% Battery Charge @ ? Battery Voltage

Ron Hill · February 06, 2021, 04:49:34 PM

Guys : I thought it important enough to take this post and make it a separate topic.

Back in the early 1990s I published a chart that showed: Voltage Reading Approx. State of Charge Hydrometer Reading
12.75 100% 1.265
12.50 75% 1.225
12.25 50% 1.190
12.00 25% 1.155
11.75 00% 1.120
I've just listed 5 of the 14 points of that chart.
So each .05 volts represents approx. 5% of charge.
This is battery steady state at rest!! FYI

A thought

PaulJacobs · February 06, 2021, 08:54:55 PM

Hi Ron,

There must be a typo, since you list both 75% and 50% state of charge at 12.5 volts.

Paul

Ron Hill · February 07, 2021, 01:35:43 PM

Paul : You are so correct - I'm just a lousy typist.

Sorry bout that. Now corrected, Thanks

A thought

steveg · February 07, 2021, 04:58:05 PM

If I remember it correctly, the voltage is 12.06 V for 50% charge and 10.5 V for 00% charge for a lead acid battery. AGM batteries are similar and lithium batteries are different since it holds its voltage until it drops at around 10% charge.

Ron Hill · February 08, 2021, 02:36:23 PM

Steveg : The numbers I quoted are form an article in Practical Sailor. They are for lead acid "flooded" batteries. I found that those numbers also hold for AGM and Gel batteries. I don't know about lithium??

A thought

steveg · February 10, 2021, 02:45:22 PM

Ron, I should have clarified it better. The voltages I put up is what is used in the manufacturing testing. The one you have is set up to keep batteries from getting to low where sulphate occurs if recharge is not done in a timely fashion. I was a chemical engineer and worked at a company that made Flooded, AGM and Gel batteries. A very messy process and one that can cause all sort of issues.

Ron Hill · February 10, 2021, 03:32:57 PM

Guys : The % of charge chart above was meant to give you an idea of the state of your battery/s when you check their voltage.

Steve : If you have a better chart, please publish it!!

A thought

KWKloeber · February 10, 2021, 06:27:15 PM

Rod's caution on generic SoC charts:

A Voltage to SoC chart should only come directly from the manufacturer of your batteries, or through physical testing of your own batteries, as I have done here. You should avoid the use of use generic charts off the internet. Why? Because every battery manufacturer will have a slightly different voltage performance curve under load. Deep Cycle AGM will be slightly different than dual purpose AGM or TPPL AGM or GEL or flooded etc..

Some charts you find on the internet do represent a "resting voltage" , but you'll also find some for a 20 hour discharge rate or an under-load voltage. If you're using voltage as an indicator of SoC, the rate of discharge, as well as battery temperature, can also affect your SoC to voltage points. Cyclic On/Off loads, like we have on a boat, can also tweak this relationship. Using the wrong Voltage to SoC chart only exacerbates issues related to using voltage for SoC estimation. Using the wrong chart can actually accelerate the murdering of your batteries.

Link to his full article:
https://marinehowto.com/under-load-battery-voltage-vs-soc/

PaulJacobs · February 10, 2021, 07:01:00 PM

Years ago I learned two important numbers for lead-acid batteries, specifically 12.8 volts and 12.0 volts.

The former is the open circuit voltage when the batteries state-of-charge, or SOC = 100%. During charging the voltage will be higher. But when NOT being charged if your Lead acid battery (or AGM) is at 12.8 volts, that is GREAT!

On the other extreme, when the open circuit voltage drops to 12.0 volts, then SOC is approximately 50%. As many know, letting the SOC get substantially below 50% will adversely affect battery life. Thus, while a battery voltage at or below 11.9 volts is not instantaneously "terminal" you do NOT want your batteries to live in that neighborhood very long.

Pleiades has a 100 W solar panel mounted 10 feet above the cockpit sole on her split backstay. Nancy and I visited her this afternoon just to be sure all was well. With two recent snowstorms her winter cover was under a heavy blanket of snow, and just unzipping the transom zipper was a struggle! Finally aboard, I checked the bilge. It was "bright pink" with polypropylene glycol anti freeze in a "semi-slushy" state, but not hard enough to crack fiberglass.

Despite being currently in the dead of winter, with relatively short daylight hours, and a rather low sun angle, and with the panel not being optimally oriented while Pleiades is on the hard, as well as numerous very overcast winter skies, the house battery bank was nonetheless at 12.6 V and the engine battery was at 12.5 V!

I know this at a glance, because two of the best things I ever did were to install a small (about 1.5" x 3") digital voltmeter for the house bank, with a nice teak surround, on the panel just forward of the trash bin / hot water heater, and a second digital voltmeter for the engine battery, also with its own teak surround, just above the sole at the aft end of the sole on the small transverse quarterberth panel. Both of these voltmeters have LED displays (one green the other blue) that draw about 1 milliamp. In addition to providing battery status at a glance, they also act like perfect, soft glowing nightlights, and have helped both Nancy and myself avoid stubbing our toes when heading to the head in the wee hours.

mainesail · February 11, 2021, 04:03:12 AM

Points to Ponder:

*Always get your Voltage to SoC data directly from the actual manufacturer of your batteries, not from the net.

*Make sure you know what your voltage to SoC chart represents; eg: "resting voltage" or "under-load voltage". They both exist and are not the same.

*Study and learn your batteries voltage behavior, under your average on-board loads. Run experiments if you need to.

*Use a known accurate voltmeter that displays to the hundredths position.

*Your voltmeter should only sense voltage (positive & negative dedicated wires) directly at the battery bank terminals.

*Discharge rate affects your Voltage to SoC relationship.

*PSoC cycling affects your Voltage to SoC relationship.

*Battery temperature affects your Voltage to SoC relationship.

*Aged batteries affect your Voltage to SoC relationship

*As your batteries age the voltage behavior under load will also change.

*Battery temperature directly impacts the time it takes to attain an accurate resting voltage. In cold weather, it can take many weeks.

*Keep it simple, don't discharge your bank below 12.15V to 12.20V even under your average house loads.

*Recharge your batteries to 100% SoC as soon as possible after each discharge.

*The easiest solution is to use a self-learning battery monitor to track SoC, such as the Balmar SG200