How To Size Wire, Fuses, And Nickel Strip For A Lithium Battery PackPosted on: Tue Apr 05 2022
When designing a battery-powered system from scratch, there are many factors to take into consideration. How do you size a nickel strip for building a battery? How do you know which wire size to use? If you are wondering these things, then you are in the right place.
We will explain everything you need to know about fuses, wire, and nickel strips. We will also discuss how to find out the perfect wire size to prevent your voltages from dropping beyond an acceptable degree.
The negative impacts that improper wire size can have on an electrical system are often misunderstood and underestimated. In this article, we will explain how to find the correct wire, fuse, and nickel strip for a battery-powered project.
How To Size Wire For Lithium-Ion Battery Pack
When designing low-voltage, battery-powered systems, using the wrong wire size can have a significant impact on battery life and your project’s overall performance. If your wires, nickel strips, busbars, are too small, these things can themselves become a significant load.
This situation can cause batteries to charge slower and battery powered devices to not get their full rated power. It can also cause your batteries and wires to heat up more than they normally would, which could lead to fire.
If the conductors and fuses you use to build a battery pack are too small, then your motor wont have as much torque as it should, or your lights will be dimmer than they otherwise need to be.
Why Is Wire Size Important For DC?
When it comes to high voltage AC wiring, having the right wire size helps keep the risk of fires low. In contrast, power loss is the main problem with using the wrong wire size in the low voltage DC systems that we will be covering in this article.
At these voltages, you can't go on ampacity alone. This is because lower voltage systems simply have less of a tolerance to voltage drop because the drop in voltage is a considerable portion of the total.
Determining The Total Amperage Of Your Circuit
Current is measured in units called Amps, which are abbreviated as the letter A. There are 1000 mA (milliamps) in 1 amp.
For example, an LED strip that has 30 LEDs that draw 80mA each consumes a total of 2400 mA, which is or 2.4 amps.
In another example, you may be running a 500W device that runs on 12 volts. Watts divided by volts equals amps. So, that means your circuit will require 41.6 amps.
Why Use Nickel Strip?
Lithium ion batteries can store quite a bit of energy. To be able to access that energy, a conductor must be used to connect the cells together in the best way for a given project. Nickel is the preferred material to connect lithium ion battery cells together.
Nickel strip is the most common material used in lithium ion battery construction because it is easy to spot weld and has excellent anti-corrosive properties while having a relatively low cost.
Make Sure Its Pure Nickel
Pure nickel is around twice as conductive as nickel plated steel. Nickel plated steel has its use cases, but nickel plated steel should never be used for battery construction. The real problem is the fact that many online vendors sell nickel plated steel as pure nickel.
When it comes to pure nickel strips, the thickness can vary from 0.1mm to 0.3mm. Most low cost welders have a hard time around 0.15mm, and most cannot even work with 0.20mm, even on the highest settings. So, keep that in mind when shopping for nickel strips.
The most commonly available pure nickel strips are 7mm wide, though 10mm is readily available. There are some 47mm strips, but there is a large air gap in the middle. If you need more current, you could always spot-weld 2 to 3 layers of nickel strips on top of each other for more capacity.
How To Determine Proper Wire Size For Battery Pack
So, how do you know what size wires to use for your battery project? It can be confusing, but it can also be dangerous. If you don't use a large enough wire, the wires will become excessively hot under the intended load. And while we do recommend over-sizing wires, if you oversize by too much your build will be more expensive than necessary.
Tables & Charts For Proper Cable & Wire Sizes
Most people don’t realize that the amount of current a given wire can carry is dependent on the type of insulation it has. That's right, the same amount of copper can carry more current if its insulator is rated for higher temperatures.
For example, Non-UF-B (Metallic Sheathed Cable) can only carry 15 amps at 14 gauge because of the type of insulator used. In contrast, THHN and XHHW-2 insulators have high heat resistance, enabling 14 gauge cable to comfortably carry 25 amps.
You can use the table above for sizing the wire for the charge and discharge connectors for your battery pack. All you have to do is cross reference the type of wire you want to use with your battery’s peak current.
It’s important to not run anything at its limit, so, whatever the highest current discharge your BMS supports, use wire that has about 20% extra current carrying capacity.
Wires used internally in the battery pack construction are relatively short, so ampacity is all that matters. You don’t really have to worry about voltage drop so much at these lengths.
Voltage Drop In Cables
You might assume that cables transfer power without consuming it, but in practical application, all wires and cables have some amount of resistance, and therefore use some amount of power.
This resistance of a given conductor is directly proportional to its length, but inversely proportional to the conductor's diameter. This means as a given wire’s length increases, its resistance increases. It also means that larger gauge wires will have lower resistance.
