Interesting "trick" to improve dual alternator charging performance

My CAT 3208TA engines come from the factory with 51 amp, small-frame Leece Neville alternators that are pre-set to 13.5 volts output. They do not have an adjustable voltage. There is no room to install large frame alternators. Both alternators were originally connected to the start battery and the house bank was connected to start bank through a battery isolator. This was not ideal since the house batteries were only getting 12.8 volts from the alternators due to the .7 volt drop in the isolator. I replaced the battery isolator with an ACR (with a remote manual control switch at the helm) and that helped a lot, but once the house bank got to 80% SOC or so, the charge current really tailed off and I was never able to run the main engines long enough to fully charge the house bank, even on 6+ hour trips.

One of my alternators failed a couple of years ago and I replaced it with a 95 amp Leece Neville that has adjustable voltage. Rather than connect it it the same way as before, I connected the output directly to the house bank, increased the output voltage to 13.8V and left everything else the same. This means that the field coil is fed by the start battery and it also means that the reference voltage for the regulator is also fed by the start battery.

When the engines have been running for a few minutes and the start battery is recovered from the big (but short duration) starting loads, the ACR mentioned in paragraph one combines the start bank to the house bank. If the house bank is low, the voltage will be pretty low while charging, and both alternators will provide plenty of charge current. As the house bank refills, the charging voltage will increase up to 13.5 volts and at this point the 13.5 volt alternator is not doing much. Once the charge voltage gets to 13.8 volts, the charge current will really drop and then it will take forever to get the house bank up to 100%. When this happens, I manually disconnect the house bank and the start bank with my ACR helm switch. This means that the 95 amp alternator output is only connected to the house bank. However, because the field coil/regulator reference is still connected to the start bank, the regulator sees 13.5 volts. Since the regulator is set to 13.8 volts, it will force the 95 amp alternator to try to increase voltage but the house batteries are still not fully charged so they absorb the excess voltage and draw more current which dramatically improves charging time. As the house batteries charge, the voltage keeps increasing and the charge current keeps dropping. Once they get to 100% SOC (which coincides with roughly 14.4 volts and 20ish amps), I manually re-connect the ACR, the charge voltage will drop back to 13.8 volts, the charge current drops to 1 or 2 amps, and the house batteries will remain charged.

There is a danger of over-charging the house batteries if the ACR is not used to reconnect the house and start batteries because the 95 amp alternator can climb to 15.2 volts, however it is not exactly difficult to manage.

I realize I can accomplish the same thing without any risk of over-charging with externally regulated alternators and external regulators, but there's not a lot of incentive to do so when I am limited to such tiny alternators. I posted this solely in case it might help someone else out.

With my two engines the setup was similar. One alternator to start batteries and the other to the house bank. A blue sea’s ACR did the seamless balancing trick with no helmsman input required.

Good job faking the alternator to do what you want.

But couldn’t you do much the same by connecting your alternator’s field coil reference to the house bank?

David

DavidM said:

Good job faking the alternator to do what you want.

But couldn’t you do much the same by connecting your alternator’s field coil reference to the house bank?

David

Click to expand...

I realize I left out an important fact: The alternators are set to such low voltages (13.5 and 13.8) to keep them from overheating when re-charging a big house bank in automatic mode. This is no big deal for the bulk charge, but to get from 80% to 100% SOC, 13.8V is extremely slow. With my trick, I can temporarily bump the alternator voltage and current up once the batteries stop accepting much current at 13.8 volts. It's just a simple and cheap way to go from absorption to float in 2-3 hours instead of 10+ hours, while underway. That extra charge in the house bank comes in handy when I get to an anchorage for an overnight stay.