Battery % isn't a very real number, as I had to explain to someone else yesterday. Over time, these batteries lose capacity. What the % reflects as much as anything is voltage, not capacity, and 50% of not much is 'half not much'. The only way to put together a half meaningful % is to do what is often done with laptops and do a full charge/discharge/charge cycle to see what the present capacity really is, and guess from that based upon usage.
If you're running on your original battery in that 6100, it may well be about knackered by now. There's a good chance that it's about 8 years old.
The real question is how long the device continues to run if you manually pull the plug on it and require it to continue operating (vs. selecting sleep mode or power down). Please advice number rough number of minutes you get. That will tell us a lot.