Due to this unavoidable, intrinsic resistance, a voltage drop occurs when current flows through any conductor. When working on small scales or with low power projects, this voltage drop is negligible. If you are, however, building a larger system, or something that consumes a high amount of current, then these small drops can add up to big power losses.
The amount of voltage drop between your battery and your load can result in a significant loss of power and battery life.
How To Determine The Proper Cable & Wire Size For Given Load?
When it comes to determining the correct cable and wire size, you must take several factors into consideration. One of which is circuit load. If all else is equal, the smaller the load the smaller wires you need.
Making sure you have the right wire size for your DC electrical project is crucial. Remember, if a wire is too small it will heat up and could eventually ignite. Something as simple as using the right fuse can reduce the chances of a fire to nearly zero.
Make sure to avoid solid wire. This may seem counter-intuitive, but stranded wire is usually the best option. Solid wire is stiff and hard to bend compared to stranded wire. It’s also a lot harder to solder. Also, their added stiffness makes solid wire very difficult to splice compared to stranded wire.
Universal Wire Sizing Chart
This chart effectively replaces several pages of sizing charts. This can be applied to literally any working voltage, at the acceptable voltage drop of your choosing. This chart will work for any voltage and voltage drop and works with AWG and metric sizing.
VDI stands for Voltage Drop Index and is a reference number based on a wire’s total resistance.
You can find your VDI by performing the following calculation:
When doing this calculation, make sure to only consider the one way distance between the battery and the load. For the %VOLT DROP, choose your acceptable voltage drop percentage.
Consider this example:
You can also simply multiply your calculated VDI by 1.1 to find out what size metric cable you need for your project.
NOTE: Metric standard wire sizes are available in 1, 1.5, 2.5, 4, 6, 10, 16, 25, 35, 50, 70, 95, and 120 mm².
It's important to keep in mind that while this calculation does tell you what size cable you need to maintain a certain voltage at a certain current, it does not account for the ampacity of the cable itself. So, for best results in determining cable size, the VDI and ampacity ratings should be compared and the larger cable of the two should be chosen.
How To Determine Acceptable Voltage Drop For Various Electrical Loads
IEEE rule B-23 states that no more than a 2.5% voltage drop should be measurable between any two points between the power supply and the load. That is for building wiring, but it's a good figure to go on.
So, that means 2 to 3% voltage drop is acceptable for some situations. It is important to remember, however, that not every load is the same. Different types of electrical circuits have different tolerances to voltage drop.
Below are some examples of low voltage DC projects and how voltage drop affects each one.
Legacy lighting such as incandescent and quartz halogen systems can exhibit up to a 10% drop in brightness from a 5% drop in voltage. This is because in these types of lights, the voltage drop results in a cooler filament. As the filament’s temperature lowers, its color temperature also lowers, moving from white-hot towards red-hot, which emits far less light in the visible spectrum.
In the case of fluorescent lights, voltage drop results in an almost proportional drop in brightness. This is linked to the fact that fluorescent lights use up to 66% less current to produce the same amount of light as an incandescent.
DC motors typically operate at much higher efficiencies than AC motors. Another benefit of DC motors is they completely eliminate the need for an inverter to run them from battery based systems. While they do have some amount of inrush current, DC motors don't require the excessive current required to start an AC induction motor.
AC motors, which are used in appliances and large power tools, can fail to start if the voltage drop is too high. This is because AC induction motors have very high surge demands when starting.
When it comes to Solar Powered battery systems, wire size is critical. This is because a voltage drop can cause a massive loss of charge current. To be able to charge a battery, the charging system must be able to apply a voltage to the battery that is higher than the battery voltage.
Most photovoltaic modules have a 16V to 18V peak power point, so a voltage drop over 5% will reduce this necessary voltage difference, which can reduce the charge current to the battery by a much greater degree.
Voltage Drop Per 100 FT Run Of Paired Wire
Why Do You Need A Fuse?
A fuse's main function is to prevent damage to components, but they also prevent fires. For a fuse to effectively serve its purpose, it has to be the right size. If it's too low, the fuse will blow every time and if it's too high, everything will work fine until it doesn't. If there is any issue, it will be a big one.
A fuse ensures that only a certain amount of current can make its way through your device. If there is a 20 amp fuse in the circuit, then no matter what happens and regardless of what component fails and in what way, a maximum of 20 amp of current will flow through the device. This greatly reduces the chances of component failure.
Another benefit of using fuses is that if anything goes wrong when you are working on your project, the chances of catastrophic failure is greatly diminished. This is because once the fuse blows, the current is completely removed from the line. This makes it so the line only experiences a set level of overload for a fraction of a second. There is simply no time to heat up and catch fire.
