Battery DIS-charger?

[Ozbozz57]

Went to start the bus on April 20th to go camping for a few days, and the starting batteries were D E A D. Two Group 31 batteries. Like, about 1 volt. They were new in Jan 2022. Connections were all fine. I had permanently installed a 4 Amp Battery Tender about 2 years ago, and plugged it in about April 1st just to keep the batteries topped up. I had left nothing on in the bus. I replaced both batteries, and when I connected the new ones, quite a few LEDs lit up in the fuse/relay panels. The bus started easily, and when I checked the status of the LEDs, they were all out, except for "DOOR OPEN".

I managed to find a couple of batteries on Easter Sunday, and we went camping.
To play it safe, I disconnected the NEG cable while we were out at camp. No problem starting the bus to come home.

Seems very weird, but I'm wondering if the Battery Tender became a battery
DIS-charger somehow. Have any of you ever had something like this happen with a Battery Tender?

Puzzled...

 

[Jolly Roger bus 223]

if your batteries were already low on april 1st then the maintainer would not have done anything.
it only holds a good charge it will not charge a battery from low juice back up

 

[Timeline]

This 04 Bluebird has a slow battery drain even with the battery switch on off. Here in Minnesota where it goes sub-zero they only way to save the batteries is by keeping them indoors.

 

[fo4imtippin]

Went to start the bus on April 20th to go camping for a few days, and the starting batteries were D E A D. Two Group 31 batteries. Like, about 1 volt. They were new in Jan 2022. Connections were all fine. I had permanently installed a 4 Amp Battery Tender about 2 years ago, and plugged it in about April 1st just to keep the batteries topped up. I had left nothing on in the bus. I replaced both batteries, and when I connected the new ones, quite a few LEDs lit up in the fuse/relay panels. The bus started easily, and when I checked the status of the LEDs, they were all out, except for "DOOR OPEN".

I managed to find a couple of batteries on Easter Sunday, and we went camping.
To play it safe, I disconnected the NEG cable while we were out at camp. No problem starting the bus to come home.

Seems very weird, but I'm wondering if the Battery Tender became a battery
DIS-charger somehow. Have any of you ever had something like this happen with a Battery Tender?

Puzzled...

I had a harbor freight high frequency charger that i once put on a battery just to fully top it up. It was over 12v when i started, when i got back, the battery was under 10 volts. I never did figure out what happened, i put it on a dumb charger and it was all good. After parking that HF charger, i got it back out and havent had the problem again.

 

[Ozbozz57]

if your batteries were already low on april 1st then the maintainer would not have done anything.
it only holds a good charge it will not charge a battery from low juice back up

Batteries were fine, not low. I've been traveling most of the winter, starting just fine. Had the bus out on March 16th for a car show (won a trophy!) and started one other time to move it prior to April 1st. I generally have kept the starting batteries on the Battery Tender continuously, but I hadn't run the power cord back to the bus since March 16th. The completely weird thing is how severely discharged the batteries were. Barely 1 volt, both batteries.

 

[nikitis]

I know using some chargers on certain types of batteries can have negative effects if you select the wrong charging type on the charger. AGM batteries need AGM selection on the charger, for example. Using STD selection on AGM can have similar consequences. I killed a battery similary to you by just leaving it on STD when it was an AGM battery.