My P85D is drawing 1.7 kWh / day or about 70 watts. Even if I used a real charger to keep it from being drained, my peak power cost is 42 cents / kWh. Given the average number of hours per day during peak that I'm home, this would cost me more in electricity than the cost to replace the battery in just a year.
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Any reason not to hook up a battery tender to the 12 volt battery ?
- Thread starter edgrzy
- Start date
snort
Member
then why wouldn't providing a similar singular link from the 12V power supply provide exactly the same safety?
the 3G, WiFi and Bluetooth are exactly the same sort of radio as is in my phone, and the keyfob is even lower power. It's able to idle for days on a 5whr LiPo battery, even if I use it for a few minutes a day. so that's probably not it, although they could have foolishly chosen a high power design.
the center console (several processors, not just a cpu) uses more power but again, unless they made a foolish decision, not much more when idling.
If I understand what it is, the battery management system is a relatively simple computer controlling a bunch of big relays. if the design is such that the electricity is fighting a big spring at all times, that might be doing it. it shouldn't be necessary, although they may have chosen an off the shelf contactor for proven reliability. on the scale of a thing that's plugged in, 1-2kwh is not a big drain, but it is a lot for a battery...it's probably more than the entire capacity of the battery.
the thing is, the car should be able to keep this thing topped up by turning on the trickle charger every hour or so. it may be that they have to turn too much of the car "on" to do this frequently and are trying to save power. but if it's plugged into a charger, this shouldn't matter.
SteveS0353
Member
Assuming we maintain the HV contactor architecture for safety, what you are proposing is this; HV battery via contactors -> 400V -> DC-DC converter -> 12V used amongst myriad other things to close the contactors. Sure, it would be safe for first responders to sever the 12V which would de-energize the contactors to open them. That would be safe. But when the car is powered down under normal conditions, the contactors open (for safety) which would mean no 12V to close them -- that's a critical deadlock. That's the function of the 12V battery; provide an independent power source to close the HV contacts to begin operation of the car. It's a classic bootstrap, 12V battery provides power to the electronics which closes the contacts, which powers the DC-DC converter, which provides 12V from the much larger capacity HV battery to recharge the 12V battery and power the electronics. Without the 12V battery, it would not be possible to power up the car in that architecture.
It's pure speculation on my part, but I think that in the rush to get the Model S released, the engineering team didn't pay the necessary attention to the electronics power consumption. As you point out, it should be possible to design a modern hardware / software system, including sophisticated battery management, including the necessary safety features, for a whole lot less power than 50W on average.
The key to improving the 12V battery longevity is to significantly reduce the vampire drain, which will significantly reduce the need to discharge - charge cycle the 12V battery.
I don't have any info on the Tesla, but the Ford Energi uses a similar configuration of a 12V lead-acid battery charged with a DC/DC converter from the HVB. In the case of the Ford the DC/DC converter is capable of supplying up to 145A, quite a bit more than a trickle charger. Seems like overkill, but maybe with subwoofer blasting away...
Thinking out loud (before finishing my second cup of coffee) - maybe move the DC-DC converter inside the battery pack, allowing it to stay powered up continuously without energizing exterior HVDC connections.
Or a small, secondary DC-DC inside the pack, with sufficient capacity to power the vampire load and energize the main contactor, thereby powering the main (higher power) external DC-DC when the car is running.
The pack could have a small access panel to replace the internal DC-DC without complete pack teardown.
Or a small, secondary DC-DC inside the pack, with sufficient capacity to power the vampire load and energize the main contactor, thereby powering the main (higher power) external DC-DC when the car is running.
The pack could have a small access panel to replace the internal DC-DC without complete pack teardown.
gavine
Petrol Head turned EV Enthusiast
It maybe a stupid question (I do not know enough of batteries, I'm a software architect), but wouldn't the problem be solvable by using a Li-ion or NiMH based 12V battery instead of the Lead-acid ?
SteveS0353
Member
It would change the situation, but not resolve it. All rechargeable batteries have a wear-out cycle count, even Li-Ion; that's why main pack degradation is not covered by the 8-year unlimited mile warranty. Deploying a Li-Ion or NiMH battery would move the wear out point, but not eliminate it, and at a cost. The real solution to 12V battery longevity is to significantly (order of magnitude or more) reduce the vampire drain on it.
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Moving the DC-DC inside the pack, or adding another smaller DC-DC to it, and powering whatever is in there all the time would simply increase the vampire drain because nobody has yet designed a DC-DC converter that is 100% efficient.
The way to resolve the 12V battery longevity is to significantly reduce the vampire drain on it, in line with what we see in other EVs (Nissan Leaf, Ford Energi, ...).
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Yes, of course I do as most do. I tried it the other way for a while and it's a pain to wait so long for the app to finally connect. The reason I tried it the other way was to see if it made a difference and it didn't. The draw was exactly the same. There was a huge thread on this and mine is exactly the same as many other posted. When I first got my PD, it was in the 40 watt range but then about 3 updates in 6.2 land it changed and made a big jump and has stayed there ever since.
I had thought about the whole DC-DC inside the pack thing previously. A few things came to mind though.
