Balance of System

Balance of System is a fancy term that refers to “and everything else”.

In an electric vehicle, you already know about the main components, like the motor and batteries, but it can sometimes be the little things that people don’t talk about, and can be the most confusing.

In this case, we will talk about the on/off key, main fuse, contactor, battery disconnect key, pre-charge resistor, shunt and ammeter, instrumentation, power indicator light, and DC/DC converter. Most of these components are shown right on the wiring diagram, along with their specs.

On/Off Key
When I got my cycle frame, the ignition key was broken. I replaced it with a simple keyed electrical switch. It’s a “double-pole, double-throw” switch, which means that it completes two separate circuits at the same time. That’s great, because with one switch, I can turn on both the 12V accessory system and the 48V drive system at the same time.
I built a mounting bracket by cutting down a piece of metal from a recycled computer case. I drilled a hole to mount the key switch, two holes for bolts to mount the bracket to the cycle frame, and painted it black. The switch gets two sets of wires to the back of it, both with small crimp-on ring terminals. One set goes to the DC/DC converter to run the 12V accessory system, and the other pair activates the main contactor for the 48V drive system.

On a motorcycle that has an existing, working ignition key, you can route 12V power from the key to activate a relay that will turn on the main contactor and motor controller.

Battery Disconnect
The battery disconnect is just a big kill switch. It completely disconnects the batteries from the rest of the system. It’s an easy way to disconnect power for when you are working on the cycle, and acts as an emergency backup in case the main contactor ever failed. Both the On/Off Key and the Battery Disconnect are mounted on the left side of the motorcycle, not far from where the “Emergency Reserve” switch would be on a typical cycle’s gas tank. Since there is no clutch or other left hand control, these are mounted on the left side in easy reach of the rider.

The batteries are connected or disconnected with a removable red “key” plunger. Make sure to get a disconnect rated for high amperage. The full current of the vehicle goes directly through this component. All battery cables, fuses, connectors, shunts, shut-offs, and anything else carrying current needs to be sized correctly. Since I’m using a 300 amp motor controller, sizing everything to 300 amps makes sense.

Main Fuse
The bike needs a fuse that will blow and protect the system if anything shorts or otherwise draws too much current (such as a blown motor controller.) I used a fancy-looking fuse holder with a 300 amp fuse in it. Make sure to mount this in such a location that the fuse is easy to access and replace. (If you want to get really wild, make it so it can be easily replaced on the side of the road, in the middle of nowhere, at 4AM in a rainstorm. Because you just KNOW that’s when you are going to have a problem….)

Main Contactor
The Main Contactor is a large, remotely-activated, high-power relay. When I turn the on/off key, it sends 12V to the main contactor, which closes, and completes the 48V drive circuit. The contactor is spring-loaded, so that if it no longer gets that small amount of 12v power, it opens and shuts down the cycle. This works well as a safety feature. For example you could wire a switch in series with the 12V power to the contactor from the kickstand. If the kickstand is down, the main contactor won’t close, and you can’t turn the cycle on.

Pre-charge resistor
Most motor controllers require a “pre-charge resistor”. That’s a way to allow power to slowly go into the motor controller to charge up the capacitors. If power was suddenly applied to the motor controller (such as just flipping a switch) the capacitors internal to the controller would suck up power almost instantly. Do that too many times and the capacitors will blow and wreck the controller. If you called the manufacturer for warranty work, the first thing they will ask you is about the pre-charge resistor.
The resistor simply bypasses the main contactor. When the battery disconnect is turn on, current will flow from the batteries, through the resistor, and into the controller. As it does, the voltage internal to the controller will raise to match that of the batteries. Once it does, you can turn the key to on, which activates the main contactor. The contactor is now a less resistive path, and when you twist the throttle, high current can go from the batteries, through the contactor, controller, and motor, and drive the cycle. Pre-charging the controller also prevents any arcing internal to the main contactor and prolongs its life.

Ammeter and Shunt
The ammeter is a display of how much current (measured in amps) that you are using at any given moment. Think of it as a real-time energy meter. In general, you want to minimize amperage while cruising (to maximize range and battery life) but it would also be nice to know how much power you use for burn-outs and powering up hills.
This is usually a matched set. The Ammeter is the display itself, mounted on the handlebars or other location for easy viewing, and the shunt, which is a calibrated piece of metal that the current flows through. Two wires (one on either end of the shunt) go to the ammeter. The needle on the ammeter varies directly with the amount of current through the shunt.

