Using Model S to jump another vehicle

Two different scenarios to consider here -

1. Using the DC-DC converter to charge another vehicle's battery: doesn't seem like it would be too big of a deal, although Tesla calls it a no-no. The reports are that the DC-DC converter is rated ~200A/12V or so, and that should be enough to provide a reasonable charging supply to another battery. You'd probably want the main contactor closed, so having your door open and the car awake is probably the most important point here.

2. Leaving it connected while you attempted to start the other vehicle. This is a BIG no-no, IMO! Current draws of several hundred amps from a traditional ICE starter motor could end up damaging the DC-DC converter or blowing the fuse. I suspect that if you did start the lawnmower while it was still connected to the Model S, you didn't risk too much because those motors are much smaller. Do not do this with another car!

There is a 50A fuse on the battery connecting the front battery posts, you cannot jump another car with that setup... See this thread: Is the battery post under the nose cone fused?

Although FlasherZ blows me away on electrical knowledge, I know enough to say that I agree with him .

The Model S doesn't need any massive currents from the aux battery for any reason, whereas ICE batteries are designed for large cranking currents.

I wouldn't attempt to charge another battery with the S battery. Maybe it would be ok for a smaller battery, but if the battery has a large capacity and is at a low state of charge, it could draw a lot of current and blow a fuse. That's probably why Tesla doesn't condone it.

Attempting to crank while connected will definitely try to draw cranking amps from the aux battery on the S, which ought to blow a fuse...you might luck out with a smaller motor, but you don't know what's big enough to blow that fuse or cause damage. I'm keeping my S battery safely tucked away.

Go get that new battery, yobig . It's 5 minutes at a hardware store, and they'll pay you for the old battery (just like Tesla! haha)

Kalud said:

There is a 50A fuse on the battery connecting the front battery posts, you cannot jump another car with that setup... See this thread: Is the battery post under the nose cone fused?

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I have successfully boosted a vehicle through a 30A fuse in the past, so I wouldn't say you can't do it with 50A, (it all depends on just how close to "enough" their battery still has) but you are right in principle, there's a high chance of blowing that fuse.
My suggestion would be, as others have said, use it to charge, but not to start. So if your friend's car is dead, hook up cables, let it sit for a while, (5-10 mins?) and then disconnect the cables and let them try to start. You can always try again with a longer wait if it fails the first time. That said, there is still a risk to this procedure, and you will find at this point that almost all manufacturers advise against boosting another vehicle (not just Tesla), if the other vehicle just has a dead battery and no other problem, there's very little risk, however you have to wonder why the battery is dead, if there's a short, or marginal short in the wiring of that other vehicle there's a risk of blowing that 50A fuse and/or damaging your 12V system. There's also always the risks associated with reverse polarity if you aren't paying enough attention hooking things up.

green1 said:

There's also always the risks associated with reverse polarity if you aren't paying enough attention hooking things up.

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My Dad did that once jumping an ICE off another ICE. The dead car's battery cables had been replaced, and the idiot had swapped colors (so the black went pos to starter, and red neg to ground). My Dad's car was running, and he happily connect red to red and black to black. There was a bit more arcing than usual, and the his engine bogged down, but he figured it was just because the battery was really dead and drawing a lot of charge current. Then the cable melted at the clamp and popped out of the crimp! I suppose the fact that the battery was really dead may have limited the current flow somewhat.

They figured out what had happened, got new cables, and got the other car started. Surprisingly enough, all was fine (the dead car was old enough that it probably pre-dated on-board electronics).

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yobigd20 said:

lol ok I won't do it again. It worked though haha yea I'll go get a new lawnmower battery. It's for a zero turn john Deere. That's my gas guzzler now.

