I've got a cat and each
engine has a 110 amp
alternator hooked up to it.
As far as I can tell, each
alternator is hooked directly to the start
battery for that side.
There is also a connection, via a
battery isolator switch, that connects each start battery to the house bank (so I have a "port bypass" and "starboard bypass" battery isolator switch).
The PO installed
cheap solar controllers from the house bank to the start
batteries to act as
cheap DC-DC chargers to keep the start
batteries topped up.
I have a 720 AH house bank, comprised of 6 120 AH
AGM batteries. Over the last week, I was on the
boat and drew the batteries to -200 AH (according to the shunt, which seems to be fairly accurate. This assumption about the shunt could be wrong).
Solar wasn't really bringing in enough charge, so I decided to run an
engine to bring a bit of
power to the house bank and tide me over.
When I turned on the "starboard bypass", I immediately saw my battery
monitor indicate that 40 amps or so was flowing into the house bank. That number quickly went to 30, then 25, and eventually settled around 15 amps going into the house bank. This happened over the course of about a minute.
Before I go throwing an amp clamp around different
parts of the system, I just wanted to check that my basic understanding of
charging batteries was correct.
The house bank and the starter battery are connected in parallel via that isolation switch. When I close that circuit and connect the batteries, I would expect the house bank to have quite a low resistance to accepting
current (as it was at about 70% charge) and the start battery to have to have quite a high resistance to charge (as it should have been at essentially 100% charge). I would assume, therefore, that the house bank would gladly take quite a bit of amperage from the alternator (much more than 15 amps).
At the same time, I understand that the voltage of the start battery would be one value, the voltage of the house bank would be another value, and the voltage output by the alternator would be a third value, so those three values would have to come together and that could be messing with what the alternator is attempting to do.
Is my assumption that 2 batteries in parallel, where one battery is in a low SOC and the other is in a high SOC, would allow the low SOC battery to charge efficiently incorrect? Is there a factor in my system that I'm not considering?
An interesting thing that I noted was that the solar charge controllers never reported putting in more than 20 amps (combined) into the battery. When I left the
boat one afternoon I was at -215 AH. I cam back the next morning and the battery
monitor reported the battery as full. This fact leads me to think that the shunt may be inaccurate and my house bank may not have been as low as I thought, and that is why the alternator was only putting 15 amps into the house bank.