Help with battery monitor needed

I've just recently replaced and upgraded my battery. I'd like to try some advice I found, namely to keep your Li-ion battery always withing 20--80% in order to make it last longer. Now I'm a little annoyed that there seems to be no energy settings option in order that a message and a distinctive sound signal appears when either of these values are reached. This would help me to unplug or plug in, respectively. I'm sure there is a solution that my GNU/Linux ignorance made me not find, so I'm hoping for advice.
Thanks in advance!

P.S.
Even tried changing the settings in dconf/org/mate/power-manager but it doesn't even have an effect!