Jump my car

It's been really cold here lately. So cold my car, no I did not plug in the block heater, died.  I got a jump and all well, this time.  But that got me thinking, since the car battery is a 12V and my lawn mower runs on 12 V batteries (3 of them).  Would one of those be able to jump the car?

Able to? Maybe. Good for the batteries? Probably not.

I presume you are talking about 12V Lithium ion for the mower? The current draw for starting a motor from cold like a car would likely be massive, and might exceed the safe rating for the batteries. At that point you don't know what will happen. The batteries might have a BMS circuit to protect the cells, in which case current draw may be safely regulated. If not you may permanently damage the cells, or burn out circuits.

My understanding is that car batteries use lead/acid in large part because they can cope with massive current draws over a short time.

Also, your mower batteries are likely to be well under nominal 12v rating if it is as cold as you say.

I agree with Michael. Batteries of different chemistry need the correct protective electronics between them to prevent damage.

If you are close to grid power, you are much better off buying a plug-in battery charger with a boost setting.

Also, plugging in the block heater for an hour or preferably two is usually enough to make the difference. If the car is stored outside, wrap a cheap tarp around the hood to keep the breeze from sucking away the heat.

We just went through a record cold snap, so I know what you're facing. Luck!

A lot of the new jump boxes use lithium-ion batteries. I have been in the woods and left a dome light on and started my truck with a Dewalt 18-volt battery. You sometimes have to improvise.
The new jump boxes are an incredible addition to what I carry in my vehicle. I can run my laptop and charge my cell phone. I don't think charging an LI battery in the cold is a good idea, so I wouldn't charge it my car in the winter.

Thanks for the quick  Replies!  

I put 2 of these super capacitors in an ammo can and use that for jump starting.  2 gauge wire works well.  These put together give it about 90 farad at 16v.  Getting the initial charge is a little tricky, but doable with a homemade choke.  I keep it plugged into our 12v solar system and it is always ready to go.  Had to use it today on the old 1.9 tdi vw.

Robert Ray wrote: I have been in the woods and left a dome light on and started my truck with a Dewalt 18-volt battery. You sometimes have to improvise.

Fair comment. In an emergency, I would fry an external battery to get a vehicle going. I think I would charge the vehicle battery for a little while before turning the key.