When jump starting a car A using a car B, can one connect the chassis of both cars together instead of connecting the negative battery terminal of car B to the chassis of car A. In theory this seems to work as the negative battery terminals of both cars are connected to the chassis but wouldn't there be extra electrical resistance ?
The intent here is that the last connection to be made is not connected directly to the battery. One of the byproducts of the operation of a lead-acid battery is hydrogen gas, which may accumulate within and around the battery. When you make the last connection and complete the circuit, there will probably be a spark. Sparks and hydrogen seldom end well.
Safety. Connecting the second cable makes a spark. You want it away from the battery. A depleted and over-discharged battery may outgas hydrogen. Hydrogen is explosive.
Lower resistance (not much, but still) - when starting, the current flows in general from the running alternator of the working car to the starter of the dead car. An exposed metallic part of the engine (if any) is even better place for a connection than the chassis.
Battery management system - a lot of modern cars have a current measuring shunt somewhere between the minus battery terminal and the chassis. The system expects no current flowing around the shunt and may react in unpleasant ways if that happens (up to and including shutting down the engine and requiring service).
In theory it will work. In practice, it will also work. Yes there is extra resistance as you have increased the path to ground. In a perfect system each ground would be equal, but it is not a perfect system. Due to the large surface area it is minimal but there will be some resistance. And due to the large current needed in charging and turning over a motor, the voltage droop/rise over that resistance will be non-zero. It will also likely be negligible in the use case provided.
So yes you can connect to the chassis, frame, or engine block of the donor car as long as it is clean and grounded. Obviously there are edge cases if you have a positive chassis vehicle (unlikely, antiquated setup), 6 or 24v vehicles (antique bikes and commercial trucks, helicopters, RVs and boats). But for your average car it's fine.
Car batteries are extremely well-connected to chassis
Almost all cars use the chassis of the car as the (negative) ground.
Chassis is extremely well bonded to the battery negative terminal, since this is the normal current return path for the starter, which can pull up to 1000 amps.
So no, there will be as little resistance as you can find.
Your biggest problem will be paint or body rust; I solve that on my cars by bolting a lead terminal to somewhere appropriate on the frame. Such lead terminals are sold as "top post converter kits" intended to give a top-style post to a side-post battery.
Doesn't matter. You don't pull 1000A down a jumper cable.
That's not how jump-starting works. The real power of jump starting happens in the minute or two before you attempt to crank. You're either doing one of two things:
- You have a battery that is merely flat (discharged) but otherwise perfectly capable of doing its job. You are refilling its energy "tank", and the start energy actually comes 90% from this battery.
- You have a battery that is "at the cusp of end-of-life". It can still store enough energy needed for a start, just not for any length of time. So you are both boosting that battery's charge temporarily, and also, heating it up - batteries store more and perform better when they are warm.
A lot of people just dive right in, hook up and crank; but really, the heavy lifting is being done in the minute or so after they've connected but before they crank. It's better to rev up the donor engine (just a little bit; 1500 RPM is plenty; don't throw a piston rod!) and hold it like that for about one song on the radio (3-5 minutes). At that point you could even unhook and still get a start.
If you're trying to start a long-dead car whose battery hasn't started an engine in 2 years and is basically a rock, then yeah, you're depending on the jumper cables to carry all starting current. But most people do that kind of thing in nice weather :) When temperatures are warm, oil is thinner, and engines and motors turn easier. Cranking amps are in the low 100's typically, and it's conceivable you could get that out of jumper cables.
The current path of interest is the battery to the starter motor on the car being started.
Each battery has a cable running from the negative terminal to the engine block or transmission near the starter motor. If you attach the cable to the engine block on the non-running car you can bypass one of the cables (which can only be better).
I usually attach the cable to the negative terminal of the battery or to a piece of metal on the engine block like a bracket (assuming you can find something made of metal).
To add to the other reasons given, there's one more important reason why you should connect to the chassis, and use the proscribed connection order: to avoid shorts.
Yes, there's a risk of sparks igniting hydrogen as noted above, but a car battery is also capable of delivering 200+ amps for the starter motor. You don't want to short it; the jump leads will quickly get extremely hot, potentially melting the wires and setting the insulation on fire. I suspect it might also damage the car's electrical systems, as the voltage is likely to do odd things for a moment.
It's usually pretty cramped getting big clips to the battery, and using the chassis for the negative terminal avoids getting the clips close together, reducing the risk of a short.
The proscribed connection order (connect +ve wire to both cars, then the -ve wire to both cars) means avoids a number of potential risks:
If you connect both wires to one car, there's a risk of them touching when you carry them over to the other car. Some idiot I'd given a jump start to nearly shorted out my car when he disconnected both wires from his car and carried them to me. They were swinging from his hand, and came within seconds of touching before I managed to remove the negative from my car.
If you connect the negative first, then drop the positive wire when trying to connect it to the second vehicle, or accidentally touch the chassis when trying to manoeuvre it into position, you'll get a short.
By connecting to the chassis, it reminds you that the chassis is part of the electrical system, something it's easy to ignore generally.
Connect it to the terminal on the full battery, so the current doesn't have to go through battery -> chassis. But if the recipient car has the starter motor connected to chassis, then the path with least resistance is the chassis near the starter or the starter motor terminal.
Then again, usually you need the recipient battery to be charged a bit, so it helps in providing current. Depends on the setup, but some cars won't start if the empty battery is drawing current and you need to rotate the motor too.
This works in theory and may work in practice if there's enough current getting to the starter.
On many (most?) cars, the starter is grounded through the engine block and subsequently the chassis. For example, here is a wiring diagram for a Miata starter - there is no wire going directly from battery ground to the starter:
In theory, if you are, say, jump starting one Miata with another you could connect both chassis instead of connecting battery grounds. In practice there are three issues with connecting chassis:
- You need to get a good hold of the connection point with the cable clamp.
- The connection point must be unpainted.
- Whatever part the connection point is attached to must be well connected to the rest of the chassis (this is also why engine grounding is important, and why many no-start troubleshooting guides instruct looking at engine ground wires/terminals and ensuring they are in good condition).
If you try jump starting a Miata with a truck in the summer, you'd probably be fine connecting chassis together. If you try jump starting a Miata with another Miata (smaller battery capacity) in winter, chassis to chassis connection may not be sufficient.