Choosing the right battery for your e-bike. The “C” factor.

CBA 4

Because I have been a very good boy, Santa brought me a great present for Xmas, a professional computerized  battery discharger: West Mountain Radio – CBA 4.

I made some test on my cells, and here below there are some graphs that explain how batteries are affected by heavy loads.

These are real tests and not some data taken from a data sheet.

All the test in this page have been performed at an average ambient temperature of 21°C.

In the next two graphs the same battery was discharged a 1C, then recharged and discharged at 2C. These cells are rated for discharge at 1C continuous with occasional 2C short burst.

The battery is a Li-Co 18650 Panasonic (used in laptop batteries) 1S2P 4400mAh.

The vertical red line represent 75% of the nominal capacity. The cut-off was set to 2.8v to avoid any damage to the cells.

Graph Voltage vs Ah

LiCo_1C_vs_2C_Ah

Graph Voltage vs Time

LiCo_1C_vs_2C_Min

In the next two graphs the same battery was discharged a 1C, then recharged and discharged at 3CThese cells can support a 3-4C discharge continuous with occasional 10C short burst.

Those are Sony Konion cells,  LiMn cells, 18650 format 3.7v 1500mAh for each cell.

The cells are connected in parallel, so the battery is a 1s2p 3.7v 3000mAh nominal.

Graph Voltage vs Ah

LiMn_1C_vs_3C_Ah

Graph Voltage vs Time

LiMn_1C_vs_3C_Min

As you can see from the graphs the Sony Konion cells are more suitable for an electric vehicle. For the same size (cell format 18650) they store less energy compared to the laptop cells (1500mAh vs 2200mAh) but can deliver more current.

In other words LiCo laptop cells can still be used for electric bikes but have to be discharged at less than 1C to avoid damage  and shortening dramatically the cycle life of the battery pack.This means building a bigger and heavier pack compared to cells with high discharge rate.

Batteries with a high continuous discharge rate can be used to build small capacity packs that can deliver high current.

It’s always best to choose the battery according to the specifications of the vehicle (Amps continuous and peak) and needs of the user (range for example).

Advertisements

2 thoughts on “Choosing the right battery for your e-bike. The “C” factor.

  1. Thank you for this information, I adore these graph because is the only way to check how real are the published datasheet the company shows on their site to convince us to buy!!

    I suppose this test has been done at 25°,, I sure near 0° you will see the peformance fall down.

    • You are welcome Ccriss!
      I just wanted to share this info because people that are not familiar with batteries just don’t consider/are not aware if the importance of the “C” rate.
      Moreover especially laptop cells (1C continuous discharge max) are very expensive (it’s not uncommon a price of 10USD per cell). It would be a very bad surprise to invest a lot of money for those cells and then ruin the pack after a few discharges because those cells can’t cope with the load.

      Yes the tests were made at 21°C on average, thank you for reminding me 🙂

      Maybe I will make discharge test at low temperature just to see how the cells behave!

Leave a Reply

Fill in your details below or click an icon to log in:

WordPress.com Logo

You are commenting using your WordPress.com account. Log Out / Change )

Twitter picture

You are commenting using your Twitter account. Log Out / Change )

Facebook photo

You are commenting using your Facebook account. Log Out / Change )

Google+ photo

You are commenting using your Google+ account. Log Out / Change )

Connecting to %s