Knowledge base – Batteries for e-bikes

Before you start to build an e-bike or purchase a complete bicycle the first thing you will see is the question of battery, because that is the one item, which will determinate the power and performance of your bike. I hope you will not run flat before you read this short summary… 😉

First there was the lead-acid battery and you were quite happy to charge it 200 times before you throw it away. That was very heavy but cheap too, so the 60-70% of the old ebikes which are still running using that technology. Later on we saw nickel batteries, which were able to store twice so much energy as the old ones. And we are at present times, when you can see only lithium batteries. From those I would mention 2 types: LiMn, LifePo4. If I use the LiMn as the reference, the battery of LiFePo4 with the same weight would store 20% more energy. The lifetime of the battery is arround 500-1000 cycles.

So, “Li Ion battery for all ebikes”! That is not so easy if I count with the cost of the battery too.The LifePO4 is a little bit more expensive as the LiMn, and more dangerous because of the chemistry of the cells. So if your preference is the low weight and the money doesn’t matter, than your choice should be the LiMn. That is the type  BionX is using in their new models of 2011. But it’s really expensive….

Ok, now we have the battery type, but how can we know, what is the distance we could ride with it? Well, I’m not able to answer this question, as this depends from so many parameters (like weight, path, bike, slope…),  but I try to help you out with some estimation. If you want to know that more detailed, go to ebikes.ca (or if you are really interested try this one: kreuzeutter ;)), but I think there should be a more easier way to translate the technical parameters to driving distance. We know, that the most important factor is the Wh of the battery, which you can calculate, if you multiply the Voltage of the battery with the Ah parameter. So if you have a 36V system and you have a 12 Ah battery than this number is 432 Wh. If you have a battery with twice so much Wh, than your driving distance will be doubled too. If we would have a constant number, which could tell us how much Wh energy we use per km, we would have the distance immediately.

I’m happy to announce, that with the support of the statistic science and extraenergy.org we have this! :p This last one is a non-profit organisation, which test ebikes every year and publish they data. Every year they review around 20 bikes on the circuit “city”, “tour” and ” hill” simulation. Here you have the 2010 track:

2010 test track

And here you can see and overview of some bikes, which includes the Wh and the distance until the battery ran flat on the different test circumstances:

Distances in km

So let’s make some averages and we have our result!

  • in the city with a lot of stops and acceleration if you ride with the speed of 15 km/h you use 13 Wh/km
  • if you go on tour with 25 km/h your usage is 7Wh/km
  • if you go to hills up and down with around 20 km/h average speed (around 5% slope) than this number is 15 Wh/km
So let’s go back to our example battery; if we would use it in city our distance with pedaling is 33 km, on tour 62km, and 29 km in the hills.
We have one more interesting question, what will be the weight of the battery? As I mentioned earlier, the usual types are the LiMn, the LifePo4 and Li Ion nowadays. There is an average energy density per battery type, which is  85 Wh/kg, 100 Wh/kg, 200 Wh/kg in the sequence, so the weight in our example is 5, 4.3 and 2.2 kg… And on the top of that if you have a battery assembled on the rack than you will have additional one and a half kilo….
What did I forget? Maybe the charging time which is not so important as this is between 3..6 hours (in case of LiMN and LiIon battery), and if you charge during the night, you will have fully operational battery in the morning.
Maybe I should mention the regenerative braking. With that if you brake the motor will start charging your battery, and the driving distance will increase by 10-15%. For that you need a far more complex electronic, so experts are saying, that it not worth to build in. Despite that we still like this function in the BionX models! 🙂