For DIYers: adding distilled water to dead OEM 12v batteries

[Harmanx]

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.

 

[Tronguy]

Interesting. My understanding is that Lead-Acid batteries naturally evolve H2 gas from the charge/discharge process, the H2 coming from the water in the battery. Hence, ye olde-tyme lead acid batteries had little vent holes on the caps on each cell, through which the H2 would leave town. Not sure where the Oxygen in the water went. So, whether it was straight water evaporation or water being disassociated into its constitute gases, one would have to add distilled water from time to time.

On sealed batteries, I had the idea that the gas pressures would build up, causing the H2 and O2 given off by batter operation ti dissolve back into the water, where they would somehow react with each other to form water again, eventually, and negating the need for adding distilled water from time to time. But that implies that there might be some serious internal pressure in the battery; very likely not enough to blow anything up, but higher than atmospheric pressure. Now, add to that two things:

1. Hydrogen likes to leak through things. It's a tiny little molecule: Ask the SLS people any day.
2. A manufacturer might get the sealing process wrong.

As it happens, I used to own a Prius which had one of those sealed lead-acid batteries. The battery failed after about a year and a half of operation, in 2011. That car is still going strong with the replacement battery in 2022, picked up at Advanced Auto, forget the brand. Implication: The brand that Toyota was using as an OEM on that battery wasn't as good as the brand that Advanced Auto was selling on the shelves.

Wonder if anybody does brand comparative longevity studies on these non-deep-discharge, sealed lead acid batteries?

 

[Harmanx]

Interesting. My understanding is that Lead-Acid batteries naturally evolve H2 gas from the charge/discharge process, the H2 coming from the water in the battery. Hence, ye olde-tyme lead acid batteries had little vent holes on the caps on each cell, through which the H2 would leave town. Not sure where the Oxygen in the water went. So, whether it was straight water evaporation or water being disassociated into its constitute gases, one would have to add distilled water from time to time.

On sealed batteries, I had the idea that the gas pressures would build up, causing the H2 and O2 given off by batter operation ti dissolve back into the water, where they would somehow react with each other to form water again, eventually, and negating the need for adding distilled water from time to time. But that implies that there might be some serious internal pressure in the battery; very likely not enough to blow anything up, but higher than atmospheric pressure. Now, add to that two things:

1. Hydrogen likes to leak through things. It's a tiny little molecule: Ask the SLS people any day.
2. A manufacturer might get the sealing process wrong.

As it happens, I used to own a Prius which had one of those sealed lead-acid batteries. The battery failed after about a year and a half of operation, in 2011. That car is still going strong with the replacement battery in 2022, picked up at Advanced Auto, forget the brand. Implication: The brand that Toyota was using as an OEM on that battery wasn't as good as the brand that Advanced Auto was selling on the shelves.

Wonder if anybody does brand comparative longevity studies on these non-deep-discharge, sealed lead acid batteries?

Model 3/Y aux batteries (not sure about S/X) have a hole on the side of the top panel that the gasses vent out of. There's a tube with a connector that gets plugged into the hole. The Prius had the same hole and tube, since the battery was located inside the cabin, essentially -- and the tube was needed to vent the gasses outside of the car. I'm not totally sure why the Tesla aux battery setup also does this -- since the battery is situated outside the cabin. Perhaps it is because the battery is near to the HVAC air intake.

On recollection, the Prius battery that I easily added water to wasn't the OEM battery, but a third-party replacement. The original battery might not have been so user-serviceable.

 

[**BatteryLow**]

Can we also add new distilled water through the venting hole on the side?