24VDC boost to 12VDC truck

[charland]

We have built some military vehicles that operate on 12V but have 24V capabilities. We've done this by using a battery bank with some of the circuits coming off of one set of batteries wired in parallel (12V) and then the other circuits coming off both of the sets of batteries wired in series (24V). Under normal conditions this works great but when all the batteries in the battery bank are dead, the truck has to be boosted by a 24 V supply (written in the spec). The problem is that the vehicle electronics and the starter system are all 12V. If you have enough time, you can run the 24V through both battery sets and wait for them to charge but in a military environment, this is not feasible. We looked at the possibility of connecting a huge resistor to simply throw half of the heat away but because the starter has a variable current draw, the voltage drop across the resistor would also be variable.

My background is in mechanical so forgive me but I think that there may somehow be a solution in this thread using zener doides?

thread237-145132

Thanks in advance
Charlie Harland

 

[OperaHouse]

RV and marine centers have an isolated inverter module that will charge a second 12V battery that is in series to make 24V.

 

[charland]

Perhaps I don't understand how this can be used. We have 24V and need to power a 12V system. The 24V system could be anything from anywhere so we have no control on that equipment.

An additional piece of info is that the starter draws from 100 to 800 amps while the rest of the vehicle system (glow plugs, dash lights, sensors) draw significanly less. Its my understanding that it all has to be energized in order for the boost to work.

 

[dpc]

Maybe a dc to dc converter?

http://en.wikipedia.org/wiki/DC_to_DC_converter

800 A is a lot to ask of a converter system, however. (Unless you want to tow it behind your rig.)

 

[SteveWag]

Why not a string of forward biased silicon diodes (ten should work), enough to drop 12 volts, mounted to a LARGE heatsink connected to the 12 volt starting battery and to a dedicated JUMPER START terminal. They provide protection against connecting the cables with reverse polarity and insure that the JUMPER START terminal won't hurt anything if shorted to ground.
Steve Wagner

 

[JStephen]

I don't understand all your post exactly. But long ago, I drove some dump trucks, and some of those had 12 volt systems and some had 24v systems with two batteries in series. What I found was that when the 24v batteries were "dead", usually one of the batteries was deader than the other, and you could jump either of the two batteries off another 12v vehicle and get the thing started. No guarantee that it'll work everytime, and it may depend on whether you actually run them down or have some sort of battery defect as well.

 

[itsmoked]

Wow what FUBAR system..

A reliable solution would be to replace your glow plugs with 24V ones. Or Rewire them putting pairs in series, then reconnect them across 24V.

Reconnect your starter across 24V.

Leave the rest of the stuff across the one series battery.

Now when normally starting you will have more starter power. The modified glow plug circuit will run them at the same power.

During jumps nothing will now be different. The instruments and lamps and other stuff will run as normal.

Yes, you can run the 12V starter on 24V. It's done all the time on millions of diesels.

Keith Cress
kcress -

http://www.flaminsystems.com

 

[OperaHouse]

You said the vehicle is a 12V starting and charging system. There are also two 12V batteries in series to provide power to the 24V systems. The first battery is charged by the 12V vehicle system and the isolated inverter maintains the charge on the second battery. These systems are out there if you just choose to google.

 

[rtronics]

Charland, Do you wonder how many posters actually read your OP.?
My suggestion is: These 24 volt vehicles that you will get the jump-start from, surely they will have two batteries in series, so just jumper from one of the batteries, voila - 12 volts!!

Alternatively, it may be possible to get a carbon pile regulator setup to do the dropping 12 volts under load. This would be an interesting retro piece of engineering.

Also the diode series string suggested earlier would work, you would get about 1.6 volt drop each, puck, or capsule diodes would easily carry the current, could be expensive.
Regards, Ray.

 

[davidbeach]

A couple of thoughts:

1. A disconnect switch for the 24V stuff so you only have the 12V and then jump from one battery of the source.

2. Rearrange the electrical system so that you have both batteries in parallel and a boost converter for the 24V stuff.

Best is just to get 12V from the source vehicle; you don't really want power electronics in the starting circuit for a large Diesel engine.

 

[itsmoked]

The 24V system could be anything from anywhere so we have no control on that equipment.

rtronics; Some of us actually read ALL of a Poster's posts NOT just the original one. He can't consider having 12V available from the jumping vehicle...



