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Any reason not to hook up a battery tender to the 12 volt battery ?

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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?

I doubt the "deep cycle" would help much unless the charging algorithm was changed to allow for it. The car would not know it could suddenly allow the battery to discharge further. Maybe a boost converter would be able to allow the battery to drain more.

The "big" part of the battery suggestion might help, as the battery would discharge more slowly, thus cycling less often. But that involves more weight or a more expensive battery technology.
 
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?

Respectfully, putting a heavy battery into the crash structure area of the car, where it could be thrown toward the occupants in a crash does not sound wise.
 
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There is a easy solution for Tesla. They could integrate a trickle charger into the charge path. When a tesla is plugged into AC it would float charge the 12v and offset the vampire.

A high quality float charger is $150 max.

They recommend keeping the car plugged in when possible anyway.

Of course addressing the vampire is the preferred solution.
 
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There is a easy solution for Tesla. They could integrate a trickle charger into the charge path. When a tesla is plugged into AC it would float charge the 12v and offset the vampire.

A high quality float charger is $150 max.

They recommend keeping the car plugged in when possible anyway.

Bingo! Having them do it rather than us, and take the HV battery totally out of the loop, seems completely reasonable.
 
How much power does the HPWC consume itself when holding the contactor closed to pass AC power to the car? I think it may be more than the vampire load. While I think the idea of integrating the 12V battery maintainer in the car is a good idea, doubling the utility power consumption to maintain the battery does not sound efficient. However, in cases where you don't actually need to charge the traction battery, a 120V EVSE may have much lower internal power consumption.
 
There is a easy solution for Tesla. They could integrate a trickle charger into the charge path. When a tesla is plugged into AC it would float charge the 12v and offset the vampire.

A high quality float charger is $150 max.

They recommend keeping the car plugged in when possible anyway.

Of course addressing the vampire is the preferred solution.
I brought this up a while ago. Someone countered with the fact that the charger commands the EVSE to kill AC power to the car when it is not charging.
 
There is a easy solution for Tesla. They could integrate a trickle charger into the charge path. When a tesla is plugged into AC it would float charge the 12v and offset the vampire.

A high quality float charger is $150 max.

They recommend keeping the car plugged in when possible anyway.

Of course addressing the vampire is the preferred solution.

The way it works now is that the car is the master of the charging circuit. The EVSE (HPWC and UMC are Tesla's versions of the J1772 standard with a proprietary connector) simply advertises how much current it can supply via the pilot circuit, and the car commands the EVSE to close it's contacts and supply power. When the car is done charging, it commands the EVSE to open it's contacts to interrupt the supply of power. There is a safety issue here; the plug at the end of the EVSE charging cable is not energized unless it's plugged into the car and the car commands the EVSE contacts to close. That's how scheduled charging works too; the car controls the EVSE to supply power at the programmed time. Just adding a trickle / float charger on the car charge port with the current charging architecture would be worthless. The trickle / float charger would not be powered unless the car is charging, but when the car is charging the DC-DC converter in the car is charging the 12V battery anyway so the trickle / float charger would be redundant.

Now, Tesla could design a change to the charging architecture. One way would be to include another set of high current contacts in the car under software control to isolate the on-board charger from the charge port. This would not require any change to the EVSE. The car could energize the EVSE charge cable the moment it's plugged in and de-energize it when it's removed (safety feature), and control charging the HV LiIon battery via the additional high-current contacts in the car. This way, the change port would be energized whenever the car is plugged in to the EVSE, but the car could control start and stop charging internally. Under those conditions, a trickle / float charger from the charge port to the 12V battery would be effective.

Since the car can control the magnitude of the charging current, it might be possible to add the trickle / float charger and change the software without adding any additional hardware to the car. In that case, energize the EVSE when the cable is plugged in to the charge port, but reduce the current to zero until the scheduled charge time is reached and when the charge is complete, but leave the EVSE contacts closed to energize the trickle / float charger while ever the EVSE is plugged in.

Of course, both of these schemes waste power in the EVSE holding the contacts closed when the car is not charging.

But, I agree wholeheartedly that significantly reducing the vampire drain is the preferred solution. 40-50W running continuously is simply enormous, and that's why the 12V battery is under such stress.
 
