Hi shack, seeing as you want to play with charging older batteries, if you have a lithium battery, anything of 50Ah or bigger, full charge the lithium battery then disconnect the charger.
Now get a set of jumper leads and connect the lithium battery in parallel ( + to + and - to - ) to any type of lead acid battery ( AGM or Wet Cell ) in any state of discharge and leave them like that for a week.
Before connecting the lithium battery, do a load test of the lead acid battery you are going to charge.
After the week, disconnect the lithium battery and load test the lead acid battery again.
You might be in for a big surprise.

