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.
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.