EV managment module

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EVguru
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Postby EVguru » Sat Aug 16, 2008 10:32 pm

And I will stick with my variac and bridge rectifier as I have for the last 6 years without toasting any batteries.


If you are fully aware of how easy it is for things to go wrong and are happy that you can operate it sucessfully, then go ahead. I wouldn't recommend one to a novice.

I could not in concience recommend a variac to anyone without pointing out the potential drawbacks. Even if used carefully so as to avoid acute damage to the batteries, I've been asked many times to diagnose problems with batteries caused by chronic miss-charging (both under and over). I also point out that a variac on it's own is technically illegal, since a power supply with an input voltage of more than 60 volt should have an isolated output.


I'm sorry if I sound negative a lot of the time, but there are many ways for things to go anoyingly, expensively, or dangerously wrong with an EV and I've seen and heard a lot of them. That includes cases invoving the destruction of a house and one of spending several hours in Hospital due to burns and lacerations. The last one was really nasty and the person involved was very lucky not to lose their sight. They were an EV builder of many years experience, who simply made a mistake with a high voltage system when they weren't wearing their safety gear.
Paul

http://www.compton.vispa.com/scirocco/
http://www.morini-mania.co.uk
http://www.compton.vispa.com/the_named

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Jeremy
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Postby Jeremy » Sun Aug 17, 2008 9:22 am

EVguru wrote:I'm sorry if I sound negative a lot of the time, but there are many ways for things to go anoyingly, expensively, or dangerously wrong with an EV and I've seen and heard a lot of them. That includes cases invoving the destruction of a house and one of spending several hours in Hospital due to burns and lacerations. The last one was really nasty and the person involved was very lucky not to lose their sight. They were an EV builder of many years experience, who simply made a mistake with a high voltage system when they weren't wearing their safety gear.


I know you mean well, Paul, but there are quite a few of us around who possess enough knowledge and experience to weigh up the hazards and make our own decisions as to the degree of risk we are prepared to accept.

In my view, if giving advice on request to someone with little or no knowledge then it's fine to point out the potential pitfalls, perhaps in some detail. On the other hand, assuming that all who are experimenting with EVs are a little short on knowledge and experience and need to be lectured on what, in your view, is the only right way to do something can be a bit irritating at times.

My own view is that there are many different, often equally right, ways of handling pretty much anything. The solution that is perfectly OK for one person's level of expertise and risk appetite, may not be appropriate for someone else. Thankfully we're all different and will inevitably want to do things our own way.

Jeremy

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Peter Eggleston
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Postby Peter Eggleston » Sun Aug 17, 2008 6:38 pm

I built my variac/ bridge rectifier charger based on the discription and diagram in Bob Brants book. I have an iscolation switch and a fuse on the output and an ammeter and voltmeter to monitor charging and I operate it using a timer.
Peter

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Postby EVguru » Mon Aug 18, 2008 5:48 pm

Peter Eggleston wrote:I built my variac/ bridge rectifier charger based on the discription and diagram in Bob Brants book.

Bob's book was one of very few reasonable sources of EV information when it was published. It is however is some places a very flawed book. In particular in the section where the conversion of a pickup is described and, 'the weight of the batteries will hold them in place'.
I have an iscolation switch and a fuse on the output and an ammeter and voltmeter to monitor charging and I operate it using a timer.
Peter


You're certinly operating much more safely than many, however when I taked about isolation, I mean that the output of the charger is directly connected to the mains when the charger is in operation. It's fairly dodgy in the US with houshold power at 110vac (220 split phase), but far worse at 230vac.

I'm not going to say don't use it, I have a variac setup myself, although it's never used unattended except when it is being used as a quasi constant current source by having a lightbulb in series with the output.

What I would say that it takes more methodical use than most people are capable of to not under/overcharge batteries. I would also say, don't use it outside where anyone could access the vehicle and definitely don't use it at a public charging point. An isolation transformer would make it safe from that point of view, but add a considerable amount of weight and bulk.
Paul

http://www.compton.vispa.com/scirocco/
http://www.morini-mania.co.uk
http://www.compton.vispa.com/the_named

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Postby EVguru » Mon Aug 18, 2008 6:16 pm

I know you mean well, Paul, but there are quite a few of us around who possess enough knowledge and experience to weigh up the hazards and make our own decisions as to the degree of risk we are prepared to accept.


I agree, but I want other people reading a thread to appreciate the difference between 'expert' solutions and 'casual user' solutions.

Humans make incredible adaptive systems, but poor monitoring ones.
Paul

http://www.compton.vispa.com/scirocco/
http://www.morini-mania.co.uk
http://www.compton.vispa.com/the_named

GregsGarage
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New features added.

Postby GregsGarage » Tue Sep 02, 2008 9:08 am

I have added 2 new functions to my managment module, a vacuum pump control circuit and a rpm limiter.

Both of these functions was on the "todo" list for the Volvo.

For the vacuum pump, I am using a pump fitted to late model Volkswagen group turbo charged petrol cars (part number of this pump is listed on my EV album entry). The pump is about the size of a Coke can and quickly charges the brake servo. It doesn't include a vacuum switch so I have wired it to the stop light circuit. This works very well but the pump runs more than it needs to. Since I already have a stop switch input I have added a simple timer routine to run the pump for a set interval. Changing the value of VACTIMER will alter this, at the moment set for 3 seconds. This setup is simpler than setting up a vacuum switch and should be more reliable, no vacuum switch to break or go out of adjustment.

