Flat battery while tender plugged in

I had a similar experience with a CTEK tender on my tractor battery. Tender showed fully charged, battery measured 12.9v with tender disconnected. Turned the key on and things just clicked, didn't even try to start the tractor. Battery voltage after turning on the key was 4.8v???? Looks like a new battery for me, this one only lasted 12 years.

 

"Went to do some detailing yesterday, my 575 is plugged in to a CTEK tender, ..."
****
I had a similar experience several months ago. 575 not driven as much as in the past so battery in on a tender, sometimes several months between starts. Went on a week long, 1k+ mile rally, no battery issues. Next day, battery completely dead. Placed on a charger over night and still stone cold dead. At the time, I wondered if long, continuous times on a tender, with out occasional real use might some how damages the battery. The battery was two years old.

 

"as a jump start may not work?"
*****
Years ago, I was living in a situation where I couldn't keep the car on a tender and battery would occasionally be depleted to 0. For some reason, with the dead battery in-situ, the car wouldn't jump start. I had to remove the battery and charge it, re-install and the car started.

 

Darius- Do not jump start her under any conditions. Very likely to give you a permanent airbag warning and possibly even worse ECU problems. AA will change the battery for you if you are a member.

If you want to try charging it with a full-size charger, disconnect the battery from the car, charge it, and reconnect. If that works, drive her to your technician and have him put in a new battery. That one is dying.

Those battery maintainers are only meant to keep excellent health batteries charged. As soon as a battery starts losing strength, the maintainer cannot keep up. Mine has not been on a maintainer for 12 years or so but gets driven a couple of times a week. Batteries have lasted me about 7 years on average.

 

Darius- A good AGM is what you want, around 800-900 cca. Not sure of the brands in the UK, but any of the big names selling there should have one.

 

Those alternators are electronically fragile. Running with chronically weak batteries kills them. My alternator rebuilder retired on what I paid him rebuilding those. It should put out 120-140 amps and if not its losing its rectifiers. Its a very common issue.

 

A few additional points:

- battery tenders don’t last forever either; I’ve had ctek chargers fail on me leaving the battery flat dead.
- “smart chargers” usually can’t revive stone dead batteries (despite many of them saying that have modes for this), but you can bring them back to life sometimes by putting an old fashioned dumb charger on for a bit just to lift the voltage high enough that the smart charger knows what to do;
- jump starting via another car is indeed a very bad idea, but there’s no reason you can’t jump start using a booster pack; these can’t induce the deadly voltage spikes that kill fragile Digitek electronics.

 

One way I was able to revive a dead battery was to hook up the Ctek to a good battery and daisy chain jumper cables to the bad battery. The Ctek sees the good battery as able to take a charge and will go through it's cycle. The jumpers will keep the bad battery in the circuit until it can be put on the charger by itself. No jump starting needed