Let's Make Robots!

Li-Po batteries - the worlds smallest fact box


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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tucker98's picture

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

fritsl's picture

Ok, Warning: There are kids on the site! 

OddBot's picture

I think he means "WARNING: do not short circuit or give to a Crazy Dane!"

Either that or:-

"WARNING: choking hazzard for young children who can't spell!"   ;-p

tucker98's picture

lol thats not what i ment

TinHead's picture


I got my BAD LiPo replaced in time ;) 

Booyakaa!

 

OddBot's picture

I'm surprised you forgot the safety tip about not shorting a LiPo out on a ball bearing motor unless you are wearing full leather welding gear including mask, gloves and boots.

webmaster's picture

- but nothing happend! Not a problem.

Ro-Bot-X's picture

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.

webmaster's picture

Very good point, I forgot that!

I'll include it in the text, thanks.

TinHead's picture

How long can you store one? 

Or better said if you charge it at 3.7 how long until you need to refresh? 

The one I found dead is less than a month old ... add to that storage at the vendor..