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  #31  
Old 07-10-2012, 07:43 AM
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Default Re: Did Jack say he needs a BMS?

Ideally you'd do some cell testing before putting them into your car. My first year I had all my cells easily accessible in my trunk so I did use the car as a test bed, and I did have a Cell log 8 that I moved around to check groups of cells under load and charge to get an idea of their behavior. I then put my weakest cells in easily accessible locations in case they might need attention.
I think cell monitoring is a good idea, I just haven't found a solution I really like.
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  #32  
Old 07-10-2012, 08:09 AM
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Default Re: Did Jack say he needs a BMS?

Coulomb,
Do you have a thread where I can get an update on your BMS progress? I hope things are coming together.

I don't debate the need of monitoring or management. It is obvious that we don't need cell level monitoring in all cases as many demonstrate by not doing it. I am also not overly concerned about the inconvenience factor of the failure occurrence but that is a real issue is some cases and it would suck.

The main reason for me to implement a monitoring system is to gather more information about battery behavior in the field to share with the effort. I see it purely as a test bench on wheels at the present time and not so much as critical operational component. However operating margins will be a moving target as technology changes. Hopefully they will move towards the forgiving side which we will be able to confirm or deny with our monitoring methods.

The data will speak for itself and the more of it shared the better. The only way to say the data is of little or no value is after we have collected and analyzed it over time.

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  #33  
Old 07-10-2012, 08:15 AM
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Default Re: Did Jack say he needs a BMS?

Quote:
Originally Posted by JRP3 View Post
Ideally you'd do some cell testing before putting them into your car. My first year I had all my cells easily accessible in my trunk so I did use the car as a test bed, and I did have a Cell log 8 that I moved around to check groups of cells under load and charge to get an idea of their behavior. I then put my weakest cells in easily accessible locations in case they might need attention.
I think cell monitoring is a good idea, I just haven't found a solution I really like.
You are doing it right IMO.

I haven't found a solution I really like either but I am giving a shot at developing one.
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  #34  
Old 07-10-2012, 12:55 PM
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Default Re: Did Jack say he needs a BMS?

"I'm thinking if you have 35 cells, you might more often get away without close monitoring than if you have 228 cells."

Is there anyone running a DIY project with a string of 228 cells in series? Not even sure if there is a controller on the market that handles ~800 volts(228s LiFePO4 on the charger set to a max of 3.5v). I'll be running 100 cells and know others running a similar number but there's little out there available that handles more than this unless it is an OEM solution or otherwise unobtainable. Running a higher number of cells in series allows you to remove a cell, jumper around it and not lose as much capacity as say 33 larger Ah cells with the same total capacity where a cell is lost. Say a 100 mile car(for simplicity), the difference is between losing 1 mile or 3.
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Old 07-11-2012, 09:48 AM
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Default Re: Did Jack say he needs a BMS?

I've got one volt meter dedicated to monitoring one cell out of the pack to see what it's doing. I've been thinking that I might move the pickup leads from one cell to another cell once a week to see how each cell is performing. I've got 46 cells in my pack. So after almost one year I will have monitored each cell for one week. When I started using the pack I had each cell balanced to 1/100th of a volt. It'll be interesting to see how they all perform. And to see if this method of monitoring helps me find cells that aren't performing well.
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  #36  
Old 07-11-2012, 10:57 AM
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Default Re: Did Jack say he needs a BMS?

Are your leads clamp-on or screw-on? I wouldn't want to mess with the terminals frequently. While an annual checkup sounds good, I don't think that will find a bad cell before there's other indicators.

If possible you could hook up two DVMs rated for half your pack voltage or greater to each half and watch that for a few days. If you see greater variance on one half you split that side etc. until you identify the gimp(s).
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Old 07-11-2012, 11:09 AM
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Default Re: Did Jack say he needs a BMS?

I know nothing about electronics, but ...

Couldn't you use an LED (+resistor) on each cell, then a splitter resistor to target a given voltage, say 2.7 volts. All cells will be lit that are over 2.7v (use a big resistor, and there will be little draw, it takes almost nothing to run an LED.

When a cell hits 2.7 earlier than the rest by a good amount, then replace just that cell? You could put the LED's anywhere, and even have an "on-off" switch for them. Every once and a while draw your batteries down to whatever target you use to check them.

Cost for this is under $10. Less if you already wire and some LED's floating around, and some resistors.
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  #38  
Old 07-11-2012, 02:00 PM
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Lightbulb Re: Did Jack say he needs a BMS?

I came up with an idea similar to yours. See:
http://www.diyelectriccar.com/forums...bms-75822.html

If the cells are easily visible, LEDs on each cell might make sense. Otherwise you need to run wires, which is a hassle and could cause a problem if there is a short. The BMS ICs are pretty reasonable at about $1/cell, and have the advantage of using a daisy-chain system for communication with a remote indicator/controller, and they also have drivers for active load and charge balancing.

I do think it would be worthwhile to build a simple low-power voltage monitor into each cell (or pack) to give a visual indication of charge if they are used as a backup system or stored on a shelf as spares.
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Old 07-12-2012, 10:57 AM
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Default Re: Did Jack say he needs a BMS?

Quote:
Originally Posted by Ziggythewiz View Post
Are your leads clamp-on or screw-on? I wouldn't want to mess with the terminals frequently. While an annual checkup sounds good, I don't think that will find a bad cell before there's other indicators.

If possible you could hook up two DVMs rated for half your pack voltage or greater to each half and watch that for a few days. If you see greater variance on one half you split that side etc. until you identify the gimp(s).

My leads have clamps to make them easy to move without unbolting the cell contacts. You're right, one checkup a year is a bit long to wait. The way my pack is stored, it wouldn't be hard to move the contacts each day, so I could check a different cell every day. Or I could get a helper, and perhaps once a month I would run through every cell with the leads while my helper watched the gauge and called out numbers. I'm thinking the once-a-month plan is easier and more productive in finding cells that aren't up to speed.
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  #40  
Old 07-12-2012, 11:41 AM
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Default Re: Did Jack say he needs a BMS?

Get a Cell log 8 and move it around, it's much easier and faster for occasional checks. Don't leave it hooked up to any one group of cells much longer than another since it can create an imbalance over time. I think it only draws power from the first 7 cells, there is a mod to make it draw from all 8, but I just check 7 cells or less at a time.
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