After a couple of months parked, your engine and transmission ECU, and perhaps the stereo radio, plus natural battery degradation, can drop the battery’s charge to below 50%. And that means a no-start is entirely possible.

A hotter issue with long-haul drivers and owner-operators is what happens overnight – or worse, over a long weekend, especially when using some kind of auxiliary heat source such as an electric blanket or a cab/coolant heater. Above and beyond listening to the radio or running reading lights, inverter use is becoming more common for powering microwaves, coffee percolators, computers, etc. After a while, all those appliances gang up on the batteries and can leave you stranded if you’re not careful about how much current you draw sitting with the engine off.

It’s important to devise strategies to minimize battery discharging, or cycling, says Mike Crull, service manager at Delco Remy. That’s because the commercial starting battery is designed to provide a sudden burst of energy for starting, not a long, slow trickle that discharges it deeply. Repeated deep cycling overnight and over weekends can send batteries to an early grave.

At TMC Transportation, Des Moines, IA, the problem of parasitic loads reached a critical `point after a successful driver education program to reduce fleet idle time.

With the engine off, drivers were using accessories in the sleeper not even available a few years ago, says Rod Simon, general supervisor at TMC. Things like cellular phones, coffeemakers, VCR’s, microwaves, and electric mattress warmers.

“The results are lost hours and jump starts, frustration for drivers and mechanics, and shortened battery life,” says Simon.

Isolated battery systems have sprung up in the past several years as one answer.
After some false starts, including an unsuccessful try at bigger batteries (900 CCA batteries overheated and failed from deep cycling, so he returned to 625s), Simon adopted a three-plus-one isolated battery system.

“It has worked very well for us,” Simon says. “Installation took an hour and a half, and it extended power to the satellite system and other accessories to 45 hours.”

Mervyn Osborne, an owner-operator based in Barriere, B.C., reports similar success with his do-it-yourself battery isolator system. He replaced one of his four batteries with a Group 31 deep-cycle battery, and isolated that one with a continuous-duty normally open solenoid – a part available at Canadian Tire for about 12 bucks.

When he’s running the engine, he closes the switch allowing current from the alternator to flow to all four batteries. When he shuts down, he opens the switch, isolating the three starting batteries from the truck’s electrical system, leaving the one deep-cycle battery to fulfill all of his electrical needs.

“I’ve left my truck for up to a week with the bunk air heater set just above freezing and still had enough power to warm up the engine with the coolant heater,” Osborne says. “I run the accessories off the deep cycle battery, keeping the other three batteries fresh for engine start-up.”

Smart Systems
Another way to address the trickle-down problem provides more electricity in the sleeper while preserving the ability to start on Monday.

Rather than taking accessory current from only one battery, some new smart systems monitor all four batteries with an ECU whose only job is to protect their charge. By sensing such starting-critical factors as ambient air temperature and engine-oil temperature, the ECU can decide when to shut down power to sleeper accessories.
Such abilities are now available as an add-on device. Also, a battery-preserving engine start-stop system has been offered by Detroit Diesel for some time (the Optimized Idle option).

When battery voltage has dropped to a predetermined level, the Detroit system will start the engine and run it to recharge the battery pack. It also starts the engine based on sleeper temperature and outside air temperature.

As use of electrical and electronic components expands, this problem of parasitic loads will become more of an issue.

Fortunately, truck manufacturers say they are actively developing their own solutions, thanks to the awareness kicked off at the meeting.

Bruce Purkey, president of Purkey’s Fleet Electrics, heads the Technology & Maintenance Council task force on key-off parasitic loads. The group’s objective is to define and develop ways to measure parasitic loads, and to develop formulae that can be used to predict their effect on battery packs.

Measuring Your Load
You or your favorite techie can determine the parasitic load on your batteries, one appliance at a time, by measuring the amount of current each one draws. Once you know what the draw is, you’re ready to calculate how many hours of that load your batteries can support.

Using a ring-type inductive ammeter, measure amperage at the battery ground cable with the door closed (eliminating drain from the dome light). If load is under 5 amperes, you’ll be better off using an in-line ammeter, which requires disconnecting the battery and hooking the ammeter in line with one of the cables.

Purkey offers several cautions: make sure to zero the meter first; if there are two ground cables, disconnect one before measuring; also, avoid magnetic interference by keeping the ring-type meter away from other cables.

In Purkey’s example, three batteries with a manufacturer’s reserve-capacity (RC) rating of 160 have a total RC of 480.

If the battery manufacturer supplies a conversion chart, use it to determine how many ampere/hours the battery pack has. Otherwise, multiply RC by a conversion factor of 0.6. Multiplying 480 by 0.6 gives 288 a/h. Reduce it by 10% to reflect a realistic state of charge, and you have 259.2 a/h available from the battery pack.

Using simple arithmetic, you can see that using up half the 259.2 a/h will reduce the battery to a 50% state of charge, an acceptable minimum for warm-weather starting

You’ve determined that you have 129.6 a/h available. Now divide that number by the parasitic load. If you discovered, for example, that your cab/coolant heater draws 3 amps on high and your fan draws 4 amps on low, you would divide 129.6 by 7.

The result is 18.5 hours, the amount of time you can draw 7 a/h continuously before the battery pack reaches a 50% state of charge.

With an accurate idea of your loads and how many hours they can be supported, you can pursue a course of corrective action.