24v charging system in a 12v vehicle

[manOwar07]

Hey guys, I'm new here so please bare with me.

I have a vendor trying to supply us with a vehicle using a dual battery system and charging at 24v. The entire vehicle including starting, operation and auxiliary is 12v. Where can I find some information that will prove my point of this being a bad system and will not work? They claim one battery will operate the vehicle and the other for aux power. We know the aux battery will have a consistent drain @ 25-30amps while the vehicle power will be much less.

Any information will be appreciated.

 

[MacGyverS2000]

For one, if you plan to charge with 24V but run on two separate 12V systems, you need a switching system that would allow the batteries to be separated during normal operation. That adds to initial cost and ongoing repair bills.


Dan - Owner

http://www.Hi-TecDesigns.com

 

[17037]

I'm not going to say I know a huge amount about this, but that sounds like an odd set up to me. Generally you would look at battery isolators when dealing with two batteries. With the system charging at 24 volts it would seem the two batteries are charged together in a series circuit. The down side to that is if there is an issue with one battery the other does not pick up the slack, rather both batteries suffer.

The other issue I'd think would be the long term cost of that system. Dealing with repairing a 12v system compared to a 24v system would have large savings.

I'd have a lot of questions on why they chose 24v, and if you have any doubts about the quality of the vendor.... stay away.

 

[rmw]

The way you phrased your question it is hard to understand what is going on. 12/24 systems were fairly common on tractor/(trailers) about 25 years ago but seemed to have gone out of vogue quite some time back. It had a switch called a series/parallel switch that switched a pair of batteries to operate in series for 24 volt starting and then back to 12 volt (parallel) for normal running. The charging system was 12 volt. Because the batteries were in parallel during 12 volt operation any variations in current drain due to attached devices or systems wasn't a problem.

The way your post is worded it almost sounds like a mirror image of the above, and if so, and the batteries are in series being charged at 24 volts, it won't work. If it is the old 12/24 volt series/parallel switch set up, they used to be as common as dirt.

They were very common in 'hard to start' situations like very cold climates.

There are also 12/24 volt alternators for fire trucks that have devices that need both. Is your system similar to that?

What is driving your situation such that the vendor wouldn't be offering a straight 12 or 24 volt system?

rmw

 

[Fabrico]

The batteries could stay hooked up in series but you would need two separate and fairly balanced electrical systems. Starting would be an odd load. As mentioned, the side with a problem would be helpless. In addition, if one battery developed a shorted cell the other battery would get cooked. It could work but the dual systems would have the unique quality of mutually assured failure.

 

[manOwar07]

Sorry for the confusion, Let me try again.

Charging system = 24volts, 2 12v batteries connected in series charged by 1 24v alternator.

Battery 1 = 12v for starting and running the vehicle. Draw will vary from AC, Lights, ect.

Battery 2 = 12v for communications power. Draw will be consistent @ 25-30amp but could peak at 40 on occasion.

 

[GregLocock]

You'll persistently overcharge one or other of the batteries.

If you use a 12V alternator, and put the batteries in parallel, they'll last longer, starting will be easier, and life will generally be more wonderful. The only downer is that you'll be able to flatten them with the comms gear, rendering you immobile. A simple isolator on the second battery would prevent this, or it could be done with a relay so that they are in parallel when the ignition is on, but isolated from each other when ignition is off.



Cheers

Greg Locock

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

 

[rmw]

What you describe ain't going to work, excuse my english. Greglocock states the reason perfectly. And whoever is offering it to you doesn't know their head from a hole in the ground. Excuse my opinion Run from them.

Both batteries can't be referenced to the same ground, so unless the communications equipment is some kind of system with its own ground that is isolated from the chassis ground they are nuts.

Unless there is something else that you aren't telling us.

rmw

 

[manOwar07]

All of you are correct. I'm looking for some supporting documentation that will prove my (our) point.

 

[MacGyverS2000]

It's up to them to prove their point (which they won't be able to do), not the other way around.

Dan - Owner

http://www.Hi-TecDesigns.com

 

[rmw]

Draw up a sketch of the charging system and do a ampere-hour balance on each battery.

rmw

 

[Thecardoc]

Being new here I didn't see this thread until today. US military used a 24v/12v on many vehicles and it worked quite well. A 24v starter on the 6.2l and 6.5l diesel (circa 86-mid 90's) really helped the thing fire up.

One such vehicle upon retirement from the military was donated to a local police force, who in turn gave it to a volunteer fire department. The 24v system had become problematic at that point, especially because of parts availability. Converting this back to a standard 12v system was actually simple. It required the typical 12v starter every civilian model used. Taking the batteries out of a series configuration, and making them parrallel. A slight change in the starter circuit eliminating a relay that switched the 12v ignition signal to a 24v starter command. A rewire of part of the fuse block was necessary including removing the second alternator bias, rewire the charge lamp #2 indicator to match charge lamp #1 and rewire the second alternator to chassis ground instead of being biased ontop of the first 12v system. The result was a normal 6.2l, with twin 150 amp alternators. The next thing we did was add invertors to the vehicle it is currently used for emergency lighting and tools. It's essentially a 4WD diesel generator.

 

[thruthefence]

I don't think this is what the original post was alluding to; the poster said every system, including starting, was run at 12 volt, but charged at 24 volt. I don't think starters really care about the voltage, there are many series-parallel switching schemes out there. Some aircraft run a 48 volt potential to the starter under certain circumstances.