How many LiFePO4 cells can be in parallel?

Question

How many lithium iron phosphate (LiFePO4) can safely be connected in parallel, in order to achieve higher power output (and capacity)? Wired directly together, without components such as resistors or power transistors limiting current flowing between parallel cells.

Precautions taken would include ensuring they're brand new cells from the same manufacturer lot, at about the same state of charge, and letting them rest for a day to equalize before charging or discharging the pack.

This chemistry is supposed to be much safer than other lithium chemistries, as it doesn't catch fire even when punctured. Is it also safe to add more cells in parallel? The most I've seen discussed for other chemistries is around 3 cells in parallel. Would it be possible to get to 10, 20, 30 cells in parallel with this chemistry? The completed pack would drive a DC motor.

If it's not safe to wire that many directly together, would it be safe if current limiting components or fuse/polyswitch were added between parallel cells? Or is it better to architect a large pack in strings of at most 3P x (however many) in series?

Note: NOT asking about other lithium chemistries such as lithium polymer. Specifically LiFePO4 in form factor of cylindrical 18650 or 26650. Advice about tradeoffs in using much larger cell sizes rather than parallel small cells welcome.

EDIT:

When cells in parallel are new and well-matched, they balance well. As the cells age, their internal resistances get progressively less matched. Somewhat self-correcting as a cell with lower internal resistance will charge & discharge at higher current than others - thus getting aged faster and catching up to its peers.

However if one cell fails to open circuit or very low internal resistance, current will now be flowing mostly through that one cell. Say the pack normally gets charged at 1C rate, now that cell is getting 3C rate. If there are 20 cells in parallel, now it's 20C rate. The more cells in parallel, the more current can flow to the most unbalanced cell. It seems dangerous not to limit this current.

@SpehroPefhany recommends self-protected cells. Are there methods which add protection circuitry around unprotected cells? Are lithium iron phosphate cells considered safe enough (no fire/explosion) not to require it, with cell failure simply leading to low pack performance or requiring pack replacement? How are dead/shorted cells normally handled in these large pack designs?

Answer 1

There really isn't a maximum!

You have already hit the major concerns including using the same state of charge, using batteries that are in the same condition, from the same manufacturer (and preferably the same lot number to get an even closer match). You also want to make sure that you never short circuit that battery pack as it will have an incredible amount of power and can release that power really quickly. Putting the cells in parallel also lowers the internal resistance.

Where did you read that 3 is the maximum for parallel for regular lithium ion? I built a battery pack from 40 - 18650 lithium ion cells in parallel and use it every day. I connected a PCB to protect against short circuit, over charge and over discharge.

It is used for relatively low current, 4 amps and less, but charges at as fast as 10 amps with no problems.

For your project I would look at the electric bicycle group. They probably have the most experience using larger LiFePo4 batteries. I am sure that they have added them in parallel and can share their experience. I agree with you that the LiFePo4 is a relatively safe chemistry. Another alternative is the lithium Manganese battery chemistry found in the Nissan Leaf. There are videos on YouTube showing people hammering nails through the battery with no fires or explosions. The Leaf's battery runs at the usual lithium voltage of 3.0 - 4.2, unlike the LiFePo4 which runs at a lower voltage.

Answer 2

I upvoted Filek's answer, as it is technically correct, and answers the question as posed - there is no electrical safety reason to limit the number of cells in parallel.

However, when designing a large scale battery, one does have to consider other kinds of safety. Stored energy is, well, stored, waiting for something to use it. So let's take this question to its logical extreme - in an infinite array of cells connected in parallel, all of which are equipped with protection hardware, all installed at the same time with the same state of charge, what would be the worst thing that could happen?

Well, eventually, the thing is going to catch fire. Out of infinite trials, one of these is going to both short circuit AND have it's protection circuit fail. Once it catches, there's really no way to stop what happens next - subsequent cells, full of energy, melt, short circuit, explode and continue a never ending cycle of Murphy's Law, because we have infinite energy waiting to jump out at us.

So we modify the array to make ensure that there is sufficient containment on all sides to endure the instantaneous discharge of a short circuit from the worst case scenario failure cannot possibly cause a primary containment failure of the next nearest cell - and there you go, we no longer have an infinite fire.

Obviously in a real application, there would only be a very finite fire, but suppressing that risk is a task largely for chemists and mechanical engineers. Over here on the electrical side, we'll specify current and voltage ratings as usual, and generally, only specify the capacity we need.

If you think I'm being silly about the fire thing, please read up on the battery fire of the USS Bonefish - it gutted the ship and killed three people. Fun fact - in an oxygen starved, burning environment, when a high pressure air line melts open, the line itself becomes a flame-thrower.

Answer 3

Suggest you use protected cells if you're going to put them in parallel- protected against overcurrent (in or out) and overcharging and overdischarging (they have a small PCB inside- at the end actually- with a bunch of parts on it).

Many cells are not protected, some are only partially protected (short-circuit only), and there are many, many, many (probably dangerous, definitely inferior) fakes around.