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SteveS0353
Member
The 12V lead-acid battery get recharged by a DC-DC converter from the high-voltage main battery.
All rechargeable batteries have a cycle limit. In other words, you can only discharge and recharge batteries a specified number of times before they "wear out" and can't hold a charge nor support the load. There is an engineering analysis a couple of us contributed earlier in this thread that explains the vampire load and calculates the theoretical longevity of the 12V lead-acid battery -> #32
Bottom line, the vampire load is way too high (IMHO) and is the cause of 12V lead-acid battery wear out in the 12-18 month time frame.
Obviously killing the vampire is the best solution, but just out of curiosity I looked into the energy density of current super capacitors, to see whether there is any hope of them helping the situation.
http://www.maxwell.com/images/documents/K2Series_DS_1015370_5_20141104.pdf
Please correct me if I'm wrong, but from what I've read above the HV contactors close every 3 hours to top off the 12V battery. If the vampire load is 40 W, then that's 120 Wh that needs to be provided. The above capacitors store 4.1 Wh/kg (minimum), which means you'd need 30 kg of them. That's, um, clearly too much.
But there's no issue with cycling, as they talk about lifetimes of up to one million cycles. 8 times a day is only 3000 per year.
Would it be feasible to open the contactor for just a brief time to feed the capacitors much more frequently? Like, say, every 30 minutes - that would bring the required storage down to just 20 Wh, or 5 kg of capacitors, and it wouldn't "wear out" the capacitors for more than 10 years. I'm not sure what 5 kg of capacitors translates into in terms of physical size, and I'm scared to ask: does anyone know how much that would cost?
I know that big banks of super capacitors are used to provide the ability of large rail vehicles to operate without overhead power, so it's certainly possible to do this sort of thing on a bigger scale.
http://www.maxwell.com/images/documents/K2Series_DS_1015370_5_20141104.pdf
Please correct me if I'm wrong, but from what I've read above the HV contactors close every 3 hours to top off the 12V battery. If the vampire load is 40 W, then that's 120 Wh that needs to be provided. The above capacitors store 4.1 Wh/kg (minimum), which means you'd need 30 kg of them. That's, um, clearly too much.
But there's no issue with cycling, as they talk about lifetimes of up to one million cycles. 8 times a day is only 3000 per year.
Would it be feasible to open the contactor for just a brief time to feed the capacitors much more frequently? Like, say, every 30 minutes - that would bring the required storage down to just 20 Wh, or 5 kg of capacitors, and it wouldn't "wear out" the capacitors for more than 10 years. I'm not sure what 5 kg of capacitors translates into in terms of physical size, and I'm scared to ask: does anyone know how much that would cost?
I know that big banks of super capacitors are used to provide the ability of large rail vehicles to operate without overhead power, so it's certainly possible to do this sort of thing on a bigger scale.
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gavine
Petrol Head turned EV Enthusiast
I didn't realize this but it makes sense. Does the car give any indication when this is happening? If not, that would explain why occasionally the contactor on my car doesn't click when I open the door....presumably because it was already engaged to feed the DC-DC converter during that time. hmmm.
I always assumed the 12v only charged while driving or when charging the HV battery. Good to know.
The contactors CLOSE to top off the 12v battery. There is only one set of contactors in the pack (one for + and one for -) that enable high-voltage to leave the pack. Once these close, the HV is made available to the DC-DC in the front of the car and it immediately begins charging the 12v battery and taking over all the 12v loads. The 12v battery is carefully instrumented with a current shunt and temperature sensor to make sure it's charged properly. In theory it should be possible for Tesla to enable the shore power and let the OBC (on-board charger) power up the high-voltage system, and thus the DC-DC while leaving the pack contactors open and thus isolated. I propose Tesla add this mode to the settings so we can have the option of not cycling the 12v if we are plugged in. Should be just some simple software additions.
The biggest culprit in vampire load seems to be the center display which actually incorporates 2 computers; the Tegra-3 based Linux touchscreen and a separate Freescale MPC5668 32-bit "Gateway" controller. Also in this beast is the Wifi, Bluetooth, and cellular modems. Externally there is also the Body controller, which is responsible for reading the fob, controlling the doors and security, etc. In theory, the center display can be completely shut down and the car can still respond to the fobs, etc, through the body controller. However, it appears Tesla keeps the center display in some degree of awake all the time, probably to keep the cellular modem awake and able to respond to external network requests. I would imagine this could be worked on to drastically reduce the vampire load if they felt it was a priority. Maybe the value of the data they collect from us is worth the price of replacing out 12v batteries and they are fine with that. (for now?)
BTW, it's a simple matter to keep the 12v outlet in the console live all the time if desired. It's just a simple relay in fuse box #2 (the one on the passenger side under the removable cowling cover - see owners manual). Simply make up a jumper consisting of short piece of 14AWG or better wire with a couple of male 1/4" spade terminals. Pull the front most passenger side relay out and insert the jumper into the now exposed 1/4" female receptacles, and you're done, the 12v outlet is now on all the time. You can quickly change it back anytime by removing the jumper and reinstalling the relay.
The biggest culprit in vampire load seems to be the center display which actually incorporates 2 computers; the Tegra-3 based Linux touchscreen and a separate Freescale MPC5668 32-bit "Gateway" controller. Also in this beast is the Wifi, Bluetooth, and cellular modems. Externally there is also the Body controller, which is responsible for reading the fob, controlling the doors and security, etc. In theory, the center display can be completely shut down and the car can still respond to the fobs, etc, through the body controller. However, it appears Tesla keeps the center display in some degree of awake all the time, probably to keep the cellular modem awake and able to respond to external network requests. I would imagine this could be worked on to drastically reduce the vampire load if they felt it was a priority. Maybe the value of the data they collect from us is worth the price of replacing out 12v batteries and they are fine with that. (for now?)
