justaperson76 said:
Regarding LVC, I would just prefer the BMS to stay out of the way. The controller also has a LVC which I would prefer not to rely on. That's why I'd rather just have a digital readout of total capacity then I can decide when to shut off.
You can certainly do that.
(It's how I use my packs on my SB Cruiser trike...but only because they're large-format EV grade cells used "lightly" vs their specs...if I was using an 18650 or other small-format cell pack I'd be using a BMS with it, at least for it's monitoring ability, even if not for actually shutting off automatically if a problem were detected).
Just remember, everything is a compromise.
In this case, total capacity doesn't tell you anything about what the cells are doing individually. So it will work as long as you don't care about that (and any possible consequences of any particular parallel group of them being run down too far), and whoever is riding this is always going to concentrate on the readout instead of just having fun riding.
(see further down for my experiences)
If you want something that is easy to use and "safe" for untrained-in-battery-maintenance people to ride and use (meaning, other than you, in this case, since you'll have built this you'll know about it), a BMS that shuts off based on cell-level LVC is easiest, with the controller's pack-level LVC as first line of defense, so the controller itself will just stop operating the motor whenever the load placed on the battery causes it's voltage to drop below that LVC. Then if an individual cell group has a problem, and drops below it's LVC, the BMS will shut off power entirely to the controller, preventing potential damage to that cell group.
If you want control over the system, rather than automated shutoff, you can use a BMS, bypass the discharge FETs to connect the controller to the battery, but add a light or an audio warning (beeper, etc), or both, on the handlebars, dashboard, etc., that is wired to be turned on by the signal from the BMS to it's FETs that would turn them off under problematic conditions. Then you can see / hear the alert and decide if you want to keep going and possibly damage cells, or stop. But you have the control, not the hardware.
That is a good point also. How would you detect this without a BMS? This thing will go pretty slow anyway so I could monitor it fairly easily(ish) in realtime. I haven't seen much discussion about that.
Without a typical BMS, you'd have to use some form of cell monitors, like the old Celllogs (I don't think they're made anymore, but there should be equivalents), that display all the voltages all the time so you can read them at a "glance" (it's actually takes significant time to *read* them and process that info in your head, but once trained to it by practice you may be able to see ones that are different from others at a glance, if they all fit in a small enough area to be in the center of your vision all at the same time).
The catch with monitors like these is that you are running wires from *inside the battery, at various battery voltages between them*, all the way up to your handlebars or dashboard, and that is a lot of wire with nasty consequences for a short inside the bundle.
If they have alarms built into them for LVC that are visible or audible without doing this, then that's not an issue...but if you want to actually read them while riding, it is.
I did actually use a set of Celllogs on my battery pack on the trike for some experiments; I couldn't run wires for them all the way up to my bars (would've been at least 10-15 feet of wire for every cell), so I put them as far as I could with a couple feet of wire, which was about at my knees, just above / in front of them, angled so I could see them with a downward "glance'. But they're small with small numbers, and my vision isn't that great anymore, so I had to look pretty hard at them to read them, and could only do this very rarely while riding since I'm on the streets, and had to be sure I was on a stretch where no one could step off a sidewalk, come off a driveway or side street, etc etc. But even if they had been right on the handlebars where the Cycle Analyst is, I would still have had a hard time quickly processing 14 voltages and comparing them to each other / a remembered LVC number in my head. So...useful for monitoring cell voltages when stopped somewhere...but not so useful for doing it while riding, under load, which is where the problems really show up.
I describe a way to do it by using a BMS above, that doesn't let you see the cells but does give you an LVC warning.
A BMS that can use bluetooth to a phone/etc, or with a hardware display for that specific BMS, could give you the cell voltages on the bars/dash without potentially hazardous wiring.
Seeing that there are no RC chargers that are >13s (anymore), could I just use something like an iCharger 206b 8s, have 6s balance cable on one half of my pack, a 7s balance cable on the other, then have a positive/negative charge cable in the middle of the pack and then charge the two groups separately without disconnecting the series connection? It would take a fair while to charge, and a minor inconvenience charging two packs but I much prefer this method. No crappy BMS app to put up with and it can balance quicker if need be as well.
