Are you ready to get into the nitty-gritty of DIY EV conversion?
In the previous installment, we focused on getting you started thinking about what your ideal DIY EV would look like. Choosing your platform and the exploring the big ticket items that go into a DIY EV conversion, along with some very high level cost estimations to help you budget for your own DIY EV conversion project.
This installment is a DEEP dive (2,600-plus words!) into what you need to know to connect all those big ticket items and actually build the thing—what to buy, where to put it, and how it all gets hooked together.
A basic, series-wound, DC system
For the moment, we will stick to the basic structure and components of a series-wound DC system. It’s what I am most familiar with and is considered the entry level for EV builders. I will be sharing some of the theory behind how the components work, what their limitations are, and common issues with their selection and installation.
Throughout, we’ll be referencing this diagram I made just for this article. Brian’s made multiple versions that highlight the bits we’re discussing so you can visualize how things are connected and work.
First of all, let’s make a distinction between what is called Electric Vehicle Service Equipment (EVSE) and a charger. Sometimes what is actually an EVSE gets called a charger.
A vehicle’s charger is onboard. The thing that hangs on the wall or pedestal is an EVSE. It is basically a set of components for connecting and disconnecting the main power supply in a safe manner.
An EVSE connects to your car through its charging cable and plug. Smaller EVSEs that use 120VAC are common accessories with OEM electric vehicles. They usually consist of the vehicle plug—usually J1772—a contactor/relay box, and a NEMA 5-15 plug for your wall outlet.
The vehicle’s charger is in essence a rectifier and DC-to-DC converter in a configuration that will take 120VAC or 240VAC, rectify it to DC current, and step it up or down to suit the battery used in the vehicle.
Many chargers today are considered “smart”, meaning they can be configured for different nominal DC voltages and battery chemistries. Chargers also have a relay input through which a battery management system, if so configured, can terminate charging.
Elcon is a popular brand of budget charger. In the large diagram below, their PFC3000 is pictured. From direct experience, I will note that this charger really likes to be kept cool. The addition of fans blowing along the bottom of the heat sinks will really aid in keeping it from thermally limiting charging current.
Battery Management System / Battery Monitor
A battery management system or BMS is a system of interconnected hardware that makes sure a single cell or sub-pack of cells in an EV battery pack doesn’t have too high or too low a voltage.
Generally speaking, for most custom EV designs that use large capacity single prismatic lithium cells (100-200ah), each cell gets a sensor board which is connected in series in a loop with all other boards to a “headboard”. The headboard can also incorporate a display.
During a charge state, each cell board can “shunt” charging current around a cell whose voltage is already reaching an upper limit of “full” (3.5-3.7v typically). This shunting of current helps balance charging between cells in the pack so some aren’t denied being fully charged because others cells charge slower or faster.
Bringing a pack into balance can involve loading or charging individual cells and then charging the pack for a few cycles so all cells reach full at approximately the same time.
The BMS will also have a low voltage disconnect (LVD) warning. Just like you never want to run your fuel tank completely dry in a conventional vehicle, you don’t want to completely drain your batteries. This can cause serious, long term performance issues. Some systems can have outputs that limit throttle range for the motor controller to help protect individual cells from being discharged past a lower limit, usually 2.6v.
A BMS, whether building a custom pack or buying OEM, is a worthwhile investment.
It helps you monitor battery pack health and performance through signal LEDs or a digital readout. It is a lot easier than keeping a log book of 48 or more individual cell voltages.
The BMS pictured in the diagram is an older type, but illustrates the idea well.
More sophisticated battery management systems incorporate CANBus interfaces and software compatibility. The OrionBMS is a good example of a system that does this and, instead of many cell boards, its topology is a single head unit with a harness and signal conductors for each cell in the pack.
If I were to build a 144V nominal pack today, I would consider the $950 for a 48-cell-configured system an absolute must in my budget.
Charging Input Adapter (J1772)
Chargers will accept any 120-240V input, but if you want to charge the vehicle at a public location you’re going to want to use the most widely available standard inlet—the J1772.
These come in different capacities. 16A, single phase and 30A split phase are probably the most common, and you definitely want a version that can do split phase, 240V 30A as a minimum. This is considered “Level 2” charging and is considerably faster. (Like 4hrs vs. 20hrs faster.)
