[C320-list] Flooded acid battery and refrigeration questions

[Scott Thompson]

Jack, the monitors are very good at measuring voltage and amperage going 
into or from the batteries. It's true that they guess about capacity and 
the state of discharge, but you can actually learn quite a lot from 
those two measures, and how they change over time. So I guess I have 
more faith in the smart charge controllers than you do.

The best charge controllers are clever enough to avoid the situation you 
describe. For example, the Victron Phoenix chargers will start a new 
bulk charge cycle due to a load turning on only if the load causes the 
charger to go to maximum amps for at least 4 seconds. See 
https://www.victronenergy.com/media/pg/Phoenix_Smart_IP43_Charger_manual/en/charging-algorithms.html 
<https://www.victronenergy.com/media/pg/Phoenix_Smart_IP43_Charger_manual/en/charging-algorithms.html>. 
Your fridge drawing 5 amps should not come close to triggering that with 
a 30 amp charger, even if it draws more briefly when the compressor 
starts. As long as the charger stays at float voltages there should be 
no gassing. And the amps going into the batteries will not go up either 
if the voltage does not rise. Instead the increased output from the 
charger will be going to the load, not the battery.  A different 
controller might not be so clever and might raise the voltage when it 
detects a load.

I'm all in favor of empirical evidence, however. So perhaps next time 
you are plugged into shore power you might check whether the voltage or 
amperage going into the battery goes up when the refrigerator is running 
to test your theory. You will need an accurate battery monitor to detect 
it, but as I said they do measure voltage and current accurately.

BTW, you are technically wrong when you say "too much current causes 
boil." It's actually too much voltage applied to a fully charged battery 
that causes "boil." If current were the problem then batteries would 
boil when charging even when highly discharged, since that's when they 
accept the highest current. On the other hand, a charger that applies a 
voltage above the gassing level will overcharge full batteries even if 
the current level is very low. That's why you want a charger that is 
clever enough to apply no more than float voltages to a fully charged 
battery, even if it appears to be drawing more current than usual.

On 11/16/2021 4:47 PM, Jack Brennan wrote:
> Hi Scott:
>
> What I worry about a little is that the electronics manufacturers still haven’t invented a decent battery monitor that can accurately tell you the bank’s state of charge. Those costly monitors you buy now are making educated guesses, nothing more, and rely partly on your input.
>
> My thinking is that if those monitors can’t tell with a great deal of accuracy, the controllers aren’t any better. So when the fridge kicks on and draws five amps, it might reduce the bank voltage by, say, a 10th of a volt. That makes the controller think the state of charge is lower and send more current to the batteries.
>
> Too much current causes boil.
>
> I base this guess partly on experience. When I take a long cruise every spring, I never have to top off the batteries because we’re using a steady flow of electricity. When I’m just day sailing and not using many amps, that’s when the batteries seem to need water.
>
> Part of my concern with the 110-volt charger is that it is a less expensive one. (I only use it occasionally.) I guess I don’t have a tremendous amount of confidence in its ability to do sophisticated charging.
>
> Jack Brennan
> Sonas, 1998 Catalina 320
> Tierra Verde, Fl.
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> Sent from Mail for Windows
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-- 
Scott Thompson
Surprise, #653