Posts Tagged ‘ah’

LiPo – Batteries Terms Explained

Electronics, technology | 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.