Remember, for a fuse to do its job properly, it needs to be able to carry about 10 to 50 percent more current than your wires are rated for. This, of course, is assuming you are using the right wire size.
What Is Fuse Rating?
If you take a look at the top of a fuse, you will see the fuse rating. This rating is given in Amps and it is the maximum amount of current that can pass through the fuse without destroying it.
Fuses are designed so that they do not get hot near the limit, but right at the limit, they blow. This means that the amount of current you want will be able to flow through the fuse without the fuse being a current limiting factor or a source of heat under normal operation.
So, this means that a fuse can handle as much current as stated. Anything more than that, and the fuse will break. When that happens, it opens the circuit and stops the flow of power.
Fuse And Other Circuit Protection Questions
What Is A Blade Fuse?
The automotive-style blade fuse is the most common type of fuse. These familiar, transparent, U-shaped fuses have the rating on the top and you can find them in everything from your car to your 3d printer. It’s also common to see removable blade fuses used with in-line fuse modules.
Technically all fuses are inline fuses. This is because for a fuse to work, it has to be installed in series with the connection you are intending to protect. Normally, however, fuses are installed somewhere within the device. When they are not, they are referred to as inline fuses. So, generally speaking, an inline fuse is one that is installed in the wire or power cable rather than the device.
What Is A Circuit Breaker?
A circuit breaker is a mechanical device that interrupts the flow of power in the event of an overcurrent condition. They function similar to a fuse in that the device breaks the flow of current when too much current is detected on the line, but unlike a standard fuse, a circuit breaker can be reset to normal operation.
Circuit Breakers Vs Fuses
Even though they serve the same overall purpose, there are some fundamental differences between Circuit Breakers and Fuses that need to be considered when designing any electronics project.
- Typically requires manual replacement
- Low cost
- Requires little-to-no maintenance
- Known to function until over-current event
- Resettable after overcurrent event
- More expensive than fuses
- Requires maintenance and testing
- Known to fail over time
- Automatically resets after over-current condition
- Takes long time to return to normal resistance
- Low cost
- Maintenance free
What Does Overcurrent Mean?
Overcurrent is exactly what it sounds like. It's when the current in a circuit is higher than what the equipment is rated for. There are several different possible sources of an over-current condition. An overload, short circuit, and ground fault are a few examples.
What Is An Overload?
An overload is when any system takes on more work than it is rated to handle. Once this happens, it usually results in an over-current condition. For example, if a motorized lift is only rated for 200 lbs, but it is made to lift 300 lbs, then the motors will have to use more current than the system is rated for, which can result in overheating and permanent damage to the equipment.
What Is A Short Circuit?
In an electric circuit, current takes a path and performs its work along the way. If positive and negative touch, however, this ‘shortens’ the circuit. A short circuit is effectively a 0 ohm load. This means as much energy as possible will flow into the short until either something bursts into flames or a fuse blows.
What Is Short Circuit Protection?
Short circuit protection is a safety feature that effectively disconnects the circuit using transistors and a feedback circuit. Short circuit protection is a form of over-current protection. In systems that support short-circuit protection, all current flows through a transistor or array of transistors. The line current is monitored and if the current reaches a particular threshold, the transistor is switched off, effectively blocking the flow of current.
What Is A Slow Blow Fuse?
Slow blow fuses take longer to blow than the more common ‘fast blow’ fuses. You could say they blow ‘slowly’. A slow blow fuse can be exposed to a current that exceeds its rating for a certain period of time. For example, you may be able to run a 15 amps motor with a 20 amp fuse, but on startup that motor may require 40 or 50 amps for a short amount of time.
What Is A Fast Blow Fuse?
What you would normally think of as a standard fuse is of the ‘fast blow’ variety. A fast blow fuse is a fuse that blows quickly, even when exposed to an overcurrent condition for just a short amount of time.
What Is A PTC Fuse?
A PTC fuse is a resettable fuse that has an internal mechanism which dramatically increases its resistance when a short circuit occurs. This increase in resistance effectively disconnects the circuit. Once the current flow is below a certain threshold, the PTC fuses automatically switch back into a low resistance state.
The one glaring drawback of a PTC fuse is that once they are triggered into the high resistance state, it can take a considerable amount of time for the fuse to go back to completely ‘normal’ resistance levels. So, these are only recommended for low-current operations.
When you are building a battery powered low voltage system, it's critical to build the battery with the right size nickel. It’s important to not overlook the wiring outside of the battery pack, as it’s just as important as the battery’s internal connections.
It’s relatively easy to build a battery, but when it comes to building high-quality battery packs and reliable low voltage DC systems that are efficient and work well, there is quite a bit of knowledge to acquire and apply. We hope this article helped you learn how to size wire, fuses, and nickel strips for your battery pack project.