It's been my experiance that with these chargers you take your chances as to if you get a good one or not. It took 3 NIB Guest units to get a good one once.
Have had the Auto mode stick on others and explode a battery in the past.
With that said....The reviews are pretty high on these. So far I've got 1 in service. [Gen 4]
I must have got lucky with mine. I used a Guest 2611 for four years and it came with the boat when I got it so no telling how old it actually was. Looks like the heat came from an external source but I’m no expert.
No heat source close by - all visible damage came from the unit. both batteries are probably ruined but fuses were blown so they may survive but both are dead. Not sure how long ago it happened. Charger is about ten year old
Sounds like a short somewhere on the battery side. Take the batterys and trickle charge them you may be able to save them but time is the killer if they set uncharged...