Lithium Battery Update

[SamoaPower]

To see the intro posting to this topic, go to: http://www.fieldlines.com/story/2009/8/12/214144/046

I've been running a 55Ah LiFePO4 battery pack in the RE system for about four months and have been watching them pretty close to see if they live up to expectations. I can only say that I wish I could have had these years ago. Of course, they have only been out a few years but I still can wish. Because of their allowed DOD (depth of discharge), this pack is equivalent to a pair of T-105s cycled at 25% DOD, which is advisable for long life.

After receiving the 100 A123 cell order from Hong Kong, the next step was to assemble them into a pack. I used 5/16" x 0.040 copper strips for interconnects and soldering these to the cells without damage proved to be the biggest challenge. I used my largest iron, 125 watts with a ½" tip, but even this wasn't enough heat to do the joint quickly. I took to supplying additional heat to the iron tip just prior to making the joint by preheating the tip in the flame from a propane torch. This worked reasonably well and was quickly followed by a cooling quench from a wet sponge. All cells survived this torture without excessive cell heating.

A simple wooden jig was made to hold 12 parallel-connected cells in a row for the soldering operation. Eight of these 12-cell rows were assembled with their parallel connecting  strips on each end of the cells. Next, four of these row assemblies were stacked in another jig and the short series connecting strips (4 per row pair) were added. This results in a 4S12P 48-cell pack. Two of these packs were then stacked and parallel connected for the final 4S24P 96-cell pack.

The final pack weight was just under 17 lb, a far cry from the 125 lb of two T-105s. The assembled pack size is 7.5 x 13 x 2.75", which fits nicely on a small shelf on my system panel. No more stumbling over batteries on the floor.

Since I'm home all the time, I intended to monitor the pack performance quite closely. I put off the BMS (Battery Management System) build until I have more data. It's been designed and the circuits have been breadboard tested. Only etching the PC board and assembly remains.

To monitor the system I built a panel with eight DPMs (Digital Panel Meter) so I could observe the important parameters simultaneously. They look at the individual series cell voltages (consisting of 24 parallel cells each), total pack voltage, source current, battery current and load current. A separate digital thermometer looks at pack temperature in two places.

How does it perform? Exactly as expected! There have been no surprises. True to the rumor that the A123 cells hold their balance quite well, I've only given them a top-off balancing charge a few times. The cells are usually within 20mV of each other. I haven't seen the pack voltage go below 13.2V even after a long evening of ceiling fans and various lighting loads. I usually take about 40-45Ah out of them per cycle. My normal daily need is for about 70Ah with the PV supplying the remainder during the day. The battery usually tops off by 10AM so I'm not short on source power. The thermometer readings are pretty boring. I've yet to see ANY temperature rise over ambient even with charge/discharge currents of 25A.

Am I a happy camper? You bet!

[zap]

Thanks for the update SamoaPower.

I've been wondering how your pack has been fairing and from the looks of things it seems to be performing wonderfully.

I would think we'll be hearing more from Commanda and her thoughts about lithium soon also.

It appears you still intend to go with a BMS or would you forgo that if the cells stay in balance?

I've read much on the A123 and Konion cells staying in balance without a BMS as long as they're treated nicely.

I have a pack I'm using on my e-trailer although I don't know much about it other than it has cylindrical cells.

Amazing stuff for sure.

For many endeavors the price of lithium is a 'no brainer'... radio control vehicles, electric cars and bikes, and all sorts of electronics... things where weight and/or high discharge rates make them a perfect fit. It will be interesting to see just how long a life span lithium gives in real world use.

[willib]

Hey , been a while!

I like your setup

I have heard nothing but good things bout the A123 batteries.

a Massachusetts company that just signed a contract with china to supply batteries to them.

That looks so cool , you did a nice job on them.

Battery technology is rapidly advancing (thank God), its about time.

Stanford U has produced a painted carbon nanotubes onto paper ,super cap .

Some companies are talking about or using already nanotubes for a electrode in Li ion batteries.

I know that we have had our differences in the past , and i'm sorry for what i have said to you.I was extremely rude and down right unfriendly. And i would be very happy if you could forgive me?

.

William

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[David HK]

How do you know which wire goes where?

David in HK

[SamoaPower]

Hi Zap,

Thanks for the comments.

Yes, this has been a rewarding project. I kinda wish I could fast-forward to see the end game.

I'm also looking forward to Amanda's report on her LiFePO4 system.

Yes, I will definately finish the BMS since I don't want to closely monitor them forever and it does more than just balance the cells.

Bill

[SamoaPower]

Bill,

Thanks for the kind words. Apology accepted, of course. I don't hold grudges. Yes, the whole nano technology thing is exciting.

