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