The adapter inlet or socket will come with a cable whip, consisting of larger conductors for charging current and some signal lines. Depending on which charger you choose, these may wire straight to the charger or interface with a module like this: http://www.evwest.com/catalog/product_info.php?products_id=107 to utilize immobilization and proximity features, ensuring the pilot signal can shut off power before the plug is disconnected.
Contactors are basically large high current relays, meaning they can pass—but also reliably interrupt—large DC currents. By far the most popular in the DIY EV community is the TYCO EV 200 or one of its several variations.
They are used as isolators in the high voltage wiring—not unlike the battery cutoff switches found on serious race cars. As shown in the diagram, it is recommended to have a minimum of two and to put them as close to the battery as possible in both the negative and positive runs.
Putting the contactors close to the pack minimizes the number of feet of conductor (wire) is energized when the contactor is open. This is important in emergency situations, so any cutting for extraction doesn’t pass through a hot conductor. (Read: The firefighter using the Jaws of Life to cut you out of a wreck doesn’t cut through a hot wire and get savagely electrocuted.)
Conductors = High Voltage Wiring
Wire size depends on what amperage your expected build will pull from a battery pack. DC setups with lower voltages generally can pull up to 1000A, and most AC/BLDC setups will pull 600A from a higher voltage pack.
Generally, wire gauge is a minimum of 2/0 and will range from something like fine stranded welding cable to specialized, orange, steel braid-jacketed cable.
Without getting too bogged down in American Wire Gauges (AWG) and capacities, you can use a simple bit of background knowledge. A 2/0 copper cable is listed as 175-300A, depending on jacket, before it’s de-rated for conditions of use or continuous duty. (It’s fine to pull 1000A through it for 10 or so seconds at peak power.)
Additionally, the continuous rated horsepower of your chosen motor will help you determine if you need larger cable if you plan to build a highway cruiser.
As an example, the Warp 9 Brushed DC motor is rated at 24kW (32hp) continuous duty. At 144V, that’s roughly 166 motor amps (figure ~200 battery amps). I have observed that, with a transmission and a 2900lb vehicle, this is enough to manage 65-70mph on level ground.
As you can see, rarely will you need bigger than 2/0 cable. Most wiring you are likely to buy has either a 300 or 600V-rated jacket. Make sure you check.
Crimp Connections & Tooling
How do I connect my fat cables to components in my EV?
Most of the time, you will be using some sort of crimper tool to attach lugs to conductors, which compress fit onto bare cable. There are a few types of crimper.
Cheapest – A hammer-set crimper. This is literally is a spring loaded punch you hit with a hammer. I used this type on my battery pack and have never had a connection loosen up. These are usually restricted in effectiveness to just a couple sizes of wire.
Still pretty cheap – A hydraulic hand crimper. Has multiple die sizes, but nothing beyond 2/0. Your hand might get tired of pumping.
Not so cheap – A mechanical crimper. Like a pair of loppers or shears, these types of crimpers use a lever action to apply crimping pressure and usually come with interchangeable dies for different wire sizes. This will do 95% of what you’ll ever touch high voltage-wise.
I don’t think anyone wants to talk about electric assist or larger hydraulic crimpers. They cost as much as a battery pack and just do things faster. (Maybe save this investment for when you’re building EV race cars for customers.)
Large wire crimps – Any copper lug will have a wire size and stud hole size. If it doesn’t list one—don’t buy it.
Tinned copper is better than bare copper (tinned is silver) if you are concerned with any possibility of corrosion.
Some manufacturers do color-keyed systems with their dies and corresponding lugs. And some some crimpers list their dies in millimeters squared (mm2) and not in American Wire Gauge (AWG), so it can be a bit annoying to figure things out without a good chart.
Low voltage work will also require a crimper and connections.
Most signal wiring is 22-18AWG, and 12V power wiring runs 14-6AWG. Crimpers and connections for this range of wiring can be had at any moderately good hardware store or even auto parts store—but the crimpers at many locations are usually garbage.
You need one of these, too.
