One of the most important and equally annoying parts of any electric vehicle is the batteries. Every other component in an EV (which is basically the motor and… that’s about it) is on-par with it’s gas counterpart or better from a safety, simplicity etc standpoint. But when it comes to batteries, the entire idea seems to be total shite.

Yes. I know Keanu. Tragic.

There are a couple of reasons for me saying this.

1. Weight: Although lithium-ion batteries are making this situation better, batteries still weigh a lot for the power they provide. A gas tank on a gas kart weighs maybe 30lbs full. The best Li-Ion can do is about 90lbs for less power. Even if it was equal power, that’s still about 3X the weight. This sucks.
2. Recharge Time: This is by far the MOST ANNOYING thing about batteries. The weight I can deal with. Yes my kart might be a bit slower, but whatever. The cost I can deal with, amortizing it over time. Recharge Time however is something I can’t do much about. Once the batteries run out, it will take 4 hours to recharge them to 90% ish capacity. Filling up a gas tank takes about a minute. So that’s about 240X faster than batteries. Yes. You can charge the batteries with a higher current, but then you risk slowly degrading their lifespan. This sucks immensely.

That aside, I had two options to consider when buying batteries – Lead Acid or Lithium Ion. As with most things in life, these choice here was weight versus cost (or speed versus cost). Lead Acid batteries are heavy, but cheap. Lithium-Ion batteries (LiFePO4)  are much lighter, but are much more expensive. I chose the latter. Heavy go-karts aren’t fun. I want fun.

Once I chose to go the Li-Ion route, there are a few more things to consider while narrowing down your options. The basics.

Cells

Li-Ion batteries are usually sold in cells which are strung together to make batteries. This is not much different from how any other battery works. In fact, that’s what the word battery actually means – a collection of cells. It’s just that when you buy Li-Ion cells, they are marketed by the cell usually.

Cell Voltage

Each cell has a nominal voltage of 3.2V-3.6V depending on the manufacturer. Stringing cells in series adds their voltages to get you something that can run your motor. My motor for example is happy when you feed it around 72V (things don’t have to be too exact. Over-volting or under-volting a motor by a small amount – 5V to 10V – is OK). This means I need about 23 cells, assuming each cell is rated for 3.2V.

Cell Capacity

Each cell also has a capacity rating. Capacity ratings are measured in Amp-Hours or simply Ah. This rating tells you how many Amps the battery can source and still last an hour. For example, a 40Ah battery will last an hour if you continuously draw 40A from it over said hour. If you only draw 20A, the battery will theoretically last 2 hours. This has caveats of course. If you discharge the battery at 80A, you might actually get 20min, instead of 30min, out of it. Always add in some overhead when you plan your numbers. Money, Batteries, Women, whatever.

C-Rating

It might be tempting to get cheaper 40Ah cells and discharge them at 400A and make them last for 6 minutes. This however, might not be very good for the cells. Each cell also comes with a C-rating. The C-rating for a battery tells you roughly how fast you can discharge it. The C rating translates to Amps roughly as A = C * Capacity. The 40Ah cells I am currently looking at have a C rating of 3C. This means that the maximum continuous discharge for each cell is 40 * 3 = 120A. It also has a burst C rating of 12C. This means a burst discharge of 12 * 40 = 480A. This means that when used on my go-kart, I can thrash the cells for a short amount of time (10 seconds maybe) at 480A.

Given this information, my current choice is http://currentevtech.com/Lithium-Batteries/CALB/CALB-40ah-cell-p28.html.

Before you buy your batteries however, you also have to worry about straps. Li-Ion cells expand because of gasses created inside when they are put under load. To keep these cells from bloating to their deaths, they have to be kept under pressure using straps.

(Stolen from http://adventure-ev.com/?p=457)

MAKE SURE your battery supplier also gives you straps and related hardware. Some good ones also offer to send you batteries pre-strapped. I’ll have more information here once I actually find a good vendor and buy some batteries.

Well, this has been long as hell. Hope this is useful to someone.

And no. Still can’t do much about recharge time. Sucks Balls.