BMS

YAVLiC - BMS blues

Last week the kit including the BMS arrived. No circuit or wiring diagrams, just a pile of parts. Hmmm...

The BMS consists of a module for each battery cell and a "control unit" if you can call it that.

The BMS components are from EV-Power. Here is a link: EV Power cell module.

The "control unit" seems to consist of a few relays and a butchered plugpack which floats around in the box. It seems that I am supposed to connect the mains to the Vectrix charger through this box. As far as I can determine without tracing the tracks on the PCB, it is supposed to switch off the mains when the battery is charged.

BMS-CM-sml.jpg

BMS Module Testing

Since I am not going to entrust a $4000 battery to a pile of components which I don't understand, I started by testing the BMS modules. I hooked up the modules to a Lab power supply. A Fluke multimeter was connected via separate leads directly to the module and a second multimeter was used to indicate the state of the Solid State Relay (SSR) on the module. Like this:
BMS-modules-testing20130826.jpg

It took about 2 hours to measure all 42 modules. Here is the data:
BMS-modules-data_0.jpg
BMS-modules-data2.jpg

  • VLL is the cell voltage lower limit (LL). Below this voltage, the SSR goes open circuit.
  • Vth is the threshold at which the module starts to shunt some of the charging current through itself. I measured the voltage at which the module starts to draw more than 10mA.
  • VUL is the cell voltage upper limit (UL). Above this voltage, the SSR goes open circuit as well.
  • IUL is the shunt current drawn by the module at VUL.

In retrospect, I should have also recorded the shunt current at maximum allowable battery voltage which is 3.65V for the CALB CA66Fi cells. However, the current rises rapidly from Vth and reaches around 0.5A at Vth+10mV.

The only parameter which approaches a normal distribution is Vth. For the other parameters the production process seems to be less well controlled.

A significant worry is VUL. As can be seen from the measured values, VUL is 3.965V (average). This is 0.315V higher than the maximum allowed as per the battery datasheet. This is also the voltage at which the SSR switches to open circuit (OC).

However, the real worry is that the control unit does not know when the first cell has reached it's maximum voltage and the BMS module starts shunting current. By the time the SSR switches to open circuit, at least one cell will have been cooked for a while.

The maximum shunt current achieved by the BMS modules is 0.8A, some only get to 0.73A.
I don't think that shunting 0.8A when charging with 10A or 15A is going to make much of a difference to a cell.

How it ought to work

To properly manage the battery and to be able to equalize the pack, the charging system needs to have at least 3 states:

  1. Nominal charging, e.g. C/4 until the first cell reaches the maximum allowable charging voltage and the BMS module starts to shunt current. The chain of SSRs must go open circuit at this time.
  2. Equalization phase. The battery charger reduces the charging current to a value not much higher than what the BMS modules can shunt, in this case 0.8A or maybe 1A.
  3. Charge complete. When the battery pack reaches VBatt = N × 3.65V, all BMS modules should be shunting current and all cells should be at SOC = 100%. The charger reduces the current to 0A.

A "real" charger should of course also have a state for the case where the initial VBatt is below the lower limit, timeouts, alarms, etc...

Conclusion

I have the wrong BMS modules. They might be designed for cells which have a Vmax=4.0V (maybe Thundersky?). They also switch at the wrong time.
The modules don't seem to be suitable for the cells I have because:

  1. The modules don't indicate when the first cell has reached Vmax and shunting starts.
  2. The SSR switches at the wrong woltage for my cells.
  3. By the time the module's SSR switches, at least one cell (but probably many) will have been cooked for a while with nominal charging current minus 0.8A

Grrr...

YAVLiC - Got the bike now what?

After having been convinced by Matt Lacey's Li conversion videos that a Li conversion is quite managable, I started looking for a 2nd hand VX1.

