Using jump starter to rescue a failed 12V battery

[rrolsbe]

Since it is possible the 12V battery could fail in a shorted mode and a the tow truck drivers jump starter might not work to wake the car, has anyone tried disconnecting the negative lead from the battery and operate the car directly connected to the portable jump starter? The jump starter should only need to be connected until the contactors close supplying HV to the DC-to-DC converter and not needed again until the car needs to be woken from the sleep state. I do not currently own a functioning jump starter or I would perform the test myself and report the results to this forum.

Thanks In Advance!
Ron

 

[rrolsbe]

I have verified a 9V battery will open the frunk so I carry one in my pocket along with my house keys anytime I drive. A better solution might be to purchase a 22AH AGM 12v battery about $45 shipped. I already own a Smart Battery charger to charge AGM batteries. I would think a 22AH battery would supply enough power to close the contactors? I always like to be ready for Murphy. It is a given the 12V battery is going to fail at some point and you never know when it going to happen. If it happens at a hiking trail head that has no cell service, accessing the emergency battery and 10MM wrench located in the frunk using the 9V battery might make for a much better day. There are other forum threads where owners are discussing their hassles getting a 12V battery replaced under warranty. The emergency spare battery might allow these owners to keep driving their car until Tesla replaces the battery under warranty.

 

[n2mb_racing]

I have verified a 9V battery will open the frunk so I carry one in my pocket along with my house keys anytime I drive. A better solution might be to purchase a 22AH AGM 12v battery about $45 shipped. I already own a Smart Battery charger to charge AGM batteries. I would think a 22AH battery would supply enough power to close the contactors? I always like to be ready for Murphy. It is a given the 12V battery is going to fail at some point and you never know when it going to happen. If it happens at a hiking trail head that has no cell service, accessing the emergency battery and 10MM wrench located in the frunk using the 9V battery might make for a much better day. There are other forum threads where owners are discussing their hassles getting a 12V battery replaced under warranty. The emergency spare battery might allow these owners to keep driving their car until Tesla replaces the battery under warranty.

Could be even bigger than you need. I'm not sure if the model 3 needs very much power at all from the 12v. Maybe one of these would work?

https://www.amazon.com/dp/B07X7MD2JK/ref=cm_sw_r_cp_apa_i_Hy7eFb9EXP02X

There's a smaller 8ah version too, but it's only good for 20A peak draw. Also, they are handy for backup emergency power. They have a BMS built in, so no need to worry about balance charging, etc.

 

[rrolsbe]

Could be even bigger than you need. I'm not sure if the model 3 needs very much power at all from the 12v. Maybe one of these would work?

https://www.amazon.com/dp/B07X7MD2JK/ref=cm_sw_r_cp_apa_i_Hy7eFb9EXP02X

There's a smaller 8ah version too, but it's only good for 20A peak draw. Also, they are handy for backup emergency power. They have a BMS built in, so no need to worry about balance charging, etc.

Thanks for the post! That battery looks like it could be a good candidate. I have used my digital MM to measure the current draw of the 12V AGM battery. While sleeping the current draw was 200ma or less when the car woke up the highest charge current I monitored was around 1A and that only lasted for less than one minute. I have since purchased a DC meter and a current shunt to perform future current measurements. If I had a digital scope, the voltage across the shunt could be measured to determine the current required to close the contactors (it appears both contactors close within about 5 second). Maybe a meter with a peak and hold function could also work? It appears the 12V AGM battery is normally only required to deliver low outbound current levels and closing the contactors is probably the most power delivered in normal operation; however, If the DC-to-DC converter failed, It would be interesting to see how the 12V systems would react especially the power steering and audio amps which can draw high current levels. Maybe you could disconnect the 12V cable connecting the DC-to-DC output under the rear seat and the Power Distribution System located on the firewall to simulate a failed DC-to-DC converter? Has anyone had the DC-to-DC converter fail? could you still drive the car? did anything not work?. how long did it take to deplete the 12V battery?

 

[camalaio]

Thanks for the post! That battery looks like it could be a good candidate. I have used my digital MM to measure the current draw of the 12V AGM battery. While sleeping the current draw was 200ma or less when the car woke up the highest charge current I monitored was around 1A and that only lasted for less than one minute. I have since purchased a DC meter and a current shunt to perform future current measurements. If I had a digital scope, the voltage across the shunt could be measured to determine the current required to close the contactors (it appears both contactors close within about 5 second). Maybe a meter with a peak and hold function could also work? It appears the 12V AGM battery is normally only required to deliver low outbound current levels and closing the contactors is probably the most power delivered in normal operation; however, If the DC-to-DC converter failed, It would be interesting to see how the 12V systems would react especially the power steering and audio amps which can draw high current levels. Maybe you could disconnect the 12V cable connecting the DC-to-DC output under the rear seat and the Power Distribution System located on the firewall to simulate a failed DC-to-DC converter? Has anyone had the DC-to-DC converter fail? could you still drive the car? did anything not work?. how long did it take to deplete the 12V battery?

If I'm recalling correctly, if the PCS (DC-to-DC converter) fails, it will choose not engage the contactors and therefore can't do much of anything besides living off the 12V for things like unlocking the car. I imagine it might have more specific failure modes than I'm thinking though, and could be a bit more nuanced than that.

It's failed for a few people on here. The result is that the next drive will not be possible. The 12V battery cannot support the car for very long without the PCS, but just how long depends on a number of factors. Some have had it die before the tow truck arrives. Some have had it die some time between parking it at night and trying to get into the car the next morning. The only firm answer is that the clock is ticking.