Electronic – wake up a sleeping Li ion battery pack without dismantling it

Lots of questions! Instead of just pointing you to a certain IC or battery, I will try to answer your questions so that you can understand the concepts and pick or build your own battery.

1. What precautions do I need to take so that there are no explosions? Certainly something that is possible with lithium ion batteries!

   A) Do not overheat. You can find charging systems with thermistors that monitor the temperature of your battery.

   B) Do not over discharge. Lithium ion should not be discharged below 3 volts, but 3.3 volts is a better goal to aim for. You Over discharging is more likely to cause the battery to lose capacity than explode, but best to keep over 3 volts regardless.

   C) Do not over charge. Very important not to charge over 4.2 volts.

   D) Do not short circuit. Allowing the positive of the battery to come in contact with the negative for longer than a few milliseconds will likely cause a fire. Usually the short circuits are accidental and there are circuits that will protect the battery if short circuit occurs

   E) Do not discharge the battery at a rate faster than the battery is rated for. Again there exist protection cicuirts that will protect against this.

   F) Cells in series must be balanced. Simply applying a current to the battery pack might result in some of the cells charging at slightly different rates leading to imbalance. As a result it is a very good idea to monitor each cell as it is being charged. There exist IC that will do this.

2. How does the use of boost and buck converters affect battery life? Boost and buck converters are quite useful, but you will lose some battery life due inefficiencies. Most of the boost and buck converters that I have seen have about an 85% efficiency although occasionally I have seen 95% efficiency claimed. Thus you lose 5-15% battery life due to inefficient conversion.

Don't forget that a fully charged 3 cell lithium ion battery is 12.6 volts - is this too high?

Good luck.

Alrighty, so there will be the answer that we can extract from the information on the datasheet/discharge curve chart. That will give you a fairly good idea of what to expect. Or if you want more that than you can run tests on your specific cells to find out. But I think you are just needing a more of a ballpark answer. Ballpark is the only thing that is really of any value, unless you know exactly what temperature the cells will be at all the time as well as some other variables.

So, li-ion cells have a non-linear discharge curve. If we look up the discharge curve for your particular cells it will look something like this one.

Now you can understand why you would need to know the current draw if you wanted more than a ballpark guess; the capacity in W-H changes with load because different loads cause different voltage sags.

So anyways, lets assume we are talking about 2A draw only. Look at the 2A curve (the black line with a negative slope) You are saying you wanted to set the cutoff at 3v. Well since we are ballparking, we can notice that the discharge curve for the region you are interested in is close enough to linear that you could just easily calculate the area under the curve without an integral.

If you aren't sure if you need the capacity in WH or just AH well ah is easily found by just finding the capacity that relates to the cutoff voltage you want in the discharge curve.

If you do want WH though: If we say the voltage range is 4.1 to 3.1, and the capacity is 3.2ah well then you get a rectangle with a triangle on top of it that you can find the area of and know your Watt Hours. WH = (3.2*3.1) + 1/2(3.2* (4.1-3.1))