Jump-Starting/Battery Charging

Just a quick question:

Does connecting car batteries together in the typical jump-starting configuration (Positive to Positive, Negative to Negative) actually charge the dead battery? My intuition tells me “heck no” but i have read on many products and online that when you connect the cables you can (or should, depending on the source) leave them on for a couple minutes to charge the battery.

How does this happen? To my understanding, if you were going to charge a battery from another battery, you would need to connect the 2 batteries in series (not parallel) and have some sort of huge current-limiting resistor in place so the batteries did not explode. Trickle chargers have electrons going in the positive side of the battery and exiting out of the negative terminal, just like the battery would normally. So how does having the postive and negative terminals connected charge the battery in any way, shape or form?

So tell me, great Dopers, what is the straight dope on this?

Inquisitively yours.

When you connect a fully-charged battery to a discharged battery, you are doing two things:

  1. You are putting a load on the fully-charged battery. This will drain some of the energy from the fully-charged battery.

  2. Current will flow into the discharged battery. This will charge the discharged battery a little bit. But in no way will it fully charge the discharged battery.

If you want to charge a battery from another (fully-charged) battery, the latter better be connected to an active energy source (e.g. alternator).

Oh, and when you electrically connect two things together, I guess they can be considered to be in both series and/or parallel. From a theoretical/modeling perspective, it’s probably more correct to say they’re in series. But from a practical/seat-of-the-pants perspective, I think it’s better to think of them in parallel.

Yes, it does charge the battery.

You can look at charging the battery as pushing current back through it in the reverse direction to when it is discharging.

Lead-acid batteries drop in voltage as they discharge, so the charged battery will be at a slightly higher voltage than the “dead” one, say 13V vs. 11V.

Hook the positives and negatives together and the charged battery will push current through the dead one. The current may actually be quite high, as in a “fast” charger, but there is no danger of overcharging because the voltage of the dead battery will gradually rise during charging and vice-versa. Eventually they will reach the same voltage and the charging will stop.

Re: series and parallel - draw it out! Connect a good battery in series with a dead battery, so that the good battery pushes current around the loop and back through the dead battery. You will see that the two batteries are connected positive-to-positive and negative-to-negative - in this case series and parallel are the same.

This is true if you model each battery as an ideal voltage source. But if you design your model to include a source impedance with each battery, the “ideal batteries” in your schematic would be in series.

You want to think that part over again? :dubious:

The reason you do not connect the batteries in series is that you would end up pushing current through a dead battery (batteries have internal resistance, remember) instead of past it, to energize the starter. You would also end up with a nominal 24 volts, not usually good for anything not large and diesel-powered. Most of the energy used to turn the starter is coming from the good battery and the running vehicle’s alternator.

Okay, clumsy phrasing. I apologise. I was thinking of hooking the batteries only to each other, without any load. In which case they can be in series proper, giving a 24V, shorted battery, or “in series” but in opposition, which is the same as being in parallel.

Probably better to just ignore my comment re series and parallel altogether.

Oh, and when you electrically connect two things together, I guess they can be considered to be in both series and/or parallel. From a theoretical/modeling perspective, it’s probably more correct to say they’re in series. But from a practical/seat-of-the-pants perspective, I think it’s better to think of them in parallel.

Just to clarify, using my (possibly flawed) terminology, Connecting in parallel would be Positve to Positive terminals, Negative to Negative terminals. Series would be Negative to Positive terminal, Negative to positive terminals. I am also assuming a load runing between the positive and negative terminals of one battery. How could either the parallel or series scenario be considered anything else?

What? Sure, the positive terminal of battery a is connected to the negative terminal of battery b… through the negative terminal of battery A… thus kind of negating the whole ‘positive’ part… right?

D’oh. Should have hit “preview” before my reply. My apologies.

Two batteries connected + to + and - to - are in series because the same current goes through both of them. That’s the definition of a series circuit. That’s also the case if you connect + to - and - to +. In the latter case you will not charge the low battery, a lot more current will flow, and it certainly isn’t recommended procedure.

