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  #31  
Old 01-05-2011, 11:09 AM
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Default Re: To BMS or not to BMS

Not even gonna touch the BMS-discussion with a ten feet pole, that topic is way too infected for me to even bother with and you seem to be quite capable of pissing each other off without my help.

However:

Quote:
Originally Posted by rwaudio View Post
I think "spagetti" wiring is the worst excuse not to use a BMS that anyone has ever come up with. Use some common sense, wire it properly and safely, secure and protect the wiring. If you aren't capable of that you shouldn't be building an EV in the first place, and that is no fault of a BMS.
I once worked for the Swedish Telephone company. All of the old electro-mechanical telephone stations had racks of hundreds or thousands of relays, all the wires were painstakingly bundled together with some kind of waxed string in perfect, pedantic order! Every single rack had hundreds of single wires running straight as arrows, cut just long enough to reach their terminal and between the racks ran thousands and thousands of wires, all color coded, bundled together and extremely easy to trace despite the sheer number of them.

Spaghetti wiring is nothing but sloppy craftsmanship. Period.
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I like the Soliton's hi-tech build and ability to deliver whopping doses of current until someone screams "Uncle!"
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  #32  
Old 01-05-2011, 11:23 AM
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Default Re: To BMS or not to BMS

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Originally Posted by gottdi View Post
Like Frodus said, this is not focused on any one person.

Now you see, this is the issue with this ongoing debate. All the professionals from what I have been reading ONLY state this is what I have heard from.... Holy crap guys. I hear that crap all the time. My brothers cousins sisters brothers, friend said...........WOW, it must be true.
I've heard of more issues DIRECTLY from the customers. Not from this person who told me that, who heard it from someone. I've actually helped these particular people deal with more issues without a BMS than not.
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  #33  
Old 01-05-2011, 11:33 AM
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Default Re: To BMS or not to BMS

I'm going to run on a bit so be prepared.

I'll throw this out again on the pro / con BMS debate and let you all decide if it is a BMS and if it's worth looking at during planning your system

Right now using off the shelf stuff we (Myself, RWaudio and a few others) are actually building this.

We are going at it with a top balancing, limited voltage charge protocal and LVC or LVW.

What we plan is using, one, reletivly inexpensive, isolated DC to DC Converter attached to each battery (large format prismatics) or battery group (small formats other then prismatics) to charge it.

The DC to DC is set to be able to only charge to a specific set voltage. The DC to DC then CAN NOT/WILL NOT over charge the cell. Since many of the DC to DC units can have a trim pot each unit can be adjusted to bring your cells in to perfect top balance.

So for top end charging we set the voltage of the DC to DC to a safe 90% for the cells. Now the DC to DC units I am using (thanks RWaudio) can pass 25 amps at 3.65 volts and will accept any voltage between 35 and 76 volts. They can be turned on and off individually and have over and undervolt and over temp protections and a lot of others I don't understand.

Chargeing power can be handled by any of a number of inexpensive units with whatever voltages within the range of the DC to DC units because of the DC to DC units built in protection protection.

Then for LVC or LVM we use a simple voltage device like the Cell Log8M to monitor each cell (large format prismatic) or battery group (small formats other then prismatics). The Cell Logs have a warning system that can be set up to monitor each cell and trigger an alarm that could be used as an audible warning or to trigger an event such as power cut back, main contactor disconnect or whatever.

So for bottom end we set the Cell Logs for some safe limit voltage say 80-90% DOD. What happens here depends on your set up It could shut you down until you fix/charge or with the proper controller back off power each time a cell log hits low voltage.

This should keep you in the fat/flat area of the charge/discharge curve and do wonders for your pack life.

As long as you plan your pack size for the middle 80-90% of your pack this shold work pretty well.

Let's face it, when you consider the above it's not unlike filling a gas tank. You should never fill a tank all the way (100%), If you fill the expansion space you're eventually going to get a boom when things heat up. Most people never use their reserve fuel, if the do they'er in trouble eventually.

