Can batteries be wired in series and in parallel? - PeachParts Mercedes-Benz Forum

The Warden · #1 01-16-2004, 12:22 AM

Hello!

I'm getting conflicting information from other boards, and am wondering if anyone here will know for sure.

I've got an electrical system that uses 2 12 volt batteries wired in series to get 24 volts. While I need the 24 volt system, I also need a basic 12 volt system. Can I run the same set of batteries in parallel to get 12 volts for these accessories?

Currently, the 12 volt accessories are hooked to only one of the batteries, and while this works, it means that that battery's being discharged at a faster rate than the other battery, and the batteries are being charged by 24 volt power (and no, a 12 volt alternator cannot feasibly be installed), so they can only be charged evenly.

Thanks!

The Warden · #2 01-16-2004, 02:07 AM

This is for marine use.

hill · #3 01-16-2004, 03:00 AM

If you are looking for 20 amps or less a 24-12V reducer can be had for $60. JCWHITNEY eve has them.

http://www.redarc.com.au/vrt.htm

Ken300D · #4 01-16-2004, 07:53 AM

Possibly the voltage down-convertor would have some efficiency loss in the down conversion, resulting in greater overall battery drain.

About the only thing you can really do, and you have to do it VERY carefully, is spread the load across the two batteries by tapping each one for 12 volts. This is dangerous though, because you end up with two grounds, one at 12 volts higher potential than the other.

So, if you touch two things together that are being powered by different 12 volt taps, you get sparks.

My recommendation is to leave it alone, or get a bigger capacity battery for the one that takes the 12V load.

It is a system of compromises. They designed it to be safe for the most part, rather than get the most out of the batteries.

Ken300D

MikeTangas · #5 01-16-2004, 08:44 AM

All of the boats I worked on that were equipped with twin 12D batteries had the 12v accessories powered off one battery. The alternator kept the batteries well charged while the engine was running. If I remember correctly, the 24v system was needed to provide sufficient power to the starter only.

G-Benz · #6 01-16-2004, 11:01 AM

While I don't have the 12V/24V issue, I run two 12V batteries in my VW...the stock 45A alternator is anemic at best, and when I demo'ed the stereo at car shows, I would run a single battery dry in no time!

So I have one battery for the stereo and the other for the operation of the other essential vehicle components (incl. the starter

).

An external regulator allowed the alternator to charge each battery seperately...otherwise, you put a tremendous load on the alternator...

Palangi · #7 01-16-2004, 12:04 PM

I would consult with the Unimog people. Some Unimogs are 24 volt systems. No doubt they have encountered this problem with adding radios or other 12 volt loads. CoachGeo could probably point you in the right direction.

My best guess would be that tapping into 12 volts between the batteries for low currrent devices would probably be OK. Using this method for high current devices (lamps, amplifiers, etc) might not be such a good idea.

Wiring batteries in parallel requires an isolating device (like on the Ford diesel pickups). Without isolation, current will flow between the two batteries and they will run each other down quickly.

bjcsc · #8 01-16-2004, 12:14 PM

I don't understand. What exactly are you trying to run? I have a twin Interstate marine battery setup in my jet boat, but nothing that requires 24v so I don't think my specific setup would be of any help. I have, however, done some pretty tricky wiring in the past. What I don't understand is that if the batteries are wired in series, anything running from either would be getting 24v, right? It shouldn't matter which battery anything is directly attached to. If this is not the case, there are other technical specs about your setup that would be helpful.

The Warden · #9 01-16-2004, 01:30 PM

Basically, the boat's electrical system is 24 volts in its entirety; that's the source of the engine starter power, lights, horn, wipers, etc. I need the 12 volt power for the VHF radio and for bilge pumps (probably 3 small ones; one for each compartment).

Also, I've got a permanent-mount battery charger that's set up to charge up to three 12 volt batteries. Can anyone forsee a problem hooking this up to the two batteries that are in series? I know that it's feasible to charge two batteries in series with two separate 12 volt chargers while drawing 24 volt power from the batteries, but do'n tknow if there's a potential problem with doing this with a single charger.

Thanks! And I'm not purposefully trying to withhold info

if any more info's needed, just let me know...thanks

bjcsc · #10 01-16-2004, 02:24 PM

OK I think I get it. The best thing to do regarding the 12v you want is to divide the use amperage between the two batteries, ie. instead of using only one for all 12v, use both, each for half by amperage. They should then discharge more evenly. The permanently mounted charger has three sets of leads each providing 12v or what?

As an aside, if I were you I would use 24v bilge pumps (attwood makes a 24v 1250gph for ~$60) and just get a converter for the vhf only, and run nothing else on 12v. You wouldn't need a very high amperage converter for just the vhf. Also, DO NOT use a converter like the one at the link in one of the previous replys. Make sure whatever you end up using is rated for marine use, like this one.

The Warden · #11 01-16-2004, 03:46 PM

Thanks! I didn't know that Attwood made a 24v bilge pump...I took a look around West Marine's site and couldn't find anything.

And yes, the charger has 3 sets of leads that each provide 12 volts.

bjcsc · #12 01-16-2004, 05:18 PM

OK, in the case of your charger, it would be fine to charge it with one set hooked to each battery. Actually, that will give you a good even charge. West Marine/BoatUS probably won't stock the 24v pumps, but they can get them for you in a day or two (around here anyway). I'm very cautious when it comes to my boating equipment. I would worry if there was anything (like a converter) between my bilge pump and its power. You'll be much better off set up like that. Your boat seems to be a pretty neat project.

Anyway, to really answer your posted question, it is possible to run the same batteries in series and parallel. This is done in 4 battery and higher setups (not in 2). Eg. You could buy two more 12v batteries, connect them together serially, and then make a parallel connection between your new pair and old pair for an end result of 24v with twice the reserve.