In this topic, I will be talking about my experience working with high power batteries and what I think to be the best cells and their configuration for an electric longboard setup. Previously, I have worked with lead acid, nickel–metal hydride, LiPo and Li-ion cells. All of the times, my application required high current. If you are unfamiliar with these batteries, let me break them down into their pros/cons.
- Good Shelf Life
- Easy Charging
- Good Amperage
- Decent Capacity
- Good Amperage
- Difficult to charge
- Low Life
- Self Discharging
- Huge Amperage
- Large Capacity
- Inexpensive (Typically)
- Explosive if ANYTHING goes wrong
- Seriously, like a thermite bomb!
- Large Capacities
- Fairly Lightweight
- Safe (Not All)
- Long Life Cycle
- Can't Pull Lots of Amps
- Packs Require Balancing
So, these are the four main groups of batteries that you can buy, and I've worked with all of them in my projects. For each project, I looked at what the requirements were, and made a list like I did above to find what would work best. For example, in my speaker project, I needed a large capacity, and decent current capabilities. I was originally going to use LiPO, however, I was not fond of putting two large potential thermite bombs in a wooden box next to $400 of electronics so I opted to use Li-Ion cells instead. One of the most common Li-Ion cells is called the 18650. It's called this because it is 18mm in diameter, and 65mm long. Makes sense. These can be charged up to 4.2 volts and typically have a capacity of 2.2-3.5Ah. They also typically have a max discharge rating of between 2-10A and range from $2-$5 a cell. In order to get the capacity and current my speaker needed, I used 28 cells in a 7s 4p (7 series, 4 parallel) configuration.
Enough backstory, lets look at the electric longboard scenario and list the requirements. To power and electric longboard effectively, you'll need a battery that is:
- Safe (A must especially since you and others are at risk)
- Lightweight (Don't want to carry a 50lb board)
- Small (To Keep the Slim Profile of the Longboard)
- Large Capacity (For a decent range)
- Powerful (Can Supply a TON of Amps)
So, with this in mind, lets go down the list and start crossing things out.
Safe: This automatically rules out the LiPOs. I personally think that anyone using LiPOs for anything other than RC vehicles are just asking for trouble or are ignorant to the risks.
Lightweight + Small: Say goodbye to lead acid. In order to get the voltage needed, you would be looking at 40lbs of batteries minimum.
Large Capacity: Although NiMH are decent, they have short life spans and their cell voltage is low. Who wants to change their batteries every 200 rides?
Powerful: Now this is where things start getting tricky as the only thing we have left, is Li-Ion cells. In order to get the voltage necessary, we must wire cells in series (typically 10-12). But most 18650 cells can only provide between 5-10A. Even the best cells only do 20A and at the cost of degrading their life span significantly. So, what must be done is packs must be put in parallel to distribute the current over the cells. Realistically, unless you want to have lame acceleration and zero hill climbing ability, you will need at least 3 packs in parallel for use on a longboard, preferably 4 packs.
Well, wait a second. How is it that Boosted Boards are able to have 2000 watts of motors on their board and their battery pack is so small. Well, the answer is a123. In the early 2000s, MIT researchers developed some nano scale materials that found their way into Li-Ion batteries. There are several different types of Li-Ion chemistries. One of the most stable and safest chemistries is called Lithium Iron Phosphate or LiFePO4 for short. This is actually the same chemistry used in Teslas batteries. Anyway, the guys over at a123 were able to apply this new nano tech to this battery chemistry and what it allowed for was HUGE amperage. I'm talking 70A continuously and 120A peak! There are however some trade offs. For one, the cells are 26650's and as you guessed it, they are 26mm in diameter. So they are a bit larger. They also have relatively low capacities of 2.4 Ah and their fully charge voltage is 3.5v instead of 4.2. But these are pretty good trade offs for what you get. Oh yeah, and almost forgot to mention the price, they are $13 a cell! Ouch! Boosted uses 12 of these cells in series to supply 40v to their two 30A motor controllers. This leaves 10A of headroom for the cells. A full charge will get you about 5 miles on their dual plus variant so 2.5Ah is nothing to sneeze at.
So back to the original task, what cell should we use? Well, it really depends what you want to do. If you just want to cruse around with very little torque for long distances, then 3 or 4 packs of 18650's should do the trick. However, if you want to build a dragster with decent range and unprecedented acceleration, then you'll want to pick up some of a123's 26650's. Perhaps even put a few of those packs in parallel and see what you can do!
Well that's about all I have to say on this matter. If you have any comments or questions, feel free to reply and I'll get back to you!
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- Josh Hess
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- Joined: Sun May 24, 2015 6:00 pm
- Location: Orlando, FL
- Areas of Knowledge: Electronics, Product Design, Manufacturing
- School/University: UCF
Science may set limits to knowledge, but should not set limits to imagination.
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