Why do modern cars need such huge batteries?

Find out why modern batteries are more complicated than you'd think

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If you’ve ever lifted the bonnet on a modern car, chances are you’ll have spotted that it has a much bigger battery than your 1.2-litre Corsa had back in the day. Or you may have found the battery isn’t even under the bonnet…

Here are just some of the reasons why modern cars need such enormous batteries – and, in some cases – more than one.

Cars have more electrical systems on the go at once

Cast your mind back 10 years and think about the ordinary cars we were running about in. Let’s take the Volkswagen Golf as our ordinary example. The Mk6 Golf was a pretty simple car – its driver-assist systems were limited to basic cruise control, and you still stared over the wheel at a pair of analogue dials. The most taxing things the car battery had to deal with were cranking over the engine and dealing with you playing Rolling in the Deep by Adele at full blast. Yep, 2010 was a horrible year for music.

Considering modern cars basically drive themselves, it's no surprise they require more from their batteries

Considering modern cars basically drive themselves, it's no surprise they require more from their batteries

Jump forward to the 2020 Mk8 Golf and things have changed dramatically. It’s still very much an everyday car, but it has more technology than the smart home section of an Amazon warehouse. The speedo and rev-counter dials have been replaced by a high-res screen powered by a small computer with a graphics processor that’s probably more powerful than the last-gen Xbox. The front bumper and windscreen hold a barrage of sensors and radars to feed the car’s complicated data network, which in turn will accelerate, brake and steer for you.

All that tech comes at a huge electric cost, and you need a big battery to keep it all ticking along.

Start-stop makes a difference

All that extra energy use is compounded by the fact that your car’s engine no longer runs the entire time you’re driving.

Start-stop is probably the one piece of modern car tech that places the most strain on your battery. It’s really quite hard to start an engine full of oil and metal parts sticking together, and this is what your battery and starter motor have to do every time you press the start button.

And modern cars don’t just start when you pull away from your driveway – they’ll stop and start automatically in traffic to reduce your fuel consumption and CO2 emissions, and this start-stop tech requires extra power from your battery. Also bear in mind that when your engine stops in traffic, your radio still plays and the air-con still keeps you cool – all without the battery being charged by the alternator.

If your car has start-stop, it’ll be fitted with either an Enhanced Flooded Battery (EFB) or an Absorbed Glass Mat (AGM) battery. These types of battery are far more stable than your old-fashioned ‘Starting, Lighting & Ignition’ (SLI) car battery, and can cope with big drains even when they’re not fully charged. In AGM batteries you’ll find a glass mat which absorbs the battery’s acid and means the battery can be more efficient for its size, while also giving it a better starting capacity.

A regular SLI car battery relies on the engine running constantly so your alternator can keep it in its ‘happy zone’ – which is fully charged. Once its charge drops its performance rapidly drops off as well, so it can’t cope with the extra drain caused by extra ignition cycles. Think of it as the battery’s performance dropping off exponentially as it gets discharged.

The main lesson here is this. Whether your start-stop car has an AGM or EFB battery, you must always replace it like-for-like.

If you’re struggling to find the perfect start-stop battery, Yuasa YBX7000 batteries and Halfords Advanced EFB batteries are specially designed and engineered for vehicles with advanced start-stop technology, and provide up to 270,000 starts. The same applies to the Yuasa and Halfords AGM batteries, but these can provide up to 360,000 starts.

Making cars more efficient also requires different batteries

Most modern cars run regenerative braking systems. These systems use energy captured when you’re braking to put charge back into the battery, saving the alternator some work. However, the battery will need to be able to accept the high currents from these systems without being damaged. While the low internal resistance of AGM and EFB batteries allows them to do this, high currents will damage a standard SLI battery and reduce its performance and service life. Again, this means you should always replace your car battery with one that has an identical spec.

Sometimes you’ll need to put more than one battery in your car

Modern campervans use a separate leisure battery to keep the lights on longer

Modern campervans use a separate leisure battery to keep the lights on longer

Quite often you’ll find that large, powerful cars and some convertibles will have two batteries. One is usually used to start and run the engine, while the other deals with the car’s other electrical systems – the infotainment, interior electricals and convertible roof mechanisms. Most campervans also run a second battery called a ‘leisure battery’, which powers the van’s lights and electrical outlets when you’re parked up for the night. Lots of big American pickup trucks will run two batteries in series to double the cranking power for their huge engines.

Want to know more about batteries? Find out about the 8 biggest battery myths.

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Comments (2)

  • Are they still lead-acid batteries? I wonder if it's possible to integrate those technologies onto lithium batteries to increase its capacity.

      17 days ago