Lifepo4 prismatic cut off voltage question

I have been using this 16S (58.4V) 20ah chinese prismatic lifepo4 (encased in aluminum) for more than 6 months now and for the most part, I like it except that the BMS that was originally installed has a rather high cut off voltage of 48V (or 3V per cell). My question is, will installing a BMS with a lower cut off voltage of around 2.0V to 2.5V per cell hurt the cells in anyway or am I over thinking this one?

I only get around 18 to 35 kms per charge out of the pack ( I am not sure why the range varies, but I just changed hub motors so I will test my range again)... the shop that sold me the lifepo4 is a local ebike shop and they do use the regular chinese brands for their batteries and bms... even the BMS that they use is the one that was rated for 10A rated output although I am not sure if they used a lithium ion or a lifepo4 BMS..... all I know is that the cut off voltage is 48V and I find this too high for a lifepo4 battery... for a lithium ion its quite normal to cutoff at 3 to 3.4V but lifepo4 I believe that 3V is a bit too high of a cut off considering its full charge voltage is just 3.6V

lifepo4 discharge nonlinear

majority around 3.3v

when close to empty

acts like cliff

3v oddv conservative yet safe

Nominal (midpoint voltage) for 16S is 51V.

You are **really** stressing the bank out taking it up to the max stress level at 58.4V, very much reducing lifespn, especially if holding any Absorb / CV time at that voltage.

Personally 55V would be my limit at low C-rate charging, maybe 56 V at higher rates say up to 0.4C.

Link to the pack specs page?

3.0Vpc LVD is **not** low, not conservative.

In fact I would only go that low with very high C-rate usage

With sub-C rates that would usually leave well under 6-8% usable in the pack, much too close to the danger point especially as the pack wears.

And going to such low DoD is murderous to your cycle count lifespan, foolishly so IMO.

So no, do not go any lower IMO.

If you test OCV a couple hours or more resting isolated after that cutoff, and see it's bounced back to over 3.15V or so, maybe.

But only if that was a normal peak amps for you, if you are treating it gently no peaking that's going to be too low.

Unless those few minutes extra range are worth having to replace the pack much more frequently.

Well coddled that chemistry can last 10000 cycles. Treated OK like that but in a high-C propulsion context maybe 1500-2000 IRL

But truly abused then maybe only 500 cycles.

Especially with "normal" crap chinese quality.

Using A123 cells, everything else equal, you'd get 3x lifespan at least maybe 10x.

I was using the charger that came with the pack and it said 58.4V... but that charger blew a fan and I bought another one that charges to 54.6V. It maxes out the charging voltage to 55+V.... so I think I am now good there. Unfortunately I have no spec sheet for the lifepo4 cells since I bought it as a fully assembled batt pack but I thinks its one of those aliexpress 20ah lifepo4 cells that have the solderable battery posts, not the screw on ones and are encased in aluminum.

You guys think that a lithium ion 18650 cell battery pack is superior to lifepo4 ones at the same capacity? I am thinking of making one myself using samsung INR18650-25R2 (the light blue ones...) for a total of either 48V (54.6V or 13S) or 60V (67V or 16S) but I still have to plan this one since no one locally sells battery packs like these. Some do sell 18650 based packs but most of them use A LOT of refurbished laptop batteries, the CGR ones (low discharge cells).... while others sell 18650 one but I cannot verify the brand that was used.... Its new but I dont know who made the cells.

Lifepo4 can take 3,6 to 3.7v but stopping at 3,5v is best as there is very little capacity above 3,5v . Your BMS may need a higher voltage to charge and balance. Plus how hard do you use your battery. You may be using it to hard making it unbalance. Maybe ? Quality matters. Link to battery please. 2.8v cutoff is as low as you should go.

ah I have no link to the cell spec sheet as I am not sure exactly what cell it is or where they got it, all I know is that these are lifepo4 cells and they (reportedly) have 20ah of capacity.... that's it.

I have been searching aliexpress but without knowing the brand or even the model or part number of the cells, I am not sure what type of cell it is. I have been seeing 20ah prismatic cells on aliexpress but mainly its two types, one that has a screw post and the other type has solder on posts.... I know I have the latter since I remember them soldering on wires to each cell when they assemble it.

