Battery bank SOC questions

[Dr. D]

My boat has four AGM batteries (70 Ah each), three grouped together as the house bank and the stand alone motor battery. The house bank and motor battery are isolated from each other in all configurations. To read the voltage I was using the control panel display.



When on the charger at the dock, both the house bank and motor battery show the voltage from the charger, about 13.7 V. Now, the interesting part:


I had work to do on the boat which required disconnecting shore power for several hours. The motor battery showed 12.8 V the whole time (well, after an hour I didn't check it again). The house bank started at 12.7 V, then after about 45 minutes showed 12.6 V. When the refrigeration cycled on the voltage would drop to 12.5 V, occaionally 12.4 V.


The refrigeration draws about 4 A. I turned on the navigation equipment to work on them. They appear to draw 2 A and the house bank would drop to 12.5 V, but not lower.


OK, why am I seeing this variation? I had thought the house bank would start at 12.8 V (maybe 12.7 V) and stay there since it was disconnected for only a few hours. The refrigeration and navigation instruments are minor draws. The bilge pumped may have cycled once, again a minor draw.


Or is measuring the house bank voltage this way faulty in itself?

[john61ct]

Well accurate measurement instruments are required for precision, but all this seems pretty normal.

Voltage does not correspond to SoC in any fixed, linear or standard way, especially when there is current flowing into or out of the battery.

Initiate a load, voltage drops. Stop the load, it bounces back. Getting an accurate "at rest" reading can take 24 or even 72hrs isolated.

Does that help?

[a64pilot]

First of all confirm that 13.7 is correct for their float voltage.
But what your seeing may be what some refer to as a surface charge, that amounts to a battery being a little overcharged if you will and the voltage drop is pretty big right at first with very little power removed from them, then after that surface charge is drained off then you get into the real battery capacity if you will and voltage drop will usually slow way down then.
I would if I were you, do a quasi capacity check, just try to see where your voltage drops to overnight. You can get an idea of what capacity is, if for example your average draw is 6 amps and the bank is down to 12 volts in 10 hours, then your banks capacity would seem to be about 60 AH, which means you have a problem and can’t rely on that bank should you want to go out for a weekend at anchor.
Just as a general statement most batteries are down to about 50% at 12V and most don’t want to discharge lower than that on a daily basis.
That is by no means a real capacity check, but will give you some idea of your banks health.

[Dr. D]

Thanks for the posts. The batteries are probably OK, but I will perform a longer test. Most likely in the slip so if the house bank is having problems I'm not running the motor every few hours.

[sdj]

The internal resistance of the battery (which varies with SoC, load, age etc) causes a voltage drop linear with current. More load = more voltage drop = lower terminal voltage.

Other various battery technologies have a completely different behavior since they have different internal resistances, for example lithium-based batteries have a very low internal resistance so they have a low to non-existant voltage depression during discharge. In the end, it's just a battery being a battery, and a different technology battery behaves differently.

If it's any reassurance, your description shows a healthy battery system.