Keith Cress
kcress -

http://www.flaminsystems.com

 

[waross]

For many years diesel trucks in North America used 12/24 volt systems.All systems except the starter were 12 volts and connected across one 12 volt battery. A series-parallel switch connected the second 12 volt battery in series for 24 volt starting and in parallel for 12 volt charging. There have probably been millions of such systems built.
A 24 volt jump would have to be directly to the starter. You could probably design a "Jumper" connection that would only apply 24 volts to the starter solenoid.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[OperaHouse]

Close, but no cigar. I've read the OP posts many times and he has done a poor job of describing his system and what he wants to accomplish.

 

[LionelHutz]

I think Keith has the best suggestion. You need to re-design your vehicle so the critical starting components run off the 24VDC. You could go so far as to install 24V to 12V converters to power the lower energy stuff such as the lights or engine computer. A bunch of diodes could be used as suggested but they'd be much smaller and more reliable if they only have to supply say 50A to the electronics instead of say 800A to a starter motor.

Then, the boost will end up suppling the correct voltage to the necessary parts to start the engine. As for the starter motor, it really doesn't care if you supply 24V to it occasionally.

I do know that many 6V vehicles have been boosted from 12V vehicles and many 12V vehicles have been boosted from 24V vehicles. Happens in industrial settings a lot. When batteries go dead whatever other source is available is used. Pick-ups or small tractors running 12V are boosted off dump-trucks or loaders running 24V. Over voltaging the whole electrical system is not a very good solution though.

 

[charland]

Thanks to everyone for their input. I think we'll switch out the starters for 24V and run the rest of the 12V stuff off of a converter as suggested.

 

[itsmoked]

Most Detroit Diesels, the two strokes, and there are millions of them, use a 12V starter that is run on 24VDC. This is because they have NO glow plug provision. They crank at about 800RPM - enough to guarantee cold starting on compression alone.

You should consider what I said about the glow plugs too, because on my measly 6.2L diesel they draw 120A.



Keith Cress
kcress -

http://www.flaminsystems.com

 

[rmw]

Those 2 stroke Detroits; if they turned more than about 3 rounds and didn't crank they weren't going to. They didn't like starting real cold if the engine block heaters hadn't been plugged up but I found that you could crank them for 5-10 seconds, stop for about 10 seconds and let them heat soak and then do it again and often they would start cold after about 2-3 rounds of that technique.

They didn't like ether either. I once had to crank a bus left for a week at around freezing and at about 10,000 ft. (left in an airport parking lot without access to plug in's) and I didn't have any ether (Heck it was June). But I did have a small propane bottle (for deicing brake lines when they froze) and I took the air filter out and put the bottle in with the valve opened and ran up front and cranked it with the gas flowing and when it busted off then I had to run back to get the propane bottle valve shut and out of the intake before it got sucked into the engine. Closest call I ever had getting one of those dogs cranked. Thanks for the memories Keith.

rmw

 

[waross]

The series parallel switch system used on highway trucks works well. I have seen it used on Detroits also, Keith. Original equipment would be a 24 volt starter, but there was no problem if it was retrofitted with a 12 volt starter.
All equipment except the starter ran on 12 volts. The batteries charged on 12 volts. One batery was re-connected in series to supply 24 volts for starting. During starting all lights and accessories were supplied with 12 volts from one battery. At all other times both batteries were paralleled for 12 volt operation. The series parallel switches are available off the shelf.
The wiring diagrams for the switches should be available from the supplier selling the series parallel switch.
Why re-invent the wheel??

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[LionelHutz]

When I think about this more, I believe running a 24V electrical system really is the only solution.

The big question is - What kind of load can you run off of a single 12V battery when it is one of two series connected 12V batteries that are dead and being boosted?

My first though was that if the batteries are just run down and need a charge that they would likely share enough to run a significant load. Probably not the starter motor but they should be able to run every thing else. So, using a 24V starter and 12V electrical system is possible.

However, if one of the batteries has a physical defect such as a dead or shorted cell then all bets are off. This is the case where boosting might not work because the 12V system would not get the correct voltage.

 

[waross]

LionelHutz, ask a trucker. 12/24 volt systems are proven technologies.
They were developed years ago when 12 volt systems would not reliably start diesel truck engines and 24 volt lamps would not stand up in vehicle service.
There are not many 24 volt accessories available compared to the numbers of 12 volt accesories.
A 24 volt boost must be applied directly to the starter circuit. A simultaneous failure of both batteries is rare. Even if both batteries are low, a twelve volt boost across one of the barreries will often start the engine.
There are hundreds of thousands if not millions of 12/24 systems in use.

Bill
--------------------
"Why not the best?"
Jimmy Carter