It's interesting there are a few sellers making LiFePO packs w/o BMS units. I haven't studied it closely enough to know how absolutely required they are, but e.g. with normal lithium ion batteries, the ones that are capable of exploding, it's always been thought to be an absolute requirement (for safety) to have a BMS. But for example Electric Motorsport is selling motorcycles using thundersky LiFePO batteries and not using a BMS.
And Cloud Electric is selling LiFePO packs (unclear if those are thundersky cells) w/o BMS
Their explanation is, ah, "interesting" ..
...A fully charged and balanced Li battery pack will remain balanced as long as you do not exceed 3C in discharge....
Lithium batteries are very obedient....For this reason it is very important that you monitor the charge on the groups at all times. A good tool for this is the PakTrakr. A 12V group of Li should not be allowed to fall below 10.8V, set your discharge cut off at this point. ...
etc.. Reading between the lines they claim that charging at the pack level is okay so long as you start with a balanced battery pack. Uh, this triggers the skeptic in me. Experience with SLA packs suggests a different behavior. But it's clear they have more experience with these cells and maybe they behave differently than SLA batteries.
Andrew posted this link: a DC-DC to charge li-ion... GREAT!! about using DC-DC converters to charge individual LiFePO cells. They're talking about this for A123 cells but I'd think the idea would be applicable to other LiFePO cells. In any case this offers a possibly good strategy for a LiFePO pack:-
Use a paktrakr for monitoring.. use the above charging method to ensure the pack starts each ride in a balanced state .. this is precisely the bank charging strategy we've discussed for SLA packs. An open question is how to handle automatic cutoff if pack voltage or individual cell voltage dips below the cutoff points.
If, while riding, one or more cells dips below the minimum 2.7v per cell.. will you notice it? I dunno about you but I rarely look at the gauges while riding a motorcycle or driving a car. So relying on the driver to see the below-minimum-voltage condition is not going to work. The driver will be likely to miss that condition leading to cases of batteries being damaged by undervoltage. It would be better to have an automatic cutoff that detects undervoltage and raises some kind of alarum and/or cutoff of the pack.