If you have batteries + to + and - to - and connect the + of an alternator between the battery +'s and the - of the alternator between the -'s then the two are in parallel as far as the alternator is concerned.

OK. now I see where you were going with that. I don’t think of parallel voltage sources as also being in series and inverted because you can run into all kinds of trouble with that kind of thinking. What makes for good shorthand in describing physical circuit layouts creates havoc in theoretical circuit diagrams where assumptions about the polarity of voltage sources are regularly made.

My bad.

My use of the word “back” was meant to imply that the current was travelling in opposition to the e.m.f. of the dead battery, which necessitates a + to +, - to - connection. “Backwards” would have been better.

Let’s say I have two black boxes – Box A and Box B. Each box has two wires – Wire 1 and Wire 2.

If I connect Wire A1 to Wire B1, and Wire A2 to Wire B2, then (on paper at least) it would be correct to say Box A and Box B are in parallel; after all, both boxes “see” the same voltage. It would also be correct to say Box A and Box B are in series; after all, the current is the same through both boxes. This is true regardless of polarity or direction of current.

In reality, however, it would be more correct to say Box A and Box B are in series rather than parallel. Why? Because each box has source impedance, and the wire have resistance. And all the boxes, sources impedances, and wires are in series, not parallel. Thus the current through each box would be the same, but the voltage across each box may be different.

With typical jumper cables available at discount, or even “auto” stores (I except NAPA here) you MUST charge the dead battery to some degree in order to start the car.

Because copper is heavy and relativly expensive, these jumper cables typically have, at best, AWG 10 conductors (which are labled “heavy duty”) surrounded by rediculously thick insulation. This limits the current to the point that a cool engine (thick oil) can’t be turned fast enough to start, if at all. However, the thin wire will pass enough current to charge the dead battery in a few minutes. Once the battery has a bit of charge, it can supply enough for a start.

The thick insulation actually hurts performance, because it traps heat, which makes the thin conductors even more resistive. Also, in cold weather, the PVC insulation used becomes very stiff, worse that if thick copper had been used.

You can buy heavier jumper cables from industrial suppliers. Another source is welding suppliers, which often have battery clamps you can attach to the welding cable they also sell and make up your own. Starting service is somewhat similar to welding service, so the ampacity tables used for welding cable are a useful guide.

Note that the thin wire does offer some protection to the alternator of the “helper” vehicle. It may also serve as a “fuse”, limiting other damage if the cables are improperly connected. When you make up a set of 4-0 jumper cables, and connect them to a dual-batteried diesel truck, extra care is warranted.

Another thing I didn’t see anyone mention is that typically you jump-start a car with the donor car’s engine running, which means that its alternator is putting out 13.5 volts. This being higher than the dead battery’s 11.5 (or so) volts, you’re gonna pump electricity into the dead battery.

No way is this true. I’ve jumped many, many cars without a minutes worth of charging. If you have good connections the dead car should fire right up.

At best 10 guage? I have never seen a jumper cable made with anything less than #6. YMMV, but I think you need at least enough capacity to carry the current of the starter motor.

Batteries do not charge in minutes.

When you think you have the jumper cables connected properly turn on the headlights of the dead car. If the lights come on bright and stay bright while trying to start the car you have a good connection. If the lights start out bright and return to their dim state after trying to start the car, you lost your connection, try again. If the lights never get bright keep trying to acquire a good connection. The headlights tell all. If you have bright headlights then you have a good connection and the car should start right away.
I always carry a good #4, 16’ set of cables in the car and I’ve jumped as many as 40 cars in my day and I’ve never had to wait for the battery to charge in the dead car, ever.

Sorry if I sounded snippy, maybe you could further explain what you meant. I now realised you referring to the #10 guage cables before you mentioned them in the next paragraph. Since I have never tried to jump a car with #10 guage cables I can’t speak of their performance, however I have never seen them and am still doubtful of their existence.

