How it works

Car Batteries – How it works

The one component that gets no love and nobody thinks about, until the freezing cold, dark and wet morning you head out for the day and your car just won’t start. Sat there upset and confused at the furious clicking noises coming from the engine bay when you turn the key. Stranded. Alone. Desolate. OK that’s a bit dramatic but it is a pain in the backside.

It’s easily done and we’re all guilty, unless you’re on your ‘A’ game and use trickle chargers for storing cars, we’re all in the same boat my friends. So we’re going to talk through what a battery does, how to keep it charged and how to test what’s gone wrong.

12v batteries that are most common on cars today are maintenance free, older batteries required electrolyte refills, thankfully this isn’t needed anymore.

They have 6 ‘cells’ than run in series, each cell is approx 2.1 Volts. So a fully charged battery will be around 12.6 Volts.

The battery in your car, is really for one thing. Starting it. Once started, the car will usually use the power generated from the Alternator to top up the battery and power all the electronics on the vehicle. To keep your battery in good condition, don’t sit in the car with the headlights on without the vehicle running for long periods of time, it’s why most vehicles shut off the infotainment system after a certain amount of time, to help prevent draining the battery too much.

It’s easy to forget the time before mobile phones, tablets, handheld consoles and laptops with Lithium Ion batteries. Some readers, will have never experienced life before this.

So the biggest thing to remember, is your car battery, is not like the batteries that go into these gadgets. 

The battery in your car, is designed differently and performs a chemical process that can get upset quite easily. Unlike in your gadgets, car batteries do not like to be discharged and they certainly don’t like to be deep discharged.

This is because, when your car batteries discharge, a chemical reaction occurs that coats the conductive plates in Sulfur which means the cells surface area gets less and less until they can no longer pass a current. If you have a flat battery even once, your battery is already part way gone and is not as efficient at passing current or storing energy. 

A 12.6v Battery, would be classed as deep discharged, at 10.5v. Yep. Just 2v discharged and it has made its first step to the recyclers in the sky.  a battery in a discharged state is about 11.5v, just 1 volt less. The good thing, is that you can usually get away with this a few times, it depends on the severity of the discharge, how long it’s been left discharged, if your car has an unusual power drain which saps energy from the battery even with the car turned off and locked up. However, only a few times could be twice, could be 3 or 4, you just don’t know, so you need to find the source of the problem as soon as it starts happening, a decent battery isn’t cheap.

Ways in which a car battery dies:
You’ve left your car for a long period.
You have a parasitic draw (somethings gone wrong and is sapping power when the car is locked and supposed to be tucked up and sleeping).
Your Alternator has stopped working.
Or its plain old and it’s just had enough, (typically this will rear its head in colder months).

Now we can tell you how to test. You can visit somewhere like Halfords who will do a battery health test for free (at time of writing) and you can also buy a tester from eBay etc.

The cheapest and easiest way, is to borrow a multimeter or buy a cheap one, can be had for around £5 from a motorfactors or ebay. Set it to DC and have the probes plugged into the correct ports (There are usually lines to show you which port) We will do a video of this exact procedure in the future.

Black lead on negative and red lead on the positive, with the car off and connections disconnected, a healthy battery will display around 12.6v and can be as high or a little higher than 13v. Somewhere above 12v is where it needs to be.

Assuming your battery is flat and you’re actually showing below 12v, all the way down to 10.5v, you now need to rule out your alternator. 

To kill 2 birds with one stone, ideally, charge your battery until full. Modern smart chargers may not even try to charge your battery if it’s so low, they know it’s too far gone. If it charges, measure the voltage after it’s done and hopefully you’re at the magical 12.6v or there abouts.

If you have time, now could be a good time to see if it holds charge not connected to anything, you will usually see a little drop but a good battery will last overnight unconnected and stay over 12v, if it doesn’t, you need a new battery.

If your battery has held it’s charge, put the battery in the car, remembering red goes to positive and black goes to negative. Start the car and now place the multimeter on the terminals like before but with the car running. You should see a reading of around 14v. Again different cars, systems etc vary but a good rule of thumb, it should be around 14v. This is because the alternator is generating and pumping electricity into the battery, it needs a couple of extra volts to essentially ‘shove’ the power in.

A healthy reading on a car with working alternator.

However, if you only measure 12 volts and it’s decreasing, your alternator isn’t charging your battery and needs to be looked at.

If your battery seems fine after a charge and your alternator is working, you’ve then got one of the other possibilities, has the car been left for a long period? Were lights left on? Has the car developed a draw on power when locked up? Was it left and the weather has been cold?

A fatigued older battery will have problems starting in the cold, if you find your car slowly chugging to start once the temperatures plummet, it’s more than likely just an older battery struggling and needs replacing.

If you have a weekend car, it’s probably best to have a trickle charger connected up if it lives in a garage, this will ensure that the vehicles immobiliser and alarm won’t drain the battery whilst sat for long periods of time. 

If you don’t have a trickle charger or are keeping a car off the road for a bit, disconnect the negative terminal so that everything is completely powered off. Of course, remember you may need your radio code when powered back up so make sure if you need one, you have it before you do this.

Hopefully this article can save you a few pennies and get ahead of battery problems.

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