Focus Mk1 - dead battery?

[LordHaveMurci]

OH went to start her 2.0 petrol Focus earlier, click click click - no lights or anything. Jump started it from my car and told her to take it for a good spin to put some charge in it, bring it back and stop it then try to re-start. Re-Started no problem until she went to go out then click click click again.

Jump started again and this time I took it out for 15-20mins to put some charge in it, left it half hour and it started again.

Seems odd it went flat, had a good run last weekend. She 'thinks' it may have been an interior light but with no meter I have no way of knowing what's happening. Or so I thought, forgotten I bought a C-Tek MXS-5.0 recently, dug that out and read the instructions, they say if charging on the vehicle to earth it on the chassis NOT the - battery terminal, am I reading this right? Thought about removing the battery just to be safe and charge it off the car, would this cause issues with battery code etc?

Sorry for the numpty questions, I can drive the bloomin things but no good at fixing 'em!

[S0 What]

TBH 20 mins is not enough time to get the battery fully recharged it really needs a 24Hr charge session on a home charger, rest it for an hour then drop test it, i'd remove the battery personly as any spikes from the charger can damage ECUs ect, TBh it doesn't normaly but best be safe.

[T1pper]

If it was a while since you had a new battery this maybe the problem

[Locknut]

I think those C-Tek chargers are safe to use on batteries while they are installed. They are a new type of intelligent charger but don't ask me to explain that!

[LordHaveMurci]

Forgot the car was booked in for MoT today so jumped started it again this morning & let the OH take it to the garage for them to deal with!

Twas a knackered battery as suspected, thanks for the replies.