SOC Battery Voltage Question

[kernal]

Originally Posted by JohnnieB

Something to remember about the SoC charts for Trojan brand batteries. They are based on testing of 6V batteries 30 or more years ago and Trojan has tweaked their design a few times since then.

Plus, the chargers commonly in use back then were the ferroresonant transformer type and shutoff at a predetermined voltage (45V +/-1V for a 36V battery pack) instead of fully charging the batteries. Modern charger use dV/dT (change in Voltage over change in Time) technology to more fully charge batteries and the finish voltage is higher.

Also, I've been unable to find out what the battery temperature of the batteries tested were and temperature effects Voltage as well as storage capacity.

I thought the tests were based on a battery at 25°C (77°F). Although it isn't stated specifically on the 'state of charge open circuit' chart, it is stated on all the other charts in the charging section of the "Trojan battery user guide". I "assumed" but "assuming" has pit falls.

[JohnnieB]

Originally Posted by kernal

I thought the tests were based on a battery at 25°C (77°F). Although it isn't stated specifically on the 'state of charge open circuit' chart, it is stated on all the other charts in the charging section of the "Trojan battery user guide". I "assumed" but "assuming" has pit falls.

The Runtime Minutes and AH ratings are at 80°F. The charging voltage compensation for temperature is based on 77°F.

My guess is the SoC voltage increase is roughly the reciprocal of the charging voltage temperature compensation, or it increases at about 0.0028VPC (volts per Cell) for every 1°F increase above 77°F.

There are 24 cells in a 48V pack, so it would be 0.0672/1°F. In other words if the batteries were at 87°F, 100% SoC would be 0.672V above what the chart said. Of course that is only speculation.

[ThreeCW]

Originally Posted by JohnnieB

My guess is the SoC voltage increase is roughly the reciprocal of the charging voltage temperature compensation, or it increases at about 0.0028VPC (volts per Cell) for every 1°F increase above 77°F

From my observations of my T-125 batteries in the winter vs. the summer, I see little change in the stabilized fully charged voltage. In the winter, my cart resides in a garage heated to approximately 40 F and I see a stabilized pack voltage of 52.0 v. In the summer, I see the same stabilized pack voltage at a temperature of 70 F (on the warm days).

If the static pack voltage fluctuated with temperature, I would expect to see delta V = 0.0028 VPC/deg F x 30 deg F x 24 cells = 2.0 V.

Since I see little to no static pack voltage change due to seasonal temperature changes, I hypothesize that it does not change with temperature.

I have seen changes to my static fully charged pack voltage under the following circumstances:
- After an equalization charge I saw an increased pack voltage of 0.1 to 0.2 V (due to mixing of stratified acid).
- After adding a larger water volume (3.5 to 4 quarts) to my battery pack, I saw a decrease of 0.1 to 0.2 V on pack voltage after later charges (due to decreased acid concentration).

[JohnnieB]

Here is a study done by Discover Energy, a Canadian battery company, with a publication update of November 2013.

All I could find on their website was AGM type cart batteries, so the voltages shown at the bottom of the chart probably isn't exactly the same as it is for flooded lead-acid, but the trend would be.

Looks like they found their batteries had 2.133VPC at 50°F and 2.153VPC at 86°F, so a change of 0.020VPC over a change of 36°F, or 0.0005555VPC per 1°F.

For a change of 30°F for 24 cells, that works out to 0.39996V or rounds to 0.4V. (about 1/5th of my guesstimate. )

Also, batteries have a huge thermal mass, so their electrolyte/plate interface temperature lags air temperature by hours.

So, it looks like temperature does effect battery voltage, but not very much.