Helping finding a switched 12v while Model 3 is charging

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Albert A

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Hello Forum. I'm new to this forum but not new to automotive forums. I'm hoping someone can help me find a switched 12v wire so I can activate a relay to power on a DC to AC inverter.

The back story:
I recently bought a powered golf cart that I carry in the trunk of my Model 3. I have to charge the cart every 2 rounds. I'd like to avoid having to remove the battery from my car and charging the cart battery in my garage. I already forgot my battery in my garage once and I don't want to do that again. Ideally I'd like to charge my golf cart in the trunk of my car while the Tesla is charging from a AC power outlet. I was planning to mount a relay, a DC to AC inverter and cart charger in my trunk. Anyone have any ideas where I can find a 12v switched wire to activate my relay while the car is charging? I have diagramed my thoughts in an attachment. I'm also open to other solutions. Thanks

TeslaSwitched12vCharging.JPG
 

JWardell

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Not sure you will find any analog wire that is only on when charging.
I can monitor CAN and switch a relay module when charging though.
Though that seems like a lot to worry about. Assuming the cart stops drawing power once it's charged, if you just plug or wire it in to your accessory port, it shouldn't hurt anything and is a lot less trouble.
 

m3_4_wifey

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The draw from the car battery to the golf cart battery is minimal. Why not just charge the golf cart battery off the 12 volt cigarette lighter? I think you can probably draw 10-12 Amps from that, and if you're not golfing every day, that should be plenty of time for recharge.

If your car is plugged in and the charge percentage drops, the wall charger will turn on and quickly get the car back up to the requested percentage.
 
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Albert A

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You guys are on to something. The battery charger will stop charging after the battery is "full." The cart battery is only 380 Wh and after one round it might get to 50% or 160Wh. That will only effect the Model 3 range by 2-3 miles. Now I'm wondering if the 12v cigarette port is powered on long enough to charge the cart battery in between rounds? I drive about an hour a day. I can plug everything in and I can find out
 

lance.bailey

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It's not a very technical answer, but if you had a second battery you could keep one in the cart and one charging the garage. When you get home, you swap batteries. no more arriving for a round with your battery back home.

load up the cart, pull the battery and put it on the seat beside you. get home and swap batteries putting the charged one in the cart.

sometimes process can beat out technology.
 

Albert A

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Lance, I have thought about a second battery but the cost of the battery is still pretty high for me at $500. Adjusting my charging habits is also an easy option that I can do now.
 

Long Ranger

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You guys are on to something. The battery charger will stop charging after the battery is "full." The cart battery is only 380 Wh and after one round it might get to 50% or 160Wh. That will only effect the Model 3 range by 2-3 miles. Now I'm wondering if the 12v cigarette port is powered on long enough to charge the cart battery in between rounds? I drive about an hour a day. I can plug everything in and I can find out
If the cart battery takes longer to charge than your car, you can put your car into Sentry mode. The car doesn't sleep while in Sentry and the 12V should remain active on the cigarette port. Another option would be to lower your charging current to make the car take longer to charge.

Just be sure to stick with charging your cart battery off of switched 12V like the cigarette port. I don't recommend finding an unswitched 12V tap, since that could put unwanted drain/cycles on your 12V lead acid battery. The 12V lead acid battery supplies the 12V while the car is asleep.
 

lance.bailey

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Yeah, battery prices are a killer. any visit to HomeDespot will find you a drill with battery cheaper than a replacement battery by itself. stupid.

but how much were you to expecting to pay to get a 12V tap off the charging system?

cig port is going to be the cheapest at the end of the day.
 

Albert A

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Yeah, battery prices are a killer. any visit to HomeDespot will find you a drill with battery cheaper than a replacement battery by itself. stupid.

but how much were you to expecting to pay to get a 12V tap off the charging system?

cig port is going to be the cheapest at the end of the day.

I was going to do the work myself. I've installed a lot of car audio in my day so I have wire, connectors and and inverter in my garage. I think the cig port will be the best option also.
 

orekart

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The 12V always-on supply for trailer lighting is well known. I haven't done so myself but on Model 3 manufactured before the trailer hitch wiring refresh it's just one spade connector in the driver's footwell to get +12Vdc.
 

Garlan Garner

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The 12V always-on supply for trailer lighting is well known. I haven't done so myself but on Model 3 manufactured before the trailer hitch wiring refresh it's just one spade connector in the driver's footwell to get +12Vdc.

There is absolutely no way to draw power from the footwell circuit. It will barely supply enough power for the light itself.
 

JasonF

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If the tiny images in the diagram are correct, the battery for the cart is 12 volts. Do you know if the battery itself has built in circuitry or is all of the charging control in the base? If you want to find out, open up the base and see if it has anything fancier than just a few capacitors and resistors and a rectifier, or just a transformer. If it’s just sending raw 12 volts to the battery and letting the battery handle the rest...you have something cool to build!

And that something cool would be that you can build a cradle that connects the battery terminals directly (after stopping at a fuse) to the 12 volt source - either the “lighter” power point or directly to the 12 volt battery. The smaller battery would only charge as long as the 12 volt bus is greater than the voltage of the battery, which means it should absorb power from the main 12V battery, then the DC-DC converter switches on and tops off both. They should both constantly level off as long as both are connected.

Of course the drawback to that would be if your car 12 volt isn’t healthy, or your DC-DC converter has problems, the car accessories would suck the tiny battery dry in a few minutes.
 

orekart

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There is absolutely no way to draw power from the footwell circuit. It will barely supply enough power for the light itself.
The footwell area anyhow for access to VCFRONT not the ambient lighting. I have since completed a run of wiring front to rear for a trailer lighting connector install, works as intended.