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Lithium batteries for eBikes come in a variety of sizes and power levels, but you can also DIY your own electric bike batteries for custom size and fitment. Here’s how to choose the right lithium-ion batteries and join them to make your own lithium battery packs for use in electric bicycles.
Time to Complete: 2-4 hours
Spot welder (preferably one designed for electronics)
Hot glue gun
Shrinkwrap or heat tape
Battery management system (BMS)
Soldering iron & solder
(Optional) battery holder
Lithium batteries for eBikes come in a variety of sizes and power levels, but you can also DIY your own electric bike batteries using off-the-shelf components.
When planning out your DIY electric bicycle batteries, you’ll want to first decide on the size of battery you need. Battery capacity requirements depend on motor type, riding habits and terrain. A larger, more powerful electric motor needs a battery pack capable of delivering more current and maintaining the correct temperature under load.
You can construct your own electric bicycle lithium battery by joining together lots of commercially available small Lithium-ion cells. The most common size for these cells is 18 x 65 mm. These rechargeable batteries are known as 18650 cells, and they typically come with a 3 Ah capacity and are rated for 3.6 volts nominal.
To make the correct voltage battery for your eBike, you’ll need to wire a number of these cells in series. A typical 36-volt pack will need 10 such cells in series, making it a “10S” pack. A 48V battery will need 14-16 cells, depending on the exact voltage specs of each cell.
To reach the necessary battery capacity, wire more rows of cells in parallel. For a 30 Amp-hour battery that may offer around 25-50 miles of range on a single charge, you would need 10 3 Ah cells in parallel.
When connecting cells in parallel, always make sure each cell is at the same level of charge; connecting a less charged cell to a more charged cell causes rapid discharge and is a heavy fire risk.
Once you’ve decided on the size of the entire pack, plan out where you’ll be mounting it on the bike. A rear rack mount works well for the smallest battery types, and under-tube mounting is often chosen for eBikes with a larger battery, due to the favorable center of gravity and weight distribution, though it may interfere with the placement of mid-drive motors.
A BMS for battery management is a good idea since it regulates the voltage during charge cycles and can help prevent uneven discharge and over-charging, which greatly reduces the lifespan and range of eBike batteries.
Choosing the right BMS depends on what features you need. A high-performance BMS may offer features like regenerative braking and connectors for a battery indicator. Make sure to only use a BMS rated for more electric power (Amps) than your DIY Ebike battery can deliver. A sophisticated battery monitoring system helps manage heat and makes the cells last longer.
After you decide on the number of cells in parallel and series connections and choose where to mount the battery pack, you’ll plan out the shape of your battery. It might be helpful to draw a wiring diagram. Common types of arrangements are the linear rectangular pack and the “honeycomb” style, with the honeycomb option being more space-efficient but also more vulnerable to heat-buildup in larger capacity packs, since there’s not much airspace in between cells.
When planning out your parallel and series connections, it might be helpful to draw a diagram of the battery, along with a wiring diagram that includes the BMS and charges and discharge wires.
Once you’ve designed the pack and gathered all the necessary materials, you can assemble the actual battery itself.
First, use a voltmeter to test the voltage of each cell and make sure it’s the same to within 1-5 percent. A dead cell is a fire hazard and will ruin the entire pack, so this is important.
Layout the battery cells you’ll be connecting in parallel and in series and cut the strips of nickel so that they’re the right size to connect the cells together. You can use a single piece of nickel or multiple, connected pieces, but having more material is generally better for heat management.
Now you’ll want to look at your diagram and arrange the cells so you’re ready to weld the connectors on. Some use hot glue for this step, whereas others use a jig to hold the cells in place and weld the connections on first. It depends on the shape of the battery. Battery cells pose an inherent fire and electric shock risk. Do not attempt to take apart or join cells without having at least basic electrical safety knowledge. Always wear non-conductive gloves.
Be sure to remove any metal jewelry before assembling your battery pack.
Use your spot welder to secure the nickel ribbon or plating to the terminals of the cells. If you have a spare cell or are using a new spot welder, you may want to do a test weld to make sure you have the current dialed in. Wear safety goggles when using a spot welder. Since batteries don’t like heat, you don’t want to overdo it.
Warning: Wear safety goggles when using a spot welder
You may wish to secure the BMS to the battery pack with some foam insulation and hot glue before you wire it up. Insulation helps prevent damage and loose connections from road bumps. Once the BMS is in place, wire up the connectors and then solder the sense wires onto the nickel strip for each parallel group.
Then, add the charge and discharge wires. The positive charge and discharge wires can be soldered to the positive terminal of the last parallel group (furthest from the BMS) and the negative wires should be connected to the appropriate terminals of the BMS. Always add the connectors first or use electrical tape to prevent the wire ends from coming into contact and shorting out the pack.
Next, seal up the battery using a heat shrink tube or a piece of heat foam and non-static tape. A proper battery holder and insulation can help prevent the cells from moving around against one another and potentially causing a short.
There are different sizes of heat shrink available, but a general rule is that they’ll shrink about 50% in one dimension and 10% in the other when heated with a heat gun.
For additional impact protection and prolonged battery life, you can encase your eBike battery in a waterproof and shock-resistant case or junction box. For outdoor storage, make sure to look for an IP68 rating on the case. “Potting” the battery in resin inside the case adds impact resistance and is sometimes used for off-road bikes.
Lithium-ion cells must be within two percent of the average voltage when connected.
Global demand for lithium hit 150 thousand metric tons as of 2018.