It used to be when you turned the vehicle off, all electrical loads went away.  Now that today’s trucks utilize electronics full time including the extra hotel loads for driver comfort, these electrical loads (parasitic loads) increase the demands on the vehicle electrical system.  If the engine is not running, the alternator has no output and the batteries supply all the energy to power these parasitic loads.  The extent of the total load is determined by two factors:

• Length of time the parasitic load is on.
• The amount of current the parasitic load is drawing.

For example, a small cooler plugged into a 12V power plug could draw four amps per hour and be left on for the entire week-end (72 hours).  Four amps multiplied by 72 hours is 288 Amp/hours.  However a dc-ac power inverter can pull 150 amps to power an 800 Watt (Cooking power) microwave oven.  It takes five minutes to cook the item.  Five minutes equals 0.0833 hour. 150 amps multiplied by 0.0833 is 12.495 Amp/hours.

As you can see by the examples above, a small load over an extended period of time requires much more energy from the battery pack than a heavy load over a short period of time.  Most four-battery packs contain approximately 400 Amp/hours if they are in good condition and at 100% state of charge. As a rule, the depth of discharge must be kept less than 30% to improve battery life.  The deeper the depth of discharge, the shorter the life will be for all batteries regardless of the type or manufacturer. The small cooler left on over the weekend results in a 72% depth of discharge—much more than the 30% rule of thumb.

One of the biggest misconceptions in Heavy Duty Trucking is that batteries charge the same throughout the year.  They simply don’t!  In warm weather (above 40F) batteries are very easy to recharge. They can normally take as much charge that the vehicle charging system can provide. Even deeply discharged battery packs can be completely recharged in a few hours if the only issue is a discharged battery not defective batteries.

However, the colder it is outside, the less charge the battery pack wants to accept.  When it is very cold, the electrolyte in a flooded cell battery becomes like jelly and the molecular action in the battery slows down.  The battery is similar to a tube of toothpaste, difficult to get out and almost impossible to put back in.  The operator may become fooled by the way the vehicle responds in cold weather.  The voltmeter on the dash displays a correct charging range and the lights are bright so it is assumed that the batteries are charging as normal.  What is really happening is that the vehicles alternator is at the correct voltage and is powering the vehicle loads, but because the batteries are cold, the charge acceptance of the batteries is very low, sometimes as low as 2 amps per hour.  Driving times of up to 40 hours can be required to charge deeply discharged cold batteries.  While AGM batteries have up to 40% better recharge capacity across all temperature ranges they will still require more time in cold weather to recharge than driving laws allow.

Things to remember in cold weather operation:

• Discharged batteries can freeze in cold weather.
• Never try to jump-start a vehicle that has frozen batteries.
• Cold batteries will not accept a normal charge until the batteries reach an ideal temperature or extensive charging time is available.
• Cold batteries can take up to 30 hours to warm back up to room temperature.
• While jump-starts can get the vehicle started, many hours of charging may be required to recharge the battery pack.  It might be a more prudent path to change batteries as normal vehicle operation cannot recharge the battery pack adequately during a normal workday.

As you can see in the chart below, a battery that is at a full state of charge is almost never going to freeze.  However, if a battery is discharged, it will freeze very easily.  With all of the electronic components on a truck, there are two things that should be done before parking a truck for an extended period of time:

1.  Make sure the batteries are at a fully charged state and test good.

2.  Disconnect the ground cable from the battery pack.  This will eliminate parasitic loads.

Good batteries should be capable of holding a good state of charge for at least six (6) months.  If the ground cable is left connected to the battery pack, the pack will be discharged in one month.

When a battery freezes, the water in the electrolyte expands and can break connections and/or cause other damage inside the battery.  Never attempt to charge a frozen battery.  If you are suspicious of a frozen battery (sides will be “puffed out”) or you cannot hear the electrolyte slosh around when you shake the battery, you must allow the battery to thaw out before attempting to charge the battery.  When hooking up a charger, make the connections to the battery with the charger unplugged, then plug the charger in away from the battery.