Lithium batteries in parallel

[2camjohn]

I have two lithium polymer batteries, each is two 3.7V cells in series.


I want to run one of my projects from two of these in parallel so I have more current capacity.


My question is can you run lithium batteries in parallel like that? How about charging in parallel?


Thanks in advance, normally id just go ahead and do it and see what happens but im a little wary of lithiums.


John

 

[on1aag]

Hi John,

I removed the cover of my laptop and molested the battery pack
a little bit.
So what do you think ?

on1aag.

 

[dknguyen]

The balancing of cells between two LiPo packs is no different than the balancing of cells within an individual pack. Do it, and it'll work fine.

POTENTIALLY USELESS BACKGROUND INFO:
When using multiple LiPo cells (parallel or series), they have to be charged with a special charger just for them that has been calibrated and is accurate. Then they need to be balanced out with a balancer (some chargers are able to balance themselves or support balancer attachements). So LiPo packs often have a special balancing plug that allows access to all individual cells inside a pack that is only used during charging. The charging is done through the primary terminals, and then the balancer plug is used to top off all the cells to the same voltage. As long as you use a balancer on all the cells somewhere along the line (preferably simulatenously).

It's best to balance all the cells as close to simultaneous as possible. Not a problem if all cells are in the same pack since the charger and balancer do them all simulatenously (duh!). If you use two packs you could charge them separately (I suppose you could charge them simulateously by connecting them in parallel also... as long as they don't get overcharged), and then top of them off separately. But it's probably best to connect all the cells in both packs to the balancer simulateously via the balancer plugs so the balancer monitors and tops off all the individual cells in both packs simutaneously. Depends on what the charger can do.

Still...I'd stick a diode on each pack to prevent one from charging into the other and read up more about it (you never do seem to see Lipo packs where cells are in parallel to increase capacity. They just use bigger cells. There may be a technical reason why they are never seen in parallel within a pack. Possibly due to cells charging other cells. I'd google more about it from RC truck and plane websites.

 

[justDIY]

for my MintyXL I wired two 2.6aH lithium ion cells in parallel. Each cell has its own individual protection module, and I charge them in parallel using a Maxim pulse charger. So far so good.

 

[dknguyen]

for my MintyXL I wired two 2.6aH lithium ion cells in parallel. Each cell has its own individual protection module, and I charge them in parallel using a Maxim pulse charger. So far so good.

THe protection module, do you know what it is? Is it a protection module or is it a balancing circuit integrated into the battery?

 

[justDIY]

THe protection module, do you know what it is? Is it a protection module or is it a balancing circuit integrated into the battery?

it is just a protection module; protects from over charge, over current, over discharge, short circuit, and some other things.

https://www.batteryjunction.com/prcimopfor3l.html

 

[audioguru]

I saw a pretty big electric RC airplane begin smoking in flight. It was quickly landed then its owner ran to it and plucked out its burning Li-Po battery that was brand new and cost $500.00CAN. It smoked a lot for about half an hour buried in sand. I didn't see any white hot flames like burning magnesium.

 

[Hero999]

I recommend you speak to the manufacturer; they are the only people who can really advise you.

 

[sam2]

You can charge lipoly batts safely in parallel as long as you dont go over 4.2 volts max , dont go over 1C charging current and the voltage of each cell should be resonably close( dont charge a half full cell with a very low one).

You need a CC/CV ( constant current / constant voltage) charger or a lab power supply where you can set the voltage and current limit.

They can be charged in series 4.2, 8.4, 12.6 volts but you should use a ballancer. Without it, sooner or later the cells in series will charge to different voltages some higher than 4.2, and lipoly's have a bad habit of igniting if over charged( or over discharged, below 3 volts ruins lipolys , or discharged at to high a rate), and they burn very violently.

This isnt a maybe. If you abuse these you will have a fire!
sam
P.S. If the batteries puff up at all while you using them( they are junk) discharge them with a resistor and them drop them in salt water for a couple of days. Always charge them in a flame proof container and never unattended.( no joke)

 

[audioguru]

I think water must stay away from lithium.

 

[dknguyen]

I think water must stay away from lithium.

It's debate whether the salt water actually does anything. Some people dispose of them that way, other don't.

 

[2camjohn]

Thanks for all the replys.


Without taking my batteries apart, I cant access the connection between the two cells.

From what Ive read thats gonna make balancing quite hard.




Currently Ive been successfully charging my lipolys individually using this little IC.
It seems to do everything right but I always keep an eye on the current draw just in case....

I was hoping I would be able to use this IC to charge the two sets of my batteries in parallel. Each battery contains two cells in series.

It seems from what dknguyen and sam2 are saying that charging them in this manner would not be a good idea without tapping in to the packs for balancing?
Is that correct?


How about actually using them? This may sound dumb but is it safe to paralell the two packs and then connect them to my device? Of course they will be both fully charged.


Cheers
John

 

[dknguyen]

It seems from what dknguyen and sam2 are saying that charging them in this manner would not be a good idea without tapping in to the packs for balancing?
Is that correct?

Correct. You might want to note though that balancers have not always been around. And the cells didn't get too out of balance unless abused...or so I've read about RC LiPos yesterday when I was trying to find an answer for you (and for me).

How about actually using them? This may sound dumb but is it safe to paralell the two packs and then connect them to my device? Of course they will be both fully charged.

I read an article yesterday saying two CELLS in parallel will balance each other out (assuming that their initial voltages were reasonable enough that both cells would be under the maximum allowable voltage after balancing). They then compared this to two PACKS in parallel and said it shouldn't be a problem (assuming the cells in each pack were balanced). I guess the reasoning is that although two parallel packs won't balance as nicely as two cells since the individual series cells in each pack can still get out of balance, if both packs are initially balanced and are at roughly equal charge levels, the imbalance produced won't be big enough to cause a problem.

 

[sam2]

It's debate whether the salt water actually does anything. Some people dispose of them that way, other don't.

The only reason for discharging damaged packs is so they dont light your garbage( or garbage truck) on fire.

I have discharged them both ways, but I usually use a resistor to discharge them quickly, and then throw them in some salt water to finish.

Even at very low voltage, they are still a fire hazard and bubble for quite awhile in the salt water( after the resistor).

If you throw them in salt water without discharging them first, the high current disolves the terminals down into the battery and they stop discharging.

Cells were charged in series for years before people started putting taps on them and ballancing. I did it with many two and three cell packs, and never had a problem, but I dont abuse my packs either( over charge/ discharge at high rates / over heat.)

With all the info now, I charge them the safer way.