Can I use 1 protected cell in a pack to do the job?

[tanasit]

I wondered if it was possible to use a single protected cell in series with unprotected ones to take advantage of the low-voltage cutout.

I first PM a member who then passed along the question to LuxLuthor who gave me the answer below and I'd like to thanks LuxLuthor here for his expert advice.

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I would not recommend it for several reasons off the top of my head:

1) You would need exactly the same battery brand, capacity, chemistry, and batch date of manufacture, since there are differences in current flow rates, internal cell resistance, and other characteristics that need to be matched. This is also why battery packs have cell balancer and/or charge balancing to make sure cells are ongoingly monitored to remain in same state of voltage charge.

2) The protection PCB's have very low cutoffs for low voltage--many as low as 2.5V before it breaks circuit. For many applications it is too low.

That low voltage protection function is best served when there is a reasonable amp draw and end of charge is reached, there is a rapid dip down to 2.5V breaking circuit, then a rapid rebound back to a healthier voltage.

In lower amp draw applications, you will have a slower, more gradual dipping down of the discharge curve, and more time will be spent with cell in lower damage-inducing voltage range (below 3.0V).

In your friend's proposed scenario, there will be other cells that may be dipping down more or less, until the one with protection hits the 2.5V signal to break circuit.

When AlanB gets his regulation switch, it will have an ability to set a more reasonable low voltage cutoff of 3.0 to 3.2V to avoid damaging cells.

3) Even if you matched exact cell type, the protection circuit will give a different total resistance, and thereby not deliver the same level of amperage.

4) If this was a safe strategy, you would have seen battery packs using this cheaper strategy. They always have a protection setup for each cell, since any single cell in a pack can drift beyond desired range...so the only safe way is to monitor all of them individually.

 

[mdocod]

The only way it would well in my opinion, is if you had a way to know for certain that the protected cell in the batch truly had the lowest capacity of the bunch. This condition would need to be met through the packs entire service life, so cells would need to be re-tested to confirm configuration compatibility through their life. This would not be practical in most applications, and I certainly wouldn't recommend it.

To address #3 from Luxluthor; provided the cells are in series, variations in cell resistance wouldn't really matter, in a series configuration, the same current flows through the circuit as a whole, one cell can't be drained at any higher rate than any other, it's impossible from what I understand. This is sort of beside the point, as it's still not a good idea to rely on a single cells PCB to protect the whole pack.

Eric

 

[LuxLuthor]

Eric,

This was a brief "off the top of my head" private message response to my friend, Led Zeppelin. If I had known it was going to be passed on and posted as a public thread, I would have done a bit of research and spent more time verifying my thoughts.

My brief thinking about #3 was that even if the cells were exactly matched brands/mAh/size/manufacturing batch, once you added a protection PCB with thin nickel strips, solder joints to PCB, & PCB components, you would change the amp behavior due to the added resistance at the very least of that protected cell--relative to the other non-protected cells. In essence my point in #3 was the PCB would make an identical cell no longer identical. I would have done some work to verify if this thought was accurate before posting it publicly.

 

[LED Zeppelin]

LuxLuthor, thank you for your prompt reply, quoted above, to my inquiry.

Tanasit had asked me originally, and though I had an idea why it would not be a good idea to mix cell types, I thought it best to ask an expert and suggested posting a thread as it is a good question. I then consulted you in order to provide a definitive answer to Tanasit.

I forwarded your reply to Tanasit in lieu of paraphrasing, and with good intention he posted this thread as an answer to anyone else with the same question.

I fully understand that as a public reply you would have liked to confirm your off-the-cuff ideas, but your answer is more than sufficient.

I hope there is no offense taken and am certain none was intended. It is a valuable lesson for myself in that posting and forwarding of PMs should be done with utmost respect for the person being quoted.

Thank you in advance for accepting my apology as I accept full responsibility.

 

[tanasit]

I have to say that I have the most respect for LuxLuthor for a very long time and always 'cut & paste" many many of his posted information onto my notes for future references.

I also want to apologize to both Luxluthor and LEDZeppelin for staritng the post in this manner.

Tanasit

 

[Raoul_Duke]

I've only just seen this, and have thought about this before now; I dont see that its such a bad thing ~ to try out anyway, and see wht happens.

I'd rather have (one of the ) ( same brand and matched) unprotected cells shut down at 2.5V, than not at all, or below 2.5V

(actually this is supposed to be the lowest cells like Emoli etc can be taken...Look at how they measure the discharge graphs... down to 2.5V)

Something that I ( and others) have noticed is that the "front cell" may start off at the same voltage as the others, and the cappacity be almost identical..but always ends up lower than the other.

if the protection method of a pack is 12V cutoff for 4 series IMR's then one cell may be way below the others.

I don't know if its the heat from the head...or conventional current flow :thinking: or what.:sigh:
I cant even definatley remember, not absolutley certain, that its always the front cell, for sure, I will have to dig out some hotwires and start playing again.

Trouble is I dont think they do after market protection circuits that are good for 10~12A