State of Charge, SoC or it's opposite Depth of Discharge, DoD have been mentioned some what here. The first term is used more then the second.
Two ways to get a rough idea of your battery charge condition are NO LOAD resting battery voltage, resting is typically 4 to 6 hours, or the NO LOAD Specific Gravity, S.G. readings. While both will give you a ball park idea, both need to be temperature corrected to get you within the infield.
All of this varies from one manufacturer to another and for the various battery construction.
For a more in depth detail on this here are three references. They also explain why voltage or S.G. readings aren't very good indicators of SoC.
1.
Smart Gauge This also wants to sell you a battery monitoring system but their write up is good.
2.
Battery FAQ's Good reference, and not trying to sell you anything.
3.
Battery University another good reference and no axes (selling) to grind.
Hopefully this will help with understanding your TM battery(s) a little bit more.