Leisure battery Tests

Hi there, I'm hoping somebody here can help me out, as I'm struggling with the maths myself. My incoherence scribbles have revealed nothing! I have a leisure battery, which I discovered had a faulty voltmeter attached and this is what I was using to make sure I didn't run it down to low. I wanted to check how healthy the battery is now as a result of this so have run some tests.

I ran a 100 W bulb using an inverter for one hour. The voltage dropped from 12.81 V to 12.66 V (measured several hours later a I found it took a long time to settle). The battery is 110 AH. This one: https://www.tayna.co.uk/leisure-batteries/numax/xv31mf/

I also tested its ability to hold charge. After a full charge from the alternator it read 12.93 V. I measured it again every day for four days and it dropped 1st to 12.86 V then to 12.83 V then to 12.81 V, which it held on the fourth day.

As a separate test roughly over a week it has dropped from 12.66 V to 12.63 V.

It doesn't seem completely knackered, but I have no idea if it's holding 110 AH worth of charge. Thanks for taking a look!

You need to get it properly tested using a good battery tester. Most garages will have one and many will check for free in the hope of getting a sale out of it.

An inverter will more than likely not be running at 100% efficiency so your maths and results would be skewed.

This place may help http://batteryuniversity.com/

Hey guys, thanks for the help. I'm positive that with my tests I can get a fairly good idea of the batteries condition. Even if there is a slight tolerance with the inverter. I'd just like to know roughly what AH my battery is working to. I have followed some instructions online in regards to working this out, but something just isn't clicking. Battery University looks great, but I don't need to be an expert, I just need to work out this one sum.

AH ratings are a bit lettuced anyway. The thing might well churn out 1 amp for 110 hours, but will never do 110 amps for an hour..

Anyway, so far as i can remember, "flat" in the world of car batteries, is 1.8 volts per cell. You need to hook your inverter up when the thing is fully charged and leave it untill the terminal voltage drops to 10.8V and then do your maths..

john..

AH ratings are based on a 20 hour discharge

Hi Andy,

Yes i knew there was a specified discharge rate but could not remember what it was!! The OP needs about a 66 watt load then??

john..

yes, 66w (5.5a) over 20 hours