The display bars on the Battery MK2 (and possibly Battery MK1) are really short, so you can't properly see what's their charge level.
Also, steam engines have bars of different lengths, so it's really hard to compare the available and current performance.
The issues are present since 0.13.0. The steam engines bar display was much better in the 0.12 version.
[0.13.x] Battery loading status messed up
Re: [0.13.x] Bar display messed up
Fixed for 0.13.4.
If you want to get ahold of me I'm almost always on Discord.