Skip to Content


Monthly Archives: December 2013


Electric Scooter Battery Upgrade – Pt. 3 – Building the Pack

In the last post, I decided on the 15Ah LiFePo4 cylinders in the 40152 size. Since my bike is a 48v system, I needed an equivalent voltage for this. Looking around on Taobao and the net, it seems like 16 LiFePo4 batteries in series is the standard for 48v. I already had one of these batteries from testing, but I ordered 16 new ones anyway in case I got a dud. At first I was a bit disappointed because the batteries I got were not brand new – there was rust / cruft on some of the terminals.


Since I had my handy-dandy battery capacity tester, I took 4 of the worst batteries and tested them. They all measured out to an average of 14.5 Ah, which assuaged my concerns – these batteries were either lightly used or just sitting in storage for a couple years. For the price I got them, I can’t really complain and I decided to go forward with the build.


The first thing I did before assembly was to take some black electrical tape and use it to mark the negative terminals. You can tell the difference between negative and positive with a multimeter or by looking carefully, but I wanted to know which was with with a simple glance. This turned out to be a really good idea.


These particular batteries are really cool in that they have a built-in screw terminal on each battery. This makes constructing the pack really easy – all I had to do was make some connector pieces and then screw it all together like some sort of more dangerous version of Ikea furniture. I ordered some aluminum strip (2mm * 15mm) and cut them into 55mm long pieces. I drilled 2 6mm holes 40mm apart from each other and then a 3.5mm hole in the middle. The middle hole was tapped to M4 for the balancing cable.


You can build the pack in a variety of configurations (2×8, 3×6, 4×4, etc) but I decided to go with 4×4 to make it easy to fix in the existing battery space. This also meant that the final terminals ended up on the same side of the pack. I found it was easiest to build one row at a time, then attach that to the previous rows to build it up layer by layer.


As I bolted each pack together, I made sure to use some thread locker on each of the bolts. This pack is going to be used in an electric bike, and I’m not exactly gentle with how I ride it. I did not want anything to vibrate loose during a drive and this will help ensure that the pack holds together.


After the pack was fully assembled, I measured the total voltage – at 53v it checks out, and so I taped the pack together so I could start adding the balancing cables.


The protection board came with pigtails for the balancing cables, and I picked up some wires with screw-down connectors to attach them to the battery. My first step was to solder these together and use shrink tubing to protect the joint.


After that was done, I attached each of the balance cables to the appropriate spot on the battery. My method was to first measure and check each voltage, and then mark on the aluminum bus bar with a Sharpie which line it was. Then wiring up the connectors was as simple as looking for the right number and screwing it down. Again, I used Loctite on the screws to prevent anything from coming loose. The result was rather bomb-like but hopefully it acts less like a bomb and more like a battery. Once all the balance wires were in place, I checked each cable with a multimeter. Going in order, I confirmed that they were each roughly 3.3v apart.


Next was preparing the protection board. The positive and negative terminals of the battery each had M6 screws, so I attached some M6 ring terminals to those leads. For the connection to charger and ebike power, I used some awesome XT60 connectors which can handle at least 60 amps – way more than I need, but they were cheap and polarized. The biggest challenge was the 8 gauge wire was a major pain in the ass to solder and I really had to crank up the temp on my iron to get it right.


Next I plugged in each of the balance cables to the protection board according to the instructions and then taped the board in place with a couple passes of tape. The next step was to charge up the battery and make sure it was all balanced.


Unfortunately it was not balanced. If I was doing this again, I would probably do something like put all the cells in parallel first and then charge them fully. Instead, I had to individually balance the cells. I originally had thought the protection board would do this, and it probably would if you let it sit for eternity, but some of the cells were almost fully discharged. You see, the way the balancing works on this board is by slowly discharging the excess voltage through an LED until it drops below the predefined voltage. This is great if your cells are only off by a few hundred milliamps and you’re willing to wait a couple hours. Not so good if you have 10amp difference to make up.


My approach was pretty simple – charge up all batteries through the main charger until some of the lights came on, unplug the pack from the charger (important), and then individually charge each cell with a single-cell charger using alligator clips. After a few cycles of doing this, the whole pack was fully charged and all 16 balance lights were lit. Awww yeah!


Next, I wanted to verify the capacity of the pack. I hooked it up to the battery capacity tester and set the amperage to 10A. After about 90 minutes the results were in: it tested out as 15.18Ah – slightly more than the advertised capacity of these batteries! I was quite pleased. You’ll note the power resistors were in a water bath – they were putting out a total of about 500 watts during testing and got quite hot – I even had to change to colder water about halfway through otherwise it might have started boiling. Will definitely need to get a better setup later as water + heaters isn’t exactly a brilliant idea.


Once my pack was verified and good to go, I did the final layer of wrapping to prepare it for going into the bike. I was gluttonous and used an entire role of Kapton tape. Sometimes I just love living in Shenzhen where its about a buck a roll. I left air holes in case the batteries need to vent and left the cables out. Since the kapton is translucent, I was still able to see the balance indicators through the opening while protecting the electronics from dust and road particles.


