Twice now my battery has gone flat due to a lot of short journeys during lockdown. Problem is that one minute it's almost spinning the starter motor out of the engine & the next minute it won't start the car. A clue is that the stop / start stops working but it's a long time before the battery fails altogether. Is there any way of knowing when the battery capacity is critically low? 

Measure the voltage between the battery posts before putting the keys in the ignition. 

Then compare the numbers with those here to see roughly how much there is left in the battery

https://www.google.co.uk/search?source=hp&ei=NFCkX4fyCIiSlwTVhaPwBg&q=lead+acid+battery+state+of+charge&oq=lead+acid+battery+state+of+charge&gs_lcp=CgZwc3ktYWIQAzIFCAAQyQMyAggAMgIIADICCAAyAggAMgYIABAWEB4yBggAEBYQHjIGCAAQFhAeMgYIABAWEB4yBggAEBYQHjoICAAQsQMQgwE6DgguELEDEIMBEMcBEKMCOgUILhCxAzoLCC4QsQMQxwEQowI6DgguELEDEIMBEMkDEJMCOgUIABCxAzoICC4QxwEQowI6CwguELEDEMkDEJMCOgIILjoRCC4QsQMQxwEQowIQyQMQkwI6CwguELEDEMcBEK8BOggIABCxAxDJAzoICC4QxwEQrwE6DggAEOoCELQCEJoBEOUCUMgYWIv4A2DAhQRoI3AAeACAAdUBiAHOEpIBBjMxLjEuMZgBAKABAaoBB2d3cy13aXqwAQY&sclient=psy-ab&ved=0ahUKEwiH3L7Gj-zsAhUIyYUKHdXCCG4Q4dUDCAk&uact=5

 

1 hour ago, Wino said:

Measure the voltage between the battery posts before putting the keys in the ignition. 

Then compare the numbers with those here to see roughly how much there is left in the battery

https://www.google.co.uk/search?source=hp&ei=NFCkX4fyCIiSlwTVhaPwBg&q=lead+acid+battery+state+of+charge&oq=lead+acid+battery+state+of+charge&gs_lcp=CgZwc3ktYWIQAzIFCAAQyQMyAggAMgIIADICCAAyAggAMgYIABAWEB4yBggAEBYQHjIGCAAQFhAeMgYIABAWEB4yBggAEBYQHjoICAAQsQMQgwE6DgguELEDEIMBEMcBEKMCOgUILhCxAzoLCC4QsQMQxwEQowI6DgguELEDEIMBEMkDEJMCOgUIABCxAzoICC4QxwEQowI6CwguELEDEMkDEJMCOgIILjoRCC4QsQMQxwEQowIQyQMQkwI6CwguELEDEMcBEK8BOggIABCxAxDJAzoICC4QxwEQrwE6DggAEOoCELQCEJoBEOUCUMgYWIv4A2DAhQRoI3AAeACAAdUBiAHOEpIBBjMxLjEuMZgBAKABAaoBB2d3cy13aXqwAQY&sclient=psy-ab&ved=0ahUKEwiH3L7Gj-zsAhUIyYUKHdXCCG4Q4dUDCAk&uact=5

 

Measure? How? 

https://www.amazon.co.uk/ULTRICSÂ-Digital-Multimeter-Voltmeter-Ammeter/dp/B00TM0W8ZY/ref=mp_s_a_1_3?dchild=1&keywords=multimeter&qid=1604611196&sr=8-3

measure even without open a bonnet

~7 euros from Ali

similar devices, with voltage only, will be cheaper, i searched one with Q3.0 Fast Charge

 

and shown values are correct

https://www.briskoda.net/forums/topic/477834-agm-batteries-not-efb’s/?do=findComment&comment=5362969

 

https://www.briskoda.net/forums/topic/454623-what-have-you-done-to-your-superb-iii-today/page/38/?tab=comments#comment-5370984

 

Edited 6 November, 20205 yr by MartiniB

Be aware that if you leave such things plugged into a permanently powered accessory socket, they will speed up battery discharge slightly (but at least you'll see the problem if there is one). Another member recently measured his as adding 10mA (about 50%) to the car's 'sleeping' current, ^ that one will be more with the USB sockets included. Do you leave that in place 24/7 @MartiniB?

11.5V is rather worrying.

 

4 minutes ago, Wino said:

Do you leave that in place 24/7

no, in 90%

 

5 minutes ago, Wino said:

11.5V is rather worrying.

battery were replaced under warranty, links attached above

 

On 05/11/2020 at 21:20, Wino said:

https://www.amazon.co.uk/ULTRICSÂ-Digital-Multimeter-Voltmeter-Ammeter/dp/B00TM0W8ZY/ref=mp_s_a_1_3?dchild=1&keywords=multimeter&qid=1604611196&sr=8-3

Thank you

Just bought this https://www.amazon.co.uk/Cigarette-Lighter-Voltage-Voltmeter-BuyinCoins/dp/B0092KVYGI/ref=sr_1_1?crid=22DFCFE616XC1&dchild=1&keywords=cigarette+lighter+voltage+meter&qid=1604849576&refinements=p_76%3A419158031&rnid=419157031&rps=1&sprefix=cigarette+lighter+voltas%2Caps%2C162&sr=8-1

No idea they existed.

 

Thanks folks.