Electric Vehicle Conversion Help -

Speed Traps for the Beginner

by Mark E. Hazen

Are you all excited about starting your EV conversion? Sure - so was I. I've been involved helping beginners with their EV conversion for a long time now. Along the way, I've gathered some pretty good insights into the 'speed traps' that beginners get caught in.

Speed traps are those details in the building process and use of your vehicle about which you are unaware - and before you know it, they've got you! So, here I am flashing my lights, warning you to slow down and learn. Take time to ask questions and assume nothing. The following speed traps are not presented in any special order. They are all important.

Speed Trap #1: Controller Installation

Today's pulse-width modulation (PWM) power controllers for electric vehicle conversions are built well, but are not impossible to destroy. It's important to slow down and learn about how to properly and safely install the controller of your choice.

Reverse Polarity Blowout

One of the problems beginners run into is moving too fast to install the controller and accidentally connecting it with the cables on the wrong terminal. DC controllers have 3 main high-current terminals as shown in the diagram below: (see also the Wiring page)

+B, positive battery terminal (on the controller) to which the positive cable coming from the heavy-duty contactor is connected
-B, negative battery terminal (on the controller) to which the negative cable coming from the negative terminal of the high-voltage battery bank is connected
-M, negative-side motor feed to which a short cable from the motor is connected

If the heavy cables are connected incorrectly to these terminals, the controller will blow as soon as the heavy-duty contactor closes for the first time. When a beginner gets on the blogs and talks about how his controller blew with pieces of capacitors all over the place, I know right away that he got in a hurry and connected the cables incorrectly. Of course, he blames the manufacturer of the controller. Nice!

Miles of Separation

Another problem beginners run into is assuming that there are other ways the controller can be installed besides what is shown in the manual. Controllers must be installed within a couple feet of the motor to minimize voltage spikes induced in the cabling between the motor and controller. These voltage spikes can destroy the controller. A beginner that I know decided to mount his controller on the other end of the vehicle from the motor, interconnecting the two with long cables. Naturally, destruction followed.

I don't want the positive cable connected there!

All DC controller manuals, and any other wiring diagrams shown on the Internet, clearly show that the positive cable from the heavy-duty contactor must be connect directly to the +B terminal on the controller ALONG WITH the positive cable that goes from controller to motor. Some beginners assume this is optional and connect the positive cable from the heavy-duty contactor directly to the motor, then a cable from that motor terminal back to the controller. This is a very bad mistake because now the controller is seeing a widely varying positive voltage on its +B terminal caused by rapidly changing motor polarity, which is an involved topic in itself.

Installation Notes: (1) Some people use a much smaller 250 A fuse instead of a 400 A or 600 A fuse. That's fine as long as you don't experience nuisance blowing. Basically, a fuse responds to average current, which is usually much less than 250 A. (2) The heavy-duty circuit breaker should be closer to the batteries than it is to the contactor to protect more of the cabling. I have mine behind the driver's seat where I can still reach it. (3) The heavy-duty contactor must be enclosed to keep dust out. Dust will coat the moving linear armature and may impede its free movement. Dust will also prevent solid closure of the contacts. Gravity helps pull the contacts open when the contactor is mounted vertically as shown. Some folks add a second contactor at the negative terminal of the battery bank for added safety in case of an accident - but, if a short does occur, the fuse will blow regardless of a second contactor. (4) All heavy cabling must be installed so its insulation does not wear through and cause a short. It should also be 'run' where it is less likely to be pinched in an accident.

Speed Trap #2: Motor, Controller and Batteries Abuse

Beginners read things on the blogs and have no way of knowing if the information is accurate or wise. Everyday, bloggers talk about connecting the motor directly to the differential without using a transmission. This is something like a super golf cart configuration. The beginner thinks, 'Wow, this sounds good because I can eliminate the transmission, which probably wastes power - and it's simpler!"

