Regen BMS?

[gromike]

Lately, I've been researching building an electric Grom and I've come across this BMS specifically labeled with "Regen". It's +two times the price of a comparable (for what I know) BMS. I can't find anything more specific about it than what is on the website.
As I want to have robust regen for this bike this has caught my eye. What's so special about it?


LFP Regen BMS

 

[amberwolf]

It seems to be a common-port BMS with high current capability for both charge and discharge, and capacitive balancing (rather than passive draining of high cells). Guessing it is using an IGBT module rather than FETs. Might be misusing an SSR intended for 3-phase switching instead.

I'ts also rather huge, 1.2kg for the main module and more than half that again for the completely separate balancing module.

Most BMS i see don't have high charge current capability, or aren't common-port, so are not suitable for high-current regen, so at least this one could do that. The specs don't indicate that there is any current limit against overcharge current, however, so you'd have to setup your controller to stay below the cell limit and the BMS limit. (it does have a time limit of 3 seconds for this but there isn't a current limit so I don't know how it would deal with the time limit-- just always turning off charging current after 3 seconds doesn't make sense...so dunno how charge overcurrent protection works or if it does; would have to be experimentally tested).

But any contactor-based BMS like those from JBD can do as high a charge current as discharge, as long as the monitoring system has the ability to detect / etc the same current in both directions and the software has the right settings available.


However....there's few cells out there that can handle charging currents anywhere near as high as their discharge currents, so there's little point to most BMS having the ability.

Which cells were you thinking of using?



Specs copied below, wiring diagram linked and attached:

https://www.evcomponents.com/waredetail/bms/EVC%2024S-200A-300A%20(Regen)%20BMS.pdf

1. Single Cell Over Charge Protection Voltage	3.9V±50mV(Release voltage：3.8V±50mV)
2.Battery Pack Over Charge Protection Voltage	Full charge early alarm：88.0±0.5V，slow alarm(di di di...)；Protection：91V±0.5V,Fast alarm then cut off automatically
3.Battery Pack Over Charge Release Voltage	89.5V±0.5V
4.Single Cell Over Discharge Protection Voltage	2.10V±50mV(Release voltage：2.3V±50mV)
5.Battery Pack Over Discharge Protection Voltage	Low power early alarm：70.5±0.5V，slow alarm；Low voltage early alarm：57V±0.5V,fast alarm and protection cut off, alarm is just for your information and you can detach alarm if you don't want the sound
6.Battery Pack Over Charge/Discharge Protection Release Condition	Charge battery pack to recover from over discharge：Minimum 62.5±0.5V，Over discharge(low voltage cut-off),Too high current, Short circuit protection can be recovered by charge.
7.Maximal Continuous Discharging Current	150A-200A
8.Maximal Pulse Discharging Current	300A
9.Maximal Charging Current	150A compatible with electric regenerative braking
10.Battery Pack Over Discharge Current Protection Detection Current	625A±25A
11.Battery Pack Over Charge Protection Detection Maximal Delay Time	3sec
12.Battery Pack Over Discharge Protection Detection Maximal Delay Time	1.0sec
13.Battery Pack Over Discharge Current Protection Detection Maximal Delay Time	50milliseconds
14.Balancing System Working Static State Consume Current	3.6mA(The voltage of each cells is 3.2V above.)
15.Balancing System Working Static State Consume Current	1500uA（The voltage of each cells is 2.0V below.）
16.Impedance	0.5mΩ(The voltage of each cells is 2.5V above.)
17.Dimensions	1xBalance module(L)238X(W)80X(H)48mm，1xMajor control module(L)238X(W)86X(H)85mm
18.Weight	Balance module:660g；Major control module:1200g
19.Apply to	24 LiFePO4 Cells in Series 20-200AH
20.Feature	Advanced capacitor balancing technique which provides more efficient cells balance function. It balances cells by transfering energy from higher cell to lower cell(Efficient to 60Ah-200Ah battery pack). Save energy and reduce heat emission in balance process.
21.Installation method	Double Faced Adhesive Tape or screw
22.The voltage of each cells equalization precision	±5mV---±35mV(The coherence of each cells capacity between±3%0,0.2C discharge testing method)
23.The maximal temperature ascent of balancing system module	Less than25℃
24.Working temperature range	—20℃-----+65℃

 

[dominik h]

The JK B2A24S20P active Balancer BMS has the same specs for half the price and is smaller.
It can do 200A continious charge and discharge, 350A for up to 5 Minutes. 2A Balance curent.

 

[gromike]

Which cells were you thinking of using?

At the moment 24 Calb prismatic 3.2v 72ah cells.

 

[amberwolf]

AFAIK those are 1C max charge current, and 2C max discharge (and are probably being pushed pretty hard at either of those rates).

How much regen current are you expecting to require for the braking force you need? You'll need cells that can handle that much charge current, as often and as long as necessary for the worst-case conditions you'll require regen to work for you.

For high regen currents, it may require more cells in parallel (or bigger / better ones) just to absorb the regen current than it will to get the range and/or discharge current you require.