But it cannot provide an LVC function during discharge, or tell you about the cells while you're riding (unless you carry the chargers and their power source too). That is the main point of using a BMS with an app or other display; being able to monitor discharge at the cell level.
If you only care about knowing what is going on with the cells during charging, it will do that job.
Regarding the charging using the icharger, if you use two of them, then as long as the power supplies you run them from are separate and isolated, there is no need to break any pack connections. Just have the charging wiring built into the pack separate from the discharging wiring, and make sure it is built to handle the full current that you will ever put thru it during any charging event.
Another reason for using something like an iCharger is I can balance at a lower voltage (say 4v) than a BMS, where I have to wait for it to charge to the full capacity before it charges. I would prefer to prolong the life of the battery, so may not want to charge to 4.2v or so.
Some programmable BMSs let you change the balance points (and all the other trigger points, like LVC, HVC, etc).
amberwolf said:
Mostly, though...if you use a pack of well-matched cells that are well-within their limits, never used close to their limits, never run full or never run empty, never run hard, etc., the pack will stay balanced on it's own until it ages enough for the cells to no longer be matched. You don't typically see this with cylindrical cells...but large-format EV-specific cells can certainly behave this way (mine do).
All batteries (cells) will be the same. No hodge podge mismatch between cells or groups.
Keep in mind that "well-matched" doesn't just mean all the same cell model/brand, it means actually testing them for capacity, internal resistance, etc. Then using only the cells that are all "identical" within some pretty tight tolerances. Since that often is not practical or is too expensive to toss out that many cells and buy more and hope they match what you still have, a less perfect way of doing that is doing the testing, but then just putting groups together that *average* all the same values between groups.
YOu can build serviceable packs withotu doing that, they can just perform better for longer with less work (balancing, etc) if you do. Depends on how closely matched the cells you start with are (they can vary a fair bit even within the same box from the same lot of the same brand and part number, going by the results various people here on ES have gotten when they have done and posted these tests).
Actually the whole reason why it's taken me so long to plan this is because of safety. It's taken me days to work all this out in the name of safety. Is that how long people usually take to plan all this?
Some take much much longer. Depends partly on how much they want to learn about things before they choose their parts and build something. Some just build something, then learn from that and build another, and repeat this till they get what works for them.
I was working on an 18650 type build plan for a long time (couple years? so long ago I don't remember); bought the cells, was working on collecting the rest, but never got to build it because of intervening life-changing events (documented elsewhere on ES). Even before that I'd gotten my first EV-grade cells and used that pack and it worked so well that I delayed the build of the 18650 pack....
I even thought about putting the battery in an ammo box or something. I see on youtube batteries in an ammo box which kind of suppress the explosion were something bad to happen.
If there's an actual explosion, it's probably not going to stop that (it will probably make it worse by allowing higher pressure build up from multiple cells' pressure, rather than just singles going off one at a time). But it might prevent the spread of a fire. I used them for my RC-lipo pack batteries, and my first EV-cell packs, etc. Now I don't use a case and just keep the pack built into the trike. (I also used the ammocans to make the packs water-resistant since I had them just mounted on the bike frames...the trike is built to not need that under normal conditions).
Charging is where the problems usually occur.
It is often where the problems are made obvious in catastrophic ways.
However...unless something is wrong with the charger itself, where it goes way overvoltage (and has no system like a BMS to shut it off), then the real source of the problems is the cells themselves, and the things that create the problems in them usually happen during *usage*, during discharge. So not having anything preventing problems from happening in the first place means a charging problem is more likely to occur.
The chances of something going wrong at any point are pretty small, even with "abused" cells, even without any protections...but they're less with them than without them.
And the catch with battery problems is that they are a high-consequence, if low-probability, type of failure.
I've seen all sorts of videos showing massive abuse of various cells in various ways, where (nearly) nothing happens (made some myself)...but every so often, that's not the case.
I just try to give enough info and point enough things out for battery builders / etc to make informed decisions, so you may find me pointing things out more than once, in different ways; if it gets annoying, well...better annoyed than sorry.
