when is a battery "dead"

It depends on a number of factors....not the least of which is "how old is the battery?"

For example, if you took a brand new battery and drained it down to that low level, then it is entirely possible that it could be brought back up and live to start another day.

However, the older the battery is (and hence at some advanced stage of it's life cycle), then the less chance there would be of resurrecting it.

There is no black and white answer to that question (IMHO).

Roachie

My wife has a bad habit of leaving the headlights on our Hilux (no warning buzzer).
Usually not noticed for a few days at a time so the battery is well & truly discharged.

If its dead flat my multi stage "smart charger" will report an error & not start the charge cycle. In this case I leave the battery on my older style basic charger for an hour or so to get a bit of life into it then swap to the multi stage charger to give it a full charge cycle.

If it gets flattened when she goes to work I just go down & jump start it then when she gets home it gets connected straight to the multi stage charger.

Either way the battery is still going strong.

Cheers
Stu

Hi Oz Doc,

1.4v doesn't sound right to me.
10.4v is dead flat. Perhaps this was the reading you thought you saw?
Each cell in a lead acid battery should supply about 2.14v in good nick.
A 12v battery is made up of 6 cells giving a total of 12.8v fully charged.

If you were reading 1.4 volts, it was more than likely a bad connection of the meter probe on one of the terminals. That is the only explanation I can think of as to why the battery would recover from a "jump start" (ie being subsequently charged by the vehicle alternator). Perhaps the terminals had been moved and a better resistance connection subsequently provided.
A high resistance connection of the original cable connections would be the most likely cause if the battery is now working OK.
The Ctek would just see a poor battery condition and would not initiate a charging current. The built in logic would register a fault condition.
The multimeter across the terminals would also be though the high resistance connection a give an erroneous reading, unless you placed the probes on the lead centre poles themselves, in which case ignore all the logic I have stated above:-)

Had one do that on my sons car when brake light switch went bad. Got 1 .6 V on my meter. Confirmed by Battery World Presumed it was stuffed but Battery World slow charged it for 3 days. Put it back in the car and its still going Ok after at least 15 months.

The Ctek did nothing because it thought there was a fault because the battery voltage was 1.4V.
What I would do is simply hook the battery up to my 25A regulated power supply I have in the shed.
After the battery has got some charge and voltage back, then hook it up to the CTek.

Alternatively, if you want to jump start it, I would run the donor vehicle at fast idle for a while so the battery gets some charge back. Then turn switch off the donor vehicle and attempt to start the one with the flat battery.