Battery percentage question

[Barrie]

I have 870 amp hour wet house batteries new last year connected to a generous solar system and the batteries are also charged when connected to shore power.
My MPPT solar charge controller has a meter that shows battery percentage. Over night, with just the fridge running the percentage drops to 56 but the battery voltage remains constant at 12.6 to 12.7. By 11am the next day, the solar has brought the percent back up to 100. The meter at this point is showing between 13 to 14 volts going through the system. Today I tested the system buy shutting down everything and I watch the % drop like an elevator.
I find this baffling and want to understand if this sudden drop in percent is worth worrying about or do I ignore it. The % reading was my way of understanding how much "gas is in the tank" so to speak.
Many thanks in advance for your insight. Barrie

 

[Shrew]

Charging voltage and Passive voltage are two different things.

With charge, fully charged is 14.4 VDC. At rest full voltage is 12.9-13.00 VDC.

 

[Shrew]

I have 870 amp hour wet house batteries ........................ Over night, with just the fridge running the percentage drops to 56 but the battery voltage remains constant at 12.6 to 12.7. By 11am the next day, the solar has brought the percent back up to 100. The meter at this point is showing between 13 to 14 volts going through the system.

12.65 doesn't seem like 56% for either AGM or FLA batteries. Then again, 13.5 VDC under a charge wouldn't be full either.

 

[DavidM]

I suspect your charge controller is only using battery voltage to infer state of charge. It is wildly inaccurate depending on battery use and charging. The data you showed above is entirely consistent with that view.

Get a shunt based battery monitor if you want real SOC data. Victron is a good one but expensive. Renogy is also good and cheaper. I have a really cheap but very workable QWorks battery monitor at $50 on Amazon.

David

 

[Shrew]

I suspect your charge controller is only using battery voltage to infer state of charge. It is wildly inaccurate depending on battery use and charging. The data you showed above is entirely consistent with that view.

Get a shunt based battery monitor if you want real SOC data. Victron is a good one but expensive. Renogy is also good and cheaper. I have a really cheap but very workable QWorks battery monitor at $50 on Amazon.

David

Balmar is another. I bought one but have yet to install it.

 

[Hippocampus]

On most boats it’s really difficult to eliminate all charging sources and all draws (particularly parasitic draws) other than physically disconnecting the batteries. Ideally you then want them to sit awhile and have each disconnected independently from everything before checking resting voltage or doing a load test. That’s why folks add shunts and meters to judge SOC. Mainsail has written a bunch about this and choosing good products for monitoring. Think even after his stroke he’s the guru and worth a read.

https://marinehowto.com/under-load-battery-voltage-vs-soc/

 

[DDW]

The Victron Smart Shunt is pretty inexpensive ($130), very good quality, and will do everything you need. A typical solar controller has really no clue what the state of charge is for the battery it is charging, except under very specific and limited circumstances.

 

[mvweebles]

Last weekend, I was with a friend helping with his off-grid cabin in Wyoming. I installed a basic solar system for him a few years ago, septic and drain lines plus upgrades to his elecrical system last summer, and this year I installed his propane system. He's pretty cheap so has too few batteries for his solar, and no way to accurately understand consumption (solar MPPTs show decent data on input, but no way to tell consumption). What drives me nuts is his new Victron Quatto inverter can deliver a LOT of data, but you need a $300 Cerbo 50 interface. A Victron SmartShunt at $125 is decent too.

In the end, the only way to gauge consumption vs input is via a shunt-based system. The Victron is a good basic start. Balmar and Blue Sea also have gauges and displays. The Victron is easy to install. Readout is Bluetooth to your smartphone so you don't even need a display gauge.

Peter

 

[Barrie]

Thank you all for your detailed response. Looks like there are some good systems out there that will serve my needs. I shunt look into them. Oh shunt the front door.
Barrie