# Why does open circuit voltage differ from specific gravity?

Solar Expert Posts: 228✭✭
So I have a surette 550 2 year old battery. It has been disconnected for a week.
I was just reading how to check open circuit voltage as stated here http://hespv.ca/blog/deep-cycle-battery-maintenance-part-3-checking-battery-voltage/
The open circuit voltage is 6.3 which is close to 100%. But the SG readings are 1.245, 1.240, 1.245 which my charts say correlate to about 80%charged.

What gives?
thanks
Willy

Mate, VFX 2812, FM 60 & MX 60, 2 Full River AGM 400 6v, 1400 watt Solar Array, Yamaha 3000iSEB inverter gen
12 volt Flojet water pump
off grid summer home in northern Ontario

From this post:

http://forum.solar-electric.com/discussion/comment/209119#Comment_209119

I had found an equation that estimated state of charge to voltage:

Specific gravity = single-cell open-circuit voltage - 0.845 (example: 2.13v – 0.845 = 1.285)
Or
Single-cell open circuit voltage = specific gravity + 0.845.

Battery temperature, additives to battery plates, voltage measured when battery has not "rested" for 3 or more hour, etc. can all affect the readings.

Using your numbers and the above formula for figuring out resting voltage, I get 6.265 Volts--Which is very close to what you reported (6.3 volts). I don't know the temperature of your battery bank (hot battery, lower voltage) and I guess the SG readings are temperature compensated (or taken at ~75F).

And, what is full charge does depend on the initial SG of the electrolyte fill:
Specific Gravity vs Applications
1.285 Heavily cycled batteries such as for forklifts (traction).
1.260 Automotive (SLI)
1.250 UPS – Standby with high momentary discharge current requirement.
1.215 General applications such as power utility and telephone.

As mentioned earlier, the specific gravity (spgr.) of a fully charged industrial battery, or traction battery, is generally 1.285, depending on the manufacturer and type. Some manufacturers use specific gravities as high as 1.320 in an attempt to gain additional Ah capacity, but at the cost of a shorter cycle life.

...

Higher Gravity = vs Lower Gravity =
More capacity / Less capacity
Shorter life / Longer life
Higher momentary discharge rates / Lower momentary discharge rates
More standing loss / Less standing loss
And as the battery ages (sulfates) the 100% full specific gravity will slowly drop due to sulfation (after equalization, what is the resting SG and voltage readings--temperature corrected).

My two cents--Full charge is the SG reading after you have "equalized" battery bank (no increases in SG when readings are taken 30-60 minutes apart)--That is the "new" 100% state of charge.

-Bill
Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
• Solar Expert Posts: 3,830✭✭✭✭
I'll bet your SG readings would bounce right back after a charge. You are comparing your SG reading to batteries that are likely currently under charge.

You could call it a surface charge without a lot of depth or whatever. But a freshly charged or charging battery will display significantly higher numbers. I think you already knew that though.
First Bank:16 180 watt Grape Solar with FM80 controller and 3648 Inverter....Fullriver 8D AGM solar batteries. Second Bank/MacGyver Special: 10 165(?) watt BP Solar with Renogy MPPT 40A controller/ and Xantrex C-35 PWM controller/ and Morningstar PWM controller...Cotek 24V PSW inverter....forklift and diesel locomotive batteries
• Solar Expert Posts: 228✭✭

It is 75 degrees.

This is important to me as I have been reading SG 1.245 as only partly charged and working to get the batteries higher....should I be accepting that when I see 1.250 it is fully charged?
Probably not coincidentally, these batteries have been impossible to get higher than 1.250 since new....although Surette says they should go to 1.265 or better....
Mate, VFX 2812, FM 60 & MX 60, 2 Full River AGM 400 6v, 1400 watt Solar Array, Yamaha 3000iSEB inverter gen
12 volt Flojet water pump
off grid summer home in northern Ontario
• Solar Expert Posts: 3,830✭✭✭✭
Having worked with dozens of saltwater S.G. meters for the past 23 years, I will attest that meters are usually off by a fairly significant amount. I had one that was perfect. I wisely gave it away.
First Bank:16 180 watt Grape Solar with FM80 controller and 3648 Inverter....Fullriver 8D AGM solar batteries. Second Bank/MacGyver Special: 10 165(?) watt BP Solar with Renogy MPPT 40A controller/ and Xantrex C-35 PWM controller/ and Morningstar PWM controller...Cotek 24V PSW inverter....forklift and diesel locomotive batteries

If you had three hydrometers--You probably will get three different SG readings. Just make sure you rinse out your meter with a few squirts of clean/fresh distilled water after every measurement session--Otherwise, the internal parts tend to get sticky over time (and ruin the accuracy/repeatability of your hydrometer).

-Bill
Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
• Solar Expert Posts: 228✭✭
I have 2 different hydrometers and they read the same.
That being said, when I got the batteries brand new they tested 1.240 to 1.250.
Interestingly, I corresponded many times with Steve Higgins, head at Surette and he never suggested (that I recall) that 1.250 would be a fully charged battery.
I have spent 2 years working to get them up to 1.265, which only ever occurs during EQ cycle....
An example of why I am so confused...I looked back at my notes and saw that I ran through the absorb cycle and the SG reading was 1.230 (while charging)....should it not have been 1.260???

Mate, VFX 2812, FM 60 & MX 60, 2 Full River AGM 400 6v, 1400 watt Solar Array, Yamaha 3000iSEB inverter gen
12 volt Flojet water pump
off grid summer home in northern Ontario
• Solar Expert Posts: 3,830✭✭✭✭
edited September 2016 #8
At least 95% of my saltwater SG meters have read low. That seems like their default reading. There was a time when I had half a dozen, all giving me false readings. Even if you had six hydrometers giving you the same low reading, I would tend to fault the hydrometers.

Like I said, I had one unit out of dozens that gave a perfect read out. This board loves SG readings. Well...they have experience in that area. There are probably few people in the world who have purchased more SG meters than me.

Even the high tech electrical meters with the probes are prone to false readings. I used to use three meters at a time. Imagine the confusion when they all gave different read outs....which was usually the case.

The odds of your meters reading low are much better than Surrette selling a batch of batteries with very low SG parameters.

I tried buying a top of the line measuring unit back in the 90's. It arrived...broken. It would be pretty easy to be damaged but still seem to work.
First Bank:16 180 watt Grape Solar with FM80 controller and 3648 Inverter....Fullriver 8D AGM solar batteries. Second Bank/MacGyver Special: 10 165(?) watt BP Solar with Renogy MPPT 40A controller/ and Xantrex C-35 PWM controller/ and Morningstar PWM controller...Cotek 24V PSW inverter....forklift and diesel locomotive batteries