Tuesday, 20 March 2018

18650 Li-Ion Cell - The Iconic Battery!

18650 - This is the most common standard for a Li-Ion battery (or Cell in correct technical terms).  In your Laptop, usually multiple 18650 cells are connected in series to provide a higher voltage.  A 2-cell battery (Multiple cells connected together is technically called a battery), will have 2 cells in series and produce 7.2 to 7.4 Volts.

The Power Bank that we talked about in my earlier post usually comprises or 1 or more 18650 cells - But these are connected in parallel.

There are wild claims regarding the capacity of power banks and 18650 cells as well. So lets make it clear here - The Size of a 18650 cell is 18x65mm.  In that space there is little room for wild swings in capacity - The Chemistry is nearly the same.  So we have real capacities ranging from 2,000mAH to 3,100mAH.
Cell capacities are Colour coded (You can check the details in one of the links below).  The Purple Colour Cell manufactured by Samsung is one very commonly available battery and has a capacity of 2,600mAH - You can take this as a benchmark for checking most of your products.

Hence if you have a Power Bank with 2600mAH it will have one cell.
For 2 Cells you can get a capacity of 5200mAH
The 10,400mAH Power bank will have 4 cells in parallel.

In Parallel - the cells will have the same voltage of 3.6 to 3.7 V - They have a full charge voltage of 4.2 to 4.3V.  The Capacity will add up according to the number of cells.

Note that In series connected batteries like in your laptop the capacity will remain same but voltage will add up.  Another important aspect is that if you have to use multiple cells in series, then a BMS or Battery Management System is usually used to prevent a single cell in the series from getting overcharged or undercharged.  Such a system monitors each individual cell in a battery with multiple cells in series.

A note about your power bank - The Input as well as output of a power bank is 5V - This is standardized as per USB standard.  The Battery Voltage is upconverted for the output using a DC-DC converter and for charging a special IC takes care of providing the correct Voltage and Current to charge the battery.

Links for further reading;
1) Battery Chemistry Explained
2) How to Identify 18650 cell capacity by Colour
3) Understanding Tesla's Lithium Ion Battery

Thursday, 15 March 2018

USB Meter - For Measuring Capacity of a USB Power Bank & Other 5V Devices

You have bought a power bank and are wondering if it really measures up to the advertised Capacity?

Check with this Power Meter.
USB Meter

  • You can connect it to any USB Power Source like your Power Bank.
  • It will Show you the Output Voltage of the Power Bank - This should be 5V
  • On the USB Female side you connect your Load.
  • You can then see the Current that the Device Consumes.
  • Leave it on for a while and it can give you an indication of the Power Consumed.  This is also the Power delivered by the source (Could be your Power Bank)

It measures Voltage & Current - Power = Voltage x Current (For DC Voltages like this one)
The Total Energy that can be delivered by your Power Bank = The Capacity of your Power Bank.
Energy (Or Capacity) is the ability for the Power Bank to give X Watts for T Time before it runs out of juice.
Hence TIME is the factor that comes into play to measure the Total Energy that can be delivered by your Power Bank or The Capacity of your Power Bank

Capacity = Total Energy that your Power Bank can deliver = Watts x Time = V x I x Time

Since a Power Bank is usually 5V it is sometimes mentioned as AH - = I x Time (Assuming Voltage is fixed at 5V)

To measure the capacity you can do the following;

  1. Charge your Power Bank till it is full  - Indicated by the LED or display on your Power Bank.
  2. Disconnect it from the charging source
  3. Connect the USB Meter to the Power Bank - Ensure that the AH display reads 0 - else press the button for a few seconds to reset the reading to Zero).
  4. On the output side connect a simple USB LED which draws (These usually consume between 0.5 to 1W - the exact value does not matter)
  5. You can see the current drawn by the device (LED).
  6. Leave it on until the Power Bank exhausts its energy/capacity.
  7. Check the AH directly on the display. - After reading you can reset it back to zero for the next reading - Some Meters can store multiple readings and cycle through them numbered as 1,2...
For Video details check this youtube video