Over voltage and regenerative braking

[Thuktun]

I have a question about over voltage and regenerative braking. I'm building a 14s battery so fully charged will be 58.8v. when I brake and the voltage goes above the allowed 63v what happens?

Does the controller fry? Does the regenerative braking suddenly cut out, dropping the voltage, then cut in and continue cutting in and out as the voltage rises and falls? Does it apply only enough braking force to keep the voltage at 63v?

Is it better to just turn it off?

[Sam.Vanratt]

Hi Thuktun
you have to differ between the Storage side (LiIon) and the electronic side.
The Battery when fully charged is easy to overload which leads to a fast decay of their capacity. LiPo starts to drop metallic Lithium on the cathode (which is a very active metal!!!) when overcharged and typical burn off instantly. I never had LiMn for testing (I used one from Sony) but I guess they loose capacity quite fast. A BMS generally acts when overcharged with seperating the charger (Regen) from the battery itselfs which forces the BLDC controller to waste the generated energy in it FETs which works quite OK as long as the energy is below the thermal load it could dissipate.
The controller is build for 48V blocks (12S typ) and when reached 63V a) the condensators might blow (especially when they get older and dry out) or b) the FETs are overstressed and fail (continues ON or off).

Regen braking is very functional and I love it enormously (if it would work in my controller yet) as I had in the past, but the Energy saved in a typical situation is only a few percent on a normal (not too hilly) ride. I did a Matlab simulation four years ago on my typ. daily ride with about 10 traffic lights I surely have to stop a few bridges and the about 40m altitude difference and the 23km single distance ride. I later compared it to my (onboard computer) datas in the Battery and it was (sadly) a 2-3% energy conservation instead of wasting the energy in the brakes.
Cheers
Sam

[Thuktun]

Okay, but that didn't answer what I asked lol.

I just want to know what happens to the controller and how the feature behaves, I know what can happen to the battery if over charged. I know I don't get any usable return in energy, but I do get extra stopping power which is always a good thing.

Thank you though.

[Sam.Vanratt]

Hi Thuktun
it's exactly as I've written:
the BMS will cut off the connect which means the Controller have to burn the energy and therefor gets hot until it's damaged. This could happen on the first try or work for years. The controller itself has no protection build into against misuse or overvoltage, while it features an overcurrent protection (current limitation) for the engine, unluckily not on the 5V line. Nice features like wrong polarity protection, overtemp. protection, Overvoltage shutdown, RPM Limits where never integrated (maybe in the newer MP5 as I haven't had one yet, but the MP3 and MP4 controller does not feature such "fancy stuff". The same goes for the low voltage level the MP series allow by design.
Alan could surely say more about the usecase, as I see only the electronic/construction side.
Cheers
Sam

[Morgen 3Eman]

Hi Sam.

I want to start out by saying I'm sorry for hurting your feelings.  I'm just a bad tempered evil old geezer who clearly hates all form of life, and disagrees with everything said by anyone.  So please don't ask to get me kicked off the board.  Again.

Whoever explained regen to you had no idea how a generator/battery set works.  Find a different source.

Hi Thunkun
It is not good engineering practice to subject components , particularly electrolytic capacitors,  to voltages near their rated values.  If your circuit is using 63VDC rated caps, I suggest you build a lower voltage  battery set as a power source.  Regen is a current producing event.  The output voltage is a result of that current, mutliplied by the impedance of whatever circuit is connected to the generating coil. (I x R=V)   An open circuit or very high resistance circuit will cause a very high voltage to be developed to make that current flow, while a very low resistance circuit will cause a very low voltage to be developed.   (I x R=V)  Using a 58VDC battery set will result in a regen voltage close to 58VDC, while using a 50VDC battery set will keep the regen voltage closer to 50VDC.    Since you are building your own battery set, you can pick the voltage….

I personally do not like the variable effect regen has on braking performance. 

Again, Sam, I apologize for being such a terrible person.  Please don't hate me.


TTFN,
Dennis

[Bikemad]

Hi Thunkun
It is not good engineering practice to subject components , particularly electrolytic capacitors,  to voltages near their rated values.  If your circuit is using 63VDC rated caps, I suggest you build a lower voltage  battery set as a power source.  Regen is a current producing event.

Dennis, the battery he is building is similar in voltage to your GM 48V LiFePO₄ battery.

Your battery is 51.2V nominal and 58.4 fully charged, whereas Thunkun's will be 51.8V nominal and 58.8V fully charged, so the 0.6V difference in the nominal voltage and the 0.4V difference in the fully charged voltage is nothing to worry about.