Fuses are always a good idea in low voltage systems and high voltage traction applications. A high voltage cable may be cut or nicked while energized, and a fuse will not only prevent electric shock potential, but also minimize arc flash and thermal hazards. (That’s fire, y’all.)
As pictured in the diagram, it is recommended to have at least one fuse near the battery pack or even in the middle of it. This helps prevent excess energized cable, but also potentially limiting damage from a servicing accident resulting from a short. If its a whole pack short, a mid-pack fuse will limit the potential damage.
Tesla battery packs have a fuse on each cell for protection. This is feasible because cells are paralleled from 6-7 series cells (~24V) 74 times, so current from each cell remains small.
In packs with larger form factor series cells (bigger packs), this isn’t as feasible because you would need a lot of higher current fuses in series.
This is the recommended fuse, with a continuous duty rating of 500A and a peak rating of 1200A.
Vehicle Control Units, or VCUs, accept one of two types of input—either a standard potentiometer (POT) setup (usually a 0-5kohm range) or a Hall effect sensor (0-5V output range).
Most Hall effect throttles come in a factory pedal assembly. You might see them mentioned as “HEPA”s, or Hall Effect Pedal Assemblies. This can be a setback if you keep factory pedals or already have something else you like.
For controllers that require a Hall effect input, you can purchase a converter to convert the resistive input into a voltage output, thereby using the factory throttle cable and pedal with an appropriately built POT assembly.
Some people have used pedal assemblies hacked up and mounted to accept the factory throttle cable. I’m not fond of this technique, but each to their own.
When building any sort of electrical system, it is advisable to include a DMM or digital multi-meter. Since you’ll be working on high power DC systems, you want a meeting with a direct current clamp.
Most electrician’s meters are AC current only, so make sure you check the specifications and get one that reads DC too. 600V AC/DC for voltage is fine, and 0-400/400-1000A DC is recommended for current.
These meters can be bought for under $100. They may not be as accurate as a $500 Fluke meter, but they really don’t need to be.
In addition to all the sensors that make up a battery monitoring system—some which will also incorporate a temperature sensor—it’s a good idea to have a temperature sensor on your EV’s motor and controller. Some controllers have these built in. OEM vehicles certainly will, in addition to any VCU hardware, which can also display it.
Netgain motors (Warp 9) come with a simple snap switch that opens a contact at a preset temperature to let you know the motor is hot. This can easily be wired to a display or simple warning LED.
Some common sense rules apply when working with electrical systems.
Always check for voltage and always avoid skin contact with potentially energized parts wherever possible. Internal combustion engines are full of fire when they’re running. EVs are full of lightning—and often deadly silent.
Battery safety includes the use of insulated wrenches and safety glasses at all times.
Speaking from experience, these are a great first step. A $5 pair of safety glasses is better than getting molten copper in the eye as a result of an arc flash. The danger of batteries is not so much voltage as almost infinite current release in the case of a short which will vaporize metal.
Have you ever had a wrench slip while working on the battery terminals on a conventional vehicle? Made a decent spark, maybe even melted a spot on the wrench, too, didn’t it. Now imagine what could happen if there were 20 of those batteries connected together when you did that.
Having a hard time imagining? Here’s an interesting video where a guy connects 244—used—9V batteries together. The pack makes almost 2,000V. There’s sparks around the 5-minute mark.
In a pinch, a regular wrench can be safeguarded to a degree by plasti-dipping the handle. Or you could spring for an insulated ratchet like the one pictured here.
Well worth the money if you are going to spend time building battery packs or ripping apart OEM ones.
Wrapping this one up…
The last installment was focused on getting you started thinking about what your ideal DIY EV would look like and the big ticket items that go into a DIY EV conversion. This installment was meant to be a deep dive into what you need to know to connect all those big ticket items and actually build the thing.
If it seems daunting, I’d suggest taking things one step at a time. Please feel free to leave a question below. Brian will make sure I see it and help you figure things out. Or get in touch through the contact form and ask Brian to introduce us via email.
As is the case with any custom vehicle build, there’s a lot of variables in play. In addition to learning more about EVs and what components to use, I also recommend anyone interested in building his or her own EV get a good grip on basic electrical theory, Ohms Law, and how to correctly calculate and convert power and efficiencies are a useful set of mental skills you can use on any vehicle.