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Old 12-27-2008, 02:49 AM
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Smile Personal Experience with Prismatic LiFePO4 Cells

Folks, I wanted to share some experience, I along with several other club members have had with Prismatic LiFePO4 cells, over the past year. In the hopes it helps keep others from damaging precious EXPENSIVE batteries:

1. When you build up a pack of cells, say a 12.8volt nominal battery of 4ea cells. Be sure, and place restraining plates on the end faces, bind the whole assembly with strapping to keep the cells from ever having the chance to bulge. If they ever bulge, and I've had brand new unused cells do this on their own, the internal plates start to spread apart, dry spots will develop and your capacity will slowly dwindle, to the point the battery will fail. I know this from dissecting a failed battery.

2. Interconnecting busbars, should be sized large enough that there is very, very little noticable change in the busbar's temperature after your typical EV drive. NTEAA members have progressed to using 1" wide x 1/2" thick aluminum bars, which accomplishes several things. The posts of the LiFePO4 cells are aluminum, therefore using aluminum threaded studs screwed into the cells, with aluminum busbars, held down with a double nut system, all lathered up with "No-Ox ID A" for electrical connections. There is very little heat generation at the busbar, and the batteries operate nice and cool. Keeping the cell studs and busbars aluminum also eliminates dissimiliar metal reaction or galvanic corrosion.

3. Charge regulators, are installed on each individual cell, which once a cell reaches 3.65vdc, that particular cell stops taking on energy, shunting the majority of it over the the remaining cells. The experience has been, once one regulator comes on, showing a cell is fully charged, the remaining regulators for the rest of the pack, usually engauge within just a few minutes. If you remain within the nominal discharge C rating of your LiFePO4 cells, and you truely balance them during every charge, they want to stay balanced during the discharge cycle. Use your controller's internal cutoff setting to keep from over discharging the pack, and stay within limits of your pack discharge C rating. For the most part drive sensibly.

4. Capacitor Bank, I've only just started testing with this, but so far the results look good. What I'm proposing, is build up a large capacitor bank for the pack voltage you are using, place it between the controller and traction pack. For the most part, paralleled onto your traction pack, as close to the pack as possible. What it does, is it smooths out the pulsing of the power extraction from the batteries, that occurs when the controller requests energy. The O'Scope shows the power extraction to be in pulses, similar to what is being sent to the motor, and the capacitor bank, helps smooth out these pulses so the traction pack doesn't receive such a constant electrical shock. It keeps the energy extraction at a more constant level, thus in theory, extending the life of the traction pack, controller, along with the traction pack running cooler under load, and extending the range just a smidgen.......

For photos of what I've along with others in our www.NTEAA.org club are doing with LiFePO4 batteries, charge regulators, restraining, pack build, monitoring, etc. look at:


http://www.flickr.com/photos/mbarkle...7609086829232/

http://www.flickr.com/photos/mbarkle...7604137306905/

I hope this helps you with your LiFePO4 pack, and would be grateful for any information that you've experienced as well.
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Last edited by TexomaEV; 12-30-2008 at 08:48 PM.
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  #2  
Old 12-27-2008, 06:48 PM
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Default Re: Personal Experience with Prismatic LiFePO4 Cells

Great info, thanks!

Do you mind making that a "Maintaining LiFePO4 Batteries" wiki page (or mind if I do it?)?

We could probably include david85's testing as well.
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Old 12-30-2008, 10:43 AM
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Default Re: Personal Experience with Prismatic LiFePO4 Cells

Mike,

thanks for excellent info! Can we get more details about those cell regulators? I have been keeping an eye on available options, currently there are a few, with different features and cost, perhaps these can be a pretty good competition when the time comes to make a purchase. Is there a web site or a contact Email or phone number to get those regulators?

Thanks
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Old 12-30-2008, 02:27 PM
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Smile Re: Personal Experience with Prismatic LiFePO4 Cells

Quote:
Originally Posted by dimitri View Post
Mike,

thanks for excellent info! Can we get more details about those cell regulators? I have been keeping an eye on available options, currently there are a few, with different features and cost, perhaps these can be a pretty good competition when the time comes to make a purchase. Is there a web site or a contact Email or phone number to get those regulators?