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OK one last post. I hooked my car back up to the trickle charger for 24 hours. The needs of the car are surprisingly low. With the car in the energy saving mode, the charger consumed 265 watt hours of electricity (from the wall) over the 24 hour period. At my Utility rate here in Florida about 4 cents per day. The cost would have been lower if I was using a better charger as the power factor at the light load was only .45. The battery was maintained at 13.3 volts, the HPWC was disconnected. DC draw on the charger was a maximum of .49 amps to a minimum of .35 amps. Doesn't sound like much but withdrawing that amount per hour from a small 12 volt battery will deplete it over time. The car never "came on-line" to check the 12 volt level nor did I loose any range overnight---as I expected. One point of interest, the 12 battery was in a high state of charge when I connected the trickle charger. This was after it was sitting for a while. The battery was almost 14.5 volts and who knows what it was when the DC-DC converter was running. Seems high for a gel battery that should be on "float".
 
OK one last post. I hooked my car back up to the trickle charger for 24 hours. The needs of the car are surprisingly low. With the car in the energy saving mode, the charger consumed 265 watt hours of electricity (from the wall) over the 24 hour period. At my Utility rate here in Florida about 4 cents per day. The cost would have been lower if I was using a better charger as the power factor at the light load was only .45. The battery was maintained at 13.3 volts, the HPWC was disconnected. DC draw on the charger was a maximum of .49 amps to a minimum of .35 amps. Doesn't sound like much but withdrawing that amount per hour from a small 12 volt battery will deplete it over time. The car never "came on-line" to check the 12 volt level nor did I loose any range overnight---as I expected. One point of interest, the 12 battery was in a high state of charge when I connected the trickle charger. This was after it was sitting for a while. The battery was almost 14.5 volts and who knows what it was when the DC-DC converter was running. Seems high for a gel battery that should be on "float".
Are you sure the DC-DC wasn't energized? The battery won't show 14.5 if it's not being charged. Should be around 12.6-12.7 no load, less if the vampire drain is running, more if charging. 14.5 w/out the DC-DC energized doesn't make sense.
 
The DC-DC was not energized. The car had been shut down about 30 minutes. The car wasn't connected to the wall connector, doors closed, nothing running. I'll check it again but that seemed abnormal--as is the number of 12 gel battery failures on the model S. I am going to pick up a connector so I can more easily connect the charger on a daily basis (not having to remove the nosecone). I found that many trickle chargers use what is known as a "SAE" connector which is a two prong molded rubber connector with a cap on it. You can get them with either clamp ends or ring terminals. On the S, without drilling any holes, you can attach the ring terminals to the posts behind the nosecone, fish the wire down to the grille and pull the end out under the nosecone. Not the best solution but good enough for confirming everything without modifying the car in any way.
 
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OK one last post. I hooked my car back up to the trickle charger for 24 hours. The needs of the car are surprisingly low. With the car in the energy saving mode, the charger consumed 265 watt hours of electricity (from the wall) over the 24 hour period. At my Utility rate here in Florida about 4 cents per day. The cost would have been lower if I was using a better charger as the power factor at the light load was only .45. The battery was maintained at 13.3 volts, the HPWC was disconnected. DC draw on the charger was a maximum of .49 amps to a minimum of .35 amps. Doesn't sound like much but withdrawing that amount per hour from a small 12 volt battery will deplete it over time. The car never "came on-line" to check the 12 volt level nor did I loose any range overnight---as I expected. One point of interest, the 12 battery was in a high state of charge when I connected the trickle charger. This was after it was sitting for a while. The battery was almost 14.5 volts and who knows what it was when the DC-DC converter was running. Seems high for a gel battery that should be on "float".

265Wh over 24 hours is an average of 11W continuous, and .35A to .49A is 4.2W to 5.9W (@12V) The difference probably represents the energy loss in the charger itself.

However, less than 6W draw from the charger to supply the vampire load is a surprise, and raises more questions than it answers. A single 24 hour period may not be long enough to establish an average. The average vampire drain for a lot of cars is 1.2kWh per day, but there is a pretty large variation to that number depending on various factors (energy saving mode on or off, temperature, age and state of the 12V battery,...). That would make the vampire load more like 4A. You are reporting currents a tenth of that. What is your car's normal vampire loss range? Can you repeat the experiment with energy saving mode off?