As a EE, I take note of retailers preying on lay ignorance WRT things electrical.
Like the exagerated HP specs on vacuum cleaners and air compressors, I note these things for my own amusement. I have seen jumper cables as heavy as #2 being sold, but that was, I think, at NAPA, and/or McMaster-Carr or maybe Grainger’s. I’ve yet to see anything heavier than #10 at wallyworld, poop-boys, Checkered-past, etc. The insulation will be like 1/2" OD. There will be large print saying “tangle proof, color coded clamps for safety, Heavy duty, etc.” you’ll have to use a magnifying glass to read the fine print that tells you they areonly #10 wire. Oh yes, be sure to look at how the wires connect to the clamps.

Better yet, ever seen those “booster cables” that plug between two cigar lighters? All the cigar lighters I’ve ever seen are fused at 30A. Sure, you have to wait half an hour to start the car, but, hey, no need to figure out where the hood release is.

AWG-6 cables will work noticeably better than the ASG-10 junk that are widely sold, and yes, they will start an engine directly from the “helper” battery, if that battery is in good shape, you have good connections, and the dead car’s engine isn’t really cold. I’ve carried a set of #6 cables my dad made in my SI vehicles for many years. The Diesel got a 2-0 welding cable based set. I tracked down those #2 cables I mentioned above when a co-worker became a new diesel owner and was seeking my advice…I just can’t recall exactly where we found them.

On several (4 or 5 over ~25 yrs… and I wasn’t counting) occasions, I’ve used those #6 cables to start vehicles where a jump was already set up and not working with #10 wallyworld cables. In these cases, only the cables were different…the cars were already positioned, I just swapped in my cables. This was usually met with disbelief, because these cables have fairly thin insulation, perhaps 3/8" OD…they LOOK smaller than the crap cables.

A discharged, but otherwise healthy battery will draw heavy charging current. I’ve measured 40-45A going into a not dead deep-cycle battery on my camper. That is with about 10’ of AWG-8, and return via a single vehicle’s frame. Starting batterys are designed for maximum plate area, so I would expect them to have lower internal resistance, resulting in higher initial charging surges yet.

If 10ga wire, and alternator limits the charging current to 50A, that won’t turn a cold engine, but given 10 minutes will put enough charge on a battery, that when combined with what the 10 AWG cables will supply with a 6V drop (maybe 100A?) WILL start the car.

IANAEE, but IAAGWTAR (I am not an electrical engineer, but I am a guy who teaches auto repair) The definitions we use for series and parallel go like this.
Series If there is only one path for current to flow through the circuit, then it is a series circuit.
Parallel If there is more than one path for current to flow then it is a parallel circuit.
The acid test is Christmas tree lights. If one bulb goes out and they all go out, you have a cheap set that is wired in series. If one or more bulbs can go out and the rest stay lit, the set is wired in parallel.
The following assumes two fully charged batteries (12V 500CCA):
If you hook the two batteries + on the car to battery 1 +, to battery 2 +, and minus on the car to battery 1 – to battery 2 - it would be a parallel circuit. Total circuit voltage would be 12V and 1,000 CCA. If one battery were to be disconnected, still a 12V, but only 500CCA available.
If on the other hand you hook the two batteries + on the car to battery 1 +, and battery 1 - to battery 2+, and battery 2- to - on car. You now have a 24 V circuit with 500 CCA available. Disconnecting either battery would leave an open circuit with 0 volts available.

That certainly doesn’t give me any heartburn and is one way of looking at it. However, the fact remains that a series combination of elements is one in which the same current passes through all of the elements. A parallel circuit is one in which all of the elements have the same applied voltage. With just two elements it would appear that either definition would apply to the circuit.

As a digression, the same sort of thing arises in connection with series and parallel resonant circuits. These are circuits in which there is a capacitor and an inductance. In point of fact the series and parallel resonant circuits are one and the same circuit in that in both cases a large current exists in both the capacitor and the inductor at the resonant frequency and a smaller current at other frequencies. The difference is that the series circuit is driven by a low impedance source in series with the elements and the parallel one is driven by a high resistance circuit in parallel with them. The difference is not in the resonant circuit elements and their behavior, but in the method of driving them.