Working with a mixed 12/24 V system

[Jens]

Thumper ran my battery dry again today without starting and I am kinda sick of this.
I would like to run two batteries in series to give my 12V starter 24V. The rest of the system runs on 12V (fuel solenoids, fuel pump, glow plugs etc ...)
I currently have a dedicated 12V charger that runs 24/7 to keep the one battery topped up. When I go to a mixed voltage system I will be drawing different currents from the two batteries. I am wondering if I can get away with a single 24V charger across both batteries or if I should use two 12V chargers instead ?

Any suggestions what to use for a charger(s) that is cheap and cheerful but doesn't overcharge the batteries ?
I have had the current battery in service for about a year - can I simply get a second one to make it a 24V system or should I get two new batteries so they are better matched ?

BTW, the reason I have starting issues is because Thumper slows down quite a bit as it goes over TDC and because the starter turns the generator, there is a 2:1 speed reduction to the engine. So, the engine is already turning over at half speed and then slows down at TDC and IMHO is just going too slow for reliable starting unless the engine is still warm from a previous run. I missed my run this morning so the engine was at room temperature and although she made some half-hearted attempts at starting she never caught completely.

[mobile_bob]

for what you have in mind, use two chargers and charge them seperately
or better yet get a series parallel switch

delco made a nice one, and they should still be available
it would charge both batteries in parallel with a single 12volt source
and when you hit the starter it would reconnect to series and provide the 24volt boost.

they are actually quite effective and should last forever in your application,

you might even find one in a truck bone yard in your area, as the canadian's really liked 24 volt starting

bob g

[mobile_bob]

i think you would do well with the single stage unit from delco, it is much cheaper than the
double unit.

the double unit completely isolates the starter from the batteries, and provides a level of protection
for the charging contacts should one want to jump start the engine.

i don't see you needing that level of protection personally

there were  10's of thouands of trucks made with the single units, they did just fine, so i cannot imagine
how your installation could kill one.

i am familiar with the single and doubles, have never seen a triple, and not sure what would be a magnetic switch that would be different
than the first three.

you can probably replicate the operation with separate relays if you wanted to, probably at much lower cost.

bob g

[TimSR2]

ask for a SP switch at any truck boneyard. They are pretty expensive new.

[vdubnut62]

Here's a link to how John Deere handled the problem on the first diesel electric start models,
scroll down till you find the correct diagram.
Ron

http://www.jdhpubs.com/pdf/WiringDiagrams/JD-720.pdf

[XYZER]

Does anyone know what the intended application for this is? I believe it came out of a coach to get 12v out of a 24v system but not sure.

[veggie]

Jens,

Is it time to revisit the air motor starter system? The one that makes music to your ears ?

veggie

[TimSR2]

Xyzer,

That looks a lot like a conventional diode splitter but has both 24 and 12 volt markings.  From the look of all those fins I would say it has some pretty heavy diodes and a voltage divider circuit in there. The vehicle that used it had a 24 volt alternator, but some 12 volt loads in it  like radios, lights, etc. This item would allow charging and isolating the 24 volt 'start bank' batteries from the 12 volt 'house bank' , while using only a single 24 volt alternator. Thus the vehicle could completely avoid the dreaded series /parrallel switch, and have a dedicated Start battery that could never be accidentally run down. Nice find.

Tim

[XYZER]

Tim
Thanks, I figured that is what it was for. I know they came out of a motorhome that had a 24v main engine charge system. I was assuming they could get 12v off of the 24v to run the 12v system in the coach. 100amps should run a 12v starter so then I could build a 24v system and still start my oid with the 12v starter. I will have to see what the draw is on my starter.

[TimSR2]

Your starter draw will be 250 amps minimum. Don't worry about it. The start battery can  supply many times that. The 100a rating applies  to the maximum load the diodes can handle under charging, CONTINUOUSLY! . Probably there is some fudge factor in there too.   If your alternator can't supply, or 12 v battery bank doesn't demand 100a then you will be just fine. If you are just charging one or two 24 series start batteries on the 12 volt side you have nothing to worry about. I would confidently hook up a 24v/130a Leece neville to that bad boy and charge a 650 CCA 12v start battery, or 2 450's The high current draw of an SLI battery only lasts for a minute of two after starting.

If you find the charge voltage on the 12v side exceeds 14.5 v you can just use one side of an off the shelf 30 or 50 amp battery isolator to give you another .7v voltage drop to control the current. I'll bet it is set at about 13.9-14.0; this should automatically limit the current well within it's rating.

Tim