But that's as far as I know regarding these cells... I can try to get the dimension of the cells, maybe different brands have different dimensions for their cells even though they are all 20ah.

The different makers have wildly different quality, consistency and longevity.

But LFP is LFP, and best to ignore the spec sheets wrt to voltage levels anyway, those are max/min stressful "don't go near" numbers not to be used as actual setpoints if you want good longevity.

For C-rates maybe useful, but best to stay lower the better anyway, use temperature rise to see absolute limits if you must go over 2C.

redwing0001 said:

It maxes out the charging voltage to 55+V.... so I think I am now good there.

Click to expand...

Much better, perfect in fact.

> You guys think that a lithium ion 18650 cell battery pack is superior to lifepo4 ones at the same capacity?

There is no single criterion for "superior". LFP is **much** safer and 10x longer lived if treated well.

But higher weight and volume per Ah depending on pack build tech.

Of course yours could be crap, need to check actual capacities of each cell, match your groups and do at least pack-level c-rate test if you need lots of power.

john61ct said:

redwing0001 said:

It maxes out the charging voltage to 55+V.... so I think I am now good there.

Click to expand...

Much better, perfect in fact.

Click to expand...

???
At 55V max. charging voltage it is possible that the BMS is not balancing yet as that is under 3.45V per cell. That might explain the widely varying range. My LiFePo4 16S BMS starts balancing at appr. 3.50V per cell. My charger outputs close to 60V open voltage. That obviously drops to the pack voltage when in first stage (CC= constant current) of charging. As said the BMS starts balancing when the pack voltage goes above 56V and the charger is then in CV (constant voltage mode). As the top balanced voltage is 3.55V per cell with my BMS the total pack voltage is 56.8V.

The cut off voltage of my 16S BMS is appr. 42V or around 2.6V per cell. 48V (3.0V per cell) cut off does seem very high and only using 60% of the packs capacity.

that 55V charger is new, I just started using it. Prior to that I was using the old charger but range seemed to be cut drastically.... maybe the charger is failing and not really charging the pack as it was when it was new... I need to test the new charger more to find out if I at least get back some of the range before.

Oh, one more question... do these lifepo4 batteries need a..... "break in" period like they used to tell us for cellphones? Meaning you charge it then use it until it cuts off several times or is that an old wive's tale?

Old wives tale. What you do need to do is leave it on the charger for many many hours after the light is turning green because bmsb down the high cell which will return on your charger to fill up the rest of the path but it takes a long time over and over again.

999zip999 said:

What you do need to do is leave it on the charger for many many hours after the light is turning green because bms down the high cell which will return on your charger to fill up the rest of the path but it takes a long time over and over again.

Click to expand...

But that won't work if he really has a 55V charger (is that even a charger meant for 16S LiFePo4?) and a BMS that only starts balancing at 3.50V per cell. As at 55V (<3.45V/cell) it means it will never balance the pack no matter how long he leaves it charging.
And looking at the discharge curve for LiFePo4 it is also strange that it seems the BMS cuts off at 3.0V (48V for 16S) per cell. Because both combined (charging voltage too low and cut off voltage too high) means you are using less than 40% of the packs capacity.

Yes, you need to get direct access to the per-cell balance points, put in leads if not already there

in order to find out if your cells are out of balance.

If you want to use your BMS to rebalance the cells

you need to discover what the BMS start-balance voltage is, and either adjust it lower so it matches your desired charging setpoints

or increase your charger setpoints to get just over that start-balance voltage.

Also see how long the rebalancing takes, if the BMS balance current rate is very low, when the cells are greatly imbalanced, it can take many hours or even days

which is very unhealthy for the pack longevity. A charger that lets you reduce its current to match the BMS balance current will help a bit,

but in some cases better to just forget using that BMS for balancing, put in a better one, or use a dedicated balance rig, or use a "hobby charger" for balancing.

But many of these issues require digging into the pack and to some extent rebuilding it.

For those ordering or building new packs, make sure to specify what you want, the gear should accommodate your preferences not the other way around.

For me, it is stupid for BMS limitations to dictate the charging profile. That should be optimized for the health of the cells, and the balancing algorithms adjustable to fit within the normal charging routine.

Agree that he needs the right charger for his battery 58.6 or 59 volt 60 volt. Cc CV charger 16s. I like a kingpan metal case 5,amp charger from Ping or ?