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A project log for Making a better motorcycle battery with LiFePo4

Massive problems with even $150 batteries on my motocycle frustrated me to the point to build my own.

Timo BirnscheinTimo Birnschein 02/28/2021 at 05:060 Comments

I have been looking for a better motorcycle battery for a while and was severely disappointed when I came to the realization that good batteries no matter how small are very expensive. Motorcycle batteries definitely count as small in my book - in comparison to, say, boat batteries.

For the DIYer, there are several interesting building blocks out there, some more, some less <del>explosive</del> volatile.

First, I looked at LiPo 18650s. Typical laptop batteries, or in recent years also known as Tesla electric vehicle batteries. The market is huge but so is the danger in case something goes wrong with your battery pack. Overcharging, overdischarging, shorts, overheating, the list is long and result is almost always: fire, a lot of smoke and usually the total capital loss of the device they powered. Apart from the very good power to weight ratio, LiPos are quite terrible to work with. On top of that, the fully charged voltage is too high:

4.2*4 = 16.8 Volts (fully charged 4S)
4.2*3 = 12.6 Volts (fully charged 3S)

Both is incompatible with automotive battery management systems.

Here comes the LiFePo4 battery chemistry. It's a bit different than LiPo in that it usually does not burst into flames at all. Or at least, it is considerably harder to get them into this undesired state. Also, their voltage levels is completely different, making them entirely incompatible with LiPo batteries:

3.65*4 = 14.6 Volts (fully charged 4S) !!
3.2*4 = 12.8 Volts (nominal voltage at 4S) !!

As can be seen from this quick math, four cells in series give almost exactly the same voltage range as normal automotive batteries and can therefore be used with no issue. The difference is 0.1V at the max voltage end but according to their datasheet, LiFePo4 batteries are being tested up 1.5V overcharge and still no fire.

Similar to LiPo batteries, LiFePo4 also requires a balancing circuit. Nowadays, these exist in passive, as before, and active which supposedly a couple percent more efficient as it doesn't burn energy to equalize the cells but transfers the energy instead. We will see how that works and how precise this is but that's a topic for later.

I did some math and calculated the following:

I compared two different sizes, the typical 18650 cell and the bigger, higher capacity, 26650 cell. In theory, both cells work equally fine in the motorcycle environment but there is one thing that stood out that made the difference to me:

Cranking the engine is fine in both cases but leaving the lights on will kill the 18650 battery in less than a work day while the 26650 battery would last much longer in this scenario providing much more buffer for me to notice that I forgot to turn the lights off. And yes, this is extremely easy on some motorcycles.

The next thing I needed to figure out was of the cells even fit into the packaging space of a motorcycle battery. To my big surprise, the answer is yes!

Length, width, and height are no issue. Even wiring and nickel-strips are no problem. Space for the balancer on top of the cells is also available. All I need now is a frame that holds the cells in place and the terminals to allow connecting the 4S6P LiFePo4 battery to my motorcycle.

One last mystery is, though, how the charge controller on the motorcycle will react to the battery and vice versa. I will have to take lots of measurements to ensure the battery is being treated well and never leaves 14.6 to 14.7 volts.