Jumping a car battery argument

[Zenmervolt]

And even then, most of what I run across on forums is that someone will fry an ECU, blow a fuse, or melt a fusible link before the battery explodes.

I'm not afraid to jump a car with my personal car, but I'm not letting someone else hook up the cables.

Yup. Usually the problem of effectively creating a 24 volt battery on a 12 volt system shows up faster than the batteries could heat up and explode when wired anti-parallel.

ZV

 

[BUTCH1]

Wow. Something under the hood of a car, next to the engine, was too hot to touch with the engine running. What a shock. I've yet to come across a car where the alternator was cool when the engine was running.



ZV[/QUOTE}

Let me be more specific, my car was off overnight and my alt. got very hot, very fast. I'm not a mechanic just IMO I won't jump anyone anymore..

 

[ShawnD1]

Hydrogen gas, being lighter than air, dissipates relatively quickly. It only vents while being generated because that's the only time it creates enough pressure.

The amount of hydrogen in a working battery is almost zero. What you're talking about hasn't been true for half a century.
In battery backup systems we had in the 1960s, lots of battery systems were constant current chargers like you describe. They would overcharge batteries like mad and it would generate large amounts of hydrogen. The level of electrolyte would constantly decrease because the water was being converted to oxygen and hydrogen. Back then, people would put distilled water in car batteries because that was a regular part of maintaining them. These days, lots of car batteries are sealed and you can't add water even if you wanted to. They don't need water because they don't generate large amounts of gas like you describe. We don't use constant current chargers anymore. Modern chargers are constant voltage chargers.

Bullshit. There is always a load on the battery of a modern car. There's a spark when replacing a battery as well. There's a spark when connecting a bench charger (if you're fool enough to have the charger plugged in before connecting the leads). When connecting a battery to a live circuit, there's always going to be a spark

I will make a video of this tomorrow.

If it weren't limited, the battery would never stop charging and you'd have an overcharged and destroyed battery after a single road trip. You also seem confused about voltage and amperage. A car's electrics are regulated to a nominal value of 13.5 volts. Extra load will pull more amps, but it won't mess with the voltage unless the voltage regulator is malfunctioning.

It sounds like you don't understand how a battery charging system works, specifically the part about what causes electric current. Again, just like before, what you're describing is a constant current charger. We don't use those. Nobody uses those. Even people in Africa will laugh at you if you use a constant current charger.
Let's go back to grade 10 physics for a minute and review ohm's law. Current = voltage / resistance. Voltage in this case is the voltage difference between the alternator and the battery. The alternator has a voltage that is tightly controlled by having a voltage controller adjust the amount of field current in the alternator. The alternator's voltage is constant because of this. The battery is not constant. Batteries have different open circuit voltage based on how charged the battery is, and batteries also have different closed circuit voltage based on how much load is connected. Connecting a high current load to a battery causes the battery's terminal voltage to drop because the battery has internal resistance; the terminal voltage drops by (current) x (internal resistance). When not connected to a charger, a battery might start at 13V when nothing is connected then drop to 12V when powering a bunch of lights and fans.
What determines alternator current is very simple. Alternator voltage is a certain amount, battery voltage is a certain amount, and this inequality of voltages allows current to flow between them. The greater the voltage inequality, the greater the current. This is why constant voltage chargers charge at different rates based on how charged the battery is. When a battery is low, a constant voltage charger will charge at a very high rate. When a battery is almost full, a constant voltage charger becomes a trickle charger because the tiny voltage difference only allows a tiny amount of current to flow. Your car battery will never overcharge because eventually the voltages are equal and no current is flowing. This is why you can run a battery for 5 years and it never needs to have water added.

The alternator and its voltage controller are not intelligent. It maintains a constant voltage and nothing else. If you put a huge load on the alternator and it causes a voltage sag, all the thing sees is a voltage sag, so it increases the field current until the voltage is back to where it should be. If you increase the engine rpm and the alternator's voltage goes up, all it sees is a high voltage and it responds by lowering the field current. It can't tell the difference between 70A going to some battery you jumpered to it vs 70A going to the headlights and fans. If you jumper a dead battery to your own battery and the voltage sag is enough to support 50 or 100 amps, then that's exactly what happens. Connecting a completely dead battery to a car alternator will permanently damage the battery. That process of charging the battery way too fast creates a “sulfated” battery. Google that term if you want.

