It appears this may be the most complex part of this project. Thus far everything is very much plug and play. That is, each major component be it the motor, controller etc can be attached and plugged directly to each other.
This will not be the case for the LiPo batteries I have decided to use.
The batteries I am using have their origins in electric RC (remote control) cars, planes and helicopters. As a result of this they are extremely light weight, have high capacity’s and can take large current draws of up to 30C **1. This makes them perfect for ebikes and as a result have become quite popular.
However, like all good things in life there is also a downside. That downside is that for normal RC applications only one or two battery packs are used at at time. As such, most chargers can only charge 1 to 4 packs at a time,and quite slowly at that. With the 4pack chargers costing upwards of $150 USD this becomes a very expensive operation quite quickly, especially when I would be needing two of them to charge a possible 8 packs. Then there is also the hassle of having to disconnects 8 batteries, one at a time and plug in both charging and balancing leads for each pack and then reconnecting them after.**2.
So whats the solution thats quick, safe, reasonably priced and easy to connect? Well this may be one option.
Thanks to a couple of very avid endless-sphere forum members, there now exists a product that allows you to charge up to 24 cells at a time. They have a website which can be found here. With 8 separate packs going to be used on the trike and 6 cells in each (a total of 48 cells) only two of these charges would be required. Whats more, they are well priced and light weight which means they can be connected and left attached to the bike with only a external power supply needed for recharging. This means only 1 plug is needed to charge.
Again, there are some downsides. The most obvious is that this charger needs to be built from the board level up.
This is both a good and bad thing. It is good in that I can place the finished products into their own boxes of my choice. However it adds complexity and allows room for error when mounting components. Still this shouldnt be anything I cant handle as it comes with a detailed set of instructions.
Now, the reason these boards is required is that lithium batteries require special care. Every one of the 48 cells can never be charged above a absolute maximum of 4.2V per cell or dire consequences will follow. eg: burnt batteries, burnt trike or worst case, a burnt house.
Once completed, the board is connected to the packs and to a power supply. The power supply must be of greater voltage than the combined sum of all the cells at full charge. So in this case thats 4V max per cell by 24 totaling 96V. This is a rather high voltage for an off the shelf regulated power supply and to make matters a high current is also needed to charge them in a reasonable amount of time.
As I said, this will be the most complex part of the whole project.
How and if I can use these boards is still something I am actively researching.
**1: 1C refers to discharging the battery completely in 1 hour. So a 30C discharge would completely drain a battery in 2 minuets.
**2: Each battery pack contains 6 cells in series. If they were charged in series then individual cells may become “out of balance” where they are above or below the desired voltage. If they become too far out they can vent “with flame”. Not good. To get around this problem each battery pack is wired with balancing leads. These leads just connect to each cell and allow them to be charged individuality.