Build a 1S or 3.7 Volt Lithium Ion Battery Pack

After experimenting with many different cell configurations, I found a 3.7 volt pack (one without serial cell configurations) to be exceedingly simple and inexpensive.  It's more simple because you don't have to worry about different cells draining at different rates. In a 4S pack for example, one cell might drain long before the others, leaving a lot of unused energy in the other cells.  You don't have to balance charge with 1S packs.  It is cheaper because 1S PCBs are much cheaper.  The only drawback to a 1S pack is that the voltage range (2.5 to 4.2 volts) isn't as useful as higher voltages.  In spite of this, I have found this pack to be very useful and in this instructable I will show you how to make a pack and what to do with it.  

This project assumes you already know how to solder and splice wires together.  Also, lithium batteries can be dangerous if not handled properly.  Go to this page of my other instructable for a detailed warning of everything which could go wrong.  

Step 1 PCB

This is the PCB I used from batteryspace.  Even the one rated for the highest current is only $2.50.  Compare that to the cheapest 5S PCB at $21.  Wiring it up is very simple.  Leads for the batteries are on one side and leads for charging and for drawing power (sharing the same contact point) are on the other side.  I used European style terminal blocks to ensure I would only have to solder onto the PCB once.  

Because Lithium ion batteries are so volatile, a PCB must be used to protect the batteries.  Lithium-ion cells can be ruined or even catch fire if they are over charged or if the current limit were exceeded.  Their most useful function in my opinion it to prevent the cell voltage from dropping below 2.5 volts.  Lithium-ion cells would be ruined if they were allowed to discharge below 2.5 volts.  The PCB shuts off the circuit once that lower limit is reached.  

If you short the pack the PCB will close the circuit and will not turn back on unless you fix the short and plug the charger into mains power.  Also, the first time you put the pack together it may not work until you apply 4.2 volts to the charge/power leads on the PCB.  This may be necessary even if the batteries are most of the way charged.    Applying 4.2 volts will turn the PCB on.  

The PCB protects against shorts coming out of the PCB itself, but it is still possible to cause a short from the wires coming directly out of the cells.  The PCB would not save the pack in that instance.  

Comments

[dasimpson1981]

if these cells are of about 2200mah and you have all 20 cells connected i work out more then 12 amps

[dasimpson1981]

how many amp hours is a single cell in your unit was just woundering exactly how many amp hours you have there