# Now BMS free!



## EVfun (Mar 14, 2010)

Today I pulled the BMS off the pack in the Buggy. I prepared by charging last night to 3.65 vpc average, hold for 15 minutes, then the charger shut off. Today I snuck back up very slowly until the current was low (about the same 1/2 amp the shunt can pass) and the cells where all between 3.62 and 3.66 volts. 

I pulled out the seats and began pulling the cell modules row by row, while reconnecting the pack as I went (don't want to leave the aluminum positive terminals exposed to air.) It took about 90 minutes. During this time the cells have a slightly different load as the BMS modules draw about 3 ma until pulled. When done I checked and the 1/2 of the pack disconnected first was 0.1 volt higher than the other half (all cells still coasting down from the charge and in the 3.4x range.) So, I put a 240 ohm resistor on the higher half for 30 seconds to roughly correct for it (about 0.002 amp hour.) Then I turned on the headlights for 10 minutes and measured the voltage. Every cell was 3.36 or 3.37 volts and the 2 halves read the same voltage. I'm sure I will have a little more tuning to do.

My next Lithium experiment begins. The EVworks BMS worked fine and never caused me any grief, but I wanted to see how tight the pack will keep itself. I plan to lower the charging voltage to 3.5 vpc.


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## GizmoEV (Nov 28, 2009)

Great, another person testing this out. If you put a half pack monitor in place you can see if something starts to go bad before it is too late. I documented mine at http://2003gizmo.blogspot.com/2011/07/battery-pack-balance-monitor.html. It is very simple to build.

I took voltage measurements at the end of charge just before the charger shut off around the first of each month. I then plotted the individual cell voltages on a graph to see what they did. I discovered they bounced around a little and swapped places but after a short while the high cell and low cell stayed in its relative position. I have since rebalanced my pack and removed my BMS boards, which only draw ~1mA, and am running the test again.

Are your batteries all in one box?


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## EVfun (Mar 14, 2010)

My cells are in 4 rows of 8 cells each. 2 rows start under the drivers seat and extend back. The other 2 rows are under the passenger seat and extend back. Temperature wise, they could all be considered together since the 2 groups are so close and mirror images of each other. 

The floor was sunk 3 inches for the height to fit the cells under the seats. The rows of cells are tightly bound using the Thunder Sky end plates and straps I made from stainless steel banding. When I had the factory banding all apart (I got them in groups of 4 and 6) I added some extra drilled and threaded holes to the end plates which is how I hold them down.

I took at look at your balance meter. Very nicely done, I may do something similar for the buggy. After range testing I know I have the largest cell in one half of the pack and the smallest in the other half. This would make detection easy. My only thought is to check the voltage across the 10k center tap resistor when the meter to see if you have any microamp level shifting over time. Of course, that would be easy to take back out with a 100 ohm power resistor across the high half of the pack for a few seconds a couple times a year.


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## steven4601 (Nov 11, 2010)

Hope it works out good. I plan to do the same but with the back up plan of having a balance lead attached to each group of cells. A 1kw RC model charger should help me correcting drift once or twice a season or year if lucky.


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## GizmoEV (Nov 28, 2009)

EVfun said:


> I took at look at your balance meter. Very nicely done, I may do something similar for the buggy. After range testing I know I have the largest cell in one half of the pack and the smallest in the other half. This would make detection easy. My only thought is to check the voltage across the 10k center tap resistor when the meter to see if you have any microamp level shifting over time. Of course, that would be easy to take back out with a 100 ohm power resistor across the high half of the pack for a few seconds a couple times a year.


Thanks. I've been thinking about your question of any current across the 10k center tap. There shouldn't be any unless there is some through the volt meter. The volt meter goes between the center of the pack and the center of the calibrating potentiometer. The only reason I installed the 10k resistor was so I didn't have to come up with a high voltage low current fuse. With the resistor a short to any other part of the pack will result in very low current and not melt anything.


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## GizmoEV (Nov 28, 2009)

steven4601 said:


> Hope it works out good. I plan to do the same but with the back up plan of having a balance lead attached to each group of cells. A 1kw RC model charger should help me correcting drift once or twice a season or year if lucky.


If your experience is like mine you won't have to worry about it but once a year or less. Now that I don't have my BMS boards on I'm curious to see if things stay in balance longer even though there was very little out of balance before I took them off.


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## EVfun (Mar 14, 2010)

GizmoEV said:


> Thanks. I've been thinking about your question of any current across the 10k center tap. There shouldn't be any unless there is some through the volt meter.


If you can get to that resistor you can put a regular DMM across the 10k resistor. Even just 1 microamp would show up as 10 millivolts. You are correct, it should be none, but I wonder if the meters have a little offset of their own that gets taken out by your adjustable center point.


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## GizmoEV (Nov 28, 2009)

EVfun said:


> If you can get to that resistor you can put a regular DMM across the 10k resistor. Even just 1 microamp would show up as 10 millivolts. You are correct, it should be none, but I wonder if the meters have a little offset of their own that gets taken out by your adjustable center point.


I calibrated the center by measuring each half pack with my DMM and then adjusted the potentiometer until the meter in the dash read the difference measured between the half packs. A few days after setting it up initially I checked again and each half pack was at the same voltage, out to the hundredths of a volt, so I made sure I got a reading of 0V across the center tap and pot wiper. I did this to see if the meter in the dash was consistent with my DMM.

