I briefly brought up the idea before of using a Zener Diode as a crude BMS, when a proper BMS can't be found.
I'm still hoping to build a 55S LTO battery, but finding any BMS that is programmable for LTO is rare, and up to 55S, as hens teeth.
I was told before that low voltage Zeners can't really act as BMSes for batteries, because the knee is too soft. Since then, I've thought - The knee may be soft for the diode, but it's very hard for the battery, and LTOs tolerate over-voltage very well. I've had one at 5v for weeks, with no long term damage.
So my new thought is to pick a Zener diode which has the reverse voltage over the max charge voltage, and use the leakage as a FEATURE, not a bug. Hear me out.
My base Zener would be 3.0v
According to the spec sheet, at 1v, the leakage is in the micro-amp range. So unless I am leaving the bike uncharged for weeks, no chance of completely flattening the battery.
At 3.0v, the LTO battery is still within safety limits, but may be very slowly losing its life. But at this point, it's fully open, with an impedance of 20ohm. 3.0v @ 20ohm = 0.15a - about the same as most BMSes bleed off, and adequate given there's almost no capacity above 2.55v. A drain of 0.15ma would bring the pack down fairly quickly: 5% of a 30Ah pack = 1.5Ah, @ 0.15A, = 10 hours, or basically overnight.
Hopefully, above 2.8v, the leakage is starting to get serious. Whatever it is, doesn't matter that much, as long as it's somewhere between the 0.15ma above, and nothing. So, we can actually use the knee as part of the design.
But will it keep draining during the bulk of the nominal voltage range. Being pessimistic, if the voltage leak at 2.6v is half of its rating, I'd be leaking 0.065A - it'd take me over two weeks to drain the battery flat. I ride my bike almost daily, and recharge very 2-3 days.
So... I've half convinced myself that while not ideal, it's more workable than with other battery chemistries. What do you think?
The only concern I have is that Zeners fail closed (short circuit), but a 1 amp fuse is easy and cheap enough, so if the Zener fails closed, the fuse blows.
Does anyone have any comments on this idea?
Thanks in advance.
eBike: Q100H on 16S with Phaserunner FOC Controller
eMotorscooter: Vectrix VX-1 on 36S
eCar: Mitsubishi Outlander PHEV... Waiting for warranty to expire
eHouse: Still on grid, but with LTO batteries and 3kw LF inverter...
After 5 builds, the best advice I can give, is start with high quality products. I prefer http://www.ebikes.ca