By good fortune, I ended up with a VX1 from the former Australian Vectrix importer and distributor. The bike is ex-demo/showroom and I have been told it has done less than 500km. It has a few marks and the chrome plated surfaces have brown corrosion spots. The battery could find a new application as a zero-volt reference if it were not so heavy, and since the plan always was to convert to Lithium, I did not even attempt to revive it.

My choice was to go for the largest capacity battery possible. Partly because I don't want to have to worry about making it to the next charging point and partly because the disacharge capability of the battery is proportional to the capacity. At the same discharge current, a larger capacity battery will be stressed less.

A local distributor still had stock of CALB 66Ah cells so I ordered 42 of them. As of July 2013. CALB does not make any 66Ah cells, only the 60Ah ones. The datasheet states a max discharge current of 2*C (132A) so a peak discharge with up to 200A for a few seconds should be well within the battery's capability. Internal resistance (Ri) is quoted as < 1 mOhm. At that Ri, drawing 50A would dissipate 2.5W of heat per cell or 105W for the entire pack, at 100A that would rise to 10W and 420W respectively. Average power dissipation of the battery will probably be closer to 105W than to 420W, depending on how I use the bike. In any case, with the LiFePO4 cells, I am not too concerned about battery cooling and the 2 battery compartment impellers will be permanently removed to make room for more cells.

Electric Motorcycle Fire: Was It Battery or BMS Problem--Seeking Explanations

How can we determine if our electric motorcycle had a BMS? We imported 2 electric motorcycles in late 2010. They were ordered with a 5000W motor and 72V90AH LiFePO4 battery packs with a BMS. They were routinely charged and test ridden. One bike had about 500 miles. A few weeks ago, it was ridden 20 miles and parked. Approximately 50 minutes after the ignition was turned off, the bike burst into flames and was demolished. Fire marshall determined no signs of foul play. We trying to determine the cause. We are speculating that the battery pack was NOT LiFePO4. We are also speculating that there was no BMS installed, because we cannot find any circuitry between the + and - terminals. How can we verify if there is a BMS? Is there any other way to determine by examining the other bike? We are looking for answers to this safety issue.

Ordering Components

After talking to a fella at EV works about various components I have settled on parts for my bike.

Motor: This is the less complex and slightly more powerful out of the two 48V motors. http://www.evworks.com.au/index.php?product=MOT-MARS-ME0708

Batteries: At the moment I'm getting 11 of these cells and will update later to 16 so I get the full 48V system. These will take about 6 weeks to ship so lots of time to draw up some concepts for attaching them. http://www.evworks.com.au/index.php?product=BAT-LFP60AHA

Controller: This one gives the best acceleration for my price range. http://www.evworks.com.au/index.php?product=CTL-AXE4844

Contactor: Good economical option. http://www.evworks.com.au/index.php?product=REL-ZJW400A

Charger: KP3612EL is not currently stocked so will have to wait for that. The charger will be the only component I will need to replace when changing up to 48V.

Battery Management System: Another component I need to wait for. EV works are developing a new, more compact BMS which should be ready in a couple of weeks.

Braided Cell Interconnectors: http://www.evworks.com.au/index.php?product=BAT-EVW-BCI-60-6

Fuse: http://www.evworks.com.au/index.php?product=FUS-ANN-250

That's all I'm ordering at the moment and these parts, including shipping from Australia to NZ, should cost about NZ$3800 (US$2800). I'll be getting other things like DC-DC converter, gauges, power cable and lugs from an electronics store in NZ once the bike starts to take shape.

Can I run my LiFePO4 Battery with no BMS?

Hello,
I am a newbie who just purchased my first e-bike conversion kit. I picked up the Phoenix 5304 hub motor with a 40 amp controller. The battery been used is a duct tape 48v 20ah pack. I converted my bike and everything worked fine my first time out........until I started to smell smoke :(. I looked over the battery pack it was fine, not even warm. I then inspected the BMS which basically melted. The solder was dripping off the BMS like water!

I did some testing, and believe it or not the BMS still works. After my testing I decided to do the following.

