Lithium Polymer batteries are everywhere nowadays where high amounts of energy must be stored in small compact form factors. So when my friend bought a brand new mountain bike, it came with a Chinese 36V Phylion 11.6Ah 417.6W/h battery pack as sold by akkuzentrum.de
Unfortunately, his bike came with the wrong charger to recharge the battery. He ordered a new one, but by the time it arrived from China, this brand, spanking, new battery had lowered it's charge to a dangerous 2.8V per cell.
For those of you familiar with LiPo batteries, the lower end of the scale is 3.2V. A battery in standby charge, should be around 3.6V and a fully charge battery around 4.2V.
Attaching the battery to the new charger, showed that the battery was too low to be charged.
At this point I would like to say, if you are following along at home and read this blog wishing to save your LiPo that has dropped below it's operating voltage, then you do so at your own risk. If you have no experience with LiPo batteries and with electronics, then please replace the battery now and do not attempt to repeat what I am going to do in this article. LiPo batteries contain HUGE amounts of energy and can explode in your face and burn down half your house. If you are un-sure, stop now. Buy a replacement. On the long term, it will be cheaper than repairing your house!
So can a LiPo, whos voltage has dropped down too low, really be restored back to life? The answer is: well sometimes. Once a LiPo has dropped too far it will begin to gas out and a chemical reaction takes places within the cell that cannot be reversed. If it has gone that far, then it is toast. However, if it is just on the borderline, it can be brought back to life but very, very, slowly and with a lot of care and attention. So it was time to un-box the battery.
I use my universal charger and set it to the Nickel Cadmium Auto Charge mode.
This allows me to set a maximum current of say around 0.5A and the charger does the rest. It ramps slowly up to 0.5A and measures the voltage. Then it ramps down the current again and looks to see if the battery is maintaining the voltage. If not, it slowly repeats the sequence until the battery begins to hold a charge. This allows me to charge the battery, very slowly. After every volt increase, I stop the charger and measure the individual cell voltages using a multimeter.
In the spreadsheet above, you can see the individual cell voltages in column B and later in column F. Column C (and G), show the difference in voltages between the series wired cells. This is the individual cell voltage. Excel then calculates the minimum and maximum cell voltages and calculates the difference. Cell voltages should be around 10mV and should never exceed 20mV.
If, during charging, these cell voltages drift more than 20mV apart, then the attempt to revitalize the battery should be stopped immediately. If it becomes more, the cells work against each other and chemical reactions can occur that produce enormous amounts of energy. If left un-checked the cell will explode in a dramatic way. Once one pops, they all go. You will have sustained flames 2-3 meters high for several minutes, producing noxious gas, smoke and heat.
Do Not Do This At Home Kids! You Have Been Warned!
If the cells are all ok and in tolerance, the battery will come back to life. A good test is to stop charging and see how quickly the voltage falls again. If it falls too quickly, then all hopes are lost. An effective re-vitalization will show stable voltages after charging.
The battery was then re-assembled and read to go. 486 Euros saved!