BTW, to use regen *all* the time at full capability you'll need to never fully charge the pack, so that the cells have room for the regen current without coming near HVC (or else the BMS would turn off the input, voltage will spike in the controller with no load, and possibly blow up the controller before the controller can stop regen).

You'll also need to ensure the controller's regen voltage range can be set so it doesn't disable regen within the voltage range you require the pack to be able to do this, but does disable it if the pack is too high or too low to safely charge, especially at that rate.

 

[gromike]

The plan is a 6k hub motor. I would be happy with 3 or 4k regen. With a Grom, battery space is limited.

 

[amberwolf]

6k? 3 or 4k? Not sure what you mean--watts? amps? Nm? ft-lbs? (when figuring stuff out like this it's important to use units, and the right ones for the particular thing being discussed, to minimize confusion )

How much braking force (torque) do you need? How much torque does your motor produce for a given current? (it will produce the same braking force given the same current in the reverse direction).

For example, if you need more force for braking than for acceleration, it will take higher (charge) currents than it does for accleration (discharge).


Either way you should use cells that (in however many P your groups will be) have a continuous charge current capability *at least* equal to the current you want to regen at.

If the regen will *never* be more than a very short peak of less than a few seconds, *and* your cells can handle a peak current that high, that would also be ok.

If you end up with regen currents higher or longer or both than your cells are designed to charge at, then it will heat them up more than they should on each such braking event, and voltage will rise further than normal during the event, which could trigger HVC on the BMS and disconnect the battery from the controller during the regen, which can cause taht voltage spike previously mentioned. It could also damage the cells over time (probably will; how much depends on the amount of overcurrent and its' duration as well as the SoC at the time, and the design of the cell and it's manufacturing quality).


Cells like LiTi can charge very fast, but have low energy density and sometimes low discharge rates (relative to charge rates); they might be better suited to this but will have lower range / etc. for the same size pack

 

[gromike]

The plan is a QS 268 6kw hub motor with 3-4kw regen on bursts. I'm just upping the ratio from my bike, which max's at 1.5kw input with 7-800w of regen. My rides are lots of short ups and downs.
The devil is in the batteries, I guess. Getting something to fit and do the job is quite the problem. Heretofore, I just screwed a battery pack onto the downtube.

 

[gromike]

I should up my watts. The bicycle has a 500w motor that draws up to 1500w. So that planned 6kw hub motor might draw 10-12kw, and 6kw+ on regen. That would only be for short bursts up and down.
And thanks amberwolf for your help. If I can build a capable motorbike for under $3000 on my existing motorbike it just might be ahead of spending $6000+ on a production bike.

 

[amberwolf]

BTW, here's a JBD contactor-based BMS; since it doesn't use FETs for switching access to the cells it should be able to do whatever regen current you want. (I haven't read it's manual, so you'll have to verify it's actually able to do what you want).

JBD Smart BMS 3S 4S 12V 60A 80A 100A 120A 150A LiFePO4 Battery PCB for Freeshipping - Jiabaida BMS

Jiabaida BMS JBD Smart BMS 3S 4S 12V 60A 80A 100A 120A 150A LiFePO4 Battery PCB With Balance Uart Optional RS485. High quality better service and favorable price with free shipping. Battery Management System for NMC/Li-ion Lifepo4 battery

jiabaidabms.com

So that planned 6kw hub motor might draw 10-12kw, and 6kw+ on regen. That would only be for short bursts up and down.

I hope you are going to use a controller with variable (controllable) braking force via your brake lever or throttle. If you use one that is just on/off, I'd bet on snapping the motor axle or frame or dropouts at some point from the shock loads.


Going to be a hell of a battery that can do that kind of power level for charging / regen current.

Let's say that's at 72v, you'd need one that can handle 166A discharge current, and 84A charging current.

How long a runtime do you need? At those wattages, if used anywhere near those rates, I expect you're looking at high wh/mile usages, like 100wh/mile or more. Let's assume you need to go 20 miles; that's 2kwh (call it 30Ah), call it 35-40lbs of battery depending on cells, casing, supports, etc, and probably a block as big as a good stack of trade paperback books. (in whatever shape the cells allow). If you need 40 that's 4kwh (call it 60Ah), and twice as large, heavy, etc.

That's assuming you can find cells at that low a capacity that can accept multi-C-rate charging. If the cells can only handle 0.5C charging or less as is common enough, then to get 84A of regen current forced into them without stressing them, you'd need cells that have twice that as capacity, or 168Ah or larger cell (or parallel groups of smaller cells equalling that).

If you're stuck with that size of cell(group), that leaves you with a 72v x 168Ah = 12kwh pack. Good news is that would give you a one hour runtime at full discharge rate of 12kW, but bad news is that's a gigantic battery, probably weighing upwards of 100lbs (or more including enclosure, etc).

 

[gromike]

At most, all I need to go is 10 miles one way and 10 back, with some short spurts at 60+ mph. Variable regen is on my list for the right brake lever. As I have said, my experience is with a 500w hub motor that draws up to 1500w. That, and a 125cc gasser motorbike, which I seek to emulate in electric in the most economical way possible.