There are many things here that the beginner doesn't realize. First, knowledgeable people who do this, understand that the differential gear ratio must be changed to be in the range of 1:6 to 1:5 to give the motor 'some' mechanical advantage. Otherwise, the motor is working too hard to accelerate the vehicle. Golf carts have similar gear ratios. Also, the practice of 'direct drive' is more prevalent in racing applications in which components are beefed-up and cost much more than street components. Racers spend time on blogs discussing topics that are totally out of focus for the beginner who's wanting to convert a street vehicle for practical transportation, adding to the beginner's confusion.

Motor Strain

If the motor is not able to accelerate quickly without a lot of laboring, the life of the motor will be shortened. Heavy mechanical loading of the motor causes much higher currents to flow through the motor and its brushes. This causes both motor windings and brushes to heat more quickly to a higher temperature. As a result, the life of the brushes and the windings insulation is shortened.

Controller Strain

In addition, the controller is faced with this exceptionally high current level. The controller must handle a higher average current over longer periods of time. This causes the controller to heat up more quickly and to a higher temperature. At some point, the controller may fail because the internal MOSFETs or IGBTs can't handle as much current at higher temperatures. This can occur even if the controller has over-temperature cut-out. Also, if the controller does not have excellent peak current limiting, the high peak motor currents can destroy the controller. Peak motor current can be higher than 10,000 A (amps) for very brief durations during the first part of acceleration and pedal travel. (For more on this, see Electric Vehicle Controller Operation.)

Another type of controller strain is poor ventilation around the controller and lack of heat sinking to allow the controller to get rid of its heat. Beginners often overlook this and it's very important. Air-cooled controllers require air and lots of it. They also require a heatsink to spread the heat out, exposing it to cooler air. The larger the heatsink, the better. Spreading the heat out over a larger surface lowers the temperature. Even mounting the controller on a 1' by 1' aluminum plate that is 1/4" thick is a big help when there is good airflow, although an actual finned heatsink is better.

Battery Strain

The batteries also suffer when the motor is being overloaded. The chemical reaction in the batteries is producing this high current with increased deterioration of the battery plates. Also, the constantly high current can cause shorting in battery cells and terminal meltdowns.

This overloading of the motor also happens when an EV owner, who's vehicle does have a transmission, decides not to use the 'low' gears and always starts out in third gear or higher. Not good!

All of the above adds up to high costs for the beginner and a lot of discouragement.

Speed Trap #3: Over-revving

If direct drive is used, as discussed above, and a reasonable gear ratio in the differential is chosen to allow for rapid acceleration and minimal motor loading, there is still a major problem that will turn out to be a teaching moment for the beginner - over-revving.

Without a Transmission - Direct Drive

Over-revving means that the motor is forced to operate at speeds (RPMs) that are above the maximum rating for the motor. If adhered to, this maximum RPM rating limits the safe top speed of the vehicle, given the gear ratio in the differential. Consequently, beginners force the motor to a higher RPM to gain more speed. In so doing, the centrifugal force eventually pulls the motor windings out enough to touch the casing and stators. Of course, this causes the motor to fail, and maybe the controller too..

With a Transmission

Even when the vehicle has a transmission, over-revving also occurs when the beginner decides to only use 1st or 2nd gear and ignore the higher gears. I recently received an email from a beginner who blew two motors and wanted to blame the company and the guy who rebuilt his first motor. When he described to me how he drove his vehicle, using no gear higher than 2nd, I immediately sided with the manufacturer and the motor rebuilder. As a beginner, he did not understand what over-revving was, but he was sure that he wasn't doing it.

Speed Trap #4: Bad Batteries

Beginners often get batteries donated to them or get a few here and a few there to build up a battery bank. The result is a very 'soft' battery bank whose voltage bounces up and down over a wide range opposite the current being drawn by the motor. In other words, as the current drain increases, the voltage decreases. As the batteries discharge during use, the voltage during acceleration drops lower and lower and as the batteries age, the problem becomes worse. The result is a drastic loss of power and it may jeopardize the health of the controller, depending on controller design.

Don't Mix'em

There are many things that beginners don't understand about batteries. You can't mix them in terms of make, model or age, unless you have a distributed charging system (more on this in Speed Trap #5). Series connected batteries are like the chain that is only as strong as the weakest link. The least capable, or least healthy, battery in the chain will determine the amount of charge all of the batteries receive and will limit your overall driving range.