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Avoid the Deere batteries. They have a nasty reputation for leaking around the terminals. When the one in my tractor died, I replaced it with a sealed AGM battery for a Miata (U1R, regular post terminals). No leaks, no checking water, low off-season self discharge. I got it here:

Mazda MIATA Battery - Free Shipping: BatteryMart.com

I had to fabricate a different clamp (threaded rod, with a bar over the top of the battery), as it didn't have the side clamps "wings" molded into the case like the original.

If yours uses the through-bolt terminals, this one won't work. The commercial tractors use automotive posts, but I don't know what garden tractors/riding mowers use.

The correct answer is: Purchase a jumpstart kit.

If it's a real emergency, you should be able to get by with the following (Note: that this is the same method used for the Prius and would presumably be the same for any car with a small 12V battery):

1. Take the keys from the other vehicle's owner and make sure s/he doesn't have a spare set. This is vital. If they attempt to start the car while it's connected it can do severe damage to the Tesla.

2. Pop the nosecone and locate the jumper terminals.

3. Connect positive to positive.

4. Connect an unpainted metal surface on the dead battery car to the negative jumper terminal on the Tesla.

5. Have the Tesla turned fully on so that the 12V battery remains charged.

6. Wait 15 to 30 minutes for the dead battery to charge.

7. Remove the jumper cables in the opposite order (negative first).

8. Give the dead battery owner back their keys and see if the other car will start.

9. This is an emergency procedure, not a recommendation. Once told they will have to wait 30 minutes, most will find another solution.

I came into this thread expecting Dukes of Hazzard or KITT type jumping. Not this talk about electricity. thumbs down

Kipernicus said:

I came into this thread expecting Dukes of Hazzard or KITT type jumping. Not this talk about electricity. thumbs down

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In a Waylon Jennings voice:

"Looks like Kipernicus has got himself twisted around, don't you think?"

(LMB spouse)

As I described in an earlier thread about this, I did the following to start our slightly dead ICE from LMB's Tesla last winter. No nosecone popping involved.

1) Connect cheap 6A battery charger to dead battery hot and dead vehicle frame.

2) Connect cheap 150 watt inverter to 12 volt socket between Model S front seats.

3) With Model S on, plug charger into inverter (note: charger too cheap to have power switch).

4) Wait 10 minutes.

5) Disconnect in reverse order.

6) Start dead ICE vehicle.

Worked a treat.

jerry33 said:

The correct answer is: Purchase a jumpstart kit.

If it's a real emergency, you should be able to get by with the following (Note: that this is the same method used for the Prius and would presumably be the same for any car with a small 12V battery):
1. Take the keys from the other vehicle's owner and make sure s/he doesn't have a spare set. This is vital. If they attempt to start the car while it's connected it can do severe damage to the Tesla. 2. Pop the nosecone and locate the jumper terminals. 3. Connect positive to positive. 4. Connect an unpainted metal surface on the dead battery car to the negative jumper terminal on the Tesla. 5. Have the Tesla turned fully on so that the 12V battery remains charged. 6. Wait 15 to 30 minutes for the dead battery to charge. 7. Remove the jumper cables in the opposite order (negative first).
8. Give the dead battery owner back their keys and see if the other car will start. 9. This is an emergency procedure, not a recommendation. Once told they will have to wait 30 minutes, most will find another solution.

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For me the correct procedure is: "I have a very hard to get to 50A fuse in the circuit that will blow if your battery draws more current. Since you can't guarantee that, we'll have to wave down a gasser with an easily accessible battery, and a 100A alternator."

jerry33 said:

This is an emergency procedure, not a recommendation. Once told they will have to wait 30 minutes, most will find another solution.

Click to expand...

This method worked just now to charge the 12V battery on my Smart Fortwo Electric Drive.
The Smart had a low 12V and refused to charge the high voltage battery.
The Smart has been very reliable charging from the included 12A 120V EVSE, but since I installed a FLO 7kW charger, the FLO hasn't reliably charged my Smart ED and because we hadn't driven the Smart for a few days, and the FLO didn't charge the car, the 12V went low.