Batteries SOC - State Of Charge

[Garbonz]

In ascending order of accuracy and usefulness for SLA.

Voltage under charging or discharging (useless)
Resting voltage (using MFG discharge voltage curve not the generic ones listed here)
Simple AH counter (Victron) when zeroed to 100% frequently
Complex AH counter (SG200)

No simple way to do this, regardless of what is said. You gotta know your batteries, perform full 20 tests at least annually and reset your charts, capacity and etc, as things age.. AND they do age.. And if you spend much time at PSOC then they age fast.

Best idea is to toss the conventional SLA and get carbon foam or LI.

Can't squeeze blood from a turnip.

[caffel]

Measuring State of charge while charging is a shortcut that has limitations as does simply checking the no load voltage.
Testing state of charge and battery quality requires isolating the battery (or at least the bank) and then recording the no load voltage plus the voltage under a known load to arrive at the internal resistance.
This plus a check of each cell's specific gravity will allow tracking of each unit's usefulness and make it easy to weed out the weak sisters in the bank.
We use this inexpensive tool to insure good performance on our robotics team:
https://www.andymark.com/search?utf8...MCTnt5Ew%3D%3D

[sailcrazy]

Following. As a graduate engineer,(MSCE-not Electronics), now retired, and a cruising sailor since 1999, it feels like the old adage....lets measure with a micrometer, mark it with a grease pencil, and cut it with an axe...is actively being practiced! Considering all the variables, I'm not sure there is any way to really know exactly where your batt bank is at.
We use a mounted digital voltmeter, a Victron 712, and a SG(1).....look at/monitor all 3, and "guestimate". We have solar and a windgen, and run our genset regularly. And every time we do we have the batt chrgr on. We equalize our 6T-105s (LA) about 2/yr, check the water level at least every other month, and then reset the variables (amp hrs capacity, etc) as we can, degrading for age and use, and never try to go below 75% SOC (however you chose to "measure" it). Multiple shallow cycles are far easier on a bank than fewer deep cycles. Remembering the first and Primary goal of Balmar, Victron, and all the others is to sell products/make a profit (and they write their manuals to be as all-promising, non-specific, and keeping as many secrets as they can), we don't fully trust any one of them.

[goboatingnow]

SOC is simple

lights dont come on , battery flat :SOC 0%
shore power plugged in for 24 hours , battery at 100% SOC

everything in between is a mystery , get used to it

[Mickeyrouse]

Originally Posted by Moontide

Not cheap but the semi-auto filling system is worth its weight in gold. Have special caps that replace the standard ones which are connected by tubing. I just put the end of a tube in distilled water and pump a squeeze bulb until there is back pressure.

Moon tide, can you point me the source of this semi-automatic filling system? I have killed more batteries than I care to think about because topping up the water is so damned PITA on our boat.

[Yerffoeg1]

Originally Posted by Dockhead

I beg to disagree. I would say on the contrary that system voltage IS the most scientific and accurate way to measure SOC.

You don't need to leave the batts standing open circuit -- small loads create small errors which are all on the conservative side.

If the battery is aging and has reduced capacity, this is EXACTLY where voltage shines -- when you calculate 50%, it's 50%, and if you have some loads on, it's "not less than 50%" -- which is EXACTLY what you need to know. It's 50% of a different number than with a new battery, so it's not telling you how much power you have left, but you care about the percentage, not how many amp/hours you have left, when making decisions about when to charge.

An amp-counting meter is what becomes completely useless when batteries age. It tells you how many amp-hours have gone out of the batt, but tells you NEITHER how many amp-hours you have left NOR what % SOC you are at. It will give you can increasingly erroneous % SOC which is an error all on the harmful side.

This is where the intelligence steps in. It tracks battery aging and detects battery capacity over the life of the battery. Over many battery cycles it can accurately determine the charge left in a battery. That is what your phone does. (CELL phone that is)

[moseriw]

If you have solar that is difficult to disconnect then the 'No charge' will only occur at night.

Yes, that is the point.

[Dockhead]

Originally Posted by Garbonz

In ascending order of accuracy and usefulness for SLA.

Voltage under charging or discharging (useless)
Resting voltage (using MFG discharge voltage curve not the generic ones listed here)
Simple AH counter (Victron) when zeroed to 100% frequently
Complex AH counter (SG200)

. . .

I agree about voltage while charging but strongly disagree about voltage while discharging. The manufacturers give voltage vs SOC at different discharge loads, BUT in any case, except in a surface charge situation (which you won't have if the batts haven't been charged in a while AND they are not nearly dead), voltage vs. an open circuit voltage vs SOC chart will give you NOT LESS THAN that SOC, which is supremely useful. With large banks and/or small discharge rates, the difference to open circuit voltage anyway is small.

And that is something which a simple AH counter CANNOT tell you if you have been at partial SOC OR if you don't know pretty close to perfectly the actual real capacity of the bank AND you've properly reset to 100% after a real full charge. Since it is unrealistic to know this, and since errors from simple AH counters are unknown and all on the harmful side, it is simple AH counters which belong at the top of the list as the most useless (I would say utterly useless) approaches.

Since you care about knowing SOC most of all to know when you need to charge, voltage under moderate discharge is really useful and is probably all you really need to know (micrometer-grease pencil - axe scenario) to adequately manage a FLA battery bank.

Resting voltage is also really useful, as being highly accurate, more accurate than we need to manage our batteries, however it is unrealistic to obtain in real life cruising conditions, so I would scratch that off.

Complex AH counter which takes voltage into account (or probably more accurate about the SG200, voltage analyzer/damper like original SG, but which takes current and counted AH into account, is probably the best, so I would agree with you there.

Do you need to know SOC to the last %? No, you don't. You need to know, mostly, when to charge. =/- 5% or whatever is plenty accurate. You are cutting with an axe; you don't need a micrometer. And especially if the measuring device errs on the harmless side, like voltage under moderate discharge condition.

[Joli]

We use a Balmar Smart Guage. We don't care about amps in and out only the SOC. It's very accurate after a few days and let's us know if we need to run the genset to bring the banks back up. We have a monitor in the 12 volt bank and one in the 24 volt bank.

https://marinehowto.com/smartgauge-battery-monitoring-unit/

We also use water mizer battery caps to minimize water loss. Our banks are built from golf cart batteries, both 6 and 8 volt in parallel and series. We swap out the batteries every 6~7 years. So far this seems to be a good, inexpensive and robust way for us to power the boat.