Originally Posted By: jaj
Well, that may have been true at one time, but modern cars have controllable alternators and a complex alternator management routine built into the ECU. It controls of the alternator output voltage and charging strategy, with particular attention to making sure that there's the correct voltage for the fuel pump (in particular) at heavy loads.
too much cool-aid?
a lot of these strategies are not about charging! Alternator charging strategy is pretty simple. Maintain a voltage set point that is biased by temperature, period. the intelligence you've read about is more likely an integrated systems approach that older units do not have, such as:
delayed engagement- some systems may let a cold engine stabilize for a few seconds before allowing the alt to go full-tilt
dampened engagement- while the alt will cut back immediately to overvoltage, such as when a big load powers off, immediately reducing FC strength, it won't immediately respond to a sharp undervoltage. rather, it will ramp up the field coils over a couple of seconds. this reduces shock loading and associated vibrations in the accessory drives.
beyond that, they all charge a little hotter when cold, and the temp sense mechanisms aren't usually nasa spec. the 4 chryco vehicles I've owned (2 jeeps, 2 vans) all read the engine compartment warm-up quicker than that big chunk of lead-acid, which has a much higher thermal mass.
In short, with a temperature bias, it will maintain a set point unless its overwhelmed by a) loads or b) low rpm, with a few management routines to be kinder to engine behavior and passenger comfort.
fuel pump? why single that guy out. They'll spin at 10 volts. I've seen several ECUs that stop behaving around 11.5. Radiator fans can need 60 amps to start and 30 to spin.
It's also misleading to call a vehicle charging system single-stage. Yes, it is, but NOT in the same terms as a multi-stage household charger. A household charger switches between bulk, float, etc., based on a sensed voltage. A *really* nice charger will add a temp probe to the battery, but that's not what most of us have. When V is lower, charger engages heavier current coils. After a setpoint is reached, it switches to a lower V potential and lower current winding. 2 or three times, depending on the model. In a car, the voltage itself is regulated, so the battery absorbs whatever the potential difference dictates. in my experience, with a 65 amp alternator (back when I had a car with an afternmarket gauge), that was 50amps after start, quickly falling to 15, sitting on 15 for a few minutes, then twindling down to 5-ish.... (in the noise... its a needle after all)... very close to 0. Along those lines of thinking, it did everything a multi-stage charger would do, but it did not have multiple modes that were being followed. so, for either argument to work, you have to define what constitutes multiple modes...
I didn't read the article, but I'll agree that no way is 14-16 hours needed, in a vehicle, for a normal start. not even on a bench. a completely drained battery, typical ~50AH, sitting on a multi-stage charger will probably accept a bulk charge of 10A over 4 hours for 40AH, then 5A for another 90 minutes, and move to float after that. there's 6 hours, on a bench.
batteries are cool-- and the interactions to a charging system have some neat details--- but to really understand what is happening, you've got to meter, measure, observe.
I'll also point out that, in general terms, they are still rather crude. They are noisy, and relatively slow to respond to changes. The inductance of that big field coil is slow to respond to drops in demand, resulting in those crazy spikes that everyone reads about, that the battery absorbs. Vehicle oems put a good number of parts into power control on every single circuit component to filter out the goblins. they are also still rather wasteful. Bench test an alt with 15 amps on it. you might be surprised to see how much energy escapes in heat... from the stator itself. there's a MPG advantage to using a larger capacity alt, larger windings, less resistance under load.
my $0.02.
m
Well, that may have been true at one time, but modern cars have controllable alternators and a complex alternator management routine built into the ECU. It controls of the alternator output voltage and charging strategy, with particular attention to making sure that there's the correct voltage for the fuel pump (in particular) at heavy loads.
too much cool-aid?
a lot of these strategies are not about charging! Alternator charging strategy is pretty simple. Maintain a voltage set point that is biased by temperature, period. the intelligence you've read about is more likely an integrated systems approach that older units do not have, such as:
delayed engagement- some systems may let a cold engine stabilize for a few seconds before allowing the alt to go full-tilt
dampened engagement- while the alt will cut back immediately to overvoltage, such as when a big load powers off, immediately reducing FC strength, it won't immediately respond to a sharp undervoltage. rather, it will ramp up the field coils over a couple of seconds. this reduces shock loading and associated vibrations in the accessory drives.
beyond that, they all charge a little hotter when cold, and the temp sense mechanisms aren't usually nasa spec. the 4 chryco vehicles I've owned (2 jeeps, 2 vans) all read the engine compartment warm-up quicker than that big chunk of lead-acid, which has a much higher thermal mass.
In short, with a temperature bias, it will maintain a set point unless its overwhelmed by a) loads or b) low rpm, with a few management routines to be kinder to engine behavior and passenger comfort.
fuel pump? why single that guy out. They'll spin at 10 volts. I've seen several ECUs that stop behaving around 11.5. Radiator fans can need 60 amps to start and 30 to spin.
It's also misleading to call a vehicle charging system single-stage. Yes, it is, but NOT in the same terms as a multi-stage household charger. A household charger switches between bulk, float, etc., based on a sensed voltage. A *really* nice charger will add a temp probe to the battery, but that's not what most of us have. When V is lower, charger engages heavier current coils. After a setpoint is reached, it switches to a lower V potential and lower current winding. 2 or three times, depending on the model. In a car, the voltage itself is regulated, so the battery absorbs whatever the potential difference dictates. in my experience, with a 65 amp alternator (back when I had a car with an afternmarket gauge), that was 50amps after start, quickly falling to 15, sitting on 15 for a few minutes, then twindling down to 5-ish.... (in the noise... its a needle after all)... very close to 0. Along those lines of thinking, it did everything a multi-stage charger would do, but it did not have multiple modes that were being followed. so, for either argument to work, you have to define what constitutes multiple modes...
I didn't read the article, but I'll agree that no way is 14-16 hours needed, in a vehicle, for a normal start. not even on a bench. a completely drained battery, typical ~50AH, sitting on a multi-stage charger will probably accept a bulk charge of 10A over 4 hours for 40AH, then 5A for another 90 minutes, and move to float after that. there's 6 hours, on a bench.
batteries are cool-- and the interactions to a charging system have some neat details--- but to really understand what is happening, you've got to meter, measure, observe.
I'll also point out that, in general terms, they are still rather crude. They are noisy, and relatively slow to respond to changes. The inductance of that big field coil is slow to respond to drops in demand, resulting in those crazy spikes that everyone reads about, that the battery absorbs. Vehicle oems put a good number of parts into power control on every single circuit component to filter out the goblins. they are also still rather wasteful. Bench test an alt with 15 amps on it. you might be surprised to see how much energy escapes in heat... from the stator itself. there's a MPG advantage to using a larger capacity alt, larger windings, less resistance under load.
my $0.02.
m
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