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The ins and outs of dual-battery systems

Brad Nelson Published on 06 May 2015

batteries

I was in Portland, Oregon, sometime in the 1980s at Jubitz Truck Stop. I had spent the night in the sleeper of my hay truck, and when I pushed the start button, I got less than a weak grunt from the system. I had recently refurbished the battery cables, and the truck had been starting with vigor. Four hundred miles from home was not a nice place to be with a vehicle that would not start.

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I had not two, but four batteries – all 6-volt. Each pair of two batteries was wired together in series, that is, the positive post of one battery was connected to the negative post of its mate with a short battery cable that only connected the two batteries together.

The remaining posts of that pair of batteries, one positive and the other negative, now produced 12 volts of electricity. The other pair of 6-volt batteries was wired together in the same manner. The bare positive posts of each pair of batteries then connected, via two long battery cables, to the starter and alternator of the truck. Another pair of battery cables connected the bare negative posts of each pair to the ground side of the truck’s starter.

At one time, it was common for heavy trucks to have a 12-volt system with a 24-volt starter. A series-parallel switch was used to reroute the current so that 24 volts was supplied to the starter.

If four 6-volt batteries are wired together with all wires that connect one battery to another are positive to negative (three short cables that only connect one battery to another), then the current from the bare negative post and the bare positive post would produce 24 volts.

The series-parallel switch would change the flow of the electricity to produce 24 volts for the starter and then let the system revert to 12 volts. As adequate 12-volt starters became available, the repair for issues with the usually troublesome series-parallel switch was to eliminate the switch and replace the 24-volt starter with a 12-volt starter.

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As battery technology improved, the change away from multiple 6-volt batteries wired some in series and then the pair wired parallel to a bank of 12-volt batteries wired parallel became the industry standard.

Many loaders and other heavy equipment found on dairy farms are full 24-volt systems. Most of these operate with multiple 12-volt batteries wired together in series. When servicing, charging or jump-starting one of these systems, it is critical to identify the polarity (positive or negative) before attaching anything to the batteries or the battery cables.

To use jumper cables from a 24-volt system (that uses 12-volt batteries) to a 12-volt system, run one jumper cable from the positive post of one battery on the vehicle being used as the power source. Attach the other end of this jumper cable to the positive post of the vehicle being jump-started.

Attach the other jumper cable to the negative post of the same battery on the power source vehicle or machine to which the other jumper cable is attached. This way, you are only tapping into 12 volts.

If you jump-start a 12-volt system from a 24-volt system with 24 volts, you risk melting down the entire electrical system on the unit with the 12-volt system.

The reverse is true. If your 24-volt loader won’t start because of a weak battery, and it uses a pair of 12-volt batteries wired in series, you can jump it from a 12-volt system, but only one 12-volt battery at a time. Two units operating 12-volt systems can team up to jump start a 24-volt unit with two sets of jumper cables. One 12-volt unit would connect to 12-volt battery on the unit being started, the other unit to the other 12-volt battery.

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This usually takes a few minutes to get your mind around what you are doing using jumper cables from 12-volt systems to 24-volt systems, and vice versa. Mistakes made may cause the term “spark idiot” to be used.

It’s less trouble to keep the charging and starting systems in good repair so every piece of equipment will start by itself. (This statement was made by a fellow who never experienced having to start a loader at -20ºF on the morning after the power had been off for 12 hours and the block heater used to keep the engine on the loader at an easy-starting temperature was now also at -20ºF.)

Multiple battery systems have their own set of antics. My Dodge diesel pickup with dual batteries declined to start one day. One of the four battery cable ends was loose on one of the batteries. Tightening it and some time on the battery charger brought it back to operating like new.

Then it did it again – this time at a campground close to 100 miles from home. Did you know that it takes a borrowed 10-amp battery charger the better part of three hours to charge up a pair of 12-volt batteries to where they will start a diesel pickup in warm weather?

The battery cable end that kept coming loose was developing a crack that was unnoticed. Once it was found, and pricing a new battery cable, I made a steel clamp helper that has solved the problem now for at least a year.

A loose connection at a battery post will prevent the battery from taking a charge while the vehicle or loader it is on is running. Electricity is like water. It will flow in the direction of least resistance.

Electricity will flow from a battery, through a loose connection, to the rest of the vehicle easier than current from the alternator will flow back into the battery to recharge it. When this situation shows up, it is usually in the form of batteries that are near totally discharged.

The usual repair is to pitch a perfectly good pair of batteries and replace them with new batteries. If the change also corrects the loose connection, it will appear to all that the batteries were the issue. One loose connection anywhere in the system can foul the whole works.

My situation in Portland, Oregon, many years back turned out to be one battery that had developed a dead short internally. I found this by using a $1 battery hydrometer and found that one cell of the offending battery had a much different reading than did the other battery cells. I replaced that one battery, and all was well.

Heat and sparks are the indicators of loose connections. We once solved an intermittent starting problem by having one person operate the starter while others watched battery cable connections in the dark. We had a very slight spark where the cable fastened to the starter. When we tightened that one, all was once more sunshine and roses. PD

Brad Nelson is certified by Carlisle Motion Control Industries Inc. in brake maintenance and is a trucking consultant, with more than 30 years experience in the trucking industry.

PHOTO
These two 12-volt batteries are wired in series. The output from the pair is 24 volts. Photo courtesy of Brad Nelson.

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