Battery - State of Charge

[David HK]

A friend has recently given me one wet lead acid battery and two sealed batteries with little or no information printed on the latter. I have charged them up and have been conducting daily State of Charge readings with a digital multi meter.

I am interested in understanding State of Charge tables and after searching the web have found a few which are causing me a headache. For example, let's say 12.70 volts equals 100%. According to my calculations 95% would equate to 12.07 volts and 90% would be 11.43 volts.

A table from Trojan batteries provides a different figure which suggest that:- 12.70 volts equals 100%; 95% is equal to 12.64 volts, and 12.58 volts is approximately 90%.

I know that Trojan is in the battery business and should know a thing or two about batteries, but can anyone tell me where I am going wrong with mathematical calculations.

I am fully aware of cell changes due to temperatures, problems with individual cells and so on.

All I want is a simple table that's good enough for approximation purposes.

David HK

[dbcollen]

A 12 Volt battery is effectively 100% dead at around 10 volts, so the scale goes between 12.8ish and 10ish Volts.

Dustin

[dnix71]

The exact type of lead acid battery and it's temperature factor in. Trojan makes sla's.

Calcium and antimony change the SOC as well.

http://www.batteryuniversity.com/partone-13.htm   explains it thoroughly. You don't want to go below 12.3 if you can at all help it. Going that low pretty much guarantees sulfation. If you buy a battery with an amp-hour rating you should only use about 1/2 of that in practice or the battery life will be greatly shortened.

is the simple table for a regular car battery with antimony with removeable filler caps for adding water.

[wooferhound]

This is for the Flooded Lead Acid batteries. The sealed ones will be less voltage.

State of Charge (SOC) Chart:

% of Charge- - - - - - - - Charging- - - - - - - At Rest - - - Discharging

100. - - - - - - - - - - - - - 14.75 - - - - - - - - 12.70 - - - - - - 12.50
     
90. - - - - - - - - - - - - - 13.75 - - - - - - - - 12.58 - - - - - - 12.40
    
80. - - - - - - - - - - - - - 13.45 - - - - - - - - 12.46 - - - - - - 12.30
    
70. - - - - - - - - - - - - - 13.30 - - - - - - - - 12.36 - - - - - - 12.25
    
60. - - - - - - - - - - - - - 13.20 - - - - - - - - 12.28 - - - - - - 12.15
    
50. - - - - - - - - - - - - - 13.10 - - - - - - - - 12.20 - - - - - - 12.00
    
40. - - - - - - - - - - - - - 12.95 - - - - - - - - 12.12 - - - - - - 11.90
    
30. - - - - - - - - - - - - - 12.75 - - - - - - - - 12.02 - - - - - - 11.70
    
20. - - - - - - - - - - - - - 12.55 - - - - - - - - 11.88 - - - - - - 11.50
    
10. - - - - - - - - - - - - - 12.25 - - - - - - - - 11.72 - - - - - - 11.25
    
    

http://solarjohn.blogspot.com/2007/03/measuring-battery-state-of-charge.html

[TAH]

What size load is considered to be discharging? I never have less than a 10AMP load and it varies as high as a couple hundred AMPs at times. If I am getting 24.4 or higher during moderate loading I consider the batteries to be in pretty good shape and know that I can get 25 or 30 kWH out of them if I needed to without draining them too far. Are they really only at 50%? (lead acid forklift batteries)

[Flux]

Personally I regard those tables as a waste of time. They are fine if you have a battery that does nothing and at any point in time you want to know its rough state of charge so that you can decide if you need to recharge it.

For normal operation they at best are only a rough indicator. The figures are only relevant when neither being charged or discharged and the surface charge has been removed. Invariably on charge the volts you see will be above the fully charged figure and if you have other than a very small load on discharge the volts you see will be below the fully discharged figure. If you are on discharge and there is no charge coming in then if you can sit in the dark for 10 minutes with no load on then the figure it settles to will sort of follow the chart ( if you have the right one for temperature and battery type). Similarly if you are charging then removing the charging source, applying a small discharge and then waiting for 5 mins will again bring you somewhere near the chart ( Really convenient and useful isn't it?)

Typically I find that if on charge you are below 14v then you are not fully charged. If on low levels of discharge you are above 12v then you are reasonably ok. If on high rates of discharge you are probably ok if above 10.5v If you drop below 12v on light discharge or below 10v on heavy discharge you are flat and need to stop discharge.

Flux

[Jeff]

Thanks Wooferhound! Looks like I'm going from 100% to 70% (the 70% being under what I consider the full load, in other words, the fridge) while running puts it at 12.25 to 12.35 volts.