Car Batteries Are Made to Die

But discharging a car battery too far can really kill it

Everything is born or created with an expiration date. Living things die, non-living things wear out, and that can of creamed corn you’ve had sitting in the back of your pantry since the Clinton administration isn’t bulging out just because it’s happy to see you.

None of this is to say that you can’t hold back the tide of entropy for a time. Eating right and exercising can help you live a longer, healthier life, and, in the same way, proper care and maintenance of your car’s battery can help it last longer than it might otherwise.

Car mechanic jump starting an automotive battery
Lifewire / Tim Liedtke

Of course, that’s a sword that cuts both ways. In the same way that an actuary might be able to tell you the exact number of minutes another draw on that cigarette will shave off your life, every time you discharge a car’s battery you shorten its operational lifespan in a way that is simply impossible to undo. It's just a function of the science of how car batteries work.

Duty Cycles and Dead Cells

The operational lifespan of a battery is typically expressed in duty cycles. This same term is used for all sorts of batteries, so it doesn’t have a concrete definition across every application. For instance, some batteries are designed to be completely discharged, while others are designed to always have some level of charge.

Since traditional lead-acid batteries fall into the second category, a “duty cycle” for your car battery consists of a given percentage of the drain, followed by a full charge, and life goes on.

None of that should ever be an issue if everything is working properly under your hood. Under normal circumstances, starting your car will drain the battery a little, but the alternator will charge it back up as you drive. In that same way, any power that your car’s accessories use while you are driving should be supplied by the alternator, so the battery never “cycles” deeper than it is designed to.

When things aren’t working properly, and the battery discharges more than it is designed to, that is when issues arise.

For example, if you leave your headlights on overnight, and you come back to a car that won’t start, that is an example of a battery that has been discharged too far.

In a similar vein, if you notice your headlights or dash lights dimming, a charge warning light flicks on, or the voltage meter on your dash drops below 14.2 volts, those are all indicators that the alternator isn’t charging the way it’s supposed to, which can also lead, very quickly, to an overly-discharged battery.

What Happens When a Lead Acid Battery Discharges?

Lead-acid batteries aren’t particularly impressive or efficient at what they do, and they haven’t changed a whole lot in the last century and a half or so since they were invented. The basic technology is incredibly simple. Lead plates are suspended in pairs in a bath of sulfuric acid, which acts as an electrolyte.

Each pair of plates has one that is coated in lead dioxide, and when a voltage is applied, a chemical reaction occurs.

When a lead-acid battery discharges, which happens any time it provides power to start an engine, illuminate headlights or run your fancy car stereo, the plates are slowly coated in lead sulfate. This is a normal process, and under normal circumstances, it is reversible.

For instance, if you listen to the radio in your car with the engine off while your passenger jumps out to run an errand, the plates inside your battery will undergo a small amount of sulfation. Then, when you start your engine, the battery will recharge and the sulfation will reverse.

Going Deeper Than Designed

Traditional car batteries are sometimes referred to as “starting batteries,” because that is what they are primarily designed to do. Starter motors require a tremendous amount of amperage, and it has to be delivered fast.

With that in mind, the lead plates in normal car batteries are designed to be as thin as possible, which allows for the greatest amount of surface area. This, of course, is also what makes the plates so susceptible to damage from sulfation.

Car charging systems normally hover at around 14 volts, and car batteries will often read about 13 volts when fully and recently charged. With that in mind, normal car batteries are considered to be “fully discharged” at 10.5 volts, which is only about 80 percent of full.

Why Is Discharging a Car Battery Too Far So Bad?

Even though 80 percent of the capacity remains when a car battery dips to around 10.5 volts, the battery is considered to be fully discharged because taking the cycle any deeper will cause irreversible damage to the plates through excessive sulfation.

While normal sulfation is reversible, excessively draining a battery, or leaving it in a state of discharge, will allow the soft lead sulfate to crystallize. At that point, charging the battery will still cause some of the sulfation to reverse, but any crystallized lead sulfate will remain on the plates. This sulfate cannot, under normal circumstances, return to a solution in the electrolyte, which permanently reduces the available output of the battery.

The other detrimental effect of allowing crystallized lead sulfate to form is that it effectively shortens the lifespan of the battery in an empirically measurable way. If too much of this crystallization is allowed to occur, the battery will no longer be able to provide enough amperage to start the engine, and it will have to be replaced.

What Should You Do With a Drained Battery

Once a car battery has been drained below a state of full discharge, the damage has been done. All you can do is check the electrolyte and put it on a trickle charger. If this is the first time that it has been discharged, you should be able to fully charge the battery and continue using it, but every time it is discharged below that threshold of 10.5 volts, the damage is done.

It’s also important to note that jump-starting and then driving a vehicle that has a fully discharged battery isn’t good for the battery or the alternator. Even if you drive it for a long time and keep the engine revved up, it is extremely unlikely that you will be able to fully charge the battery like that.

In that way, you will end up operating the battery at or near a state of discharge, which risks further sulfation. It’s also hard on an alternator to do that since they aren’t designed to charge batteries from a state of full discharge. Alternator voltage regulators also require a 12-volt input to operate properly.

How to Avoid Draining a Battery

The best way to avoid draining your battery to the point of damaging it is to perform regular care and maintenance, which will often allow you to catch problems before they have a chance to snowball. Parasitic drains should also be dealt with immediately and not allowed to persist.

For instance, if you notice that your car is hard to start one morning, but you didn’t leave the headlights on, there may be a drain somewhere in the system. Fixing it before the battery goes dead—or before the battery goes dead multiple times—will save you money in the long run.

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