Thanks
I have a PDF, datasheet on the "Bat-Guard" if anyone wants it. I don't know of a way to upload it here on the Forum, it's 81K, to large to attach to this post. They are manufactured in Plano, Texas. They can be customized for AGM's or any other chemistry, voltage as well. I'll see what I can do about getting more direct info posted on the Forum.

Here's what I was able to extract from the Preliminary DataSheet:

BAT-GUARD Preliminary Datasheet

Features:

* Stable and accurate Voltage shunt to protect LiFePO batteries from overcharging.
* Heat sink capable of bypassing 15 Watts of energy.
* Reverse Voltage capability of 5.0 Volts DC without damaging internal circuitry.
* Automotive Blade fuse to protect regulator if DC current is too high.
* Solid State electronics encapsulated for long life and protection from debris.
* 0.375” Ring Lug with 5” 18 gauge red and black color coded wires for connection to battery.
* Amber LED that comes on when Regulator is absorbing energy.
* Red LED that comes on if fuse is blown.
Description

The Battery Regulator incorporates precision solid state circuitry to prevent over-voltage conditions when
charging LiFePO batteries with charging currents of 4 Amps. It contains an ultrastable voltage reference and
shunt circuit that will prevent LiFePO batteries from reaching an overcharged state when used within design
limitations listed. Other voltages available upon request.

Ordering Information: Model Number: BatReg3.67

Absolute Maximums Minimum Nominal Maximum Units Test Conditions
DC Current 4 Amps Notes 1,2
DC Voltage -5 3.75
Ambient Temperature 40 C 4 Amps Current
Power Disipation 15 Watts

Specifications: Minimum Nominal Maximum Units Test Conditions
Clamp Voltage 3.648 3.670 3.692 Volts 1 Amp DC current
Current if not in Clamp Mode 225 uA 3.5 Volts

Notes:
1) Adequate free air space to allow dissipation of power
2) Heat Sink will get hot to the touch, and may cause burns
3) If fuse blows, Battery Regulator will not be able to protect battery against over-charging.


LL Labs reserves the right to make changes at any time in order to improve design and to supply the best product possible.


LL Labs
Issue A, Oct x, 2008 2908 White Oak Dr.
Plano, TX 75074

Mechanical: Dimensions: 2.36” W x 1.38” D x 2.85” H (60mm x 35mm x 73mm)
Storage Temperature -40 to 85 Celsius
Operating Temperature -40 to 85 Celsius Note: derate by 0.2 Watts/C above 40C Amb.
Mount using double sided tape Not included
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Last edited by TexomaEV; 01-03-2009 at 07:09 PM.
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Old 01-01-2009, 04:49 PM
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Default Re: Personal Experience with Prismatic LiFePO4 Cells

"The Battery Regulator incorporates precision solid state circuitry to prevent over-voltage conditions when
charging LiFePO batteries with charging currents of 4 Amps."

4amps is all we can charge the cell at? Maybe I am not understanding...

What is the maximum AH I can charge the pack at, say I have a portable generator that can push out 20Amps at 120V???
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Old 01-01-2009, 06:38 PM
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Smile Re: Personal Experience with Prismatic LiFePO4 Cells

Quote:
Originally Posted by Bowser330 View Post
"The Battery Regulator incorporates precision solid state circuitry to prevent over-voltage conditions when
charging LiFePO batteries with charging currents of 4 Amps."

4amps is all we can charge the cell at? Maybe I am not understanding...

What is the maximum AH I can charge the pack at, say I have a portable generator that can push out 20Amps at 120V???
You can charge at as high of a amp rate your cells are rated to take, but as the pack voltage climbs and you are reaching it's charge out voltage, you want to taper off to 4amps or less for the final charge/balance, so at that rate the regulators will kick in and keep the cells that fill up first from overcharging while the others catch up, and stay withing the operating range of the regulators.

