How Long Do Car Batteries Last?

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Your car’s battery is often the only thing between you and a safe ride home, so knowing some basics of car batteries can keep you from being stranded — or being left at the mercy of roadside assistance delays.

by Kelly Taylor

How batteries work: a quick breakdown

A car battery contains six cells, each of which delivers 2.1 volts of electricity for 12.6 volts at full charge. Each cell has two lead plates, an anode and a cathode, immersed in an electrolyte, which is typically sulphuric acid.

A chemical reaction between the plates and the acid deposits lead sulphate on the surfaces of each plate, giving off free electrons — electricity — in the process.

Charging a battery is the reverse of that process: applying a reverse current to the battery terminals separates the lead sulphate from the plates and it reforms back into sulphuric acid.

A battery fails when it can no longer separate the lead sulphate from the plates and thus “Won’t hold a charge.”

What does that mean for battery life?

If you guessed that the depositing of acid onto the plates and the transfer back to the electrolyte can occur a limited number of times, you’re right. One of the key rules in physics is there can be no “perfect” machine: every system, including the chemical process inside a battery, will experience some loss.

So a battery failure is always only a question of when, not if. Also, lead sludge can form at the bottom of a cell, eventually shorting out the plates.

Estimates of battery life vary, from as short as six months to as long as 48, and some batteries are marketed with four- or five-year warranties. If your battery is out of warranty, congratulations, you’ve treated it well. But keep an eye out for the signs of its demise.

Can I buy my battery more time?

Yes, you can, and one of the key ways to do that is also the simplest: keep the battery in a vehicle that is driven regularly. Batteries don’t react well to being allowed to discharge to 50 per cent or less of capacity, and your car’s alternator is ill-equipped to deal with such a drastic discharge.

Since batteries will naturally discharge over time, a vehicle that sits for a week or more between drives will put more strain on its battery than a vehicle driven daily.

If you park outside in a cold winter climate, consider the use of an electric battery blanket. Keeping the battery warm helps slow the natural discharge process, which speeds up in the cold. That helps to leave you both with enough cranking power to start the engine in the morning and with less overnight discharge.

electric car battery blanket
Above: Here’s what an electric battery blanket looks like (that’s alright, we didn’t know what one looked like either). They sell for about US$70 or so and help ensure reliable starting all winter long while improving your battery’s life.

This next step won’t necessarily buy your battery time, but it will help avoid a false indication your battery is dead: keep the terminals clean. The chemical process can leave acid deposits on the terminals, which can interfere with a good connection. Baking soda and water works, followed by a smear of petroleum jelly.

If your battery is completely toast and you’re shopping for a new one, check out some car battery options here.

What’s this AGM stuff about?

Car batteries will either be a wet cell or an absorbed glass mat (AGM) type. Wet-cell batteries keep the electrolyte in liquid form between the plates. AGM batteries suspend the electrolyte in a mat of glass fibres.

The AGM batteries are considered maintenance-free, while some wet-cell batteries require periodic top-up with distilled water. Most wet-cell batteries sold today are maintenance-free. AGM type batteries are more expensive, but typically come with longer warranties, too. Your exact pricing will depend on your local retailer’s marketing efforts at any given time.

How do I know my battery’s on borrowed time?

If your car loses its typical crank-crank-kaching! kind of quick starts, replaced by lug-lug-lug-lug-OK-already-I’ll-start, but then runs fine once it’s started, there’s a good chance last rites are in store for your battery.

If it’s hard to start but then runs well, it’s likely it is your battery and not your alternator, since the battery is primarily there to start the car and then let the alternator take over.

Don’t be too hasty to rush off to the parts store, though. Check that the terminals are clean, and, if it’s not maintenance-free and has filling caps to add water, see if the water is low. If it is, add only distilled water. If these check out, your battery is about to, as well.

What if I messed up, and my battery’s already dead?

Boosting a battery is an emergency technique, and should get you back on the road right away, but it’s not the ideal way to breathe life back into a battery. Such a deep discharge can cause your alternator to overcharge the battery, which only compounds the problem of having let it get so low in the first place.

If it’s hard to start but then runs well, it’s likely it is your battery and not your alternator, since the battery is primarily there to start the car and then let the alternator take over.

If you can, remove the battery from the vehicle and either use a computer-controlled three-stage charger to recharge it, or find a mechanic who can do this for you. This form of charging is the safest way to bring a battery up from a deep discharge.

What about storage?

If you plan to store a vehicle, baby your battery by removing it from the vehicle and storing it indoors attached to an intelligent battery charger. Keeping a charge in the battery and preventing it from freezing are important considerations for storage. Look for a charger that’s computer controlled to switch from charging to maintenance as needed.

Keep it off a concrete floor, as well: placing it on a piece of wood or some other insulator is effective.

On that note, check out our 10 Experts Tips to Store Your Car for Winter Like a Pro.

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