Take care.

Bill

[SamoaPower]

David,

Yes indeed, it's really kind of a rat's nest. Since the system is still somewhat experimental - hardly a week goes by that I don't make some changes, I opted for easy access and short leads instead of neat and pretty. Nice metal boxes with wiring in conduit doesn't seem to be in my future.

A schematic certainly helps.

Bill

[independent]

It's a really tidy setup. Inspiring even. Makes me think about sorting out my mess of a RE system.

Really like the panel meters. Wouldn't mind an explain of what all the boxes are for? I figured out the breakers and emergency cut-off switches.

[dnix71]

If you are going to do that many cells for $1 a tab there used to be a store near me that would spot weld them professionally.

How much more would solder tabs have cost if you bought them that way from the Hong Kong supplier?

[SamoaPower]

dnix71,

Thanks for the input. Actually, the cells did come with welded tabs since the cells were removed from new Dewalt tool power packs. However, I learned some time ago when assembling packs for RC from the same source, that the spot welds are quite tiny (about 1 mm^2 x 4) which I deemed not to be adaquate for high current applications. Also, the welds were quite unreliable with about 30% of the welds breaking with just a light tug and a fair number of them were broken as received.

A buck a tab would add almost $200 to the cost of the pack which is already pretty pricey at over %500. To me, it was worth the trouble to solder them and I sleep better.

Bill

[willib]

Thanks

Yes it is exciting , i get daily updates on my google homepage.

.

Bill

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[SamoaPower]

Thanks for the flowers but I think you're just being kind.

Okay, here we go.

Starting at the upper left and moving right: A Xantrex C40 charge controller connected for diversion (dump) load, the DPM monitor panel, another C40 connected as a low voltage disconnect load controller, nine DC load circuit breakers of various values mounted on a brass buss bar, below them is the negative load buss bar, below that are the 400W dump load resistors in a shield box (they do get hot), going back toward the left is a terminal block (gray) for source and dump load connections, then source and battery bank switches (red), below and to the left of the left red switch is the plus battery buss bar with mounted source circuit breakers, hanging below the buss bar is a 100A replacable link cartridge fuse in the battery plus line (hard to see), back to the right are three precision shunts for the current measurements. The battery is on a small shelf at lower left and to it's right is a Heart Interface 1kW inverter/charger.

I know that some are going to question the use of a fuse in the battery line and breakers and switches in the source lines since there has been recent discussion about it in wind systems. I firmly believe these are necessary to protect ALL the involved components. I handle the issue of the WT running away without load by another method located at the outside WT junction box.

Thanks for your interest.

Bill

[dnix71]

There are some guys in the UK who make custom ebike packs from Dewalt batteries.

They solder tabs like you did.

http://www.youtube.com/v/vkDgoXikI_8&rel=0

No sound in the video, but it's clear what they are doing.

[taylorp035]

I have been using these cells in my battlebot, which I have a 4s2p configuration.  As for the balancing, these cells don't unbalance themselves as long as you don't discharge them all the way down (like past 95% DOD).  You could benefit from a $20 balancer sold by astroflight which I use myself.  The question would be if the balancing discharge rate of 0.15 amps would be enough....

One thing to applaud you on was that you put the cells in parallel first, which helps keep the cells balanced.

I was using my battery pack this past week to do some tests with my 400/750 watt inverter, and the cells worked very well.  They pulled 40 amps for 7 minutes before going dead.  The nice thing is that you can charge these cells in less than 6 minutes if you want to, and 15 minutes by the manufactures recommended maximum rate.  I have seen people charge the cells in 6 minutes for 400 cycles and they still had 91% capacity left!!

Things that I have learned about these cells that you should follow:

1. Never twist the cell tabs (if you have them) as they are connected by a ribbon inside.
   
   
2. Inside there is some plastic, so don't overheat the cells while soldering, but I have yet to destroy a cell by doing this and I have heated the cells with my soldering guns for over a minute with out damage.
   
   
3. Don't charge past 4v/ cell.  You should cut off at 3.6 as you probably know.  I once charged a pack to 5.4 volts/ cell, and the next time I used them, all of the crimped aluminum seals (the end of the cells) exploded off (6 of 6).  They all sounded like a lawnmower backfiring when this happens, so I imagine that 96 cells would be quite violent.
   
   
4. Don't discharge them below the 2v / cell recommendation.  I also let some cells get to 0.7 volts/ cell.  12 hours later, they only had 25% capacity left.
   
   
5. Be careful!!! At 230 amps discharge PER cell, shorting your pliers on the connections makes for an exciting afternoon (same goes with your solder spool).
   
   
6. 2300 mah is a stretch most of the time.
   
