How do they test them? They start with some kind of a dummy load, usually built from an old industrial electric heater. They then hook one end to one side of the battery, then hook the other end of the battery to somewhere on the heater element to get the required test current. This usually only needs to be set once. Now they record the voltage and current every 5 minutes or so until the battery voltage is down to about 10.5V. They repeat this for every battery they have.
When draining a hot battery, usually at about 120 degrees fahrenheit, a good one will start at around 50 Amps, and last 50 minutes, or more. Each car will get two of the best batteries in the batch. Battery selection usually occurs a few days before a race. Other batteries can be used for practice.
You would like to know how many Amp/Hours you can get out of your pack. Once you have a good idea of how many Amp/Hours your batteries can produce, then you need to know how many of those you can expect to get out during a particular race. On a constant speed track, like an oval, you will be able to use more of the total Amp/Hours than on a variable speed road course.
Two years ago, at the Entek Gran Prix in Corvallis, Oregon, Cloud Racing was happy to get 34 Amp/Hours out of the batteries during a race. On a tiny track in Woodburn, Oregon this year, they got 39.15AH from one of their cars, and 33.16AH from the other. In Lacy, Washington they got 38.27AH from car #41, on a fairly tight, parking lot course. In Yakima, Washington, they got between 40.43 and 43.7AH from the 2 cars on each day, on a 1/2 mile oval. Early last summer, one Cloud Racing car used over 44AH during one heat of the America Cup in Portland, Oregon.
Where do these numbers come from, and how can the driver control his battery usage during the race? Cloud Racing started using simple current shunts with cheap Dgital Volt Meter's (DVM's) to monitor battery current and voltage. This worked fine in the garage, but the DVM's would go all black on a hot race day, and be useless. They then progressed to using the Cruising Equipment EMeter which although it had plenty of information, it was impossible to read in sunlight, and could only display one thing at a time. Then they created a display to hook to the EMeter RS-232 port, which would display current, voltage, amp/hours, and KiloWatt/hours, simultaneously on a large LCD. They used this for more than 1 season. BSDesigns then came up with their BatteryManager. This unit displays current, voltage, Amp/Hours, and time since the start of the race, all without an EMeter! These units can be used for drain testing, in addition to giving the driver a very good idea of where he is during the race.
For instance, During the Woodburn, Oregon race, Cloud car #41 had used 3AH 3 minutes into the race. At 5 minutes, they were at 5AH. This was clearly too much for a 40AH race, so the driver chose to slow down a bit. At 26 minutes into the race he had used 18AH! This appeared much better, then at 36 minutes, or 6 tenths of the race done he had used 24AH, or 6 tenths of a 40AH race, perfect! The final result, 39.15AH, only slowing slightly during the last few laps of a 55 lap race! Cloud car #41 wins!
So how do you improve your teams performance during an Electrathon race? Heat your batteries, as 120 degree batteries can store about 20% more energy than 75 degree batteries. Know what your batteries can do on the bench, plan how much you think you can get out of them during your race, and drive your plan. If you are ahead of schedule, slow down. If you haven't used enough, go faster. Do your best to use ALL that your batteries can produce.
Let's go racing!
March 20, 2001