After that, I put the batteries in the bike and strapped it down using the old parts that held the SLA batteries in place. I took it out for a spin and while I don’t have hard data to back it up, it felt quicker than the old setup. I measured a top speed of 48kph and definitely like how it handles now. One major improvement is that the battery pack went from about 28kg in weight down to 8kg in weight. Dropping 20kg from the weight of the bike is a definite improvement and seems to make a difference.


An awesome feature of the bike is that there is a little hatch from the cargo compartment to the battery compartment which means I can easily check the status of the battery after a charge – by looking to make sure all the LED balance lights are lit, I can know if all the cells are ready to go or not.


Here’s the finished bike. Of course no upgrade would be complete without washing it until it was sparkly clean and gorgeous again.


Electric Scooter Battery Upgrade – Pt. 2 – Battery Selection

After getting my tools ready in part 1, I was ready to start deciding on which battery to use. I scoured Taobao for many different batteries, including the infamous ‘Ultrafire’ batteries which claim a ridiculous 5500mAh capacity. I ordered a couple of each different kind of batteries from a bunch of different vendors.


Testing the batteries was pretty straightforward – the testing unit can be configured with a minimum voltage cutoff that signifies that the battery is fully drained, as well as a programmable current so that the battery drains at precisely the rate you want. I tried to use a 0.5C discharge rate across the board for all the batteries. I made sure to fully charge each battery before testing and set the discharge voltage to 2.75v. During testing the meter can be cycled between voltage, amperage, wattage, and amp-hour, and duration displays and at the end will report the total amp-hours for this battery. It comes with 4 pieces of 200-watt 1 ohm resistors which are the load for the testing.


The results were very interesting, and I’m really glad I got a tester to get quantitative data as some of them were complete bullshit – most notably the Ultrafire batteries (measured at ~1500mAh). Surprisingly, most of the batteries turned out to be legit.


Here is a list of the batteries that actually tested true:

* Sanyo 18650 @ 2600mAh – Tested 2510 mAh
* Panasonic NCR18650A @ 3100mAh – Tested 3010 mAh
* Panasonic NCR18650B @ 3400mAh – Tested 3260 mAh
* A123 20Ah LiFePo4 Pouch – Tested 19.40 Ah
* Unknown (A123?) LiFePo4 40152 @ 15Ah – Tested 14.50 Ah
* Unknown (A123?) LiFePo 38120 @ 10Ah – 9.32 Ah
* Unbranded Lipo @ 36Ah – 35.37 Ah


Of course all of those different batteries had different sizes, weights, densities, and prices. There were 2 main chemistries as well – the 3.2v LiFePo4 and the 3.6v Li-on. In order to really compare apples-to-apples I made a spreadsheet (which you can download here). On the spreadsheet I entered all the critical information – voltage, capacity, price, weight and dimensions. From that information I was able to calculate volume, watt-hours, cost-per-watt-hour, “Volumetric Energy Density” aka watt-hours per liter, and “Gravimetric Energy Density” aka watt-hours per kilogram.

Microsoft Excel-1

I also threw in an entry for the SLA battery that I was replacing. Not surprisingly it had low scores for energy density, but it was a decent contender for cost, which is probably why people use these sorts of batteries – cheap and extremely easy to use (no protection circuitry required usually – just wire and go!)

With this information in hand I was able to narrow down on what sort of battery pack to build. This was my first time around the block, so I wanted to make it very simple. Additionally, I wanted to make sure that I built a very safe battery pack as well. Lithium ion batteries have a tendency to catch on fire if you don’t treat them right and I absolutely do not want that to happen to me or my bike. For that reason I leaned towards the LiFePo4 chemistry, which is supposed to be a bit more abuse tolerant than the other chemistries which I really like. Another added benefit on top of all this is that the lifetime cycle count of LiFePo4 batteries is pretty high (~1500 charges before you get to 80% capacity). The downside is that they have a relatively low energy density for the volume and weight. I narrowed it down to the 20Ah pouch or the 15Ah cylinders. The real clincher for me was price – the 15Ah cylinders were about 60% the cost of the 20Ah pouch – the total cost for the batteries and protection circuit ended up being about $100. As this was a complete first stime, I wasn’t even sure it would end up working.

If cost is no concern, and you wanted the best energy density available – your choice is simple. Copy Tesla Motors and use the Panasonic NCR18650B cells which are 3.4Ah in a tiny 18650 package. You’ll need a spot welder and patience to assemble a few hundred of these cells into a pack, but you’ll end up with a huge amount of energy in a small package. The next project I’m going to attempt is a 100Ah bike, and this is the battery I’ll be looking to for that.


Last, but not least there are some other things that need to be used for building a Li-on battery pack. You need a charger and a protection board and ideally some way to balance the cells. I opted for a simple CV/CC charger and a protection board with built-in balancing. That means it is simple to connect up the charger (2 wires, one connector), and the protection board will handle all the overvoltage/undervoltage/overcurrent/balancing issues. Both of these cost about $20-$30 on Taobao.


The protection board I chose has some nice features – a nicely heatsinked and protective aluminum case, separate connections for battery load and charging, as well as LED indicators for the balance charging. All in a nice little package with simple wiring instructions that even though they were in Chinese were very easy to understand.


Check out the next step, building and installing the battery pack.