The current vehicle who has the "Hot Juice Electric" BEQ1 regulators, can handle around 2amps or so, and his charger cuts back to that or less, towards the latter part of the charge cycle. The "LL Labs" Batt-Guard regulators can handle 4amps.

For instance, we're starting to work with a 100Ah LiFePO4 & 110Ah AGM hybrid pack, which is to be charged with a Zivan NG3 charger, it's programmed so that when the pack reaches it's knee voltage setpoint, it drops down to around 2amps for the final charge/balance of the pack. There will be Batt-Guard regulators on the LiFePO4 Cells, which are ganged together in 12.8vdc battery modules, and paralleled onto each individual 12vdc AGM battery. Attached to this hybrid pack will be a 44000uf 200vdc rated capacitor bank as well.

With the charger set to drop well below the regulator current limit one could just about plug in the EV, and forget about it until the next morning. I say this just because I've left a 4cell pack fully charged, on a charger for well over 10hours one day, and every LiFePO4 cell was protected during that session, using the Batt-Guard regulators. I'd recommend maybe a timer, so you didn't waste that extra energy, as you'll soon find out how long it takes for your pack to fully charge, after a few cycles.

Hope that helps, answers your question. If not let me know.
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Old 01-01-2009, 08:55 PM
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Default Re: Personal Experience with Prismatic LiFePO4 Cells

I threw together an article from your notes. My only change was the capacitor bank close to the controller instead of the battery pack. Please let me know if anything needs to be changed / adjusted.

http://www.diyelectriccar.com/forums...ad.php?t=25818

Thanks again!
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Old 01-01-2009, 09:07 PM
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Smile Re: Personal Experience with Prismatic LiFePO4 Cells

Clint, thanks for taking the time to put that together. I hope it helps others utilize LiFePO4 to the best, performance, the chemistry can offer them. It offers up a means of using these large format LiFePO4 cells, and not feel like they have to use hundreds of small cells.
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Old 01-02-2009, 07:46 AM
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Default Re: Personal Experience with Prismatic LiFePO4 Cells

Mike,

I have couple more questions after looking at your pictures:

- your cells are white casing, I thought TS LFP were yellow, are yours TS LFP or some other name or model?

- considering comment about sagging plates under their own weight, would it make more sense to mount cells horizontally? Even with strapping and restraining plates mounting position could make some difference over time, right? I think your fornts are horizontal but those in the back are vertical, right?

- what about temperature control? How much are they effected by ambient hot/cold weather and how much do they heat up during charge/discharge? Any consideration for insulating/venting battery boxes?

Thanks
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Old 01-02-2009, 01:01 PM
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Smile Re: Personal Experience with Prismatic LiFePO4 Cells

I have couple more questions after looking at your pictures:

- your cells are white casing, I thought TS LFP were yellow, are yours TS LFP or some other name or model?

** These cells, certainly are not TS LFP, These were purchased from LionEV & Beepscom.com

- considering comment about sagging plates under their own weight, would it make more sense to mount cells horizontally? Even with strapping and restraining plates mounting position could make some difference over time, right? I think your fornts are horizontal but those in the back are vertical, right?

** The best way, from what we can tell is they need to be installed with the terminals up. There is very little electrolyte in the cells, and it tends to settle to the point the top portion of the plates aren't saturated, as there is some distance from the top of the cell to the actual plate material in the cells. Plus there's always the chance that the terminals might not have the greatest seal, and then there's that center plug as well.

** Originally John was toying with laying the ones in the front of the Fiero down, but he's since retooled his battery trays to allow them to sit up.

- what about temperature control? How much are they effected by ambient hot/cold weather and how much do they heat up during charge/discharge? Any consideration for insulating/venting battery boxes?

** Any way you could keep them at a constant temperature, certainly couldn't hurt, that's for sure. If you could keep them as close to 70F you'd probably get the best life out of the cells, but I'd have to look at the spec sheets on the LiFePO4 cells before quoting operational temps for sure.

** I'll ask if the Fiero owner can shoot some temps of the batteries before/after a run. But the times I've been around the car, they seem relatively close to ambient temperature, with having the aluminum busbars installed.
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