Piranha battery monitor - LED readout

Sounds OK to me. I have the one with a toggle switch to check main and aux batteries. It shows 3 green bars when at rest, 8 bars when running and the main one drops to one on starting.

Check with a voltmeter if unsure.

Peter

I hope this is going to be the answer to what you mean as you may already know this.

The battery at rest is 12.5 to 13v so the charger puts in around 13.5 - 14v or even up to 15v. While on charge the battery will read the full 13.5v (it will read the output of the charger or a bit less) and will stay like that for a little bit. But a charged 12v battery at rest with no input is somewhere between 12.5 and 13. So less than the car charger. This means it cant hold the higher charge for long and reverts to it's at rest voltage over time, in fact if you put a multimeter on it as soon as you turn off the car you will see the biggest drop with in a minute or less.