Picked up a used 100Ah leisure battery for £25 — worth it or false economy?

Found a Banner 100Ah AGM at a car boot sale last weekend for £25. Bloke said it came out of a motorhome, "barely used" (yeah, I've heard that before). Took it home, chucked it on my CTEK MXS 5.0 and it charged up fine, resting voltage sitting at 12.6V. Capacity tested it with a basic load tester and it's holding around 70–75Ah by my rough reckoning, so not full capacity but not terrible either.

My plan is to use it as a secondary battery in my Transit conversion — mainly for a 12V compressor fridge (draws about 4–5A) and USB charging overnight. I've got a 200W panel on the roof going into a Victron 75/15 MPPT, so in decent weather it should be getting a good top-up each day. At £25 I figured even if it only lasts 18 months it's paid for itself compared to a new one at £120+.

What I'm less sure about is whether mixing it with my existing 110Ah flooded lead acid is a bad idea. I know mismatched batteries in parallel can cause headaches — different chemistry, different age. Thinking I might just run them separately on a battery to battery charger setup but that adds cost and complexity.

Has anyone bought secondhand AGMs on the cheap and actually had decent results? And has anyone run AGM and flooded in separate banks with a B2B — was it actually worth the faff?

ShaunTaylor94 | 47 posts

@JakeShaw96 Banner AGMs are decent quality so that's a good starting point. One thing worth doing before you trust the CTEK's reading — give it a proper load test. Resting voltage and charge acceptance can look fine on a compromised battery, but put 20A across it for 20 minutes and see how the voltage holds up. A healthy 100Ah should barely flinch; a knackered one will drop off a cliff pretty quickly.

Also worth checking the case for any swelling around the sides — AGMs can puff up if they've been overcharged or left flat for extended periods, and that's basically game over regardless of what the charger says.

At £25 you've not lost much if it turns out to be a duffer, but fingers crossed it's a genuine find. Let us know what the load test shows!

Cleggy51 | 203 posts

@JakeShaw96 The CTEK will give you a decent baseline but to really know what you've got, you want to do a proper capacity test under load. Chuck a known load on it (a 10A car bulb or similar) and time how long before it hits 12V. That'll tell you far more than resting voltage alone. At £25 for a Banner AGM, even if it's only holding 60-70Ah it's probably still decent value — those batteries new are £120+ depending on supplier. I'd say it's worth investing a bit of time testing it properly before writing it off or getting too excited. What's the resting voltage sitting at after the CTEK finished its cycle?

WillBrown81 | 312 posts

Nice find Jake! £25 for a Banner AGM is worth the gamble even if it turns out to be half-knackered. To add to what @ShaunTaylor94 and @Cleggy51 have started saying — the real test is capacity under load. Resting voltage and even a charger's assessment can flatter a tired battery. If you can get hold of a proper load tester (Halfords sometimes hire them out, or ask around locally), hit it with a decent discharge and see how many Ah you actually recover. AGMs can look fine sitting idle but collapse under any real demand. That said, I've picked up worse for more money. Even if it's only got 60Ah of usable capacity left, you're still quids in at that price. Keep us posted on what the CTEK makes of it once it finishes the cycle!

JohnMason | 891 posts

Good score @JakeShaw96 — Banner make solid AGMs so you're not starting from a terrible place. One thing nobody's mentioned yet: check the date code on the label. AGMs tend to hold up reasonably well in storage if they weren't deeply discharged, but if that battery's been sat neglected for a couple of years it may have sulphated beyond what the CTEK can recover. Once the CTEK's done its thing, put a proper load on it — even a 12V car headlight bulb — and see how it holds voltage under draw. A battery can look fine on resting voltage but collapse the moment you ask anything of it. At £25 you've not lost much if it's knackered, but if it tests well you've landed a right bargain.

JackHunt | 447 posts

@JakeShaw96 One thing worth doing that nobody's mentioned yet — once the CTEK has finished its cycle, put a proper load on it and see how it behaves under real-world conditions. A 10A load for 10 hours should give you roughly 100Ah if it's genuinely healthy, but most used batteries won't get close to that. Even 60-70Ah usable capacity at £25 is honestly decent value for occasional off-grid use. Just don't rely on it as your sole battery for anything critical until you've built up a bit of history with it. What are you planning to power with it?