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Old 23-06-2023, 15:41   #1
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Confusion around charging house bank off alternator

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.
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Old 23-06-2023, 17:19   #2
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Re: Confusion around charging house bank off alternator

I dont have a clear picture of how the alternator is connected to the house batteries. But I will say that the last 20% or so of charge going to lead acid is a slow process. Its really not that practical to charge lead acid anywhere near 100% SoC using a alternator or generator. Takes to long per Ahr. Thats what your solar should do- use the time factor to squeeze free solar into the last bit of top off charge needed to keep lead acid cells healthy.

So from what Im understanding of your set up it sounds like its doing what its supposed to. Look up pics of 3 stage charging of lead acid and note the Volts and Amps near the end stage. Should help bring some clarity.
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Old 23-06-2023, 17:41   #3
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Re: Confusion around charging house bank off alternator

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Originally Posted by workmaster2n View Post
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.
I think you have answered your own question, combining the two banks has increased the voltage the alternative see's but the house bank should also draw down the start battery quite quickly as well so I suspect your combined bank was at absorption phase after equalising with the start battery. All seems normal to me.
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Old 23-06-2023, 18:10   #4
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Re: Confusion around charging house bank off alternator

What revs were you running the engines at when charging start/house bank?
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Old 24-06-2023, 01:03   #5
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Re: Confusion around charging house bank off alternator

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I think you have answered your own question, combining the two banks has increased the voltage the alternative see's but the house bank should also draw down the start battery quite quickly as well so I suspect your combined bank was at absorption phase after equalising with the start battery. All seems normal to me.

I didn't consider the voltage difference and the fact the house would draw down the start battery. This makes more sense!
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Old 24-06-2023, 01:04   #6
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Re: Confusion around charging house bank off alternator

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What revs were you running the engines at when charging start/house bank?
I had the engine at 1200 RPM, but the same thing happened when I cruised home at 2000 RPM.
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Old 24-06-2023, 01:16   #7
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Re: Confusion around charging house bank off alternator

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I didn't consider the voltage difference and the fact the house would draw down the start battery. This makes more sense!
Run your house bank down to say 50% then try again, you should be charging in bulk phase after the start battery equalises with your alternator putting out 30 amps or more at decent revs.
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