Odd Behaviour of the 12V Battery in MHEVs

madmadmax · Mon Jul 13, 2026 6:12 pm

Just like last year, I linked the 12V battery of my 2025 CX-90 to the VIKING 4 Amp Fully Automatic Microprocessor-Controlled Battery Charger/Maintainer from Harbour Freight before my three-week summer break. On the other hand, the battery was dead and the Viking Charger/Maintainer was flashing a bad battery symbol when I returned last weekend. I had to mechanically open the driver's door with the auxiliary key since the key-fob wouldn't open the door, so I could open the hood. Following multiple unsuccessful attempts to fix the original Mazda battery using the Viking, I decided to swap it out for a Costco Interstate's Group 35 AGM battery. This is the same battery that powers my 2018 CX-5; after three weeks of sitting idle without being connected to a charger or maintenance, it started up without a hitch. I spent $180 (including tax) on a brand-new Interstate AGM battery with a 6/26 sticker. Everything is working properly now that the part is replaced. My next move was to reconnect the Viking charger/maintainer to the original battery in the Mazda. The screen miraculously indicated that the battery was charging just fine. To keep it charged, I let it sit overnight. By the following day, the battery had been completely charged. Strangely, it charges when removed from the car but not when connected to it. For the $15 core refund, I suppose I have not returned this battery to Costco just yet. Does anyone else's 12V battery act strangely?

Benjamin_RR · Mon Jul 13, 2026 6:13 pm

My best guess is that the Viking charger messed with the car's software that controls the battery life, but I have no idea what happened. It appears like these new cars have a lot of built-in diagnostics and failsafes. Might the 48V inverter be acting as an integrated trickle charger for the 12V battery? If that's the case, the Viking might have ruined it in some way.

madmadmax · Mon Jul 13, 2026 6:13 pm

However, I successfully completed a similar task last summer.

Benjamin_RR · Mon Jul 13, 2026 6:14 pm

Yeah, it's quite peculiar. I would never trust my CX-90 with a screw driver from Harbour Freight.It's likely that the Viking is establishing a satellite connection with the Chinese Communists.

2,000 miles ago, I had my oil changed and tires rotated. My 12v battery was the mechanic's main concern; he checked it and reported it was "84%" good. What the fuck does 84% even mean? Just 2,000 miles?