Is regen still possible with this kind of bms?
- Thread starter brumbrum
- Start date
Yes, regen is possible, in fact, the BMS won't be able to stop it even if the cells go over voltage. That's the issue here. With a single port BMS, the regen would get cut off if the cells went over. With a two port BMS only the charge port turns off for over voltage.
About the only way this would be a problem is if you lived on the top of a big hill, fully charged the pack, then went down the hill with regen. Many motor controllers will have a pack voltage limit on the regen, so this would protect your pack if it is set up correctly.
About the only way this would be a problem is if you lived on the top of a big hill, fully charged the pack, then went down the hill with regen. Many motor controllers will have a pack voltage limit on the regen, so this would protect your pack if it is set up correctly.
okashira
10 kW
fechter said:
Exactly. This bms cannot prevent overcharge or regen. The current will flow through the mosfet body diode.
Unless they are using a relay.
fechter said:
There are at least two potential catches with this scenario:
First is that if the BMS cuts off the battery from the controller to protect it from overcharge during regen down a hill, the bike/etc will suddenly begin accelerating down the hill. If the rider was not expecting this, it will take that person's reaction time at minimum to begin braking with the mechanical brakes. If they're not on the same lever or switch already being pressed or squeezed / etc then it'll take however much longer to switch to the mechanical lever/etc and engage the brakes. Most likely this will cause no issue, but if the rider is right behind someone else going down the hill and pacing them, it could cause a collision.
Second, if the battery is loading down the regen voltage that is seen across the FETs, then wehn the BMS shuts off the battery, the votlage across the FETs will now go up to whatever the regen voltage is when unloaded, which might be higher than the FETs (or the main battery caps) are rated for. Probably not going to be an issue, but if it's high enough long enough it could take something out.
Neither of these is likely to cause a problem in most cases, but the potential is there.
Yes, quite true. I've had the first one happen to me before. The sudden loss of regen was a surprise and almost threw me off my scooter.
The second one is also something to worry about but most controllers that do regen have some kind of upper voltage limit that would hopefully kick in and prevent it from being destructive. This voltage limit should be set lower than the trip point for the BMS so the BMS never trips. Some of the older controllers don't have any voltage limiting feature and could easily blow up if the battery suddenly disconnects.
The second one is also something to worry about but most controllers that do regen have some kind of upper voltage limit that would hopefully kick in and prevent it from being destructive. This voltage limit should be set lower than the trip point for the BMS so the BMS never trips. Some of the older controllers don't have any voltage limiting feature and could easily blow up if the battery suddenly disconnects.
Agreed - however - a two port BMS will often not protect the pack in such a manner. (i.e. the discharge port will generally not have overvoltage protection.) This is arguably worse since rather than see unexpected reduction in braking you destroy your batteries.amberwolf said:
I guess it depends on your point of view.
Personally, I'd rather destroy the batteries over time with a bit of overcharge damage than some of the things that could happen because of either of the two issues.
--sudden braking failure causing possible loss of control or collision with someone in front of me
-- sudden controller FET failure causing extreme plug braking of the wheel causing possible loss of control or collision with someone behind me
It's also possible (though unlikely) that the overcharge of a battery could cause a fire; it'd most likely take quite a lot of overcharge, which is not that likely to occur in most usage scenarios. The main way that would mostly likely happen would be on a long steep descent (with constant regen braking) at the start of a trip where the battery is already fully charged.
Personally, I'd rather destroy the batteries over time with a bit of overcharge damage than some of the things that could happen because of either of the two issues.
--sudden braking failure causing possible loss of control or collision with someone in front of me
-- sudden controller FET failure causing extreme plug braking of the wheel causing possible loss of control or collision with someone behind me
It's also possible (though unlikely) that the overcharge of a battery could cause a fire; it'd most likely take quite a lot of overcharge, which is not that likely to occur in most usage scenarios. The main way that would mostly likely happen would be on a long steep descent (with constant regen braking) at the start of a trip where the battery is already fully charged.
There's no "sudden braking failure." In unusual conditions (100% charge) you can lose regen, which is generally a small percentage of the total braking power available from your brakes. So you squeeze a little harder and get exactly the same braking. Unless you are following six inches behind someone it won't matter.amberwolf said:
This used to happen on an old EMS E+ bike I had. It was a non-issue.
This is indeed a serious problem, but is a problem whether someone uses a one terminal or two terminal BMS.
