Let's Make Robots!

Li-Po batteries - the worlds smallest fact box

LiPo's are great - and people tend to make them scary. Here's a few tips

First tip about Li-Po batteries: Make sure you are not searching images on lipo.

The net is full of tips and clever in-depth articles on Li-Po batteries, and in future LMR we will have a wiki, and that will definately have a section on the subject.

BUT.. Just because TinHead said that I promised to write a tip on this (and more - which will come .. later) - here is a few things I'd wish someone had told me, so I'd not be that scared and confused.

I am sure I'll get a digit or two wrong here, please hit me, so I can fix, tanks ;)

 

  • A Li-Po is 3.7 Volts. So they say. But it's not. At 80% charge, it's 3.7 Volts, pr cell.
    You charge each cell to 4.2 - or even 4.25 (but never above - never - stick to 4.2) Volt.
    You discharge to .. say 3 volts - but never below that.
    It does not like to be stored - short or long term - at anything but 3.7 volts!

     
  • S and / or P - wtf?
    When you buy a Li-Po, it can have an S number on it. That is telling you how many cells in Series it has.
    So an 11.1V Li-Po is in fact 3 smaller batteries, hooked up to one: 3S. And it delivers not 11.1, but 12.6V - when charged.

    It can also / or have a P number on it. P specifies cells connected in Parallel. So, a 2P pack has 2 cells connected in parallel, and a 3S2P pack has 6 cells, 2 parallel banks of 3 cells connected in series.

    Which results in:
    2P can provide power longer than "no P", but at 3.7Volt
    2S can provide power in the same amount of time as "no P", but at 7.2Volt.
    I really hope that makes sense!
    On top of this, each cell can have more or less milliamps. This all appeared simple before I started writing, not it feels so complex. But if you get the main lines here, you should be able to read the specs on the battery itself from there :)


  • If the battery has more than 1 Cell (in P and/or S), most often it also has a ballancer cable - a smaller wider cable, apart from the fat one that gives the power.
    The ballancer is Ground (Black), and then it's (at 80% charge) 3.7 volts for the first one, 7.4 for the next, and so on.
    You CAN use the ballancer to take out for instance 3.7 to your logic - right from the ballancer, while powering your robot itself with the fat wire. As long as you are not drawing a lot of current on the ballancer - you do not want to drain the cells (too) unevenly.
    The ballancer is used when charging; you are not charging through it, but the charger is learning how much each cell needs to be charged, so the whole battery is in balance. Balance is a nice thing for a Li-Po pack!


  • Larger, and specialized Li-Po's have a C number on them.
    That is how much current they allow to be drawn before exploding.
    Often they have 2 C numbers; One for "Burst", and one for "long boring draw".
    If a LiPo is 6600MaH (Milliampares), and C20, C40 in bursts, it can safely provide (20*6600 mili) or (40*6600) in bursts.


  • When you charge, normally you want to charge at "1C" or less.
    - unless specified different on the pack (some will charge fine at 2 or even 3C, but they are still better off at 1 or less) That basically means that you want to charge at the same Amp that is written on the pack - like 6600 miliamps (in my case - extreme, by the way - not normal, very, very high) Many Li-Po charges will not even be able to charge at more than 5 amps.

 

Apart from that, there's all the info easily available through Google, and by reading strange things like the manuals:

  • Only use Li-Po chargers
  • Always charge and store in fireproof bag or similar
  • Don't eat, etc etc

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Nice write-up Frits!

I have to correct you a bit, S specifies the cells connected in Series, P specifies cells connected in Parallel. So, like you said, a 3S pack has 3 cells connected in series, a 2P pack has 2 cells connected in parallel, and a 3S2P pack has 6 cells, 2 parallel banks of 3 cells connected in series.

Very good point, I forgot that!

I'll include it in the text, thanks.

fritz there are kids on this site you coild worne us abaot that

Ok, Warning: There are kids on the site! 

lol thats not what i ment