My ammeter is a 300 amp meter, mounted in a hole in the former gas tank. The shunt is mounted out of the way, near the contactor and battery disconnect. I strapped the gas tank down to a drill press with a hole saw in it to cut a hole just slightly larger than the ammeter. Since the gas tank doesn’t hold gas anymore, there’s no reason not to cut holes in it and mount instrumentation in there.

I recommend an ANALOG ammeter. A needle sweeping back and forth is easy to quickly read. Although a digital display may be more accurate, it’s not as useful and it’s difficult to read digital numbers that are constantly changing.

Power Indicator Light
On an electric motorcycle, there is no engine noise or vibration to indicate to the rider or anyone else that the motorcycle is on. You simply flip a switch, and it’s instantly ready to go. Although the headlight is on when the cycle is on, the rider typically can’t see that during the day. I wanted a great big, bright indicator to tell me when the vehicle was on. I decided that a green light mounted towards the front of the tank would work well. I found some switches, lights, and other components on an old instrument panel. One light had a sign on it saying “Power” and another one had a green lens. Both lights were for AC power, not DC power. I grabbed the components and put together the green lens, the power sign, and removed the small transformer on the bottom of the light socket so I could instead run 12V DC straight to the bulb. The bulb holder was installed through the gas tank, and 12V wiring run to it from the cycle’s 12V fuse panel.

Powering the 12V system
On a typical gasoline motorcycle, there is a 12V battery to start the engine and run the headlamp and other electrical. The battery gets recharged by the engine, through the alternator, and it is what really powers all the 12V electrical.
Without an engine and alternator, you will need some other way to run the 12V electrical.

With a 12V battery
If you mostly just use the cycle for very short trips and errands, you could just use a plain, sealed, 12V battery. That battery would need its own charger, so that every time you are done with a ride, the 12V accessory battery gets recharged right away. The battery will take up some space, add some weight, and you would most likely want the charger for it left right on the cycle as well, using up even more space. It does work, and is simple, but not ideal.

With a DC/DC converter
Instead of a dedicated 12V battery and charger, you could use a DC/DC converter. The converter is an electronic device that takes one DC voltage in, and gives a different DC voltage out. It’s a very efficient way to use a trickle of power from all four of the large drive batteries, convert the 48V to 12v, and run the headlight and other accessories.

The DC/DC converter was a computer component purchased from e-Bay for $10. It’s two-inches square by half an inch thick – very compact and lightweight. This saves considerable bulk and weight over a medium-sized battery and dedicated charger. 48V from the drive batteries is wired to the input end of the converter. The output end of the converter takes the place of a 12V battery. The + goes to the cycle fuse panel, and the – goes to the motorcycle frame ground.

The output of the converter is adjustable into the range of CHARGING a 12V battery, so another option is to use BOTH a DC/DC converter and a small lead-acid battery. The converter provides power to the battery as a trickle-charge, and the battery acts as a reservoir in case you suddenly pull more power than the converter can provide, or in case it quit working.

This DC/DC Converter is rated for 100 watts. The headlamp draws 55, leaving plenty of power for the tail-lights, turn signals, and other 12V accessories.

I crimped and soldered 1/4″ spade connectors on the converter to make it easier to quickly connect the wiring. The converter already has mounting holes in it. I mounted it with small screws to the same plate that the motor controller is mounted to.

Power Brakes
Most modern cars have power brakes. These systems use the vacuum created by the engine to amplify the power of your foot pushing on the brake pedal. On most electric car conversions, a small 12V electric vacuum pump is added to take the place of the vacuum created by the engine. Since diesel engines don’t create vacuum in the same way that a gas engine does, salvage yard diesel cars are sometimes used as a source of parts.
There are also older non-power brake components which are sometimes swapped in for powerful braking action WITHOUT the need for vacuum.


All over these various components serve important roles. Even though the motor and batteries are the first things we think of on an EV, make sure you understand the balance of system to properly and safely operate your vehicle.

{ 12 comments… read them below or add one }

1 James August 27, 2013 at 3:02 pm

This website is awesome! I am about to start my bike conversion, what are your thoughts on a kz1000 frame? Its what I have at the moment and I can’t decide if the bike will be to heavy at the end of all of this.


