Charging a 24V System

[zdas04]

I have two 12VDC batteries connected in parallel (i.e., a jumper from the positive on battery A to the negative on Battery B, then the loads coming off the positive on battery B and the negative on battery A).

I described this to my supplier and he sold me a 12V SunSaver 6 charging circuit. My electrician connected the charging circuit on the SunSaver to the same leads as the load (positive to "B" and negative to "A") and after a week the system is at 13V (both batteries are down to 6.5V).

This obviously isn't right. I thought he should connect the charging circuit to one of the batteries and let the charge equalize through the jumper, but he said that wouldn't work (he didn't say why, I am just a dumb mechanical engineer after all). Do I need to scrap the 12V SunSaver and buy a 24V or can I hook up to a single battery?



David Simpson, PE
MuleShoe Engineering

http://www.muleshoe-eng.com

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.

"It is always a poor idea to ask your Bridge Club for medical advice or a collection of geek engineers for legal advice"

 

[IRstuff]

Sadly, David, neither solution will work. Hooking the charger across both batteries will guarantee that both batteries get charged... to 6.5 V apiece.

Connecting across one battery will charge that one battery and leave the other uncharged.

Is your system offline during this time? Can you afford you charge each battery individually timewise?

You could possibly charge both batteries in parallel, but there are some risks in not getting a full charge to both batteries.

TTFN

FAQ731-376

 

[IRstuff]

Oh, and your electrician is a knucklehead... ;-)

TTFN

FAQ731-376

 

[zdas04]

You know, I'm starting to agree with the knucklehead statment.

This system has to work all the time so I can't take a battery off line to charge it. It is also pretty remote and unmanned so manual intervention wouldn't work.

The two deep discharge batteries are both brand new, I guess my best bet is to replace the 12V charger with a 24V charger and accept the risk of not charging evenly. The load is 1 amp for 3 seconds every three minutes (drawing current of about 0.4 amp-hr/day. I think that works out to about 10 W/day and I have a 40 W solar panel so I don't think that I'm stressing the batteries.

David

 

[IRstuff]

I'm confused, now. Why isn't your solar panel maintaining the charge on the batteries then?

Isn't the solar panel supposed to charge up the batteries so that you can even out the charge/discharge cycles? Ostensibly, then, the solar panels ought to be charging the batteries. If they're not, then either something is wrong with the charging circuit, or there's insufficient panel capacity, either because the load is higher than what was anticipated, or the solar input is lower than anticipated, or the panels have degraded.

TTFN

FAQ731-376

 

[zdas04]

The solar panel is maintaining 13.8 V out (it has a 12V solar controller on it). That is what the batteries are charged too (6.9 V each).

David

 

[itsmoked]

Failure to launch..

batteries connected in parallel (i.e., a jumper from the positive on battery A to the negative on Battery B, then the loads coming off the positive on battery B and the negative on battery A).

You have stated parallel then promptly describe a series hook-up.

If it is truly hooked up in series you have a 24V system and need a 24V solar charger.

I will also point out that time is of the essence. If those batteries are gel-cells you should probably just order new ones now as leaving modern GCs drained for more than a few hours destroys them.

Your supplier heard "parallel" and sent you the proper 12V charger. The new 24V charger should be hooked up just as you've described.

Keith Cress
kcress -

http://www.flaminsystems.com

 

[zdas04]

"Sparks and Magic" was a really long time ago. I got it in my mind that electricity was oposite of pumps (pumps in parallel would each have the same discharge pressure and pumps in series would have a multiple of the discharge pressure that one pump could do), but it seems that my partially recalled electrical "expertise" was incorrect.

Yes, this is a series hook up. Using two 12V batteries to power a 24V load.

David

 

[IRstuff]

Oh, OK, you should go back to your dealer and have him get you the 24V Sunsaver model, presumably something like:

http://www.affordable-solar.com/morningstar.sunsaver.10l.24v.charge.controller.htm

TTFN

FAQ731-376

 

[zdas04]

The 10L was the one I was looking at. They're in stock so I guess I'll try it. Thanks for your help.

David

 

[fuzzypeach]

I've been using a battery to battery charger to charge my 24 volt bowthruster battery bank from my 12v engine start battery. This charger is pretty slick, and it doesn't have to be plugged into 110vac to work. It runs off the 12v engine crank battery, tricks my alternator to juice my 24v batteries. I use the 12v to 24v, but if I remember correctly they had a 12v to 12v, 24v to 12v and a 24volt to 24volt battery to battery charger. Label on the product manual says "Sterling Power USA". I've had it since march of this year, still running well. I'll post if otherwise-