   
7. never leave a charge unattended without an automatic shutoff, as the cell voltage can shoot through the roof in less than 30 seconds at a high charge rate.
   
   
8. If you want to, you can "zip" charge your 4s packs with a lead acid battery.  A 4s a123 pack is the only type of battery that I know of that you can do this with.  Just simply connect a 18 gauge wire between the batteries, and when the amperage gets to a low number (like 5 amps a cell), you know they are done, but I doubt you could do this with your setup safely.

   
   
   

I seriously envy you now...

   

[taylorp035]

Also, if you ever think you damaged the cells by charging them too high (<4.2v cell), you will know they are bad if you smell something like spray paint.  If you do smell this, check the ends of the cells to see if the seals have popped off.

My batteries did this while I was doing a stress test of my 15lb battlebot.  I was probably pulling 200-300 amps out a 3s2p pack for a minute when this happened.  I was getting 3 strands of 12 gauge in parallel hot, so it had to be a lot of power!

Those factory tabs are really only designed for the dewalt 36v drill which has a 700-800 watt motor, which is about 20 amps...  

[SamoaPower]

taylorp035,

Many thanks for the advice. I, likewise have been using the A123 cells for some time in other applications (RC, tool power packs, portable vacuum, etc.) and agree with most of your comments, although I tend to be a bit more conservative with the voltage numbers.

The BMS design calls for the balancing loads (3.6A) to cut in at 3.65V/cell. If the source current exceeds this and the pack voltage still rises, a PWM dump load (whole pack) controller cuts in at an equivilent 3.7V/cell. The game's a little different when charging from variable charge sources like solar and wind compared to controlled mains chargers.

The LVC (low voltage cut off) is also handled differently than some other controllers that look at the pack voltage rather than individual cells. The pack voltage approach is only valid if the cells are matched IN CAPACITY, not just balanced in voltage at full charge. Because of this, I set the LVC at 2.7V/cell looking at each cell individually. ANY series cell going below this will cut off the pack load. Yes, you lose a little (not much) in available capacity but it's safer. Actually, the best way, which I haven't designed for yet, is to have the cut off voltage be a function of the discharge current at the moment, since the cell voltage, toward the end of capacity, varies considerablly with current. I would never use a pack LVC of 2.0V/cell with a pack like this.

Re zip charging. Yes indeed, this is one reason why a 4S A123 pack has an advantage and is why my RC packs are all 4S. For higher voltage applications I just put multiple packs in series (8S and 12S). For field charging from my car, they are connected in parallel. However, you don't get enough charge if you simply connect to a lead acid battery. This is okay for 3S (with series resistance) but too low a voltage for 4S. The answer, of course, is to run the car engine so you are charging from the alternator at around 14.4V. This works just fine.

Thanks again for the comments.

Bill

[taylorp035]

Your are right about the 4s, I was not thinking straight.

Also, as stated, there is very little power left by the time you hit 2.7 volts.

So it seems like you have everything set up perfectly!!

On a side note, what kinds of rc stuff do you do with 8s and 12s, and how much power are you using?

I participate in the Battlebots competitions and I am a big fan of a123's and brushless motors.  This year I am using an 4s2p pack with a Mamba Monster 2200kv Combo from Castle Creations for my weapon, which should put out 3000 watts.

[SamoaPower]

WOW, 3kW from 8 cells! Talk about lots of current for a short time.

I do RC planes only over quite a range in power, 50W - 2.5kW, although there's a new 20 lb plane on the bench for 3.6kW. No competition, just fun flying. Besides, there's no one else here on island to compete against. As far as I know, I'm the only RC flyer here in American Samoa.

I try to limit my battery use to about 100W/cell to give reasonable flight times of 6-10 minutes. For 2.5kW I use a 12S2P combo pack using six 4S1P packs in series/parallel and field charge in parallel from the car in two batches for about 10 minutes each recharge time.

Good luck in your competitions.

Bill

[taylorp035]

Thank you very much.

The robot during my stress tests got 3 strands of 12 gauge wire hot in parallel last year, and that was a 3s2p pack.

The battlebots I do are a high school competition, and last year was my senior year.  This year I am helping them out with the electronics and some final design decisions.

My first year robot had a 3s3p A123 pack, but it was overkill as it lasted 45 minutes and I only need 3 minutes (I designed it for max power for 3 minutes ).

[solarnetwork]

Hey that's awesome. If you'd like to expand your monitoring, let me know - we are developing an open-source solar generation and home consumption monitoring platform here:

http://www.solarnetwork.net/

Click on the live data link to see the dashboard - it's HTML5 based.

and there would be room for another solarnode.  thanks, John