Remember this is all off the shelf stuff so it's going to be a bit crude. and have some holes in it that require personal monitoring. I'll depend on you smart guys to find a way to put this all on a board, with programming to do everything including washing the dishes.

When you get down to it, I'll have a 50 cell (50, 3 cell buddy packs in my case) protective charging system with high and low cell monitoring and LVC or LVM.

You will also have very high charge rate, it could be a 48 volt dump pack with a tickle charger at home and you could carry a good variable input voltage 48 volt 20 amp charger or power supply with the car. You don't need fancy/fussy $1500.00 battery chargers. and +$1000.00 BMS.

Again using off the shelf components, I've got everything so far for $1000.00 +-10%. Want to bet what some smart EE could come up with?

Sorry, I do run on.

Jim
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  #34  
Old 01-05-2011, 11:45 AM
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Default Re: To BMS or not to BMS

Jim, Thats a great idea.... It's referred to as the power conversion type of BMS I stated above. I used pretty much the same system on my lead pack for my motorcycle several years ago. individual DC-DC converters that charged each battery. The only diff was, I didn't have Cell Logs, I used microcontrollers on each DC-DC that looked at volts and controlled the ON/OFF of the DC-DC so they would use less power in standby. They'd comm to a mother unit that could cut off the contactor if needed.

Functionally, your cell-log and dc-dc converter system is the same. I actually have 36 Vicor 3.7V DC-DC 20A converters with a 48V input sitting in my garage. I also have a few 48V DC-DC power supplies that can output 800W on 110V and 1500W at 220 and can be hooked together. Only thing missing is an HVC/LVC. The cell-log would work great.

Another thing...... your system balances the pack EVERY charge. Resistive balancers can often take several charges to balance an unbalanced pack. With yours, you never have to worry. Charge: Protected. Discharge: Protected (depending on how you "react" when you hit LVC).
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  #35  
Old 01-05-2011, 12:04 PM
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Default Re: To BMS or not to BMS

Quote:
Originally Posted by Jimdear2 View Post
What we plan is using, one, reletivly inexpensive, isolated DC to DC converter attached to each battery (large format prismatics) or battery group (small formats other then prismatics) to charge it.

The DC to DC is set to be able to only charge to a specific set voltage. The DC to DC then CAN NOT/WILL NOT over charge the cell. Since many of the DC to DC units can have a trim pot each unit can be adjusted to bring your cells in to perfect top balance.
I want to point out that you need to use very reliable cell level DC to DC Converters. If you are using units with a 100,000 hour MTBF (mean time between failure) with a 50 cell pack then you can expect to experience a cell not charged issue about once a year. It is important that your system can detect that failure ASAP, hopefully before you are 5 miles from home.
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  #36  
Old 01-05-2011, 12:17 PM
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Default Re: To BMS or not to BMS

Frodus,

Thanks for the praise, but I can't take all of the credit.

RWaudio is also on this path (a bit ahead of me in fact in actual hardware and research) and I've seen many others I think that are getting there or have been most of the way there (like you).

IMHO the OEM battery makers, at least for the prismatics should be the ones picking up on this. I.E. each battery comes with the charger built in or snapped on and a set of 2 wire connector cables to go between DC to DC units for the charger power loop. They can have voltage sensing to turn them selves off when the battery reaches charge, warning triggers, etc. etc. built in. The DC to DC I've bought cost about $5.00 each in the volumn the battery makers would need I bet that could go waaaaaay down. each board could be given a unique ID so all of the sensing could be a CAN bus loop and a generic CPU could sort out what battery is doing what for diagnostics and servicing as well as LVC or LVM, no matter what the battery count.