First, there really is just no room for a DC-DC inside the pack. That thing is jam packed already.
Second, I think that there are a bunch of issues with this all around considering that there would now need to be some kind of better way to isolate the HV to the pack. If the pack itself can generate the power needed to control the contactors without an external component then this is a potential safety issue for first responders. I can imagine an impact or damage that would cause the internal DC-DC to stay running and closing the contactors while a first responder goes to cut out a passenger only to be greeted with 400V DC.
Probably would require a full redesign of all of the HV safety systems.
First, there really is just no room for a DC-DC inside the pack. That thing is jam packed already.
Second, I think that there are a bunch of issues with this all around considering that there would now need to be some kind of better way to isolate the HV to the pack. If the pack itself can generate the power needed to control the contactors without an external component then this is a potential safety issue for first responders. I can imagine an impact or damage that would cause the internal DC-DC to stay running and closing the contactors while a first responder goes to cut out a passenger only to be greeted with 400V DC.
Probably would require a full redesign of all of the HV safety systems.
S100D
Member
I was Googling 12 volt battery replacement and found the following 2 links. I am sure that either solution would not be a total replacement for the Model S 12 volt battery, but maybe Tesla should consider putting something like this in parallel with the 12 volt battery to reduce the number of discharge / charge cycles.
https://www.youtube.com/watch?v=GPJao1xLe7w
https://www.youtube.com/watch?v=8miq6sDy0wA
https://www.youtube.com/watch?v=GPJao1xLe7w
https://www.youtube.com/watch?v=8miq6sDy0wA
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The overall amp hour capacity is too low so you'd have to close the contactors and charge it up a lot faster. Still, in a large enough configuration, I could totally see this replacing the battery. In fact, it might be a much better application than what this guy is pedaling for your car. If I replaced my regular battery with one of these in any of my cars, it would be dead in about 3 days from the system drain while the car is off.
I hope my car runs when I return but I didn't like all of the cycling of the high voltage system to keep the 12 volt battery charged. I have a summer home in the North and parked the Tesla at the end of May at my house in Florida. I pulled off the nosecone and attached a 6 amp battery charger used in a standby generator set to the terminals located behind it. Every time I poll the Tesla with the remote app, the main battery pack shows the same remaining range as when I left the car (the charge limit is set well below the remaining range and the car is plugged into the wall connector as a backup if the 12 volt charger fails for some reason). I am not returning until the fall so it will be interesting what actually happened over the summer. I only wish I had remembered to connect my "killawatt" energy monitor to the charger before I left. It would be interesting to know if you are using more energy to balance the "Vampire load" by using the car's internal charging solution for the 12 volt battery then a small external charger.
SteveS0353
Member
I think that's a smart thing to do if you are leaving the car for such a long time.
My prediction: your 6A charger will take care of the vampire load entirely, and your rated range will not reduce much, if at all, indicating that the car has not performed a charge cycle from the wall connector. Please update this thread periodically when you interrogate the car with the app, and of course, when you get get back to it. I think it will be just fine with this strategy!
Well I just got back to Florida. So from the end of May until now, the Tesla has been sitting plugged into the HPWC and also a six amp charger connected to the terminal stud located behind the nose cone. Everything worked out well. The car had never charged (from the HPWC) over the summer. The rated range when I parked the car was 199 miles and was unchanged from that level upon my return. The internal charger in the car was set to recharge below 139 miles and would only have recharged to that level so that verifies that the only support the car got (or needed) was from the six amp charger. The next time I leave the car for a extended period I will hook a watt hour recorder to it to see how much power is being used by this method vs. relying on the internal DC-DC power supply to maintain the 12 volt battery. At least by using the external battery charger, the car doesn't appear to be going through its powering up, checking and charging and powering down routine. The fact that the 12 volt battery is being float charged (not cycled charged) and the HV power system is not being cycled to charge it should be better overall for the car. For those that need to leave the car parked somewhere for just a few weeks and have access to a 120 volt outlet, this would be a great way to eliminate vampire losses from changing the vehicle range from the range remaining when you parked it.
Cottonwood
Roadster#433, Model S#S37
Great experiment and data! Thanks!!
Your results match my expectations, but it is amazing that there was zero self-discharge in the main traction battery over 6 month.
For others doing this, my only recommendation would be to get a good, multi-stage charger/maintainer that is temperature compensated, and can supply over 4 Amps.
scottf200
Well-Known Member
And from this Chevrolet Volt presentation: "Accessory Power Module (APM) converts high voltage to 12V electricity 2.2kW, Max Current ~165A @ 12V" http://nctcog.org/trans/air/programs/evnt/ContractorInspectorOutreachTexasMay2011.pdf
SteveS0353
Member
Excellent outcome! Thanks for sharing the details and the data. I'm a little surprised that the rated range did not reduce at all, but it confirms that the only reason for rated range reduction is servicing the vampire drain cycling the 12V battery.
Has anybody tried swapping out the standard 12v small battery that Tesla provides, with a big deep cycle battery (like you use on a boat)? (Yes, I expect this would not fit in the existing space behind the nosecone and would have to be strapped down in the frunk). Is that a possible solution?
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