I created harness for the BMS. I am only using the BMS during the charging process. After a complete charge I remove the charger and the BMS via the harness. i then ride the bike with just the battery and an in-line 50 amp fuse. The bike seems to run great. Is this is a safe configuration? Am I in danger of damaging the battery? I assume that the BMS is only used for charging purposes, but I could be WAY off. Thanks for helping the new guy :).

sparc5's picture

Dream GreenSaver Charger - The GreenBMS

I'm moving this from a post to a blog to keep people up to date with my progress.

For those who haven't been following the threads I've posted to here are the problems I'm trying to solve:

1) Battery charging- Make sure our batteries live long and prosper.
2) Equalizing- Make sure they discharge at an equal rate so you're range isn't limited by your weakest battery.
3) Cost- Keep the price lower than the alternatives.
4) Patents- Must do this all without violating patents.

After three weeks of heavy brainstorming here's the leading plan for what I call the GreenBMS:

More than anything, we need a good temperature compensated bank charger. So I'm building a percision, programable, multichemistry, bank charger that uses PWM to provide desulfication and active monitoring of each battery's voltage. Woah, try saying that with one breath (adjusts glasses). This is the heart of the GreenBMS. By using a chip that came to the market only a month ago, you can provide your own DC power source to the charger it can accept anything from 3-75 volts DC. (Anything above 14V means higher efficiency). It is small enough to fit inside the scooter. You can save money by using the charger that came with the bike to power it or buy something beefier. You can even connect it directly to solar panels if you wish.

Extra add-on 1 (An auxiliary battery):

The other benefit to having a power converter that takes such a large range of voltages is you can charge the batteries as you drive, like an alternator does, but it will get it's current from an auxiliary battery instead of a car's engine. You can run the 12V electronics off it and get rid of the inefficient DC-DC converter. The charger will switch to a mode that charges the weakest battery and brings it up to the average state of charge of all the batteries. If the batteries are evenly matched, the auxiliary battery will never discharge much. If all the batteries are very low, or if told to do so, it switches to an extend range mode, where the auxiliary battery charges all the batteries at a rate in which it will be depleted at the same all the other batteries are. When you plug it in, the auxiliary battery gets bank charged with all the other batteries. You can make the auxiliary battery any capacity and chemistry you want. A small cordless power tool battery might be all you need, I need to do testing once I build the prototype. The only cost of this add-on is the price of the battery and the wires to connect it.

Extra add-on 2 (a display, and user interface):

Since the microcontroller (uC) is already monitoring battery voltages, amps, and temperature, it's very simple to display this on an LCD. It will have a graphical user interface and some buttons to let you interact with the BMS' parameters.

Extra add-on 3 (wireless communications):

Everyone can change their programs if they wish to. I know everyone here loves to tinker and share their improvements. I'll offer a long range wireless (102.15.4) data communications add-on so you can program the charger, install updates, monitor the charging, and download data from your rides, from anywhere in your house. It's cheaper than bluetooth, and easier to interface.

This is all going to be opensource to keep costs for everyone to a minimum. I'm doing this project for myself, and making the knowledge on how to do it free for everyone. Early adopters of electric scooters have a soft place in my heart. I'll have extra circuit boards made up, buy enough parts to construct a dozen kits, and sell them in kit form or assembled for a bit extra.

This week I'll be drafting schematics and ordering components.

This project is much more complicated then initially planed. My fears are the cost of the parts, and the complexity of making them all work in harmony. In the past I've programed uC's. I went through a clock making phase. One clock flickered 8 leds fast when you attach it to the ceiling fan it displays the time. I've made a SMPS to power a nixie clock. It turned out well. This time I'll be programming a uC with the most sophisticated program ever. The SMPS this time is powerful enough if I put a wire in the wrong place it could start a fire. If you guys don't hear from me in a while, you can assume I forgot to ground something. :P

As a request: Everyone please take out your multimeters and tell me the resistance of your batteries and voltages before charging and after. Include the temperature as well. Thanks in advance.

-=Jeff=-

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