Bottom Line
Maintenance Q &A

by Technical Editor Paul Abelson

On my last visit to OOIDA headquarters at Grain Valley, MO, I had the opportunity to ask some of our board members (yes, I’m a dues-paying OOIDA member, too) what maintenance issues concerned them the most. Batteries made the short list. Specific questions involved knowing when to change them, how much capacity to buy and how many to use.

To get the best, most thorough answers possible, I turned to my book of Recommended Practices (RPs) from the Technology and Maintenance Council, and to some friends who helped write those RPs: Brad Van Riper, vice president of research and development at Truck-Lite; Carl Smith, vice president of Sure Power Industries; Bruce Purkey, president of Purkey’s Fleet Electrics; and Charlie Groeller, senior staff engineer at Mack Trucks. These four individuals are the “brain trust” behind the Electrical and Instruments Study Group. What follows is a synthesis of their collective wisdom and their work at TMC.

What are CCAs?
RP 109 defines Cold Cranking Amps (CCA) as the current a fully charged battery can supply at 0 degrees F for 30 seconds while maintaining at least 1.2 volts per cell (7.2 volts for a 12 volt battery). Remember, that’s for a fully charged battery. Beware of batteries rated in “CA” or Cranking Amps. CA is measured at 32 degrees, not zero. A 900 CA battery may be only 600 CCA.

We know we need about 1,800 CCA of cranking power, but we can’t just go out and say “I need 1,800 CCA, so I’ll get three 600 CCA batteries.” There is more to be considered.

How many CCAs do I need?
Because engine oils thicken in the cold, and because it takes more cranking to raise heat-of-compression to diesel ignition temperature, amperage requirements increase as temperatures get colder. Starting a cold engine takes very high amperage for very short time periods. (1,800 amps for 30 seconds or less for most truck diesels. That’s about 15 amp-hours at zero degrees F.) While power requirements increase, batteries’ efficiency falls off as temperatures get colder. Batteries are considered to be at their maximum at 80 degrees. According to TMC RP 140, a fully charged battery will be only 70 percent efficient at 40 degrees, 55 percent at 20 degrees, and 40 percent at zero.

What else do I have to consider?
Batteries are virtually never at 100 percent charge, even after you’ve been on the road for hours and hours. They run down overnight from parasitic loads and from powering your engine-off comfort and convenience items.

Let’s say it’s a mild winter night, with the temperature in the low 40s. Your engine is off to save wear and fuel and to give you peace and quiet. You’ve got an electric blanket (3 amps) plugged in, your refrigerator (8 amps) is on, you ran your TV for two hours, had two lamps on for two hours and kept your satellite communications (2 amps) on standby. And just so no one would run into you in the truckstop lot, you kept your marker lamps, all 80 of them (1 amp each unless you have LEDs), on all night.

You used 750 amp-hours (ah), so your batteries have partially run down. Now, at 40 degrees, you crank your engine with a battery set that is 70 percent efficient and deeply discharged. It may not start. You begin the day needing a jump-start. If you’ve run the batteries down with all those lights, you learned you need to idle your engine, or else you need a generator for auxiliary power.

Won’t my alternator recharge my batteries?
Let’s say you run your engine. Your alternator power may not produce full-rated power until 5,000 alternator rpm. That may be 1,000 to 1,200 engine rpm. At 600 rpm idle, you may be getting only half the rated alternator output. Your battery is down, but at least you can start. You figure you’ll get a recharge going down the road. But unlike a passenger car, your battery box is out in the cold, not warm under the hood.

According to TMC RP 139A: “Low temperature slows down battery chemical reactions and increases internal losses. In cold weather, even a partially discharged battery accepts very little current at normal, or even slightly increased, voltage regulator settings. A fully charged battery runs very little risk of freezing. However, as the battery is discharged, the battery freezing point rises. Thus, a discharged battery can freeze and/or sustain damage before it can be recharged.”

In other words, a cold, discharged battery does not easily recharge. If the cycle repeats night after night, or if temperatures fall, batteries may fail by freezing and cracking. The electrolyte isn’t strong enough to act as antifreeze. If batteries crack, they must be replaced. But if they fail to crank your engine, they may just need to be taken indoors and be fully recharged.

How do I know whether I have enough power for my accessories and lights?
The measure of a battery’s ability to accept loads is called Reserve Capacity (RC). It is calculated at 80 degrees, so you have to calculate any efficiency loss due to temperature. RC is the number of minutes a battery can sustain a 25-amp discharge while maintaining a minimum 1.75 volts per cell (10.5 v for a 12 v battery). When batteries are in parallel, as truck batteries are, RC, like CCA, is additive. If you have three 900 CCA batteries, each with 200 minutes RC, your battery bank will have 2,700 CCA (at 0 degrees) with a reserve capacity of 600 minutes (at 80 degrees).

To determine your RC needs, total the amps of each device you’re using. Multiply that by the time (in hours) each will be on. Don’t forget the parasitic loads that continue to draw power even when the truck’s key is turned off, such as current to keep computer chips warm. Determine parasitic loads by disconnecting your final ground cable and running a multimeter between the battery post and the cable. That tells you what your background current flow is in amps when all devices are turned off. When you have the total amp-hours, divide by the efficiency loss due to temperature, and multiply that by 1.67 to determine how much Reserve Capacity you need. The 1.67 is a constant that converts amp hours to reserve capacity in minutes.

How many batteries do I need?
To determine how many batteries you’ll need and how many CCAs you should have, you need to know what climate you’ll run in and what comfort and convenience items you have. This assumes you do not have a generator or use shore power. Three 1,000 CCA, 225 minute RC batteries will save weight and cost, but if you need greater reserve capacity, you’ll be better off with four 750 CCA batteries, each with 200 minutes.

What else should I look for?
Avoid bargain batteries. They are often lighter weight because they do not have as much lead inside, and thus have less RC. The incremental cost of heavier duty batteries is minimal compared with more frequent replacement and possible road service calls. This is one item where having the best possible product will pay off, in life and reliability.

How often should you replace batteries?
Typically, group 31 batteries in trucks last a minimum of three years with reasonable care. Batteries weaken with use, especially when they are deeply discharged. Too many discharges weaken the plates. When they experience shock and vibration from normal road use, some plate material flakes off. There are wells at the bottom of all lead-acid batteries to catch the flakes and to keep the plates above the residue. Eventually, either the plates wear through, or weaken and break, or the flakes build up and short-circuit the cells.

Can I test my batteries with a volt meter?
If you suspect a battery is bad, use a carbon pile battery tester to put a load on each battery. Dealers and well-equipped shops should have them. You may find your problem is not the battery after all. It could be the alternator or voltage regulator. It might also be a hidden short circuit draining the batteries. If you do find that one or more batteries are bad, and they’re all more than three or four years old, replace them as a set. If they’re fairly new, test them all, but just replace those that are bad. And, if your batteries are still under warranty, don’t forget to take advantage of it.

Have a maintenance question? 
You can write to Paul Abelson, technical editor, in care of Land Line Magazine, PO Box 1000, 1 NW OOIDA Drive, Grain Valley, MO, 64029; or you can fax information to (816) 443-2227; or you can e-mail your question to 

Please mark your message Attention: Maintenance Q&A. Although we won’t be able to publish an answer to all questions in Land Line, we will answer as many as possible.