The 14.5V is an anomaly. You should only see terminal voltage like that when the 12V battery is actively charging. A healthy 12V battery when fully charged without a load would maybe exhibit a terminal voltage of 13V, not more.
 
Maybe I'm missing something, but wouldn't the preferred solution for long term storage be to disconnect the 12v battery instead? doing so would open the pack contactor preventing draw from the pack at all, and with the 12v battery disconnected no system could drain it either.

Not sure why keeping a charger on the battery would be a better option than simply isolating it?
 
This should work fine ... used on many vehicles to prevent battery drain during storage
http://www.harborfreight.com/battery-disconnect-switch-97853.html

image_12541.jpg
 
Maybe I'm missing something, but wouldn't the preferred solution for long term storage be to disconnect the 12v battery instead? doing so would open the pack contactor preventing draw from the pack at all, and with the 12v battery disconnected no system could drain it either.

Not sure why keeping a charger on the battery would be a better option than simply isolating it?

If you disconnect the 12V battery, then the car will be totally isolated. As you say, nothing will be powered. That includes the radios (3G/LTE, WiFi, Bluetooth, key fob,...). Remember, unlike other cars, the Model S has no physical key to open a door and relies on the radio signal between the car and the key fob for physical access to the car. You would need to carefully plan access to the 12V battery in the isolated state so you can reconnect it (or close the battery isolation switch should you choose to install one of those) to power the electronics so that you can essentially "boot" the car. Also, Tesla would not be able to access the car from their network to unlock it since the 3G/LTE would not be powered. Your smart phone app would not work either.

Also, the 12V lead-acid battery has a self discharge, so without a trickle / float charger, the 12V will eventually discharge itself, and you would be faced with "jumping" the 12V anyway.

I think I would prefer to keep the car alive and "well managed".
 
For long term storage yes, a battery disconnect, why not?

But for daily driving (over-night), I keep my car "online" so I can start interior heating from inside the house, and start charging to top up 5% in the morning just to get tractor battery temps up to speed too.

- - - Updated - - -

The DC-DC was not energized. The car had been shut down about 30 minutes. The car wasn't connected to the wall connector, doors closed, nothing running. I'll check it again but that seemed abnormal--as is the number of 12 gel battery failures on the model S. I am going to pick up a connector so I can more easily connect the charger on a daily basis (not having to remove the nosecone). I found that many trickle chargers use what is known as a "SAE" connector which is a two prong molded rubber connector with a cap on it. You can get them with either clamp ends or ring terminals. On the S, without drilling any holes, you can attach the ring terminals to the posts behind the nosecone, fish the wire down to the grille and pull the end out under the nosecone. Not the best solution but good enough for confirming everything without modifying the car in any way.

What about just leaving the charger onboard the car, permanently connected to battery terminals either at the battery or nosecone, and bringing the 120V plug out ("block heater style") either at the front or rear of car. Having 120V plug hanging out the frunk lid is so-ICE... retro. I know. It might be better having the 120V plug come out near the charge port, so you do all your plugging there. Maybe slap a fake exhaust pipe under the car and have a retractable cord hanging out of it, you pull out to plug in the wall. Weird but symbolic. Given that I normally back in to parking stalls where EV charging is provided, it would be awkward to have cords on both ends of the car. At work for example, I charge rearend in first to stall that just has a normal duplex nema 5-20. One port for my mobile charger the other port for battery maintenance.


With the battery cycle benefits you're showing with a trickle charger on the 12V, I'm pretty sure I'm going to be doing something like this. Especially when it comes off-warranty.

Anybody know if the 12V battery is considered part of the 8 year unlimited drivetrain warranty? or is it excluded?
 
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So, you "disconnect" it by leaving a 15amp fuse connected? Maybe that makes sense to somebody, but 15 amps will kill a 40 amp/hr battery in just a few hours. By the way, for all the cheap folks who think this is a smart way to "save" your 12v battery, just know that the BMS works 24/7 and needs the 12 volt battery to operate.

Get a simple battery trickle charger / tender.