Can flat batteries be charged?

7 September, 2021   By: Ryan Suckling

Generally speaking, unlike your phone’s battery, conventional 12-volt lead-acid car batteries are not designed to be run almost completely flat and then charged up again.

Instead, regular 12-volt car batteries are designed to be able to produce large outputs for short periods (such as when starting the engine), and then undergo more shallow discharging and recharging cycles.

Deep cycle batteries can handle larger discharge/recharge rates.

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Due to gradual degradation and the continuous low power draw on modern cars such as from the clock and security system, batteries can lose their zap over time.

However, there are ways you can prolong your battery’s life. Firstly, make sure the top of your battery is clean and dry, and the battery connections are also clean and free of corrosion.

If your car is going to be dormant for a long time (such as two to 12 months), there are ways to maximise your car’s battery lifetime. While car batteries can sometimes be revived using an appropriate multi-stage charger, it’ll be more convenient and less costly in the long run to prevent them running down in the first place.

Being able to restore the charge in a flat battery will depend on the overall condition of the battery at that point in time and also what caused it to lose charge. If it is already two or three years old, you might have trouble getting it to make a full recovery.

But if the battery is losing charge simply because it’s reaching the end of its serviceable life, even if it has been charged, it won’t be able to hold its charge properly and it’ll be time for a replacement.

Jump starting is convenient but is not the same as recharging. It may be worthwhile checking whether there’s an underlying battery or alternator problem. In modern cars, jump starting can also cause damage to the electrical system, so unless you know exactly what you’re doing, it’s best to leave emergency recharging to an expert.

Once you’re back on the road after a jump start, the best way to bring your battery back to full power is to use a smart battery charger. You can later assess the overall health of your battery with a voltmeter.

To extend the life of your battery overall (especially if the car won’t be driven over long periods), consider getting a smart charger to charge it. And remember to be mindful of all the factors that reduce its life during normal usage.

What causes a flat battery?

Beyond the obvious like leaving your headlights on overnight, the most common causes of a flat battery are short trips and infrequent use. If your car is sitting in the garage for weeks on end, your battery’s voltage will steadily decrease and the chances of it going flat will steadily increase. That’s because the car’s alternator isn’t doing any work to generate a charge.

If you’re only making short trips to work and the local shops, it may be worth doing a longer trip every other weekend or so to fully charge the battery and give yourself a leg up when you get back to your usual routine.

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Other factors include the build-up of grime and dirt on the battery terminals. This is a sign of corrosion and will prevent a proper connection between the battery cables and your battery.

There are also natural elements beyond your control that effect the life of a battery. Extreme heat can weaken its charge and speed up the process of it coming to the end of its life. The same is true in winter - very cold temperatures can also affect your battery. Over time, exposure to these extremes can take its toll.

How long does a battery last?

This will vary considerably based on the type of battery and how the car is used, but generally you can expect between three to six years.

Infrequent use is one of the key factors that will reduce the life of your battery. Short trips and long periods where the car sits unused do little to retain your battery’s charge. It generally takes up to eight hours of continuous driving for the battery to become fully charged.

Although it’s worth looking after your battery to extend its life, it does have a limited serviceable lifespan and after a point it’s simply not possible to squeeze any more life out of it.

Testing your battery

Ideally you don’t want to leave the battery until it is flat to suddenly hook it up to a charger. That’s why a voltmeter comes in handy. A digital voltmeter will accurately measure the battery’s voltage and indicate how much charge it has left.

A fully charged battery should read between 12.4 and 12.8 volts. Another simple (but not always reliable) check is to turn on your headlights, and if they appear dim that could be a sign the battery is struggling. If you then start to rev the engine and the lights become brighter, that indicates your alternator is doing its job but not enough to get the battery fully charged.

Battery chargers

The best way to extend the life of your car battery is to periodically charge it using a smart charger.

While some chargers offer simple charging with a constant charge rate, we’d recommended you opt for a smart multi-stage charger, which detects how much charge is needed and gradually slows its output when the battery has reached capacity. A good quality smart charger, like RAC’s battery charger, can help keep your battery in good condition and extend its life.

You can purchase our compact battery Smart Charger online. Plus, RAC members save 10% off.

To stay safe when charging

• Gases emitted from lead acid car batteries (hydrogen and oxygen) are highly flammable. Great care must be taken in all situations, not just when recharging. Ensure you have plenty of ventilation and no open flames around you.
• Follow the specific charging instructions on the battery.
• Before you start, check the battery case for cracks. Don’t charge it if you find damage.
• If the battery has vents, check these are clear of fragments or other blockages.
• Always connect the charger to the battery before switching the charger on.
• Switch the battery charger off before disconnecting it from the battery.
• If the battery gets too hot stop charging immediately.
• Always use certified equipment like an RAC battery charger. This is essential due to advanced electrics in modern vehicles.

If you find your battery keeps losing its spark even with regular charging, and assuming there’s nothing else wrong with the vehicle, it’s probably time to replace the battery.

If you have a question about car batteries, visit our FAQ page for more information.