(The start of this is copied from another thread. This seems to be a repeating line of inquiry and I decided it might do better having its own thread)
Let us start with first principles:
There are only two ways to regulate an alternator - fixed set point and variable.
A fixed set point regulator increases field current when the voltage is less than the setpoint and decreases it when it is over the setpoint.
The second part of the equation is how storage batteries act. You can look up their data and find an acceptance curve. At a given state of charge and a given voltage, they only accept so many amps.
Thought experiment number 1. You have a huge alternator that can putout 1,000 amps at idle and has a fixed setpoint. You have a dead 100 AH wet cell. It will NOT be recharged in 6 minutes! What will happen is the battery charge will follow the acceptance curve, which might be somewhere between 1/4 and 1/2 C, so in this case someplace between 25 and 50 amps. As the battery charges the acceptance rate tapers off. Instead of being fully recharged in 2 to 4 hours, it might get half charged in two hours and take 4-6-8 more hours or even infinity hours to get all the way back to 100%*.
Here is the tricky part - the higher the voltage the faster it charges, but the higher the voltage the more likely the battery is to be damaged eventually if it just stays that high. Imagine filling a water balloon from a hose. If you set the valve to not have enough pressure to burst the balloon, it takes forever to fill it. If you turn it up full blast, the balloon will pop.
This is why so many sailors who bought big battery banks and big alternators back in the day found themselves with either slow charging and sulfated to death batteries or fast charging and boiled to death batteries.
More to follow.
* Easy way to think of this asymptote function - every X hours you fill up half the empty space in the battery. X hours is 50%, 2X hours is 75%, 3X hours is 87.5%, and so on. You never get to 100.
Let us start with first principles:
There are only two ways to regulate an alternator - fixed set point and variable.
A fixed set point regulator increases field current when the voltage is less than the setpoint and decreases it when it is over the setpoint.
The second part of the equation is how storage batteries act. You can look up their data and find an acceptance curve. At a given state of charge and a given voltage, they only accept so many amps.
Thought experiment number 1. You have a huge alternator that can putout 1,000 amps at idle and has a fixed setpoint. You have a dead 100 AH wet cell. It will NOT be recharged in 6 minutes! What will happen is the battery charge will follow the acceptance curve, which might be somewhere between 1/4 and 1/2 C, so in this case someplace between 25 and 50 amps. As the battery charges the acceptance rate tapers off. Instead of being fully recharged in 2 to 4 hours, it might get half charged in two hours and take 4-6-8 more hours or even infinity hours to get all the way back to 100%*.
Here is the tricky part - the higher the voltage the faster it charges, but the higher the voltage the more likely the battery is to be damaged eventually if it just stays that high. Imagine filling a water balloon from a hose. If you set the valve to not have enough pressure to burst the balloon, it takes forever to fill it. If you turn it up full blast, the balloon will pop.
This is why so many sailors who bought big battery banks and big alternators back in the day found themselves with either slow charging and sulfated to death batteries or fast charging and boiled to death batteries.
More to follow.
* Easy way to think of this asymptote function - every X hours you fill up half the empty space in the battery. X hours is 50%, 2X hours is 75%, 3X hours is 87.5%, and so on. You never get to 100.
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