The rev limiter was also on the Volvo's "todo" list. It is quite easy to forget to change up from 2nd to 3rd and so a limiter set for 5000 rpm has been added. This uses Pin 7 which has a On Board Counter that can measure up to 100khz, plenty fast enough. The counter is initialized using the command OBCA_Mode (0=disable, 1=0.1 sec time base). When the ignition key is turned on the counter is enabled. It then waits for a reading and checks to see if it is higher than the specified value. If it is then it shuts off the controller, otherwise it carries on with the rest of the functions. This also adds a handy pause to the program because it will wait till the next reading is availble in the counter register before rechecking the drive conditions.

So this same module should be able to be used on both my vehicles, I can just ignore functions that are not required and the program should run fine. So the Fiat won't use the rev limiter or vacuum pump, and the Volvo won't use the regen control or low cell voltage cutoff, although I could use the low voltage alarm output on the Volvo's e-meter for this.

Later this week I will upload the program onto the CC16 board and bench test it before putting it into the car. I have abandoned the Ping BMS boards and am hoping to use Peters slave boards in analogue ON/OFF mode.

The new program code on the 1st page of this thread has been edited with the changes.

Greg
Greg Fordyce

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Postby GregsGarage » Wed Sep 03, 2008 11:20 am

Last night loaded the program onto the board and tested. Fixed a couple of bugs. Still have to get the timer working for low voltage cutout and tempurature warning. At the moment when the low voltage cutout is triggered it turns on a warning light (battery light on dash) and buzzer. It should then cut the drive after 5 seconds. This prevents a dangerous situation which could be caused by instantly cutting power and gives the driver the chance to reduce throttle knowing that if he doesn't it will shut itself off. In the future I may look at a stepped throttle reduction as has been suggested by Paul.

Here is a picture of the board shown connected to the developers board (the top one) which has leds and dip switches for simulating inputs and outputs as well as screw terminals for connecting to real inputs and outputs.
Image
Greg Fordyce

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Timer routine fixed.

Postby GregsGarage » Mon Dec 22, 2008 5:07 pm

I have updated the code shown on the first page of this post with the software I have been running since early September. I am thinking of way to improve the charger control based on Peters findings that he has been able to control his charger without PWM output, just a simple on/off control. 8)
Greg Fordyce

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centrex
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Postby centrex » Sat Jan 31, 2009 9:51 pm

HI Greg
I have been following yours and Peter Perkins information on BMS systems.
I have been doing a fair bit of hunting on the web about the subject but have not been able to find information on the requirements for balancing the cells can you point me in the right direction for information.
I do not have an electric anything I would just like to know the requirements.
Regards
Centrex

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Postby GregsGarage » Sat Jan 31, 2009 11:06 pm

centrex wrote:HI Greg
I have been following yours and Peter Perkins information on BMS systems.
I have been doing a fair bit of hunting on the web about the subject but have not been able to find information on the requirements for balancing the cells can you point me in the right direction for information.
I do not have an electric anything I would just like to know the requirements.
Regards
Centrex


Hi Centrex,

If all cells could be guaranteed to have exactly the same manufacturing tolerances and are mounted such that they are all kept at the same temperature then balancing would be unnecessary. But this is the real world. Balancing methods depends on the cell chemistry, but I will assume we are talking lithium. I am by no means an expert, I have only been using lithium batteries for a few months.

With lead acid balancing can be done simply by overcharging the entire pack until all cells are fully charged and then topping up the water. This is not possible with Lithium cells (or indeed sealed lead acid batteries) so a alternative way has to be found. The simplest method is to monitor each individual cell and when it is fully charged, shunt the current around it so that the rest of the cells can also reach full charge. Cedric Lynch uses this method with his cell protectors and is well documented on his Agni motor site. http://agnimotors.com/home/index.php?option=com_content&task=view&id=27&Itemid=39. The shunted current is wasted as heat, but only on the order of around 1 watt per cell. Cedric's cell protectors were designed for Thundersky LCP cells, but the LFP type cells that we are using have a lower voltage requirement, which is why Peter started his BMS project.

This balancing method works very well on lithium chemistries because of the way they behave when they approach a full charge. During most of the charge the cells stay at around 3.2-3.3 volts each. As the cells reach their full charge their voltage starts to climb very rapidly, particularly when they reach around 3.5 volts. This makes it very easy to identify which cells need to be bypassed. Other possible methods of balancing could involve actively transferring current from fully charged cells to less charged cells, but whether they would be that much more efficient to justify their cost will be down to the end users requirements. For the self builder the first method works well.

So balancing just makes sure that all your cells stay at the same level of charge, but that is by no means all that is required for the long life of any battery pack. Lithium cells will quickly be damaged if they are over or undercharged. So you also need a system to monitor each individual cell voltage to insure you keep the cells within their permitted range. It is no good monitoring pack voltage, you have to do this on a individual cell level, although as a failsafe I have programmed my controller with pack upper and lower limits. The charger will also shut off when the pack reaches its required voltage, but these are fail-safes only. Cedric's and Peter's systems both directly start throttling back the charger so it doesn't overcharge any cell. What you end up with is a battery management system and because it has to monitor individual cells the cost of the system goes up the higher your pack voltage is. You probably would want to add more functions to your BMS system such as temperature, SOC and other things as well.

Lastly, sign up to the Yahoo Thundersky group, lots of good advice and recent discussions in the last week about balancing issues.
http://tech.groups.yahoo.com/group/ThunderSky/

Greg
Greg Fordyce

Daewoo Matiz
http://www.evalbum.com/4191


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