This is easier to show in a video rather than to explain so a video is below. A few weeks ago I put a couple meters on the boat I was working on to show what happens and how to measure it.
Meter #1 is measuring the house battery banks voltage at the battery terminals. (it is the one shown next to the battery in the cockpit)
Meter #2 is measuring the current flowing out of the alternator and is clamped around the output of the alternator.
Meter #3 is measuring the voltage at the output post of the alternator and what the internal voltage regulator is actually "sensing".
Most internally regulated alternators sense the voltage they are regulating at their own output post. This can be okay IF the wiring, both positive and negative, are sufficient to carry the current needed. In many boats the wiring is not sufficient. Sadly many engine manufacturers run very small wire between the alt and starter post. The alt output then picks up the larger battery cable, goes through the switch and then off to the batteries. This leaves a LOT of potential poor connections in the charging circuit, and on many boats some pretty serious distance. Some engines even utilize an ammeter in the engine panel. These are the absolute worst for adding SEVERE voltage drop to the battery bank.
Why does the voltage matter? Voltage is the pressure that allows the battery to charge faster or slower. Wet and AGM batteries need voltages well in excess of 14V to properly charge. GEL batteries require just 14.1V but even at that many factory systems just never get them there.
Low charging voltages at the battery will lead to chronic under charging. Less voltage at the batery results in less current to the battery and the net result is slower charging. This can lead to short bank life due to sulfation build up. Higher voltages can help shed some sulfate even if you;re not at an equalization voltage.
Most stock alternators that are internally regulated are set for 14.2V to 14.6V. This is the voltage the alternator is trying to get to when charging. Once the preset voltage is reached the regulator holds the voltage by varying the field current and voltage to the alternator by pulsing it on & off. While there were some older alternators set for 13.6V to 13.8V they are very rare. Can't recall the last time I came across one. Many folks assume their internal regulator is set for that because they measure the voltage at the battery and not at the back of the alternator.
Ideally you should see within 0.1 - 0.2V of the back of the alternator at the battery terminals when the alternator is in absorption or "voltage limiting". With the banks in the 95% full range there should be no difference in voltage at the alt and batteries.
Clean connections, new and larger wire, going straight to a house bank and additional ground wires can all help fix the problem. Some internally regulated alternators also allow you to sense the voltage at the bank by extending the voltage sense lead. Don't do this if you don't know how to do it properly...! Doing this will allow the internal regulator to compensate for any voltage drop and feed the batteries the proper voltage. This is how external regulators work and one reason people often see "faster charging".....
When checking alternator output voltage follow a few simple rules.
#1 Test at the batteries and the back of the alternator and compare the numbers.
#2 Test at varying current output
#3 Always run these tests with NO LOADS turned on..
The fix was to run a 4GA wire direct to the house bank with a fuse at the battery and a new 4GA neg wire also run direct to the bank. The ground wire was connected to the pivot ear of the alt after thoroughly cleaning it with a wire brush to bare aluminum. I left the factory ground wire in place, 12GA, and measured the current flowing in each when done. At 15A of charge current flowing in the + wire the new neg wire was taking 14.2A and the original ground wire taking just 0.8A.. It liked the new connections.
The bottom line is at 40A of charge current the voltage at the alt and bank only varied by a max of 0.1V compared to a full 1.0V at just 10A..... A major improvement over the piss poor factory wiring..
Meter #1 is measuring the house battery banks voltage at the battery terminals. (it is the one shown next to the battery in the cockpit)
Meter #2 is measuring the current flowing out of the alternator and is clamped around the output of the alternator.
Meter #3 is measuring the voltage at the output post of the alternator and what the internal voltage regulator is actually "sensing".
Most internally regulated alternators sense the voltage they are regulating at their own output post. This can be okay IF the wiring, both positive and negative, are sufficient to carry the current needed. In many boats the wiring is not sufficient. Sadly many engine manufacturers run very small wire between the alt and starter post. The alt output then picks up the larger battery cable, goes through the switch and then off to the batteries. This leaves a LOT of potential poor connections in the charging circuit, and on many boats some pretty serious distance. Some engines even utilize an ammeter in the engine panel. These are the absolute worst for adding SEVERE voltage drop to the battery bank.
Why does the voltage matter? Voltage is the pressure that allows the battery to charge faster or slower. Wet and AGM batteries need voltages well in excess of 14V to properly charge. GEL batteries require just 14.1V but even at that many factory systems just never get them there.
Low charging voltages at the battery will lead to chronic under charging. Less voltage at the batery results in less current to the battery and the net result is slower charging. This can lead to short bank life due to sulfation build up. Higher voltages can help shed some sulfate even if you;re not at an equalization voltage.
Most stock alternators that are internally regulated are set for 14.2V to 14.6V. This is the voltage the alternator is trying to get to when charging. Once the preset voltage is reached the regulator holds the voltage by varying the field current and voltage to the alternator by pulsing it on & off. While there were some older alternators set for 13.6V to 13.8V they are very rare. Can't recall the last time I came across one. Many folks assume their internal regulator is set for that because they measure the voltage at the battery and not at the back of the alternator.
Ideally you should see within 0.1 - 0.2V of the back of the alternator at the battery terminals when the alternator is in absorption or "voltage limiting". With the banks in the 95% full range there should be no difference in voltage at the alt and batteries.
Clean connections, new and larger wire, going straight to a house bank and additional ground wires can all help fix the problem. Some internally regulated alternators also allow you to sense the voltage at the bank by extending the voltage sense lead. Don't do this if you don't know how to do it properly...! Doing this will allow the internal regulator to compensate for any voltage drop and feed the batteries the proper voltage. This is how external regulators work and one reason people often see "faster charging".....
When checking alternator output voltage follow a few simple rules.
#1 Test at the batteries and the back of the alternator and compare the numbers.
#2 Test at varying current output
#3 Always run these tests with NO LOADS turned on..
The fix was to run a 4GA wire direct to the house bank with a fuse at the battery and a new 4GA neg wire also run direct to the bank. The ground wire was connected to the pivot ear of the alt after thoroughly cleaning it with a wire brush to bare aluminum. I left the factory ground wire in place, 12GA, and measured the current flowing in each when done. At 15A of charge current flowing in the + wire the new neg wire was taking 14.2A and the original ground wire taking just 0.8A.. It liked the new connections.
The bottom line is at 40A of charge current the voltage at the alt and bank only varied by a max of 0.1V compared to a full 1.0V at just 10A..... A major improvement over the piss poor factory wiring..
