Here at Higherwire, we’re all about maximizing the useful life of lithium-ion batteries, which we typically do by pairing our solutions with renewable energy storage and for use in backup power. But we’re naturally curious to push the boundaries and find other applications to put them in.
Did you know that the average electric vehicle battery still has 75% remaining capacity when it is removed from service?
With that in mind, we like to replace lead-acid batteries in everything we can. In fact, we’ve built a few scooters and electric bikes, including a couple we use ourselves. We’ll break down a few things to think about if you want to save a few dollars and DIY a battery for your mobility application.
As usual let’s get our typical disclaimer out of the way:
Misusing or mishandling a lithium-ion battery may cause a FIRE or EXPLOSION which can result in INJURY or DEATH. You MUST read the safety warnings and resources before purchasing, using, and/or handling batteries. Even if properly handled, lithium-ion batteries may still catch fire or explode. Buyer acknowledges these risks and absolves Higher Wire Inc. of any responsibility for damage these cells may cause.
The very first consideration is the wattage you want to run. If you’re over-volting an existing scooter or bike, we recommend keeping it at or under twice the original rating, and – this is important – increase power by upping voltage, NOT amperage. More amps means larger wires and more heat, and requires more robust components. For example, 500 watts at 24 volts is about 20 amps, which you can get by with 16-gauge wire. However, at 1000 watts, you’re looking at 40+ amps which is far above what 16 gauge wire can handle. By stepping up to 48 volts (51v from a 14S lithium battery), that same 20 amps translates to 1000 watts and you likely won’t need to switch anything but the controller. Because lithium batteries spend most of their charge/discharge cycle at nominal voltage, we use that for our calculations. Remember to multiply the nominal voltage by number of strings in series to get total battery voltage (i.e., a 14S is 51.8 volts when using 3.7 volt nominal cells).
One of our projects was a 24v Schwinn S-500 that was collecting dust. We decided to give new life to the old gal by stepping up voltage to 48 volts (51v actual) and replacing the dead batteries with some of our Second Life lithium-ion 18650s. We replaced the controller and throttle with a 48-volt 1000 watt kit similar to this one at electricscooterparts.com. These are fairly straightforward to wire and often come with way more connectors than you need. Simply follow the included wiring guide and you shouldn’t have much trouble. In our case, we connected the battery power, motor, throttle, key switch, and brake switch (to remove power when the brake is applied). You might have a separate connection for the charger, but the S-500 has an XLR connector integrated in the main harness so we left that alone.
Building the Battery
From there you’ll want to determine how much battery you can fit in your application. You’re typically limited by the enclosure on your particular chassis unless you get creative. For example, we’ve done Razor E100 and E175 kids’ scooters, which can only fit 30 cells in a 3P10S configuration. You have more options for a bike, such as a triangle battery bag or pannier rack.
With those, you can calculate amperage to make sure you’re not exceeding the capacity of your cells. The vast majority of the 18650 cells that we sell are rated to 2C max, and we highly recommend staying below that! Put simply, C-rate is the charge or discharge amperage as compared to battery capacity. For example, 1C is the amperage required to drain the battery in one hour, and 2C is the amperage required to drain the battery in ½ hour. You can add paralleled cell capacities together and use that as your total battery capacity.
C-rate is the charge or discharge amperage as compared to battery capacity. For example, 1C is the amperage required to drain the battery in one hour.
The S-500 is able to fit 84 cells in a 14S6P. We know that 1000 watts at 50 volts is 19 amps, but we’ll use 24 amp draw to account for losses. Thus, each individual 18650 cell is seeing about 4 amps at maximum power*. For this build, we used LG INR18650MH1 cells rated at 3200mAh (11 Wh) and 10 amps continuous discharge. We calculated the actual capacity of our pack at 900 watt-hours at an actual weight of 3.9kg (8.6 lb). That’s about 1.3C at max load (with losses), well below our cutoff of 2C.
From the factory, the S-500 is rated at 24 volts, with a top speed of 16 mph and 8-mile range. Power comes in the form of a pair of 12v/10Ah batteries, for a total of 240 watt-hours and 6.4 kg (14.1 lb).
We doubled the scooter’s power and more than tripled the battery capacity, while cutting battery weight by 5 and a half pounds! We’re using a Cycle Satiator to charge the battery, typically using the 85% setting to prolong battery life. We’re getting just over 10 miles of range, which includes a climb over a highway overpass every mile. The best part? We hit 31 mph(!) in ideal conditions and regularly cruise at 27 mph. Not bad for some leftover parts.
Used batteries obviously won’t give you the same performance or life as new. The fact is that most of our stock isn’t meant for the higher load seen in scooters and e-bikes. This S-500 does experience a little bit of voltage sag under load, although that hasn’t noticeably affected performance. That said, it is absolutely imperative that you use a battery management system (BMS) with any lithium battery, regardless of load or whether cells are new or used. That is especially true when pushing more amperage through them like we are doing here. That will keep things safe and ensure you don't over-charge or flat-line your pack, and cut power in case a cell-string gets out of whack.
The bottom line is that lithium-ion is a great upgrade for any scooter or e-bike, but brand new batteries are expensive. As long as you’re willing and comfortable, upgrading with our Second Life cells is a cost-effective way to a faster and longer-range ride.
*This is going to differ based on individual cell internal resistance, but we’re not diving that deep since this is still relatively low load.