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Jan 05, 2018, 06:39 PM
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lipo car jump! \_(ツ)_/

All I left running was a two amp / 24W battery pack heater in my Passat while I flew for an hour and it killed the car battery. Killed it dead! Froze my ass off. No one around to hook me up with a jump, I didn't have anything to wire up in parallel for additional discharge current, though I had an old 3S 3000mAh to offer assistance to the car battery, managed to connect them with my jumper cables. While it brightened the car's interior lights, no dice starting the engine.

Moving forward I won't leave stuff running, but this misadventure made me curious regarding whether there's a safe way to jump a car with our packs. So I combed the forums here (couldn't find anything on wattflyer). Not much of a consensus on this topic which makes me more curious.

The most common suggestion is to use a four cell battery (didn't know you can mix voltages) which is typically met with objections that that will fry the car's electronics and start fires. Some suggest a high capacity 3S, which sounds safer. I could wire up several 3Ss in parallel, would that do it? Others suggest batteries of different chemistry like NiCD, LiFe and A123, I guess because their voltage multiple can lie at a more conservative 14V.

Highlights I found; perhaps you may find one useful.

Originally Posted by seeingeyegod
What could possibly go wrong?
Originally Posted by mrforsyth
Glad that it worked out for you but as I see it, you dodged a bullet here. Reason I say this is that depending on the condition of the starter battery, subjecting a vehicle electrical system to the fully charged voltage (16.8 volts) of a 4S lipoly can wreak havoc and damage sensitive electrical components. You may have gotten away with it this time but I would strongly caution against repeating this practice.

A significantly wiser alternative to a 4S lipoly would be a 4S LiFePO4 pack. At full charge, these are far more compatible with a vehicle electrical system. I have made a 4S4P pack out of retired A123 flight packs that I keep fully charged in the back of my SUV. It's rescued several guys who've inadvertently overdischarged their starter batteries by powering their lipoly chargers.

Originally Posted by tyhollin
Actually most car components are rated at 14-16 volts. Your lipo only reads 16 volts when a on a charge or drawing high current. Its 14.6 on a standard current. Your major components like your stereo get their peak rating by testing with 14-16 volts which actually produces an inflated wattage rating as a way to make you think its more powerful than it is. I worked in car stereo shop for a while and we installed high performance batteries made for high output stereos that were 14-15 volt with over 500 amps output and peaked at upwards of 17v and 700 cranking amps. There was never an incident of damaged components. Not even a blown fuse in the stock panel.

A 14.6 volt lipo will not damage components. I would be far more worried about damage to the lipo. You don't want to try this on your favorite most expensive lipo. Maybe an old one. It's just good to know its an option in a sticky situation.
Originally Posted by 3JJ
How about 4S LiFe pack then, full charge voltage conveniently 14.4V. Can they deliver the current?
One man made a youtube video touting his success:
Originally Posted by MSBWilson
It occurred to me recently when I got a new 1800mAh NanoTech battery that with it's very high C rating, it would be able to jump start a car. So I tried it and it works a treat!

To answer some of the questions about actual current draw on the LiPo in this test case, I re-did the start test with my Turnigy WaSome commentstt meter and power analyser connected.

The results were:meter
- Peak current : 90A, which is compatible with the rated upper value for the 3Ah, 20-30C battery
- Peak power: 835.5Watts
- Minimum voltage: 9.28V

Considering that it only needs to supplement cranking for a couple of seconds, this is less demanding on this battery than a full power take off of a model plane with suitable speed controller and motor. Note: Once the engine was running and charging again, the terminal voltage rose to 13V and hence would be charging the LiPo. If you use this method for starting, disconnect the LiPo as soon as possible to prevent over charging.
I don't want to take any chances with more volts than my car's accustomed to. So based on what I found, since I'm usually packing several 3S packs with me (preheated with an eighth pack), with this badboy below on hand I should be in business, right? Though I'll run it by the redoubtable Wayne Giles meter, then make my own how-to youtube video ostensibly to demonstrate the proper way to do this but really its purpose is to show off my Powerpoles.

Oh that's right baby, I crimp Powerpoles all day long.

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