Testing dead car batteries.

[optima21]

yesterday I broke down in my car as I had a flat car battery. I called out one of the recovery companies and when they arrived I told them of the issue. I'd driven for 30 minutes to get there, I was there for a couple of hours with nothing electrical switched on and there wasn't even enough power to crank it over a a couple of times.

The recovery guy then asked if I wanted the battery testing and to be jump started. my reply was that the battery was fine when I set off so it should be fine now, but you can do an alternator test. with an output of 12.7V it looks like my diagnosis was right and it was the alternator that had gone. (really annoying as is too much voltage to illuminate the battery warning light)

so back to the reason for my post, how can you test if a dead car battery needs replacing, my way of thinking would be that you can't as there is nothing to test and its just a ploy to get me to buy a new battery from them.......with several different sized ones in their van.

I do know that if I cycle the battery that was dead it'll be fine for while longer.

 

[Shox Dr]

These new digital testers can test a dead battery. Lancol is one make.

 

[Wightsparks]

You would use a load tester. But if you are getting 12.7v with the engine running then the Alt is dead. Battery likely fine.

Also if you run out of battery driving with a dead alt, it's not pretty! Every warning light goes off followed by the power steering followed by a very hasty pull over. You can guess how I know this!

 

[gaz1]

its the amps you need to look at opitma

sometimes you can revitalize a dead battery up to full usage again

 

[123hotchef]

what vechile is this on? it is not always the alterntor it can bet the clutch in the alt pulley too

 

[optima21]

its an old car I should have said, its a 1988 VW polo, it doesnt have this new fangled technology on it, I had the same issue with the last alternator in it about 5 years ago, which is why I prompted for the alternator test. anoyingly it was less than a year after I repainted it.

I drove the car home with the recovery driver following me (30 minute dive), it was fine until I got home and then it wouldnt turn the engine over again.

with hindsight I should have said that I didnt feel safe driving it, got him to tow me to save my petrol

heres the last time I was playing about with dying batteries

 

[tom 748]

battery will read 12.2,on start up should read 14.4 ish on tic over,that tells you the alternator is working,below 12 and the battery is on its way out

 

[Rig Pig]

Modern batterys seem to last for ages then just die stone dead, not like the old ones that would slowly fade away.

 

[123hotchef]

strip the alternator might just need new brushes or reg

 

[Wightsparks]

Well that was eerie - an hour after my post I drove 8 miles down the road in the Saab and the battery light comes on! Switched everything off and it drove back home so probably the alternator or the belt.. Time to break out the meter

EDIT 12.2v at rest (after 30 mins of running the car back home with no charge) and 11.8v running, belt is good so out with the alternator. Sorry Op I'll get off your thread now!

 

[johnik]

with a voltage like that it could be your alternator is kaput, sometimes tho, it will read low if the battery is duff, connect another battery to the car and see what happens, should be high 13-low 14 volt range

 

[rikrobson]

I Got a Digital tester from ebay the other day. Does a faily good test. you simply tell it the capacity of the Battery in Ah or whatever the unit the Battery is rated in then it does its thing and gives you the status. It will Work our CCA for example and also the internal resistance,

I swapped out a Battery on the x-trail as the Mrs said it ust would not start one day. after getting a new battery with 50% off from ECP it got the tester and it indeed says replace as its not giving the full CCA.

It also does in incar test and measures the charging circuit too

 

[Shox Dr]

I Got a Digital tester from ebay the other day. Does a faily good test. you simply tell it the capacity of the Battery in Ah or whatever the unit the Battery is rated in then it does its thing and gives you the status. It will Work our CCA for example and also the internal resistance,

I swapped out a Battery on the x-trail as the Mrs said it ust would not start one day. after getting a new battery with 50% off from ECP it got the tester and it indeed says replace as its not giving the full CCA.

It also does in incar test and measures the charging circuit too

What make of testerdid you buy. Andwhat did you do with the battery from ECP

 

[slim_boy_fat]

How old is the battery? As @Rig Pig says ^^, they can literally be fine in the morning and turn their toes up by teatime.

 

[earthman]

Modern batterys seem to last for ages then just die stone dead, not like the old ones that would slowly fade away.

Yep, that's what happened to the battery in the wife's car recently.

 

[Shox Dr]

Alot of batteries have brain these days, they just shut down below manufacturer determined voltage

 

[rikrobson]

the tester was a BA101. listed as 12v cal oad barret tester. Your finest Chinesium. looks loike they put the price up to £40. I paid £35

The Battery from ECP was the good replacement. the old one was knackered

 

[Wendelspanswick]

The new digital testers use a resistance test rather then load test, they measure the internal resistance of a battery and then calculate the projected capacity compared to the original stated capacity.
As a battery sulphates up the resistance rises, a decent battery has a resistance of about <4ohms whereas I have batteries that have a voltage reading of 12.3v but an internal resistance of >40ohms, try and crank from it and the voltage drops to nothing and the starter will just click.
My digital tester was £25 on eBay, I'm slowly replacing the duff batteries on my vehicles by buying batteries off of Facebook, I bought a Yuasa battery that has a 94% capacity of it's original stated capacity for £20 last Saturday, new price was about £85.

 

[rikrobson]

I spent £56 on a brand new one from ECP with their random discount service.
It happened to drop in my email inbox just after the battery started playing up

 

[Wightsparks]

Alternator testing - this has to be one of the best videos like this I have seen.
Useful, concise, in depth video made by a professional that knows what he is talking about and how to convey it.