Spring weather finally arrives in my area.
So, break out the mower.
Turn key, and its dead of course...
I got my old time charger (the sort with a transformer, rectifier, circuit breaker, and ammeter w/o anything else or smarts, etc).
Connect it up. No current is initially drawn. Then, slowly the current rises, and rises, and rises! Eventually pegs the ammeter and then the circuit breaker trips. It has an auto-reset breaker so it came back on and did the same thing. Let it go a few times before I disconnected it.
I then checked the battery voltage with my DMM and it was something like 2-3V.
I went and got a current limited DC bench supply from my home lab and set that for about 1-2A. It did not take too long before the supply switched from CC to CV (supply set at 13V). Took off the supply and checked the battery voltage. Now at something like 10-11V. So, I put the bench supply back on and left it for about 30 minutes. I then removed it and put the battery charger back on. Charger is now sourcing ~5A with applied voltage about 15V. Left that on for another 20min or so.
Removed charger and tried to start the mower. Success! I ended up mowing for a while before other problems cropped up (failing belt) but was able to restart it a couple of times OK.
I guess I will find out in a week or 2 but wondering if this battery is EOL ? Years ago, remember a battery in my winter car (that sat all summer) was dead and took no charging current. But, for fun I left the charger on it for a few hours and when I checked it later it was taking a charge. That battery went on to last a couple of years longer.