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How to test a lead-acid battery

How do you test a lead-acid battery? Well to do it properly, you need to take it to a workshop or a battery retailer who has a specialised battery tester like the Century BT900. But if you just want an indication on whether your battery is healthy, or potentially on the way out – we can do this easily ourselves. All you need is one of these – a voltmeter.

So to begin with, a nice easy way to check the health of your battery is looking at the state of charge indicator. This provides a snapshot of your battery’s current condition. Green means it is charged and healthy, clear means it needs to be recharged, and red means it needs replacement.

To go one step further – we need to use our voltmeter to measure the battery voltage. This can be done with the battery in or out of the vehicle, but ideally the vehicle should not have been driven for at least the past 24 hours.

Grab your voltmeter and put the positive probe on the positive post, and the negative to the negative. This will give you the resting voltage of the battery – in this case 12.7 volts. So what does this tell us?

Well what you need to learn first is the voltage range in which a lead-acid battery should be operating. A healthy, fully charged battery should be sitting at 12.7 – 12.8 volts. And at the other end of the scale, a lead-acid battery is considered fully discharged when it reaches 12.0 volts. Finally, to remain healthy, a lead-acid battery should be at least above 12.5volts at all times.

So what can we learn here?

  • At 12.7 volts, this battery should be healthy and ready to go.
  • If your battery reads 12.5 volts or below, it needs to be recharged straight away before testing it again. A healthy battery should be able to maintain at least 12.5 volts, and if it eventually falls back under 12.5 volts following a recharge, it is your first warning sign that it is becoming weaker with age.
  • In truth, the resting voltage of a battery is just one part of battery testing – but it’s an easy thing to test and a great starting point.
  • What’s more important is measuring what voltage your battery falls to once it is under load – for example, when you are starting your engine. A battery monitor like the Century BM12V can measure this for you, otherwise the vehicle should be tested properly by a professional using a battery tester like the BT900.

The BT900 can perform a full in-vehicle test, which measures how the battery performs when starting your engine, and also how your vehicle’s charging system is performing once the engine is running. Because the battery relies on the vehicle to bring it back up to full charge again after it has expended the energy required to start the engine in the first place.

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