The following was observed on my original Magic Pie:

If you disconnect the battery and pedal fast enough (with brake lever pulled in so that the regen is on) the generated voltage being output does not go above ~60V which is fine, however, if you do not apply the brakes, the voltage continues to rise well above this level.
By winding the pedals as fast as I could by hand, I made the wheel spin pretty fast. I looked at the voltmeter and observed a reading of ~77V , so I immediately grabbed the brake because I was worried about damaging the controller with such a high voltage going through it.

With the battery connected, this excessive voltage rise should not occur, as the residual current being generated while regenerative braking is not activated should be easily absorbed by the battery.

We have recently enhanced the cruise controller to detect if the battery pack is in place. If the battery is switched off, or is not connected at all, the controller will check the back EMF voltage, if it exceeds the preset limit, the controller will disable the regenerative braking to protect controller from being damaged.

As of today, 22/7/2009, the day of the eclipse, all GoldenMotor controllers have a new function to deal with the issue of regen braking burning out the controller while battery is off.

Unfortunately, I have not tried the same high speed hand pedalling test on the later Vector controllers to see if they work the same.

It would be interesting to find out what happens to the controller's voltage if the battery's BMS isolates the battery from the controller during high speed regen.

Alan
 

[Morgen 3Eman]

Hi Alan

To be honest, I never even considered that someone would disconnect the battery from a motor they were using!  Open circuit regen voltage  could be just about anything. 

I agree with your comment about 0.4 VDC battery constuction difference being nothing to worry about.  My concern was using 58VDC  supply voltage  if you know you have caps that are rated at 63VDC . It just isn't a good idea. I believe GM management learned from the "48Volt" MP3 disaster, and have uprated the cap rated voltage on current production product.   

At any rate, my external controller is still working just fine:)

TTFN,
Dennis

[Thuktun]

Okay, so it won't hurt my controller, that's good and as I expected. But I still don't know how the feature behaves. Does it cut in and out? Does it give less braking force so that it only generates a certain amount of voltage/current? Someone out there must be using regen braking and a 58.8 or 58.4v battery. What happens when you use the brakes when your battery is still more less fully charged?

[Thuktun]

I'll find out myself by the end of the week anyway. I start soldering Monday, building it at work because I have a lot of downtime and the fire risk is less there lol.

I had planned to build a battery over the winter but then decided to just buy one. I'm not rich so after shopping around and googling sellers names ordered one from China on eBay. I'd seen the sellers name on a few forums and mostly good things, no shipping issues at least. You expect to see threads about something failing, people usually only post their bad experiences, but most for this guy were good. Anyway, waited almost 3 months for the battery, seller was in communication the whole time, always had a reason for the delay. Finally I asked eBay to step in and got my money back. Two weeks ago I bought a stack of new laptop batteries at a great price, works out to $1.50 per cell. So here I am well into the cycling season rushing to build a battery. I'm still riding around on my SLAs but they have almost 1000kms on them now and current delivery is lacking to say the least lol, I'm running them in parallel at 24v for the last 100kms or so. For now I'm going to charge 2 halves in parallel on a Hobby King 8s charger but I have a 14s BMS on the way and I'm building my own charger for charging the whole thing at 58.8v. I was going to buy one but I have all the parts already and like I  said,I'm not rich lol.

[Bikemad]

Hi Alan

To be honest, I never even considered that someone would disconnect the battery from a motor they were using!  Open circuit regen voltage  could be just about anything. 

Hi Dennis,

It doesn't necessarily have to be "someone" disconnecting the battery from a motor they were using, some users occasionally ride their bikes without having a battery attached, or they sometimes have it attached but leave it switched off until assistance is needed, and this caused a lot of problems with the earlier controllers.

With the battery attached and turned on, if you start a trip from the top of a steep hill with a fully charged battery, the regenerative output voltage from the controller during high speed braking could exceed the maximum charging voltage of the battery, and under these circumstances I would expect the battery's BMS to disconnect the charging supply in order to prevent the regenerative braking from overcharging the battery.

However, if a home made battery pack does not have a BMS to protect it from overcharging, the cells could possibly be overcharged, which can be very dangerous with LiPo packs!

I have my MP4 controller maximum voltage set to 59V instead of the default 63V to hopefully protect my 14S lipo pack, but I don't know whether this high voltage cutoff has ever been activated.

