Battery Voltage and Specific Gravity Question

Rngr275 · November 2012

The only thing I can promise is that this will probably NOT be my last stupid question!

Here goes:
48V system,... can the voltage of the battery bank read 50.8-50.9v (which is 100%SOC) and the Specific gravity measure at something less (say 1.25-1.26 where 1.277 is 100%SOC).

Vic · November 2012

Re: Battery Voltage and Specific Gravity Question

Hi Rngr..

Voltage, in general, is a very poor indicator of SOC. RESTING voltage is a fairly good indication of SOC, but "Resting" of 3 or more hours is required for stabilization -- this is with NO loads at all, and NO charging at all for three or more hours. This is difficult to do on an off grid system.

Furthermore, there are fairly large temperature effects on voltage measurements, so, any voltage measurements need to be compensated, as you probably know.

The Gold Standard for SOC is SG MEASUREMENT.

When your batteries have had some cycling, your FNDC bat mon will become a better tool, but it will need recalibation from time-to-time to maintain accuracy.

YES, the numbers you note are quite believable to me. Vic

BB. · November 2012

Re: Battery Voltage and Specific Gravity Question

SG factory levels are "variable":

BB. wrote: »

Here is a thread on Hydrometers and how best to use them.

I understand that some tropical locations will use a bit weaker electrolyte (batteries will have a longer service life?).

From this thread, the range of Electrolyte Specific Gravity seems to be:

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
Less adaptable to "floating: operation / More adaptable to "floating" operation
More standing loss / Less standing loss

So, where do your readings fall in the above scale... And I would guess that a difference of 0.030 SG or less is nothing to panic over.

And while there is a formula for converting from SG to Cell 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.

New poster "leaf" has a really nice set of charts that compare battery voltage against different rates of discharging and charging (as well as resting voltage readings). From here:

vtmaps wrote: »

I believe those charts are from Home Power #36, August- September 1993. Lead-Acid Battery State of Charge vs. Voltage ©1993 Richard Perez.
Here is a link: http://www.scubaengineer.com/documents/lead_acid_battery_charging_graphs.pdf

vtMaps

leaf wrote: »

Am trying to upload the charts I am using...

Attachment not found.Attachment not found.

I don't quite a agree with the resting voltage line (I think the voltage is a bit low)--But it shows how to estimate a battery's state of charge while operating.

Note, where the charts "flatten out"--the room for error estimating state of charge is pretty high.

-Bill

vtmaps · November 2012

Re: Battery Voltage and Specific Gravity Question

All these charts and formulas are for batteries that are NOT sulfated. When a battery is sulfated is SG is lower because some of the sulfer is sequestered in an insoluble form. The sulfated battery behaves like a normal battery of lower capacity. Thus (for example) a 370ah L16 that is sulfated may behave like a 300ah battery with 100% SOC at a SG of less than 1.277.

If your batteries are permanently sulfated (unlikely because they are still too new) you must tell your battery monitor that they have less than 370ah capacity and you must use a lower SG than 1.277 as your 100% SOC benchmark.

--vtMaps

Battery Voltage and Specific Gravity Question

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