Don't Take'em

Donated batteries, that have been sitting around in someone's garage, are most surely heavily sulfated and will not provide adequate service to the beginner who doesn't know what to do with them. The best advice here for the beginner, or anyone, is to start with a new fresh set of batteries.

Don't have too few

An insufficient number of batteries to meet the vehicle's power needs is also a beginner's trap. What beginners don't realize is that the fewer batteries you have, the more current the motor will demand from them for a given amount of power (horsepower). Electrical power (watts) is translated to mechanical horsepower by the motor. Electrical power is battery bank voltage times the average current delivered to the motor (P = V X A). So, if the bank voltage is low, the current must be higher for a given amount of acceleration power. Consequently, the lower the bank voltage the shorter the driving range and the shorter the life of the batteries.

Important note: If you plan to increase the number of batteries to increase the voltage, first make sure that the controller and motor are both rated to handle the higher voltage.

Speed Trap #5: Bad Charging

Most beginners know almost nothing about battery chargers and battery charging. In fact, many beginners give little thought to the charger until they are actually trying to drive the vehicle. A recent letter from a beginner revealed that he only had a 24-V charger to charge two of his 12V batteries at a time. With 10 batteries total, that's five days of charging to use the vehicle one day. Yikes! Do you think he will wait 5 days to drive his vehicle? Nope!

Battery Charger Most Important!

From my engineering background and experience, I can easily argue that a good charging system is the most important component of the entire system. A good charger will deliver full charge and long life for the batteries and top performance for the vehicle.

Charger Short Course

For flooded lead-acid batteries and sealed absorbed glass mat (AGM) lead-acid batteries, the charger must deliver between 10 and 30 amps of charging current during the 'constant-current' stage of charging (first stage, aka Bulk stage). At least two charging stages are required - constant-current then absorption during which the voltage is fairly constant and the current drops as the batteries complete their charge. A three-stage charger is best, which has a finishing stage that is referred to as the float stage or soak-in stage. Three-stage chargers can be left on for days without harm to the batteries. (See also Battery Service Life.)

Two Charging System Topologies

There are two primary charging system topologies, or configurations, used to charge electric vehicle battery banks: Bulk and Distributed. A bulk charger is a single high-voltage charger that charges the entire battery bank of series-connected batteries. A distributed charging system is one in which there are many chargers that distribute charge directly to each battery or pair of batteries.

Distributed charging systems are best for many reasons:

All batteries, or battery pairs, receive a direct charge unhindered by any other batteries in the chain.
All batteries receive their maximum charge.
Individual batteries, or pairs, can be replaced in the chain without affecting the other batteries negatively.
Each battery, or pair of batteries, can be easily monitored for health if each charger has an ammeter or there is a common ammeter that is switch selected. A common switched voltmeter is also helpful.

Some distributed charging systems use many 12 V chargers. In cases where the batteries are 6V, each charger is connected to a battery pair. In my case, I have twenty-four 6V golf cart batteries in series for a total bank voltage of about 154 V (144 V nominal). I have one 12 V charger for every two batteries. Note: A 12 V charger actually puts out around 14.4 V during the 2nd stage of charging. At some point in the charging profile, flooded lead-acid batteries require a boost charge voltage in the range of 15 to 16 V (7.5 to 8 V for 6V batteries).

By the way, if you are a beginner and want to make your own distributed charging system, make sure that the chargers you choose are 'isolated'. That means their low-voltage output is electrically separated from the high-voltage AC input. In other words, the output is 'floating'. Only isolated chargers can be wired in series, which is effectively what occurs as you connect them to batteries that are connected in series.. I have added an article to the Library that shows you how to build your own distributed charging system.

Well that's it for now. I'll add to this over time and eventually move it to the Library page.

Don't be a victim as a beginner. Open yourself up to the experience of others, many others so you can test the validity of what each is saying. Don't depend solely on blogs either. Often times things that are stated on blogs are very unwise and go unchallenged.

If you have any questions about this article, please write to me at: mail@evhelp.com

Enjoy the ride, but be careful out there. Look out for those speed traps!