2 J. JEWITT January 1, 2014 at 5:06 pm

I’m seriously thinking about your conversion on a early 1980s Suzuki bike that the motor jumped time. Is this B&S electric motor STILL available? After reading many of the pages and watching about 3 or 4 youtube clips I haven’t seen a TOTAL price breakdown for conversion MINUS the bike, where if it is on the site. I’m looking for about a 65 mile trip @ 60 mph minimum speed this is a 1 1/2 hour ride mostly level terrain, is this possible? If I mounted the same number of batteries under tanks as you then one per side as saddle bags would this give me the extra MPH & range? I assume the MAJOR weight is the batteries then motor correct? About how much weight PER batterry is it? How much does the motor weight? Thanks

3 admin January 2, 2014 at 10:31 am

Total cost of the conversion was right around $2000. Only $100 of that was the frame. The (lead acid) batteries are the heaviest part. The motor really isn’t heavy at all. I think it was 22 lbs.
For a really long-rangle EV motorcycle, you would want to use lithium batteries. Those are typically available as cells. So, instead of 4 or 6 12V batteries, you might have 16 or 24 lithium cells. There really isn’t any room under my tank for extra batteries – part of the frame is right there. Sport bikes give you a little more room for batteries because of the shape of their frame. For best top speed, go for a higher system voltage, like 72V.

4 admin January 2, 2014 at 10:55 am

I haven’t personally worked on a KZ1000, but it sounds like it would be a great frame. I did see one listed on the EV album, although that guy was only running it as a 36V zip around town vehicle.

5 Arran Gibson July 13, 2016 at 6:49 am


Ive acquired a ‘project’ ( i mean shed) of a frame and i quite fancy running an electric bike. This site is very inspiring. Im still getting my head around the whole idea but i was wondering if i use something like nissan leaf batteries or such, would that give a longer range for a full charge. Im basically thinking if i can get 60- 100 miles from a charge then i can plug her in at work and use there electricity to travel. I still need to be able to cruise at around 60mph too. Ive seen a lot of projects but most seem to have 30 mile ranges which wouldnt be viable unfortunately. Any advice would be greatly received. Thank you.

6 admin July 17, 2016 at 8:36 am

Yes, Nissan Leaf cells would be great!
I replaced the original (dead) batteries in a Vectrix Electric Motorcycle with those from a Nissan Leaf. Real world range is 60+ miles per charge, and I’ve gone just over 90 miles on a single charge one week when I was just driving in town, multiple trips short trips, and NOT topping off my charge.

7 Graham Chapman October 21, 2016 at 10:44 am

Hi Ben. Purchased your vimeo access etc. Inspiring to watch. Starting small with an electric atv conversion. Very basic. Using 36 volt, but having trouble finding a fuseholder and fuse for 36 volts and 200 amps. Found lots of Megafuse, but they only seem to operate at 32 volts. What model of fuseholder did you use on the motorcycle? I cannot seem to find the details on the site anywhere. Many thanks.

8 admin October 21, 2016 at 10:53 am

Frankly, I don’t remember where I ordered the fuse-holder from. Parts like that were just mail order. For a 36V ATV, any of those “32V” fuses will work fine. It’s not like exactly 32V is some sort of magical cut-off. Essentially, it’s more like “this fuse is for low-voltage systems”. It will work fine for your application. When you get into really high voltages, then you need special fuses that can disrupt even ionized air. But that’s a whole different ball of wax.
I used a fuse similar to one in this link. There’s a fuse holder that goes with it on the same site.

9 Aaron Wymark April 15, 2020 at 4:38 pm

Hi there i really hope you can help guide me I’m researching how i can convert my classic VW to all electric, my only concern is how can i charge the battery. would it be possible to use standard EV chargers ?? I’m based in England and any help would be amazing .
regards Aaron

10 admin April 15, 2020 at 4:49 pm

Hi Aaron,

The charger is a component in the car. The component that is in your garage or somewhere out in public is called Electric Vehicle Service Equipment, or EVSE. It safely provides electric power to your charger at the proper current.

When you convert your car to electric, you just need to purchase a charger that matches the battery voltage system. To use a public EVSE, you will also need a some device to communicate and control it. In the United States, we use a protocol called SAE J1772 as our standard. It’s easy to make the basic signal that controls that. I built a simple adapter myself. You can see that at:

In England, you use a slightly different system, and also have socketed connectors, which we don’t use here in the States.
For more details, check with a UK based electric vehicle components provider, or even Youtubers from the area, such as Damien Macguire’s channel.

11 tom pekar January 23, 2023 at 1:27 am

I am dong a DC motor conversion ona model T What si the spec for the precharge resistor to go across the main contactor, love your videos, many thanks tom pekar

12 Ben N January 23, 2023 at 9:23 am

Hi Tom,
The best source of information on a pre-charge resistor is probably going to be right with the documentation on whichever motor controller you are using.
Back in the days when I did the electric motorcycle, I used an Alltrax DC controller. With their documentation, they show a 10 watt resistor, ranging from 250 ohm to 1,000 ohm depending on what your system voltage is.


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