PREVENTING VOLTAGE DROP
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
“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 some cautions: one, make sure to zero the meter first; two, if there are two ground cables, disconnect one before measuring; and three, 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
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.
From Trickle to Drip
Moderating a recent TMC technical session on key-off/engine-off parasitic loads, Purkey gave the following example:
Overnight, a tractor with sleeper could draw as much as 15 amperes per hour (ampere-hours, or a/h), based on the following sources:
* mobile communications unit – 2 amps
* cooler/heater – 7 amps
* television – 1 amp
* electric blanket – 3 amps
* radio – I amp
Over eight hours, 15 a/h adds up to 120 a/h. A typical three-battery system has 250 a/h when starting at full charge. After eight hours, it would be down to 130 a/h,
or 52% of full charge. A four-battery system would fare somewhat better, at 64% of full charge. Both of these are considered minimums for starting: 50% charge in warm weather, and 65% in ambient temps below 45ºF.