Li-ion Batterys

alband

Senior Member
I'm thinking about using 2 li-ion batteries for a battery charger for my iPod. Because li-ion's are usual 3.7V I'm going use a step-up IC, that fine. But what about when I need to charge them? They would be wired in parallel. Would an ordinary single cell charger work or would I need a two cell charger?
Thanks in advance.
 

MBrej

Member
Its generally not a good idea to put two li-ion cells in parallel, you can do it but you must be careful. Wouldnt it be easier to buy a single battery of double the capacity of the two singles?

Anyway, if you do put two in parallel you only need a single cell charger. As far as putting them in parallel in the first place, look on google :) Some possible good sources:

link
link

Matt
 

alband

Senior Member
They are the biggest size possible I can get hold of but each would only give about an hour.
This I think is where PICAXE comes in.
Would It be possible to get a PICAXE to sample them see if one is more that 0.5V above the other (maths on a PICAXE is tricky but can be done) then if it is only use the higher one until it is the same.
The trick would be getting it to use battery 1 as VDD and sample battery 2, then use battery 2 as VDD and sample 1.
 

goom

Senior Member
Why not draw from only one battery initially, then take it out of circuit and connect the other one as soon as it drops to the minimum safe voltage (2.7V?). Relays (mechanical or solid state) or MOSFETS would likely be appropriate. A decent sized capacitor om the power supply would help to smooth out the transition.
With 2 batteries in parallel, there will be some danger of a reverse current in one of them, which will irrevocably destroy it.
Comparing the two batteries may not work too well. If they are connected in parallel a 0.5V difference will likely cause a reverse current in one of them. If one is connected, and one not, then the connected one's voltage will be lower as a result of the voltage drop due to internal resistance.
Charging in parallel does not seem like a great idea since each pack could have a different charge initially. The charging may terminate prematurely, or one pack could be overcharged, resulting in damage, or perhaps even overheating and a fire. Li-ion's are not very tolerant to abuse.
 

alband

Senior Member
What about this;

2 08M's each working off one battery and measuring the other.
Connect the negatives.
Each 08M sends the % voltage of the other battery to the other chip. (readadc only gives a percentage of the VDD with 100% represented as 255)
Perhaps use readadc10 for accuracy
When a PICAXE discovers its battery (recieved number) is lower than the other one it stops supplying power to the main circuit untill the batteries match.

Just how to control the on off, I don't want to use mechanical relays or any that can be triggered if knocked.
 

BeanieBots

Moderator
They are the biggest size possible I can get hold of but each would only give about an hour.
How big do you want them?
10Ahr from here:-
http://www.batteryspace.com/index.asp?PageAction=VIEWPROD&ProdID=4053
Or 50Ahr:eek: from here:-
http://headway-cn.en.alibaba.com/product/50269244/51871098/Lithium_Ion_Batteries/High_Capacity_LiFeO4_Lithium_Ion_Battery.html
That would give 200W for an hour!

Never a good idea to put any type of battery in parallel but can be done with caution. Your idea of two 08Ms would work but a very simple comparitor is all that's needed. The output (plus compliment) could drive two FETs to control the load. Cheaper & easier than just one 08M. A lot less power too.
 
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evanh

Senior Member
Throw us some pictures of these cells and charger. We are talking a Li-Ion single cell charger, right? How big is this biggest size possible? Are you buying them or salvaging them?
 

BeanieBots

Moderator
LM339 or similar.
Personally I don't usually bother with a "comparitor" and simply use an op-amp as a comparitor instead. LM324 has four in a 14pin chip and consumes only 750uA.
 

alband

Senior Member
How would you wire up a compritor? What power supply would it use.

OOOOO! sudden Idea! I'm going to be using a step up IC to change 3.7V into 5V. They convert current into voltage (kind of) they reduce battery life and increase voltage. If I just use two, one for each cell. They'll both give 5V out exactly. I can eaisly combine that with know worrys, diode here and there to be safe.

About pict's I would but won't for your safety. The site I'm going to get them from currently has some kind of malware threat. You can try it at your own risk though: Type "Full river" into google/ first link/ click "lipoly batteries", your in the malware zone now:cool:/ now I think it is the second tab at the top called something like "super safety batteries"/ on of that bateries at the bottom of the list. 30mm wide 40mm or so tall and bout 6mm thick.
 

BeanieBots

Moderator
"They'll both give 5V out exactly".
That's the problem, nothing is ever EXACT.
You'll need the diodes and probably a resitor or two.
 

BeanieBots

Moderator
You'll need to get familiar with it.

Regulators will give you the same problem as batteries only worse because the output impedance of a regulator is much lower than that of a battery.

If reg A is at 5.001v and B is at 5.002v then B will supply ALL and A will supply NO current. Put a resistor in series with each and... well you can do the maths.
 

alband

Senior Member
That resistor can be pretty small right? Don't want to go too far bellow 5v.

Right now that getting power out of them is sorted I need to charge the little bl...

Using two voltage regulators they are now wired up as attached.

Should they just need a single cell charger, or two single cell chargers?
 

Attachments

BeanieBots

Moderator
The resistor can be quite small.
What is your current draw?

With resistors fitted, you can use a single charger. (be aware of volt drop)
With diodes fitted as well (as per your drawing) they won't charge at all.

Without diodes, one of the cells will eventually (after quite a long time) almost fully discharge into the other cell. This is because each cell has a slightly different voltage for a specified capacity.
This is a slow process and is not a problem with packs which are charged/discharged quickly but can be disasterous with packs which are left in "storage". ie a few months or more.
 

alband

Senior Member
That is for using the batteries the small black boxes are step up IC's.
For chrging them I would use a charging IC but what kind.
Would I put one, single cell charger accross each battery or one single cell IC accross both?
 

BeanieBots

Moderator
Now you are scaring me.:eek:
If you need to ask so many questions about Li batteries, you shouldn't be playing with them. They are dangerous if not treated properly.
My advice is to do a LOT of research, then, when you've got a better understanding and have chosen a chip, come back to this PICAXE forum.

Sorry if that sounds a bit off, but I don't want to give advice regarding something which can burn your house down when I'm not sure if you will understand the answer. A bit like when somebody asks "how can a PICAXE measure mains voltage?". If they need to ask, they shouldn't be doing it.
 

alband

Senior Member
I've use li-ion before and am fully aware of the dangers. I wouldn't do any of this stuff until I was sure and even then I would test any such circuits outside on a stone.
I've also managed to find a chip to charge a load of li-ion before, install it in a circuit and it worked fine. I've just never used two in parallel before and whenever you search batteries in series and parallel you get kiddy explanations of parallel being mean voltage and series being sum.
The only two good links have been listed in this thread.
 

hippy

Technical Support
Staff member
I'll echo BenaieBots sentiments; Li-Ion are not things you want to be messing with if you do not know what you are doing.

I'm sure what you want can be done but just pulling ideas out the air isn't a sound way of going about it. There are chips which can be used to charge Li-Ion but they have to be used in the configuration they are designed for or you have to know exactly that they will do what you want them to. Just hoping or believing it should work isn't a good idea with batteries which can detonate themselves or ignite in adverse circumstances.

It's very easy to think one can put things in parallel or series and believe "nothing can go wrong", but that's not the case. There's a lot more to it than that, a lot more knowledge required to do it successfully and safely.
 
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