Something every user of the modern CTEK type battery chargers needs to know is that they will not charge a completely discharched battery. They appear to need to know the battery voltage before they set themselves up for charging. I had a completely flat battery on a previous car. My CTEK charger would not work. I had to go onto a local FaceAche page to ask if someone would lend me a traditional transformer type charger. After an overnight trickle charge I took the borrowed charger back, (along with some nice biscuits) and the CTEK charger worked ok.
You can connect a partly charged 12v battery in parallel with the completely flat one and it will start charging, ( it just needs to see at least 9 volts IIRC ) you can then disconnect the donor battery after a while. The 'voltage sensing' before power is applied is down to the anti-spark and anti-reverse charging fool-proofing safeguards built into modern chargers, but as usual 'old tech' still comers in useful ( as long as you have some idea of what you are doing and don't need wrapping in cotton wool ).
Also you can get Lithium Ion batteries for motorbikes, you can use a normal 12volt lead acid charger as long as it does not have a 'de-sulphate' program step ( like some CTEK have ) - this apparently applies over 15 volts to battery and will damage a Li-Ion battery, which should not see more than about 14.4v.
I have found that a lead acid battery that has been allowed to go completely flat is damaged anyway, and time for a new one...