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EV Tech Talk


©1998 Mike Brown

Mike Brown

©1998 Mike Brown

"When I got home from work tonight, I smelled smoke coming from the front of the EV. When I checked under the hood, I noticed that the controller was hot. Is my controller going bad?"

This time, the voice on the phone was one of our local customers. "Oh goody," I thought, "a customer with a real problem—and he's even local. I can do a hands-on diagnosis instead of a remote consultation, and I'll get to see the results myself." Sometimes an EV troubleshooter feels a little like the Maytag repairman— we get an occasional phone question, we make recommendations, and we often don't hear the final results. We don't get much chance to practice our craft at close range.


I questioned my caller a little. How were the tires? A low tire would cause more drag on the motor, and therefore more current from the batteries through the controller. Speaking of current, what was the ammeter reading? Had he checked the rest of the things under the hood, like the battery cables, battery interconnects, and terminals? He didn't have any answers to those questions, so I told him to check those things and call me back.

The answers came in a call the next day. He had driven the EV down to the foot of his hill and back up, which was about a half mile at a 10% grade. The current was the same as usual. When he checked the controller temperature, it was only warm to the touch, but he had not driven as far as he had the day before. There wasn't any smoke or burning smell this time, either. The tires were fine.


How were the battery interconnects and cables? He hadn't checked those—what could go wrong with those, anyway? He was sure something was wrong with the controller. I told him to go check them just to humor me and call back. He called back a few minutes later with the news that one of the terminals with a copper interconnect had melted. Instead of being connected with a nut and bolt, the interconnect and what was left of the terminal seemed to be soldered together. How did that happen?

There is only one thing that generates enough heat to melt a battery terminal—a loose connection. Loose battery connections have plagued the EV world, both commercial and hobbyist, since the first EV hit the road. In fact, one manufacturer went so far as to solder all of the cable interconnects to the batteries to eliminate the loose connection problem. They didn't melt any terminals, but it made replacing a failed battery a messy job.

The Wrong Battery Terminal

Most of the meltdowns I have seen in the hobbyist world have come from the use of the "universal" battery terminal. The universal terminal is a regular tapered automotive terminal with a 5/16 inch (8 mm) stud cast vertically in its center. The flat surface on the top of the terminal around the stud gives less than half a square inch (323 mm2) of contact surface. This terminal comes on 90% of the golf cart batteries that are used in hobbyist EVs.

Failures occur because the lead terminals are subject to "creep." Under sufficient pressure, lead will flow at room temperature. So let's say a copper lug is crimped to a heavy piece of 2/0 (85 mm2) cable. Then it's fastened to the terminal over the stud, using the springlock washer and nut that comes with the battery. The installer is careful to tighten the connection firmly—we don't want any terminal failures here! Over time, the stud will gradually move upward in the lead post and relieve the tension, loosening the connection. Then the connection is warmed up by some prolonged 350 to 400 Amp draws. At least one terminal meltdown is inevitable, usually at the worst possible time and place.

If you are diligent, you keep checking your connections and tightening them again. Meanwhile, the studs keep creeping upward—until one day, one creeps completely out of the terminal post. Then you're back to a puddle of lead. The safest way to use this style of battery terminal is with an automotive-style clamp on the cable fastened around the terminal post, ignoring the stud. However, this clamp for 2/0 (85 mm2) cable will cost about $5. Multiply this figure by 32 to 40 terminals, and it gets expensive in a hurry.

The Right Battery Terminal

I prefer the "L" battery terminal. It's shaped like the capital letter "L" with a hole in the center of the vertical leg. This is a piece of lead approximately one square inch (645 mm2) by 5/16 inch (8 mm) thick. The cable lug (or in our case, an interconnect made of one inch (25 mm) wide by 1/16 inch (1.6 mm) thick copper strip) is fastened to the broad contact surface of the "L" terminal. The bolt and a flat washer are inserted through the inside of the "L" terminal, then through the lug or interconnect, a Belleville washer, and a nut.

EV Tech Talk

The Belleville washer is a slightly concave precision tension washer that avoids the creep problem. It should be installed with the concave face toward the lug or interconnect. With this hardware firmly tightened, the connection is almost failure-proof. With that said, since this car's batteries had "L" terminals and the interconnect was a copper strap with the correct hardware, why did it melt?

Diagnosis & Repair

This was the easiest part of the problem. The EV was a little over 4 years old, and had worn out its first battery pack. A new pack had been ordered, and the owner had installed them. The connection that failed must not have been tightened enough when it was installed. The loose connection heated up and the terminal started to melt. Then the shrink tube insulation on the interconnect started to smoke just as he pulled into his driveway after climbing the hill. Because he shut the EV off before the terminal failed completely, the molten lead cooled off and soldered the interconnect to the remains of the terminal.

The battery distributor soldered on a new terminal. A new interconnect was also installed. All the rest of the connections were checked for tightness. What about the hot controller, you ask? The hill he climbs on his way home is similar to one I have on my way home, and my controller gets hot to the touch. He admitted that he had never paid any attention to how hot the controller got before the incident. This was, in fact, a normal temperature for those conditions—the controller was fine.

Again, the First Rule of Troubleshooting applies: Resist the urge to blame the most expensive and most mysterious part of the system until you have checked the cheap, easy parts.

Send me some more questions to write about, so I don't have to take up washing machine repair.


Mike Brown's TechTalk, Electro Automotive, POB 1113-HP, Felton, CA 95018-1113 • 831-429-1989 Fax: 831-429-1907 • [email protected] Web:

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