LiPo – Batteries Terms Explained

Posted by admin February 28th, 2011

I came across a great article on LiPo batteries on the RC Helicopter web site.  These guys have been using LiPo batteries with a lot of sucess.   Click here to read the full article.  Below are the highlights translated into values which are more to our using.

Below are some of the values you need to look at when designing a pack.

148 volt LiPo pack = 40 cells x 3.7 volts

Ah – Capacity indicates how much power the battery pack can hold and is indicated in amp hours (Ah).

C –How fast a battery can be charged or discharged. A battery with a discharge rating of 2C would mean you could discharge it at a rate 2 times more than the capacity of the pack, a 3C pack = 3 times more, a 20C pack = 20 times more, and so on.

Let’s use our 100 Ah battery as an example; if it was rated at 2C that would mean you could pull a maximum sustained load up to 200 amps or 200 amps off that battery (2 x 100 amps = 200 amps). The higher the C value the higher the cost usually.  What this means to an EV though is how much power you can effectively use.  If the C rating is low, you are going to have to use more cells to increase the voltage so that you use less amperage.  My car can easily pull up to 1000 amps at 168 volts if I am racing.  Taking off from a stop at normal driving speeds can easily require 200 amps.  Many cells will allow you to pull at a higher C rating for a short period of time without damaging the battery.  If the cell is drained too fast, this usually results in heat which can damage the cell.

Internal Resistance – Most decent higher discharge rated LiPo cells will have roughly 2 to 6 milliohms (0.002 to 0.006 ohms) of internal resistance when brand new. To calculate the total internal resistance of a series wired pack, you would then add these numbers together so a 148 cell pack with each cell having 4 milliohms of resistance will show a total internal resistance of about 592milliohms (0.592 ohms).

As packs age, the internal resistance goes up and the warmer they run. Lower discharge rated packs will generally have higher internal resistance readings.

One Response

  1. OldNick says:

    Just want to comment here. I found this site because O was looking at LiPo details

    I reckon you have made some incorrect and possibly damaging or dangerous statements above.

    “C -How fast a battery can be charged or discharged. A battery with a discharge rating of 2C would mean you could discharge it at a rate 2 times more than the capacity of the pack, a 3C pack = 3 times more, a 20C pack = 20 times more, and so on.”

    The C rating is how fast the battery can DIScharge, not be charged. Charging is usually 1 (1 hour charging time),2…. maybe 5C. There m ay be better, but to say the C rate (discharge) is the charge rate is dodgy.

    “The higher the C value the higher the cost usually. What this means to an EV though is how much power you can effectively use. If the C rating is low, you are going to have to use more cells to increase the voltage so that you use less amperage.”

    If the C rating is low, then the _last_ thing you want to do is increase the volts “to use less amperage”:
    –In fact increasing the voltage will _increase_ the amperage (which is actually what you want through the motor to make it faster). While power is a product of volts and amps, increasing volts also increases amps over a given load. You get less amps from more volts and the same power only if the load draws less current at the new volts. Only if things are balanced in that regard, then you would have the same power at less amps.
    — The C rate of batteries is not just about what they can provide the load, due to internal resistance. If you have a C rate of say 20, then that battery will only tolerate a discharge rate of 20 * its AH capacity. It will suffer and maybe burn if you extract much more. LiPos are notorious for this, although now much better protected. That is what would happen if you added more batteries (Volts) without altering the load, to a point where the C rating of the battery was exceeded. This is still caused by internal resistance, which is creating heat in the battery as current flows.
    –If you want to increase the current from batteries that have insufficient discharge rate, you need to leave the volts as they are. The paralleled batteries will share the current, not get hot, and provide easier current to the load. This results in more power for the same load.

    Nick