math :: SOC charts

In order to maintain and collect data about individual cells (each 6V battery as 3 cells) I ordered the following items:

• Freas HD98C Precision Hydrometer Set
• Fisher Scientific Company L.L.C. safety distilled water labeled 500 mL polyethylene wash bottles
• baking soda (acid neutralizer)
• MSA Safety Works 817893 adjustable faceshield
• 100 count box of latex gloves
• 1 gallon of distilled water

Safety is a primary concern when working with battery acid and welding level current. During all work on the batteries the standard attire consists of jeans and a long sleeve shirt, full face shield, latex gloves (when risk of acid exposure is present), and closed toe shoes. An outdoor water faucet is readily accessible and baking soda is kept next to the battery bank in case of a spill.

When taking specific gravity readings I draw up the electrolyte, and then flush it out prior to taking a reading. The reading is taken just as the float begins to rise in the column. Once the thermometer in the hydrometer stabilizes both readings are recorded. Once three cells have been measured (usually on different batteries), I have the minimum information to calculate an average and correct my specific gravity measurement to standard temperature (80°F).

This is the chart I use for specific gravity readings (note that low temperature skews readings low – temperatures less than 80°F will need to have .004 subtracted for every 10°F below 80°F): Specific Gravity

I record readings using a chart. Here is the chart with some data entered: Specific Gravity Log (log updated 11/18/2011 due to inaccurate specific gravity correction on original). On the top of the page there is a grid of the 14 battery bank. Each battery has 3 cells, with readings for each recorded. I’ve set up the chart to flag cells that are too far from expected specific gravity in red (there are none in this example) and to point out cells that are .005 or more away from expected specific gravity in brown (note that some values are rounded to -.005 but in actuality are less than .005 from expected). Also, I predict that there is a .003 margin of error in my readings.

Using information gathered above I am able to develop a chart that shows the current SOC at 300 Watts at multiple temperatures. This chart is still under development. So far only the 30°F and 40°F temperatures have been verified: Composite Temp Voltage 300W rev2011