BTW, it's a simple matter to keep the 12v outlet in the console live all the time if desired. It's just a simple relay in fuse box #2 (the one on the passenger side under the removable cowling cover - see owners manual). Simply make up a jumper consisting of short piece of 14AWG or better wire with a couple of male 1/4" spade terminals. Pull the front most passenger side relay out and insert the jumper into the now exposed 1/4" female receptacles, and you're done, the 12v outlet is now on all the time. You can quickly change it back anytime by removing the jumper and reinstalling the relay.
Electricfan
Active Member
That sucks! I bought a VW Jetta TDI (ran bio-diesel in it) in 2006 and traded it in on my 2011 Volt. Never had a problem with my battery.
The auto industry has wanted to go to 42 volt batteries for 20 years, but mostly for one simple reasons... start / stops ICE engines. They've been talking about it for 30 years. Aircraft and some heavy industrial equipment has been 24 volt batteries for decades (for aircraft DC power only... aircraft AC power is still 115v / 400hz).
There's one big problem with a 48v battery; it will be charged at 54 volts, and that voltage is over the arbitrarily limit of what is considered a safe DC voltage of 50 volts. In simple terms, it is NOT going to happen.
i suspect that there are easy ways to rectify this, but not cheap ways. Obviously, a new $100 battery every year will add up, not to mention upset customers, so it must get fixed. I propose that they stay with 12 volt batteries on a 14 volt system, but change the actual battery to a lithium cell 12v. It can use exactly the same cells as the main battery pack.
Lead acid batteries really don't like being below 12.7 volts, even the deep cycle ones. Lithium actually prefers the lower voltage. To operate at 16.8v max, four cells in series @ 4.2v would work perfectly. Also, I would incorporate the same cooling / heating system that works with the main battery pack... heck, this 12 volt could be incorporated into the main pack. It doesn't need to be separate. At the lowest cell voltage, 2.5v, four cells in series allow a discharge to 10v. Perfect.
At 14 volts, which is where automotive systems are designed to operate while charging, this four-in-series 18650 cell "12 volt" battery is at 3.5v per cell... nearly ideal for longest life.
For Model 3, they may have to stick with lead / acid for cost reasons, and just cutout the vampire loads.
kuttakamina
Member
Only 4 year warranty I think. And replacing the battery is ridiculously complicated. You have to take half the car apart. The only other car I know that has such bad design were saturns. But their batteries lasted years and labor was cheaper. Most other cars you can replace the battery yourself. Takes about 10 minutes.
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mknox
Well-Known Member
I'm not so sure, particularly with the early cars. I've seen some photos and it looks like a fairly big disassembly job to me.
(emphasis mine)
The bold part would certainly help with vampire drain. ;-)
Also, permanently running a liquid thermal management system for a 12V battery does seem quite overkill to me.
The 12V has to be separated from the pack. That's the whole point of it ! It is supposed to still be able to power the car electronics to allow for the traction battery to be completely disconnected (HV is dangerous.) It allows to still be able to put the car in tow mode, to engage/disengage the parking brake, and to power the emergency blinkers in case of a main battery failure.
I also do hope that the M3 (heck, the MX also) will have a significant reduction in the vampire drain.
kuttakamina
Member
I didn't know they improved the location of the battery for D cars. Do you have any online material I can read on this?
mknox
Well-Known Member
Probably around the time they moved the DC-DC converter from the wheel well to the frunk cubby. Which makes me wonder... where is the DC-DC converter now in the D models? The front motor is right where it had been before.
The vampire load has nothing to do with the lead acid battery. If the vampire load is X amps, then you'll loose 12X Wh every hour from the main pack. The loss would be the same, regardless of the type of 12V battery used (PbA, LiIon, or supercap). It's buffered by the PbA battery, but the energy ultimately comes from the main pack via the DC-DC.
The simplest long term solution would be to add a 12V charger/power supply in parallel with the DC-DC, powered by the AC inlet, thereby letting the the vampire drain be powered by the input AC vs continuing to cycle the PbA battery. I'd be surprised if the X doesn't include this; the upfront cost would be more than paid for by savings in 12V warranty replacements.
Here is a link with a picture. A Peek Under the of a Tesla Model S P85D
Notice the orange/red 12v battery front and center-ish.
Original location for battery was near the passenger side, near corner of windscreen buried under a few layers/pieces of trim if I remember correctly from looking for it over 2 years ago when I got my car.
On the Gen 2 cars, the battery is located directly under the frunk tub on the driver's side, so getting at it only requires removing the carpet and the 4 bolts that hold the tub in. Still, not easy if you just want to disconnect the 12v for a vacation. This is also why the frunk is so small compared to the original. The DC-DC on Gen 1 also was the front HVJB, and it was located in the passenger side wheel well area. On Gen 2, it's been split into 2 separate smaller modules. They are located side-by-side down low just above the battery against the firewall in the middle, regardless if there is a front motor.
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Of course the vampire load has to do with the 12v battery. Most of this load comes out of this battery, thus cycling it, and there is an efficiency hit for charge discharge. Yes, ultimately the DC-DC will replenish the 12v, but it's still doing a lot of cycling on the 12v, and each cycle takes a certain % of the life and the total energy. Especially the absorption phase, which just makes a lot of heat, but is essential if you don't want sulfation.
You can't add in a separate charger, as the AC power is not always present unless the main charger calls for charge and acknowledges the EVSE's pilot. Of course this could be changed in software, but so could my proposal of just firing up the OBC w/o closing the main contactors. Then the same hardware is always used, no extra new stuff needed other than code.
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