Christian,
When you get your battery and charging sorted, I would strongly recommend that you get a 'WattMeter' so that you can accurately determine the amount of Amp Hour discharge from the battery during a session.
Determining SOC ( state of charge ) by taking voltage readings is not very reliable.
Rob was on the money when he said your should not exceed 9 amp hours from you 18 Amp Hr battery.
This equates to an approximate maximum 50 percent discharge of the battery.
This seems to be an industry standard in order to get a reasonable life from a battery between cycles( discharge/charge).
A WattMeter will give you live voltage and discharge in either watts, watt hours, amps ( also milli amps), amp hours ( also milli amp hours ) accumulative.
They are very cheap, run from the source battery inline to the load (scope etc).
Here is an example..
http://www.ebay.com.au/itm/Digital-L...item4adb5ddd55
I do not run my gear without it being inline and at the end of a session I know exactly how many amp hours I have consumed. If I am likely to exceed 50 percent discharge during a sessions, then i know my battery is not big enough.
Steve