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This is for guys wanting to run a 12v system, with no alternater or a constant discharge situation.
This info comes from a chart showing lead acid battery voltage versus draw.
We want to figure the running voltage at two points, WOT at the top end of the run, (highest load during run) and warm cranking voltage when battery is 50 percent discharged, to see whether the ignition will still work and get the car running for the pass.
To calculate the percent of draw in amp hr rating, an example is a 10 amp continuous draw on a 100 amp/hr battery is 10 percent, or stated as c/10. A 5 percent draw would be 5amps on a 100 amp hour battery, etc. Amp/hr rating divided by draw equals c/xx, and then you can check the chart for running voltage. For our purposes, a running voltage at 50 percent discharge on a twelve volt battery is about what I figure is about as low as we can go to figure our capacity for number of runs we can make with a given load and still get back to the pits. Below 50 percent charge, voltage starts to drop much faster.
At 5 percent draw under load the running voltage is 12.3
10 percent, rv is 12.0
20 percent rv is 11.55
33 percent rv is 11.20
You can see how the voltage drops off during running as the load increases, so battery size is very important. I figure I had better not pick a battery with less than 10 times my draw at WOT, which is a water pump and MSD digital seven ignition plus the Deadenbear box. So I need to figure two things, how many passes before the battery will be at or less than 50 percent charge on the return road, and how much battery I need to stay below a 10 percent draw during a run. I will update with the draw of my stuff.
battery chart
http://www.scubaengineer.com/documents/lead_acid_battery_charging_graphs.pdf

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