Ideal series-parallel 4 battery arrangement

[Castle Bravo]

Something I've been wondering about for some time now is what the ideal arrangement for a four battery series-parallel arrangement is. Many of the larger military vehicles use four 12v batteries arranged in a series parallel arrangement and have both 12 and 24v systems in the vehicle.

A simple parallel arrangement -



Though this may be an oversimplification in order to get my point across, consider the following circumstance - You're using the above arrangement in a vehicle (such as a modern diesel pickup truck with two batteries) and you're using an accessory that is drawing 30 amps of 12v power from the system. You would expect that you're basically getting 15 amps from one battery and 15 amps from the other. However, because of the internal resistances of the batteries themselves, the resistance of the wiring and the terminal connectors, due to the electrical proximity, and maybe other concerns, more current is drawn from the top battery than the bottom battery. When the alternator charges this setup, the same problem occurs - the top battery is charged more than the bottom. This problem is compounded with additional batteries. If you had a bank of 4 or 6 batteries arranged in this "ladder" style, you could continually cause imbalance issues with the "far" batteries.

In many cases, what I'm describing can be a problem that doesn't actually have a lot of real world effect, depending on the application, but it is also very easy to counteract by arranging the batteries like this -



This arrangement allows for a much better sharing of current use and correct charging and is not terribly difficult to achieve in most setups.

So now let us examine a common series-parallel arrangement, like the M939, M915, FMTV, HET, etc...



We take four 12v batteries and make two banks of two batteries in a parallel arrangement.



Then we add another wire to complete the series-parallel circuit, hooking the two parallel banks into series with each other.



There are several places that wire could be...



Something I do not know is if there is a preferred arrangement between the two shown above. I know all will work, but I would imagine that one is superior in some way.







And where you grab the 12v from probably has an effect as well. Again, though all of these will work, there is probably a superior arrangement to be made here - and this is the information I am seeking. Anybody have any input?

It could very well be that factors like heat, age, quality of batteries, frequency of use, and other things contribute to a greater amount of "x factor" than ideal wiring arrangement, but I like getting things as close to perfect as I can, especially when it doesn't cost me anything.

 

[Castle Bravo]

No replies. Either no one knows or no one cares... hmm.

I looked into it some more and made some diagrams. The first diagram of each vehicle is what the TM shows for the battery arrangement. The second diagram is the same arrangement configured electrically the same, but physically different to match the previous diagrams in the post above.

First the primarily 24v vehicles -







They're basically all the same. They have the negative ground and 24v positive connections on the opposite corner of the 4 batteries (as opposed to the "ladder" arrangement) like I've described in the post above.

12v and 24v vehicles -







The M915 arrangement uses the "ladder" style and doesn't seem ideal to me. The LMTV and HET are pretty close to each other, with the HET having what would seem like an ideal setup for a vehicle using both 12v and 24v.

The M1070 HET uses 12v and 24v power in different parts of the truck throughout the operation of the vehicle, so perhaps it is arranged differently because of that. The M915 essentially uses 24v only to start and then 12v to run the normal operations of the vehicle. I am unsure of the electrical operation of the FMTV. I should've looked deeper into the technical manual. The M915 I am referencing here is just that, the M915 - not the later Freightliner manufactured M915A2, M916A1, etc trucks that do have a different charging system and battery arrangement.

The 12v usage on the M809 and M939 trucks are quite small. As far as I know, the only 12v use on the M809 trucks is the intake heater glow plug and the only 12v use on the M939s is the low setting on the heater. I am unsure of any 12v power usage on the HEMTT. My diagram is probably for early HEMTTs. Later models with electronic engines and transmissions likely use some 12v power and have a method (alternator or battery equalizer) of providing that power.

Something worth noting as well is the charging systems of the various vehicles. The M809, M939, and HEMTT vehicles all have 24v chargers. The M915 and FMTV trucks have dual voltage alternators (though rectified in different ways) to make 12v and 24v. The HET has a 12v and 24v alternator - the 12v alternator is hooked to the 12v "pair" of batteries and the 24v alternator is hooked to all the batteries.

 

[richingalveston]

In your first post the fist diagrams will not get you 24v unless you have 24v batteries. With two 12v. batteries you must have series circuit not parallel to get 24v.

I think it is somewhat truck specific to the best arrangement. two alts, one 24v and one 12v is probably ideal each working their own systems with their own batteries.

The best thing you can due in a cucv set up is rotate the batteries every 6 months and add a solorgizer from what I am told. I do not have one yet.

 

[Castle Bravo]

In your first post the fist diagrams will not get you 24v unless you have 24v batteries. With two 12v. batteries you must have series circuit not parallel to get 24v.

Good catch, I mistakenly labeled those 24v when they should read 12v. I have corrected the error.

I think it is somewhat truck specific to the best arrangement. two alts, one 24v and one 12v is probably ideal each working their own systems with their own batteries.

You're probably right. Furthermore, its not likely the stock arrangements in each truck were done without some thought put into them.

The best thing you can due in a cucv set up is rotate the batteries every 6 months and add a solorgizer from what I am told. I do not have one yet.

Thats what I've done with my CUCV and it has worked well so far. I have rotated the batteries in each vehicle I've had with more than one battery for some time now and have had good luck. Or at least, no discernible harm done.