What To Do With 4.35v Cells?

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TheBatteries

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What are you guys doing with your 4.35v cells? I have a few hundred of them. I don't want to put them in with my regular packs because the capacity at 4.2v is often much lower, not to mention I don't charge past 4.1v in production anyway. I can't create a separate string since the string voltages won't match up then, 58.8v vs 60.9v. I don't have a use for a second battery system, ebike pack, or anything like that.

I guess the only thing to do is sell them?
 
My first lot of cells was almost all 4.35, since I tested the same way as 4.2, I get same measure of capacity from 4.2 to 2.8 in Opus BT-C3100 so I don't see any problem, yes they are rated for some capacity and you will get much less, but isn't that what we make with 4.2 cell charging them until 4.1 or less?, You have cells that at 4.1 will last many more cycles than normal cells. At 4.1 maybe are at 70% of capacity or less. So you add cells that they will be understressed, have them in a separate string to have them controled and working at same voltage than normal ones.

Regards
 
I agree, just charging them to 4.1 instead of 4.25 (.1V less for longevity) will yield a much longer lasting cell. I would, though, make sure they are marked separate so you know that those are those cells specifically.
Or, you could just make a separate string of those, even if they are smaller packs. Maybe even make 2 half system voltage packs (7s) with them since you only have a few hundred

Might be useful for other projects other than powerwall.
 
What is a " 4.35 cell???"

I'll hazard a guess ... a cell taken out of a laptop or other pack , and found to be 4.35 V....

I'd discharge it quick , to bring it down to a more comfortable voltage that will not eat into it's life ....

All 18650's can be charged to 4.35V , unless they have some defect ... cycling at 4.35 gives 115% the 4.2V capacity but a life of only 150 cycles for new cell ( if my memory serves me correctly)
 
ozz: no, it's not a defective cell. It's a standard cell that the top voltage is 4.35V. There are several cells listed in the Cell Database that rated at 4.35V
 
ozz93666 said:
What is a " 4.35 cell???"

I'll hazard a guess ... a cell taken out of a laptop or other pack , and found to be 4.35 V....

I'd discharge it quick , to bring it down to a more comfortable voltage that will not eat into it's life ....

All 18650's can be charged to 4.35V , unless they have some defect ... cycling at 4.35 gives 115% the 4.2V capacity but a life of only 150 cycles for new cell ( if my memory serves me correctly)
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There is definitely a slight chemistry difference between them, here are 2 "sister cells" made around the same time from Sanyo:


image_udlhhp.jpg


They manage to have a similar lifespan (assuming it stays linear) despite one charging to 4.3V and getting the extra capacity from it. Really wouldn't recommend charging a 4.2V cell higher than 4.2V.
 
Ha ....well I didn't know that !!!

I'm not sure why some manufacturers have done that ??? the capacity is what counts and it still is not exceptional at 2.8Ahr

I guess these are fairly rare??? not often found in lap tops??

That chart above is very interesting because it shows the decline after normal 'expiry' and gives an indication of how much life is left in used cells .

My understanding is normal cycle life is given when capacity declines to 80% ... so a cell cycled at 4.2 is said to have a life of 400 cycles ...at this point the cell still works but only with 80% capacity .... from that chart these 4.35v cells appear to have the same cycle life ...400

so assuming we get cells at the end of their normal life 400 cycles (used at 4.20) .... they still have another 400 cycles at 4.2 but only 70% capacity or 3000 cycles if we run them at 3.93 ... Then after that we have another 3000 cycles at 3.93, but at 50% capacity ...

This is very good news for the expected life of powerwalls!!
 
mike said:
My 4.35v cells seem to all test low, for example 2800mAh cells test at 2200mAh, when testing at 4.20v. I always thought that was just because of the missing 0.15v. I guess I'll have to open an Opus and actually try one at 4.35v.
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According to battery university li-ion batteries get 10% of capacity in the last 0.7V, so maybe 20% in that 0.15V you will not charge them, 80% of 2800 is 2240, so if you got 2200 from that cells I would say are in almost perfect state.

Test some at 4.35V to get rid of doubts, but you have cells with 2200mAh at 4.2V conditions.
 
ozz, actually they are very common. They are often called LiHV, HV for High Voltage, and the idea is to get more power out of them by keeping the voltage a bit higher during discharge while still having comparable capacities. They are, for example, rated for 3.85V nominal voltage and at 2800mAh you get 10.78Wh instead of 10.08Wh or 10.36Wh at 3.6/3.7V. Laptop batteries use these for quite a while and they become increasingly popular in RC batteries as well. Many chargers even have a special LiHV mode for these cells.
 
Just charge them at 4.2V rather than 4.35V.

As an example, I recently found 3x Dell battery packs with 24 of the cells.

While they do get around 3000mAh at 4.35V, they still get a nice capacity of 2500mAh at 4.2V, and they get a noticeable boost in cycle life.

TLDR: These cells are still great, do not throw them away.
 
Just when cleaning my opus I saw there[size=small]is a small switch in the housing.[/size]



image_uegzoq.jpg
 
DarkRaven said:
ozz, actually they are very common. They are often called LiHV, HV for High Voltage, and the idea is to get more power out of them by keeping the voltage a bit higher during discharge while still having comparable capacities. They are, for example, rated for 3.85V nominal voltage and at 2800mAh you get 10.78Wh instead of 10.08Wh or 10.36Wh at 3.6/3.7V. Laptop batteries use these for quite a while and they become increasingly popular in RC batteries as well. Many chargers even have a special LiHV mode for these cells.
Click to expand...

Of course your right , Raven ....it's not the capacity in mAhrs that counts (as I saidin my previouspost) ....

it's the capacity in WHrs that matters , and that higher voltage gives you more Whrs per mAhr ... ...

One option is to start a higher voltage parallel pack just for those cells , when built you could put this higher voltage pack in series with lower voltage packs it would work fine with them if it had about the same mAhr capacity as the other parallel packs ... but balancing would have to be manual always keeping the special pack at a higher voltage ,a normal bms wouldn't work with this arrangement.

In practice it's probably sensible to treat them just like the other cells , mix them together , running at lower voltage than they're designed for means they will last longer
 
ozz93666 said:
One option is to start a higher voltage parallel pack just for those cells , when built you could put this higher voltage pack in series with lower voltage packs ...
Click to expand...

I would never ever do this. All parallel packs in a series connection must have the same voltage for proper balance and safety. Never purposefully install a higher voltage pack into a string of lower voltage packs. You are only asking for trouble. Especially as you say, it must be balanced manually. What if you forget?
 
Even at 14s the voltages just don't line up to have one operating at 4.2 and the other at 4.1. 13s for the high voltage pack would give you 4.4V fully charged. Besides that each would need to be capable of the full load capacity; you get odd current sharing effects when mixing chemistries, for example LCO in parallel with LMO can cause the LCO pack to provide 80+% of the current at low state of charge.
 
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