Model S 12V battery research...

[ElectricLove]

I spent some time doing R&D on a Model S, this R&D was specifically on the 12V AGM battery, we tracked the voltage of that battery for one continuous week and the results were surprising:

Maximum Voltage was 15.93V, Minimum Voltage was 12.23V and average was about 13.43V... Now the most interesting thing I noticed is that the battery is cycled, several times a day, it is allowed to drop down to ~12.4V and then recharged to ~14.8V, every time it does this it is effectively adding 1 cycle to the battery and reducing its life by 1 cycle...

I think we have found the reason for the 12V accessory batteries failing at a higher than expected rate!

Obviously I didn't just do this testing so I can talk about it but not do anything about it though! I have been working for months on a solution and we are getting very close to having this solution available to everyone (including Tesla if they would like to team up on this!)...

The solution is a Lithium battery pack to replace the AGM, Lithium is better suited to cycling and will last significantly longer under these usage patterns! The major drawback for most vehicles is the cost, however with the Tesla batteries being as challenging as they are to access the cost of replacing a battery (time/money) off-sets a lot of the initial investment...

If anyone is interested in this solution please contact me, I don't have pricing pegged down yet because packaging is still a question mark but I'm estimating it'll be around $400 for a 30Ah Lithium battery pack comprised of very high quality LG battery cells! (incidentally the battery will only weigh about 5lbs, we are thinking of packaging in carbon fiber)

I am not just posting to advertise, I'm posting for feedback and ideas, I've presented what data I have but I'd love to see what data everyone else out there has? (on voltages, amperages, etc) Does this seem like a good viable solution to you? Does the anyone know what the stock 12V AGM battery weighs? (I'm estimating about 25lbs) Does anyone know what Tesla warranties their 12V battery for? Cost for a 12V battery swap?

Thank you all for your help and discussion!

 

[Spyder14]

It sounds like a nice idea & plan. My first question, especially if the mother ship does not implement this or something like it, is warranty. If you place that unit into your car before the warranty expires my guess would be anything that happens related to the 12 volt, is on your shoulders. i believe the 12 volt is covered under the warranty of 4 years/50,000 miles. As posted elsewhere, since it's covered under warranty, no big deal--but when is dies, so does the rest of the vehicle & it can't be resurrected by a jump or trickle charge. Or can it?

Having said that, the 12 volt certainly seems to be a weak link in an otherwise stellar vehicle.

 

[omarsultan]

Would your Li battery pack be properly properly charged and maintained by the car, which is expecting an AGM battery?

 

[ElectricLove]

Yeah, I know other manufacturers aren't going to void a warranty if you use a different 12V battery, but Tesla is... special...

The reality is that the 12V battery can't cause undue damage, it is a relatively "dumb" device in the vehicle it holds energy until the car wants it the DC/DC converter in the car will regulate the voltage on the battery so an issue in the Auxiliary power systems (12V systems) is due to an issue in the DC/DC converter. However, that won't be Tesla's stance, I'm sure...

I think there is a law in place based on the Magnuson Moss Warranty Act that legally blocks Tesla from voiding a warranty in this manner, however, again, Tesla is special when it comes to things like this...

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Would your Li battery pack be properly properly charged and maintained by the car, which is expecting an AGM battery?

This is the biggest part of why I am not just throwing it together and releasing, we've been investigating this for a few months and at this point everything is pointing to YES!

 

[harry]

Because both our Model S's are close to out-of-Extended-Warranty, we're worried about this 12V battery issue perhaps more than most. I know that the charge taper for lithium cells is significantly different from lead acid -- much faster voltage rise at the top end of charging. Why do I know that? I destroyed a pack by a few minutes inattention at the very top of a charge cycle with my BMS disconnected! Like omarsulten, I would be concerned about that end-of-charge taper with a DC\DC converter set up for the stock battery. That said, a functioning lithium battery would be a very welcome relief from our increasing worry. The price range you mentioned would not be a deterrent if it solved the "fear of failure" we now have.