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DIY - How to Make an eBike Battery Pack from old Li-Ion 18650 laptop batteries

DIY - How to Make an eBike Battery Pack  from old Li-Ion 18650 laptop batteries

Batteries and battery packs are fairly expensive.
A laptop battery pack consists of a number of individual 18650 Li-Ion cylindrical battery cells. These cells might look like  standard AA alkaline batteries, but theyare actually slightly longer and wider than an AA battery. These are lithium cells which go by the part number “18650″ (this is usually printed on the side of the cell).

Used lithium cells like these could be still useful It may be  so it may be worth collecting old laptop batteries and harvest the cells for reuse. If one or more cells in one pack  go bad, then the pack cannot supply adequate voltage, hence the the whole pack is effectively »dead«. However, ,many of the cells are still in fine shape, as it usually happens that only one or two cells go bad and the remaining ones are still usable.
Again, It is important to note that 18650s are NOT AA batteries,therefore much more caution is required when working with them! In particular, a damaged 18650 Li-Ion cell can easily short circuit, resulting in an excessively high current , heat, fire, and other damage.


In this demonstration, I’m building an eBike or electric bike Li-Ion battery pack, but these cells could also be used for many other purposes, such as back-up power or for LED flashlights…
I collected several used or “dead” laptop batteries from computer repair service.
I opened the plastic cases and I found the cells which are shown in the photo below. The red cells are from Sanyo, the lavender cells are from Panasonic, and the light blue cells seem to be some older Panasonic cells …
The Li-Po cells wiring of the battery pack is a combination of parallel and serial circuits, in order to providethe right voltage and capacity for the laptop to operate properly.
In the picture above, we can see the cells, which were disassembled/separated. Some are still wired together in groups of 2 or 3, as that alleviates testing, charging and pack assembly.
I then determined which cells were still in good working order. “Good” Li-Ion 18650 cells present a voltage between 2.7V and 4.1V; bad Li-Ion cells read less than 2.7V. Using a digital multimeter, it is easy to identify the cells which are still in decent shape.
For those  particular battery packs, almost half  of the cells were still good.
Then I charged either a group or each individual Li-Ion cell to the full charging voltage of 4.2 volts, and then I let them rest to see how well they could sustain this charge.
I use two types of charging:

·         In the left picture above I use a Li-Ion charger for 4 cells. One can see charging different battery groups of 1,2 and 3 cells. On the charger  Charging voltage, Capacity and time can be seen. This is a better and faster solution.

·         Another way is to charge them separately with a USB Li-Ion charger from eBay. But that takes a lot of time to charge all the old cells. This is a cheaper but time -consuming solution.

It is the best, if all 18650 Li-Ion cells are from the same manufacturer, because they should have nearly the same characteristic and capacity. Li-Ion battery pack from the same Li-Ion cells will lost longer and it will be more reliable.
Once I  confirmed that I did have 30 good Li-Ion cells from the same manufacturer, I had to wire and stack them to get the 3P10S size. The wiring was 3 cells in parallel for capacity and 10 packs of 3 cells in serial to get the desired voltage of Li-Ion battery pack.
I used 20A BMS (Battery Management System) for 10 Li-Ion cells as shown in the picture below.  I also needed some connecting wires and battery pack connectors.
I soldered 3 cells in paralell and 10 packs in serial to get the 3P10S battery pack. I used old solder tabs to connect the cellls together. And I let the + and – wire plug out from the battery pack (picture left below ).
Then I soldered connectors for the BMS and the battery cells. And later red wires from each cell (positive side of  Li-Ion cell) to BMS connector, as shown on the BMS wire diagram below. (The BMS  for Li-Ion battery cells of nominal voltage per cell 3,7V. 4.2V is maximum voltage per cell when the cell is full.)

  •    P-:  means Load or Power negative connection
  •  C-: means charging negative connection

  •   B+: means battery pack positive connection

  •  B-: means battery pack negative connection


Be careful when you connect wires from  each battery cell tothe BMS. On the BMS you must solder contacts: C-, P-, B- and 10 wires to each Li-Ion battery cell.
 As it can be seen from the wiring diagram:
·         B-:  goes directly to  negative pole of the battery pack
·         C-: goes directly to negative pole of the charger
·         B+ and C+ are wired together and have no connetion to the BMS. They goe to positive pole of the battery pack and charging
·         10 wires connecting each positive pole of Li-Ion cell to the BMS
I connected everything together and tested the battery pack for charging and discharging.
After charging had finished I cheked the voltage of every battery cell had to be around 4.2V. And the whole Li-Ion battery pack was to  have a little less then 42V. After one week’s time it should have had the same voltage.
Upon discharging a Li-Ion battery pack one can determine several parameters:

  • Minimal voltage when BMS disconnects Li-Ion battery pack from load. In this case it was 32V.
  •  Maximal voltage when Li-Ion battery pack is fully charged.  In this case it was 42V.
  •  Capacity of  Li-Ion battery pack in Ah or Wh. One must have a Power meter – data analyzer. In my case it was 4Ah.
  •  Determine maximal output current » I« in Amperes.  When high high load is applied to Li-Ion battery pack, the voltage of Li-Ion battery pack goes down until BMS cuts-off  electrical load.  In my case it was 7A. This Li-Ion battery pack is not suitable for eBike or electric bikes because eBikes drain greater current (I want at least 15A).  Afterwards, I had to reconnect the Li-Ion battery pack to load, to get voltage and  get working battery Li-Ion battery pack again.
For my eBike I want a Li-Ion pack from old Li-Ion 18650 laptop batteries, wich can deliver at least 15A to motor load. BMS to support the current of up to 20A.
To meet these requrements I have to make a bigger Li-Ion battery pack  6P10S or get different battery cells. One option worth consideringa areRC punch cells.
Useful links:
18650 Battery Buying Guide (test on all from eBay below $3)
Rechargeable Battery Reviews and Tests
2012 Battery test/review summary    
18650 Battery Tests
Do not copy, cite, or distribute without writen permission of the  copyright ©



Objavljeno: 20. november 2014
Zadnja sprememba: 20. november 2014 ob 14:40

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