Some OEM lead acid auxiliary batteries on older Teslas seem to completely rejuvenate with the addition of distilled water -- the same as with many lead acid batteries. I first learned this with my previous car -- a 2004 Prius -- which had a very user-serviceable auxiliary battery. I was about to scrap the Prius battery when I read some simple instructions online that said to check the fluid levels in the cells and add distilled water up to about an inch from the top if the fluid level isn't that high. The Prius battery worked fine after doing this -- for years (and the process can be repeated when the fluid level gets low again).
With many such batteries, their six cells are easily accessed by popping off a panel on the top. My 2018 Model 3's OEM aux battery -- not so user-serviceable, however. The panel is attached with a super strong adhesive -- and I eventually managed to remove it by hammering shims along its seams. The dead battery was indeed dry as a bone, so I added distilled water. The top panel wasn't designed to pop back on, though, so I reattached it with silicone glue (my go-to brand is called Shoe Goo) applied to the perimeter of the panel -- letting it dry with some bricks holding it down for a while.
At first I thought the remedy didn't work properly, since the problem-battery-notifications didn't go away after I reinstalled the battery. After rewatching a couple Model 3/Y 12v battery replacement videos, I remembered that I needed to disconnect the 12 volt power and disconnect the high voltage power (done via a plug beneath the back seat -- which takes just a minute and requires no tools), and then reconnect them both. After that, the problem messages were gone and haven't returned. I've been using the rejuvenated battery for a few weeks and all seems well.
Given how tricky it is to get the panel off with the OEM battery (and having to use glue to reattach it), I'm not recommending this for everyone with dead aux batteries -- just more hardcore DIYers who understand potential hazards with lead acid batteries (which contain acid that isn't good for skin or eyes -- and which have terminals that should never come into contact with metal [aside from their intended battery connectors] or other conductive surfaces/liquids). If considering doing this: look for online instructions/vids about adding water to batteries -- since there are a couple additional specifics -- and also instructions on 12v battery replacement for Models 3 and Y for their specifics.
With many such batteries, their six cells are easily accessed by popping off a panel on the top. My 2018 Model 3's OEM aux battery -- not so user-serviceable, however. The panel is attached with a super strong adhesive -- and I eventually managed to remove it by hammering shims along its seams. The dead battery was indeed dry as a bone, so I added distilled water. The top panel wasn't designed to pop back on, though, so I reattached it with silicone glue (my go-to brand is called Shoe Goo) applied to the perimeter of the panel -- letting it dry with some bricks holding it down for a while.
At first I thought the remedy didn't work properly, since the problem-battery-notifications didn't go away after I reinstalled the battery. After rewatching a couple Model 3/Y 12v battery replacement videos, I remembered that I needed to disconnect the 12 volt power and disconnect the high voltage power (done via a plug beneath the back seat -- which takes just a minute and requires no tools), and then reconnect them both. After that, the problem messages were gone and haven't returned. I've been using the rejuvenated battery for a few weeks and all seems well.
Given how tricky it is to get the panel off with the OEM battery (and having to use glue to reattach it), I'm not recommending this for everyone with dead aux batteries -- just more hardcore DIYers who understand potential hazards with lead acid batteries (which contain acid that isn't good for skin or eyes -- and which have terminals that should never come into contact with metal [aside from their intended battery connectors] or other conductive surfaces/liquids). If considering doing this: look for online instructions/vids about adding water to batteries -- since there are a couple additional specifics -- and also instructions on 12v battery replacement for Models 3 and Y for their specifics.
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