Enough preaching,
Jim
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Last edited by Jimdear2; 01-05-2011 at 12:21 PM.
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  #37  
Old 01-05-2011, 12:26 PM
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Default Re: To BMS or not to BMS

Quote:
Originally Posted by Jimdear2 View Post
So for top end charging we set the voltage of the DC to DC to a safe 90% for the cells. Now the DC to DC units I am using (thanks RWaudio) can pass 25 amps at 3.65 volts and will accept any voltage between 35 and 76 volts. They can be turned on and off individually and have over and undervolt and over temp protections and a lot of others I don't understand.
No problem Jim, my personal opinion is it's the best system for my car. I won't push the system on anyone, but I'd be happy to help anyone with similar goals.

I'm designing boards for 15 converter blocks (48v nominal input, 48v nominal battery block), Onboard 20 turn pots for easy output voltage adjustment. It'll have monitoring capability but those parts could be left out if not required/wanted.
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  #38  
Old 01-05-2011, 12:31 PM
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Default Re: To BMS or not to BMS

Quote:
Originally Posted by EVfun View Post
I want to point out that you need to use very reliable cell level DC to DC converters. If you are using units with a 100,000 hour MTBF (mean time between failure) with a 50 cell pack then you can expect to experience a cell not charged issue about once a year. It is important that your system can detect that failure ASAP, hopefully before you are 5 miles from home.
EVfun,

For our setup, in competition we might see 25 runs a year and lots of persona management. For people with on the road EVs that's what the Cell Log warning systems are for.

Each Cell Has A High and Low Voltage Monitor. This should catch a bad board pretty quick. The Low Voltage Warning Buzzer would go off and you would need to go to the Cell Logs to find the bad Dc to Dc.

A professionally designed board with CAN bus would do all of this and I'm sure that some EE or programming wizbang could even figure how to bypass that cell. Im sure this could all be made modular snap togther, plug in and a lot less expensive then what's out there.

Jim
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  #39  
Old 01-05-2011, 12:42 PM
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Default Re: To BMS or not to BMS

Quote:
Originally Posted by rwaudio View Post
No problem Jim, my personal opinion is it's the best system for my car. I won't push the system on anyone, but I'd be happy to help anyone with similar goals.

I'm designing boards for 15 converter blocks (48v nominal input, 48v nominal battery block), Onboard 20 turn pots for easy output voltage adjustment. It'll have monitoring capability but those parts could be left out if not required/wanted.
RWaudio,

Lordy! I hope i'm not coming off as pushing this on everyone. If I am I'm sorry. I guess I am a bit enthuisatic, but that's my nature when I see/think of something I think is wonderful.

An up front thanks again to you.

I've been thinking about this for months, but not being in any way shape or form "electronic" I couldn't find a decient DC to DC (didn't/don't know how to look). The board you showed me tripped me over the edge and made it all come together for the setup I want for our tractor.

Jim
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  #40  
Old 01-05-2011, 12:47 PM
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Default Re: To BMS or not to BMS

Quote:
Originally Posted by EVfun View Post
I want to point out that you need to use very reliable cell level DC to DC converters. If you are using units with a 100,000 hour MTBF (mean time between failure) with a 50 cell pack then you can expect to experience a cell not charged issue about once a year. It is important that your system can detect that failure ASAP, hopefully before you are 5 miles from home.
As per the dc/dc converter data sheet:
RELIABILITY CHARACTERISTICS

Calculated MTBF 2.8 10
6 Hrs. Telcordia TR-NWT-000332; 80% load,300LFM, 40oC Ta

Calculated MTBF 1.8 10


6 Hrs. MIL-HDBK-217F; 80% load, 300LFM, 40oC Ta
Demonstrated MTBF >25 106 Hrs. Field demonstrated MTBF

Formatting didn't come through 10
6 Hrs. is 10 to the power 6 or multiples or one million hours.

Demonstrated MTBF greater than 25million hours seems pretty good to me.
The power supplies I am using have a 5 million hour MTBF.
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