My wattmeter does not store the maximum voltage, but I do have a very useful alarm and data logging function on my CellLog 8S battery monitor:



I use this to monitor one half of the pack at a time, and I have never had the overvoltage alarm activate on either half. 

Here is a graph showing the voltage variation throughout a test ride:



The high peaks show the regen voltage and the low bits show the voltage sag in the battery when the motor is under maximum load, and the highest cell voltage does not exceed 4.2V (which is the maximum permissible charging voltage for these LiPo cells).

Although I did manage to make the low voltage alarm sound a couple of times while on this ride with an old and weak 8 cell LiPo pack:



Some users occasionally ride their bikes without having a battery attached (or they sometimes have it attached, but leave switched off until needed), and this caused a lot of problems with the earlier controllers.

Alan
 

[Sam.Vanratt]

Hi Sam.

I want to start out by saying I'm sorry for hurting your feelings.  I'm just a bad tempered evil old geezer who clearly hates all form of life, and disagrees with everything said by anyone.  So please don't ask to get me kicked off the board.  Again.

Whoever explained regen to you had no idea how a generator/battery set works.  Find a different source.

Hi Dennis
the explaination comes mainly from my daily work (Automotive) with BLDC (and asynch. eMotors) and the electronic we use there.
What do you think (in particulary) wrong so we could point it out? I build the complete battery electronic (incl. the superCap storage) by myself and it works in the about ten bikes we used 2012 as a ProofOfConcept for the German Train company. It includes an extensive computer system so current/voltage is logged in every current-carrying module (so every ( LiFePO4 cell is logged, the 12 Supercaps are controlled, the max in + out, GPS position, temperature, ....).
Cheers
Sam

[Sam.Vanratt]

Hi Alan

To be honest, I never even considered that someone would disconnect the battery from a motor they were using!  Open circuit regen voltage  could be just about anything. 

I agree with your comment about 0.4 VDC battery constuction difference being nothing to worry about.  My concern was using 58VDC  supply voltage  if you know you have caps that are rated at 63VDC . It just isn't a good idea. I believe GM management learned from the "48Volt" MP III disaster, and have uprated the cap rated voltage on current production product.   

At any rate, my external controller is still working just fine:)

TTFN,
Dennis

Hi Dennis
I guess Alan means what I (tried to) describe: the BMS have to cut of the external Voltage source just to keep the cells in save range. As most BMS only can dissipate about 100-250mA this situation is easy to reach. We tested such behavior as my first project was to use the MP3 for a Downhill (or better Uphill climber) bike. The GM patched solution to prevent the controller (MP3) to burn itself was to establish a reverse diode on the fuse, but effect of the BMS is the same: it simply cuts of the battery (drain) from the motor (source) to save itself. In all my experience and what I've read the controller (the FET bridges of course) then burn the energy to safe itself (therefor the voltage does not reach infinity which it would do without a powerdrain). I think the GM tech who explained it to me was also named Denis or so. I still have the mails I exchanged in 2012 with GM and could (with lots of effort) look it up.
BUT: the battery voltage drops quite a lot (no matter what Li chemistry is used)  in the first few percent of power draining so the situation (Battery dismounts itself as power drain) and the BLDC controller has to burn the energy) is very unlikely else you live on a hill and start your day with freshly loaded cells.
Cheers
Sam

[Sam.Vanratt]

Hi Dennis,

It doesn't necessarily have to be "someone" disconnecting the battery from a motor they were using, some users occasionally ride their bikes without having a battery attached, or they sometimes have it attached but leave it switched off until assistance is needed, and this caused a lot of problems with the earlier controllers.

With the battery attached and turned on, if you start a trip from the top of a steep hill with a fully charged battery, the regenerative output voltage from the controller during high speed braking could exceed the maximum charging voltage of the battery, and under these circumstances I would expect the battery's BMS to disconnect the charging supply in order to prevent the regenerative braking from overcharging the battery.

However, if a home made battery pack does not have a BMS to protect it from overcharging, the cells could possibly be overcharged, which can be very dangerous with LiPo packs!

Alan

Hi Alan
just seen it that you already answered it. I used a Celllog8S in the first go, and it's the finest log I've seen today: low prices, very accurate, enough storage and a good tool (openSource) to analyze it.
Cheers
Sam

P.S.: did you get confused by my statements at first as well (just as Dennis). Didn't think I stated anything else than you or Dennis himself did.

[Thuktun]

That's one half done. Went quicker than I expected, plus no one asked me to do anything today so I mostly just worked on this.

[Klatuu]

So you are building a lithium ion powered 4 slice toaster??