OPTIMA® Mailbox- Parasitic Draw in Stored Vehicles

Today's OPTIMA Mailbox question is one of the most-commonly asked questions and comes to us from David K:

 

Now that I drive my wife's old car, my truck is driven perhaps 60 miles in only one trip a week. It is an all stock '08 Silverado with nothing additional hooked up to it electronically. Can it go only driven once a week with a REDTOP® without having to be charged or maintained?

Thanks for asking David! Even though your truck is basically stock, many newer vehicles are now equipped with a significant amount of electrical accessories that continue to draw current, even with the engine off. These accessories could include factory car alarms or in the case of your truck, OnStar. Fully-charged, your REDTOP will measure about 12.6-12.8 volts.

Anytime any battery (OPTIMA or otherwise) is discharged below 12.4 volts and left sitting in that state, sulfation can begin to form, which diminishes both capacity and lifespan. Additionally, most alternators are designed to maintain batteries that are near a full state of charge. Relying on an alternator to repeatedly recover batteries that have been deeply-discharged (especially if the vehicle needs a jump-start) can lead to a cycle of dead batteries and jump-starts, until either the battery or alternator fails.

There are a couple of options for determining how much current your truck draws while it is in storage. The first is to measure the voltage after you park it (you may want to leave the headlights on for a few seconds, to help dissipate the surface charge on the battery) and again, right before you drive it. If your battery measures fully-charged when parked and doesn't drop below 12.4 volts while it is in storage, you should be ok.

Performing a parasitic draw test is a little more involved, but it gives you a much more specific idea of your vehicle's electrical consumption. We explain how to measure your draw in this video. Once you know your parasitic draw, some simple math will give you an idea of how long your battery can sit before it is discharged.

For example, our Group 34/78 REDTOP has a capacity rating of 50 Ah when fully-charged. A 45-milliamp draw means a vehicle is discharging the battery at a rate of 1.08 Ah (.045 x 24 hours) per day. This means the battery will be dead (0% state of charge) in just over 46 days (50/1.08) without any charge going to the battery. These calculations assume the battery is fully-charged when it is marked (many are not) and does not take into consideration climate, which can shorten (heat) or lengthen (moderate temps) these timeframes.

It should also be noted that different vehicles will require a different minimum voltage to start, which is probably something above 0% state of charge. It's also worth mentioning that even if a vehicle doesn't sit long enough to completely discharge the battery (for example, the one week your truck typically sits), this repeated partial discharge and the sulfation that will form in the battery resting in a discharged state, will cause long-term reductions in performance and lifespan.

That is why we emphasize the importance of maintaining at least 12.4 volts and recommend a quality battery maintenance device for any vehicle that doesn't see regular use. Another alternative is the installation of a quick disconnect, like the one pictured here, which can prevent fully-charged batteries from being discharged while a vehicle is in storage (you will lose your presets though)-

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