We recently designed a barcode scanner to use with our online store. To power the scanner, we decided to use 18650 size lithium ion batteries as they are high capacity for their weight and cheap to buy. This was the first project that has used li-ion batteries so we looked on ebay for some that would be suitable and found some UltraFire batteries that were supposedly rated at 5Ah.
When the batteries arrived, we put then in the scanner and tested it to see how long they would last. The barcode scanner draws around 250mA so with a 5Ah battery it should have provided 20 hours of life. 3 hours later the batteries were discharged. Either the barcode scanner was drawing a lot more power than it should have been or the batteries were not carrying their rated capacities. We decided to test the batteries to see what their actual capacity was.
To test the batteries, we used a Maynuo M9812 electronic load which as a battery test option in the PC software. The load was set to a constant current of 1A with a cut-off voltage of 2.7V. We had four UltraFire BRC 18650 5000mAh batteries to test as well as two Samsung ICR18650-26F batteries which are rated at 2.6Ah. Each battery fully charged before being connected to the electronic load and a measurement made of the time it took to fully discharge the battery. From that the capacity of each battery was calculated and plotted on the chart below.
As you can see all four UltraFire batteries were well below their rated 5000mAh with the lowest at only 643mAh and the highest at 930mAh. The two Samsung batteries held a capacity of just over 2.5Ah so they were close to their rated capacities.
The lesson to be learned from this test? Don’t buy cheap batteries from ebay.