Agreed. It would take a very long descent to get voltages high enough for a fire. However, regular overcharge at slightly higher voltage encourages dendrite growth, and this leads to battery fires eventually.
As I noted previously, that depends on how your braking control is setup.billvon said:
If your ebrake is a separate control from your mechanical brake (like on my trike and my bike where it's a separate brake lever), then if you don't have that mechanical control already under your grip (or foot, or whatever), it will take your reaction time plus movement time to begin mechanical braking.
Some vehicles' electric braking controls are not even in the levers (or foot controls) at all, and are controlled solely by the throttle, or by the throttle in combination with a button or other control. IIRC the Vectrix uses its' throttle in a reverse twist to engage braking. Some ebike controllers with proportional regen do something similar; some require engaging the ebrake signal first. I dont' remember if any can be used without doing that, but the signal doesn't have to be on a brake lever.
As for how much braking power that regen represents, that also depends on the specific setup. If you have a motor and controller that can generate high regen currents, and a battery designed to take them, then that could be quite a lot of braking power--with the right controller, motor, and battery, it could be at least as powerful as the mechanical brakes.
Suddenly losing that could be quite the surprise to the rider, and reaction time can be increased in those types of situations.
As an example, I depend primarily on my electric braking from the rear wheels to slow and stop my trike. My front mechanical brake can also do it, by itself, but I use it mostly in situations where the electric braking isn't appropriate for one reason or another (mostly when I want to slow very gradually, or I want to hold the trike in place, like waiting at a traffic light, etc.
My electric braking is two different systems, with "EABS" powered braking on the left wheel, and "standard' regen braking on the right wheel. The current output from the regen is slightly higher (a few amps) than that used up by the EABS, but the EABS has higher braking force (enough to cause brake steer to the left). Neither is adjustable/proportional, just on/off, and both are tied to the same ebrake lever (that has no other function).
I have no BMS on my EIG NMC packs, but some long time back when I was still working out all the wiring, I had an issue a few times where electric braking would suddenly stop happening, because the main battery disconnect switch/breaker had a connection problem, The controllers and the CA would shutdown, and all motor power and electric braking would cease. Mostly this happened during acceleration, under the highest load the system would see, which was annoying (and even dangerous a couple of times), but when it appened under braking it was qutie a surprise the first time. Thankfully I had plenty of time to recover from it and use the mechanical front, but there are traffic situations in which this is not the case, where someone suddenly swerves into the lane in front of me (because I'm significantly slower than the rest of traffic on the road, so there's almost always a large gap in front of me.
Of course, a cotnroller or other failure could also cause loss of regen braking, so this kind of sudden braking loss / failure is not just possible from a BMS shutoff; that's just the only scenario I mentioned as it's relevant to this discussion.
One situation in which the (front) mechanical isn't much use is in our very infrequent heavy rains, when it's just started, and the surfaces of the road are oil-slicked. The brakes work fine, but the wheel much more easily locks up and then just skids, and I then lose control of steering with no traction on the front. The electric braking is on the rear, and is very efective in the same situation, even though it's always applied fully and suddenly, partly because it can't lockup the wheel (but it *can* slow me very quickly). If I lost the electric braking in that kind of situation, the mechanical front wouldn't be able to take over the same job.
(FWIW, on my bike, it has electric braking front and rear, as well as mechanical front and rear, and in the same situation I could use the mechanlcal rear to take over if electric failed, The trike doesnt' have that option yet. (will someday)).
I'd venture thats' the case on most bikes/etc. Just not on all of them.
That might depend on the electric braking setup; if it can generate very high voltages (without blowing controller components) currents could be higher and battery voltages could get higher; whether that's enough to cause a fire depends on the particular chemistry, I guess.
But most likely it would only happen on quite a long descent right at the start of a trip before much, if any, battery charge has been used up.
(I don't remember what results there were from such situations, but I recall discussions of this specific problem with various chemistries on DIY Electric Car forums almost a decade ago or so when I started learning EV stuff. Might've only been theoretical rather than situational discussions--cant' remember. I was going to check stuff from that era, but that forum won't let me log in for some reason.
True, but it's easier to prepare for that eventuality (design of battery containment, location of bike or vehicle parking/storage, etc) than any kind of sudden issue on the road in traffic.
I dunno about that. Suddenly having a fire between your legs might not be something most people prepare for (or _can_ prepare for . . . )amberwolf said:True, but it's easier to prepare for that eventuality (design of battery containment, location of bike or vehicle parking/storage, etc) than any kind of sudden issue on the road in traffic.
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