In our charging configuration, we parallel the house and start alternators to charge the house battery bank. Each alternator has its own individual Balmar Max Charge MC-624 regulator and we’ve been asked a few times if we balance the outputs using a product such as the Balmar Centerfielder. This is one of those issues where people work very hard to solve an apparent problem that really isn’t there. When running two alternators or two charging sources of any type, people notice that one is charging and the other one at times, hardly is charging or is not charging at all. This often is described as the alternators “fighting each other”, so people naturally want to fix the problem.
Let’s look at what is going on and then talk through why it’s really not a problem. Batteries go through three broad charging phases (Balmar divides it up into 12 stages but three phases is the level of detail we need for this). The first phase is bulk charging, where the regulators give a target voltage and the alternators produce as much current as they can (it’s just about impossible with AGMs to have so much charging capacity that you exceed the battery acceptance rate). During this phase, you get max alternator output current from both alternators until the target voltage is hit.
The next phase is acceptance. During this phase the voltage level is held constant and the amperage the batteries can accept slowly goes down until it gets down to around 1 or 2% of overall capacity, at which point they are considered charged and the system switches over to float mode.
It’s during this acceptance phase that many owners worry that the two alternators should be balanced. During this phase the batteries will accept (that’s the reason for the name) a certain amount of current. Initially it’ll be max charging system output, but this value falls continuously until the battery is charged.
As an example, if you have two 100A alternators and the batteries are accepting 80A, then most owners want to see each alternator producing 40A. But, they instead might see one producing 60A while the other produces 20A, or even 80A and 0A. It does seem like a problem but, if you think about it, the batteries are charging as fast as they would with a single 200A alternator, so the overall charging rate is fine. The only problem is one alternator is doing more work than the other.
“Who cares?” is the short answer, but it is worth looking deeper. With one alternator doing all the work, that alternator is spending much more time at 100% output so will get hotter and will wear more. This is harder on bearings and produces more heat. So there is some value in having something close to balanced output, but the value is pretty minimal. I put 4,100 hours on the previous boat with two alternators never worrying about it. And we have put on 7,800 on our current boat, again, just not worrying about it. I tune the two regulators such that they are “close enough” so that both alternators contribute something. Right now, I have one running at 158A and the other running at 64A. That’s perfectly fine from my perspective. Earlier tonight we had two air conditioners and the oven on and both alternators were running flat out — these alternators can do that for hours at a time so I don’t worry much about balance.
The important things to think about are: 1) two alternators won’t be balanced in production during the acceptance phase and it really doesn’t matter, 2) they are charging your battery bank as fast as a single alternator of the aggregate capacity of your two alternators, and 3) there is some (small) value in setting the two regulators close and this is easy to do with Balmar Max Charge system or any other fully tunable regulator.