Parasitic Loads: Assault on a battery

Believe it or not, your battery can be dying right before your eyes, and you don’t even know it. The culprit: parasitic loads. Parasitic loads are very small charges on the battery—so small that the harm won’t be noticed right away.  These loads are typically of the order of a few milliamps (or mA—one mA is equal to one-thousandth of 1 amp) and are due to a variety of on-board electronic accessories such as computers, alarms, clocks and radios that you’ve installed on your bike.

Although small, these parasitic loads can completely discharge the battery if left alone long enough.  Therefore, it’s important to use a good trickle charger on batteries that have their cables left connected in vehicles parked for extended time periods.

So, how long can a bike be left parked without having to worry that the battery will not be strong enough to crank the engine when needed?  That depends on the battery’s amp-hour (Ah) capacity and the total parasitic load imposed on the battery.

Here’s a good rule of thumb to use to ensure the battery has enough power to crank an engine after extended storage: Do not let it discharge to less than 70 percent of its capacity.

You can use the following steps to estimate how long you can leave your battery off charge and still be sure it will be ready to fire up the engine when you want it to:

• Step 1: Determine the total parasitic load on the battery in milliamps.
• Step 2: Multiply your battery’s Ah capacity by 1,000, and then multiply the result by 0.70.
• Step 3: Divide the answer in Step 2 by the answer in Step 1 to get the time in hours.

The answer you get in Step 3 is the number of hours you can leave your battery off charge and be reasonably sure it will have enough capacity left to turn the engine over.

Let’s consider an example where you have an ODYSSEY 31-PC2150S battery in your bike, and the total parasitic load the battery supports is 50mA.  This battery is rated at 100Ah.

• Step 1: Capacity: 100 x 1,000 = 100,000
• Step 2: Capacity: 0.7 x 100,000 = 70,000
• Time: 70,000/50 = 1,400 hours

In this particular case, it will take 1,400 hours, or about 58 days, for the battery’s capacity to drop to 70 percent of its rated capacity. At that point it’s time to place it on a trickle charger to counter the parasitic load!