The DEVC meeting on the June 26th 2007 was attended by around 20 interested members, many of whom took a lively interest in the ideas presented! Unfortunately none of the target audience of high school DEVRA racers were present so this report covers the discussions in considerable depth as well as Ross Smith’s previous notes and writeup. Anyone with any corrections, additions, or other suggestions is invited to forward them for incorporation! This will be incorporated into the DEVC web site as a source. Many thanks to all of the contributors!
The following is the result of Ross Smith’s writings and of my interview of him. I greatly appreciate his time and effort in preparing this guide to Battery Selection, Preparation, Charging, Use, and Maintenance. I have added my views and comments in various place and these additions are denoted by (BW) to distinguish them as my thoughts rather that Ross’s which are (RS). Additional material has been added as a result of the DEVC meeting at FRCC on Tuesday the 26th of June. Contributors are denoted by (BB) Dave Hawkins, (JM) Jim McCullogh, (GG) George Gless, (PR) Pete Robustinelli; all of whom contributed to a lively and informative meeting!
(1) (BW) The following guide is directed towards the use of flooded lead-acid batteries in competitive situations involving their discharge at rates approaching C/1 or full discharge in one hour. This is very severe service and the manufacturer would doubtless frown on it. However it can and has been done and the batteries have managed to survive this treatment and give satisfactory service over several years. But it is not per the manufacturers recommendations! Take this as a warning; you are venturing into unfamiliar territory!
(2) (BW) When selecting batteries for this use the best recommendation I have heard is to select the heaviest available. The Electrathon money racers will test a number of batteries and select the best performers. Generally we must take what we get but if you have the chance to heft several then take the heftiest, it cannot hurt! Visually inspect the electrolyte level and if low bring it to the battery vendor’s attention. (RS) Some batteries are fresher than others and will require more preparation to bring them up to potential. This is addressed below. (BB) suggests careful selection and matching of battery size to the typical driving range. And to allow for the inevitable degradation in battery performance over time and use. He uses an auxiliary propane motor/generator to extend his range on longer trips and to charge away from outlets!
(3) (RS) The first step is to discharge the battery under a light load of 15 Amperes to a standard level of 11.70 Volts. Ideally this is done with a specialized load that incorporates a voltage sensor, a time counter, and an alarm. As soon as this level is reached the battery is put on a charger that recharges the battery at 15 Amperes to 80% of fully charged followed by a taper charge to 98% of fully charged, followed by a final charge at 13.8 Volts for several days. This charger is discussed below. (BB) Suggests that for EV use a long easy break in period of up to 50 cycles is optimal for longest life. Using a hygrometer on flooded batteries allows accurate tracking of charge levels whereas voltage checks need a 24 hour period of “settling”!
(4) (RS) Now the battery is ready for testing. Discharge it at a rate of 15 Amperes to the 11.70 Volt level recording the time accurately. (BW) A load device as described above is ideal but a stopwatch, voltmeter, and diligent observer can get the job done. (RS) Immediately recharge the battery as described in (3) above. Repeat this cycle of charging and discharging to complete the “forming” the plates. The factory does this but not to completion and you are increasing the battery capacity by doing so. After two or three cycles the discharge time will stop increasing. At this point the battery is at it’s maximum capacity.
(5) (RS) The ultimate test of performance will define the rate that the battery may be discharged in a one hour race. This is a C/1 discharge test at a rate starting at 33-34 Amperes dropping to about 29.8 Amperes at the end. Again there is a timer to stop the test after one hour, an Ammeter to monitor the current draw, and a voltmeter to monitor the battery status. IMMEDIATELY recharge the battery following this test as it severely strains the battery and can PERMANENTLY damage it, if left discharged for any time. This is true at the end of a race also!
(6) (RS) While an elevated temperature will improve the performance of a lead acid battery it can also damage it. As an limit never exceed 108 degrees F electrolyte temperature and 80 degrees F is a good working temperature. Fully discharged a battery is at 10.5 Volts but a limit of 11.0 Volts will extend battery life without measurably reducing output.
(7) (BW) It is a good idea to remove the batteries as soon as possible at the end of a race and get them on charge. Just jumpering them in parallel to an automotive battery/alternator will help to bring them up. Then recharge as per (3) above when home. There are battery “Maintainer” chargers that will keep a battery charged for the long period between races and over the winter.
(8) (RS) Tools needed include a selection of good ($30) multimeters that can accurately read Voltage. Used with a ($10) current shunt Amperes can be read with precision. (PR) Provided the useful bit of information that 1 foot of #10 Ga. copper wire is closely .001 Ohm in resistance and used as a shunt with a millivolt meter will read 1 Mv/Amp! Furthermore an increase of 3 in gage number represents a doubling of resistance per foot while an increase of 6 in gage number represents a halving of the copper crossectional area.
Load resistors in the1.67 and .83 ohm ranges are needed with power capacities of 400 and 800 Watts respectively. These are not trivial devices! Also voltage monitors with automatic shutoff capabilities and timers are very useful! These are not difficult to build however. (BW) Ultimately a motor controller could do double duty as a load controller but it probably would need to be removed from the vehicle for this task! (JM) Your typical small controller must see an inductive load to perform properly. While this is possible other methods might be preferable! (BB) One system of loading batteries for discharging involves the use of a cheap 12 VDC – 117 VAC inverter. 400 Watt units are widely available at low cost and will discharge a 12 VDC battery at well over 30 A! BB uses them to run his shop lighting under these circumstances saving some money!
(9) While not strictly on topic the chargers and controllers used have a significant effect on battery performance and life. (BB) favors high current controllers and uses a “Zilla” on the street. He also uses a string of 12 VDC chargers to individually charge each battery in his pack. For faster charges he uses a full voltage, onboard, bulk charger. (JM) has used a SCR controller with his 32 HP, shunt and series wound (compound), separately excited, motor. Motor RPM is varied by this controller up to a speed of 2600 RPM, after which the speed is controlled up to 4200 RPM by field weakening! This drives through a four speed manual transmission giving a very wide operating range! Overall consumption is 3 KWH per mile.
(10) Several suggested using some kind of monitor for individual batteries and (JM) described a homebrew one built by Victor Creazzi. (BB) suggested the PakTracker monitor from Ken Hall at www.paktrakr.com as a worthwhile battery monitor. Another device extensively discussed was the “Pulse Charger” that provides a short, high voltage pulse into a battery intended to decompose sulfation on the plates! (BB), (BW), JM), (GG), and several others offered opinions. Summarizing the opinions “Pulse Charging” will restore maltreated batteries but probably does nothing that repetitive charging would do over time. But it seems to do it quicker!