer, as my poor lil falcon aint got the room of you lucky 4x4 owners =D. So will be charging by gennie and battery charger. My electrical knowledge is a little rusty, but If I recall correctly if these are hooked up in parralel, this would equal 12v 200ah. Instead of the 24v 100ah if hooked up in series (please correct if im wrong lol). Would this mean the batteries could be charged up still hooked up in parralel, with a 12v charger? Would this also mean that the output amperage of the charger would be halved between the two batteries? (30A charger 15Ah charge per battery? Is it necessary to remove devices running off batteries before charging? Sorry these may seem very basic questions to most of you but I am kind of new to all this. Thanks to the kind souls who enlighten me, in both this and my previous threads. Happy camping/caravaning to all =D

Back Expand Un-Read 0 Moderator Share

Reply By: Robin Miller - Friday, Jun 08, 2007 at 10:43

Friday, Jun 08, 2007 at 10:43

Hi TentEnkaMan

Your doing well with your reasoning - and have it all correct

Robin Miller

AnswerID: 245605

Follow Up By: Mike Harding - Friday, Jun 08, 2007 at 13:25

Friday, Jun 08, 2007 at 13:25

You know... I wonder about this charging in parallel business...?

As batteries age they will, inevitably, do so at differing rates (how similar were they to begin with?) which means a situation will be reached where one battery is being overcharged. I suppose the questions are: how dissimilar will they become and will it take so long that it doesn't matter anyway? Hmmmm... don't know but I think if I were to buy $500 or $600 worth of AGMs I'd charge them separately.

Mike Harding

FollowupID: 506645

Reply By: TentEnKaMan - Friday, Jun 08, 2007 at 10:55

Friday, Jun 08, 2007 at 10:55

Thank you robin miller for your reassurance =D. anyone know if the charging in parralel/ with devices drawing current is a problem?

AnswerID: 245607

Follow Up By: Member - Alastair D (NSW) - Friday, Jun 08, 2007 at 11:50

Friday, Jun 08, 2007 at 11:50

No problem charging with devices connected, it will just slow down the charge rate of the batteries by the amount of current drawn. Only proviso is if the generator/charger setup has some spikes etc it could damage things like computers etc. If the generator/charger is good like a Honda with a 3 stage charger then all is well.

FollowupID: 506631

Follow Up By: Member - joc45 (WA) - Friday, Jun 08, 2007 at 12:44

Friday, Jun 08, 2007 at 12:44

Yes, use a decent 3-stage charger to ensure you retain your investment in those AGMs. This should put in max current without risking over-voltage. The max equalise voltage is about 15 for a brief period before settling to about 14v, so as long as your 12v equipment you're running is capable of 15v, you should be right running it while the battery is being charged.
You have much more protection against over-voltage with a proper 3-stage charger than a cheap unregulated charger or the 12v output of your gennie.
Having said that, I recall at least one post on this site, where a guy bought a cheap 3-stage charger, which didn't perform as stated. Might pay to put the new charger thru its paces, checking it thru its cycle with a decent digital voltmeter to see it doesn't exceed its specs.
Gerry

FollowupID: 506636

Reply By: TentEnKaMan - Friday, Jun 08, 2007 at 12:40

Friday, Jun 08, 2007 at 12:40

Thanks alastair D. nah just junk i run off the gennie atm anyhows was considering taking notebook once we get PSW inverter. Will remem to unplug before charging as we have el cheapo gennie atm. (will be upgrading but hopefully dont have to yet lol) My gennie is 650w scorpion 2 stroke. Anyone know about the charging in parralel bit please?

AnswerID: 245615

Follow Up By: Member - joc45 (WA) - Friday, Jun 08, 2007 at 12:56

Friday, Jun 08, 2007 at 12:56

Just connect the batteries in parallel. No probs. If they are the same age and have been treated the same, they should charge equally. You should also discharge them while in parallel - don't separate them unless you have a specific reason to do so.

With el-cheapo gennies (and some more expensive ones), the inbuilt regulators (which incidentally only control the 240v, not the 12v outputs) will have a finite response time to fluctuations in load. The removal of a heavy load could cause a brief spike in the 240v before settling back to the proper voltage, and you need to be mindful of this. If you're running your inverter off the batteries, there shouldn't be a problem, as both the charger and the internal resistance of the batteries will absorb any spikes at the secondary (12v) side. But when charging the batteries off the gennie, you'd be better running the lappie directly off the gennie rather than thru the inverter.
Gerry

FollowupID: 506639

Reply By: TentEnKaMan - Friday, Jun 08, 2007 at 12:46

Friday, Jun 08, 2007 at 12:46

Also wondering if I have an inverter that cuts out at 10V is that going to be enough to prevent me crossing the 90% use threshold of agm batteries? Dont want to damage the lil beggars lol

AnswerID: 245616

Follow Up By: Mike Harding - Friday, Jun 08, 2007 at 13:19

Friday, Jun 08, 2007 at 13:19

It is not a good idea to, more than very occasionally, take a battery down to 10V.

Most batteries have a (usual) minimum voltage spec. of 10V5 but some tests I have recently done show that only an extra 3% of energy is obtained by going below 11V (See a recent post). My recommendation is _never_ go below 11V and try not to go below 11V5 more often than necessary.

Mike Harding

FollowupID: 506642

Follow Up By: Member - joc45 (WA) - Friday, Jun 08, 2007 at 13:58

Friday, Jun 08, 2007 at 13:58

If you're adept with a soldering iron, and have a bit of electrical nous, Oatley Electronics have a Battery monitor kit which has a low voltage cutout and bar graph voltmeter. A cheaper option is their battery combiner kit, which can be used as a low voltage cutout as well if used to feed the load rather than combining batteries; Battery combiner kit
Either of these will prevent you from killing your valuable investment.
Gerry

FollowupID: 506650

Reply By: TentEnKaMan - Friday, Jun 08, 2007 at 15:22

Friday, Jun 08, 2007 at 15:22

ah cheers for that looks like 30 bucks well spent =D wonder how much the extra mosfet is speakers switch and box ? I will research ..... Like the idea of the variable resistor to get cutout voltage perfect with a multimeter nice. Yerh I got a soldering iron and ive done kits but not for years might be time to come out of retiremnent. Cheers for this

AnswerID: 245634