If you look at your own car's charge system, you might notice that there's a single fuse on the positive wire going from the alternator to the battery. That's the only thing limiting the current. There's no intelligence in the system; it relies entirely on that fuse. If the dead battery pulls too much current and the fuse works properly, it kills the circuit and now your car needs a new fuse before you can drive again. If the fuse doesn't work and the thing pumps out a ton of current, your alternator is going to fry.

You just claimed that the charging system in a car doesn't have any intelligent circuitry... And now you're claiming that "chargers only look at battery voltage"? Which one is it? Chargers "look at battery voltage" to determine the battery's state of charge and then adjust the current flow (amperage) supplied to avoid overcharging.

You don't understand how alternators work and your statement doesn't make any sense. The voltage controller adjusts the FIELD current. You seem to be under the impression that the voltage controller directly controls the armature current, and that's simply wrong.

 

[Zenmervolt]

You seem to be under the impression that the voltage controller directly controls the armature current, and that's simply wrong.

No, I'm under the impression that the voltage controller controls the voltage. That's why I said it controlled the voltage. That's why I called it a "voltage regulator". If I were under the impression that the voltage regulator controlled current (amperage), I'd have called it the amperage regulator.

The alternator and its voltage controller are not intelligent. It maintains a constant voltage and nothing else. If you put a huge load on the alternator and it causes a voltage sag, all the thing sees is a voltage sag, so it increases the field current until the voltage is back to where it should be. If you increase the engine rpm and the alternator's voltage goes up, all it sees is a high voltage and it responds by lowering the field current. It can't tell the difference between 70A going to some battery you jumpered to it vs 70A going to the headlights and fans. If you jumper a dead battery to your own battery and the voltage sag is enough to support 50 or 100 amps, then that's exactly what happens. Connecting a completely dead battery to a car alternator will permanently damage the battery.

I've spent more time thinking about this and what it comes down to is that the battery in a car's electrical system is wired in parallel with the alternator. I've been treating this as though the battery were wired in series (as would be the case with a bench charger or a float charger). You're right that, because the battery is wired in parallel with the alternator, the battery will only draw charging current if there's a voltage differential. However, that's not the same as the charger "looking at" the battery's voltage, it's simply a case of the battery's voltage determining how much current it will draw into its parallel circuit.

If you look at your own car's charge system, you might notice that there's a single fuse on the positive wire going from the alternator to the battery. That's the only thing limiting the current. There's no intelligence in the system; it relies entirely on that fuse. If the dead battery pulls too much current and the fuse works properly, it kills the circuit and now your car needs a new fuse before you can drive again. If the fuse doesn't work and the thing pumps out a ton of current, your alternator is going to fry.

If you want to be technical, that's the main positive line for the entire electrical system, not just for the battery since the battery is wired parallel to the vehicle's electrical system. It's not as though the current flows first through the battery and then the rest of the system.

Also, the fuses are generally after the battery to protect the remainder of the car's electrical system. For example, my Volvo has 8 different "main fuses" (all between 50 and 60 amps) protecting different circuits.

If the battery were to "overload" the alternator as you say, the voltage regulator would not be able to maintain a constant current and the car would stall as the voltage dropped below functional levels. However, the chances of something like that happening are so small as to be disregarded. Automotive batteries just plain don't go down to 0 volts across the terminals as would be necessary for your doomsday scenarios to be viable. A "dead" battery is generally no worse than 11 volts at the terminals and it won't be enough to cause problems.

ZV

 

[hanoverphist]

Leave it to ATG to have 10 pages of debate over the proper way to change a lightbulb :awe:

i agree. i weep for our future.

 

[ViviTheMage]

I've jump started a lot of cars ... I have the good car on

positive + ground on the dead car
hook up ground then live on the live car
start her up ... remove positive + ground from newly live car, then the live car

I've had no issues this way ...