I'll be doing another end of charge cell level voltage reading in a couple of days and I'll check the voltage across the resistor at that point. In any case, would there really be any current if the half pack voltages were identical? And if they weren't, then the current would have to go through the 10k resistor and the X megaohm resistance of the meter.


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## EVfun (Mar 14, 2010)

If you read nothing across the resistor there is no current flowing. If you read 1 millivolt across the resistor then you have 0.1 microamp of current flowing. That is a tiny number, less than 1 milliamp hour per year, truly unimportant. 

If you regularly have an imbalance then you do regularly pull the high side down slightly, but it still most likely only amounts to a few milliamp hours per year. I was just curious and realized that your 10k resistor provided a handy way (and a very sensitive way) to check.


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## GizmoEV (Nov 28, 2009)

I went out and moved the center tap on cell over and I measured 3.3mV across the 10Kohm resistor. The voltage of the high side was 38.03V and low side was 31.13V so a difference of 6.9V caused a current of 0.33microAmps. Since the current should be linear with voltage difference that would imply a pack imbalance of 0.15V (the most I've seen under load) will induce a current of about 0.007microAmps. Only about 63 microAmp-hours/year.

I guess this is the current flowing through the meter since otherwise there is no connection between the center tap and the wiper on the potentiometer.


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## EVfun (Mar 14, 2010)

Wow, that meter has about 20,000,000 ohms impedance. 

I was looking as a Lascar SP400 digital panel meter for a pack powered volt meter. It can measure its source, if the supply is regulated to 3.0 - 7.5 volts. It only draws 350 microamps. A 0.5 watt D0-35 zener only requires 250 microamps to maintain regulation. So a 120 kohm resistor and a 5.6 volt zener could power the meter, and a 1 meg ohm plus 1 k ohm resistor could be the voltage divider (200 mv full scale), for less than 1 milliamp from the pack. An optical isolator or photo-relay that can handle 30 milliamps, with a power resistor to drop most of the pack voltage, could light the meter when the vehicle lights are on (about 4700 ohms 5 watt resistor.)


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## EVfun (Mar 14, 2010)

I took advantage of the bad weather to do a little pack testing. My pack didn't start full as I had been driving a little and testing a heater load. My pack is made of 32, 60 amp hour Thunder Sky LiFeYPO4 cells made in February 2010. Hopefully somebody can find a little helpful info in some basic, no-BMS pack cycling. 

I discharged the pack until the first cell hit 3 volts. My pack is top balanced (pretty much) so the highest cell was at 3.082 volts when the first cell hit 3.000 volts and the discharge was ended. The heater load was drawing 11.2 amps from the pack when I disconnected it. I pulled a fuse that removes all load from the traction pack and let it rest overnight.

The next day the cells all read between 3.174 and 3.213 volts. I started the recharge at 15 amps and held that for 2 hours. Then I turned the charger down to 12 amps as that is what I normally charge at. The pack charged until 56 amp hours had been returned. At that point the pack was at 111.4 volts and the Manzanita Micro charger started flashing the timer. The countdown timer is set to run for 40 minutes and the current tapered down from 12 amps down to 3.6 amps when it shut off. A total of 4.4 amp hour where added during the countdown, for a total of 60.4 amp hours of charging. The cells ranged from 3.46 to 3.52 volts just before the charger shut off. The charger was only at 111.6 volts and at 3.6 amps I wasn't sure it was a full charge.

I turned the charger right back on when it switched off and turned the voltage up just a little to 112.0 volts. The timer started within seconds of turning the charger back on so it was another 40 minute countdown to shutoff. The current started at 3.9 amps after turing it up and ended at 1.2 amps. The current dropped faster at first and much slower toward the end. This added another 1.4 amp hours to the pack (2.3%, about 0.7 miles of range), bringing the total charge to 61.8 amp hours. The cells where all between 3.47 and 3.54 volts and the pack total was 112.1 volts. I pulled the pack fuse and will check the cell voltage tomorrow after work. The cells are not quite a tightly matched as at my last check. Still, they are to close for any of them to get hurt.

Tomorrow, after checking the cell voltages I plan to do a discharge test.


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## GizmoEV (Nov 28, 2009)

I noticed my cells bounce around a bit. The high and low cells eventually settled down. Temperature appears to have some effect on them. It will be interesting to see what the voltages are like in the morning. In any case, those voltages are not very far apart.


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## EVfun (Mar 14, 2010)

I checked the cells about 16 hours after completing the charge yesterday. They had zero load on them overnight. The cells where all resting at 3.34 volts. 

I cranked up my heater load tester and started counting amp hour delivered. The discharge started at 14.5 amps and held there for 2.5 hours. I ran a voltage test at the point where 30 amp hour had been removed (1/2 of capacity), the cells where between 3.215 and 3.226 volts while delivering 14.5 amps. The pack delivered a total of 62.4 amp hours when the first cell hit 3.000 volts under load. The highest cell was at 3.084 volts, the current was at 10.6 amps and the discharge was ended.

The discharge and recharge over the last 2 days will give me some type of baseline for comparing the pack as it ages. The pack is recharging now and will be ready for some uncontrolled discharging tomorrow.


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## GizmoEV (Nov 28, 2009)

Did you let the discharged pack rest a little while to see how much bounce back there was in the voltage?

I should do a similar discharge test on my pack though 200Ah will take a while. I need a baseline. I wish I had done one at the beginning with all 40 cells and then paired them all for the best match of total Ah. Maybe during Thanksgiving or Christmas break I can do a rundown test. That will be at the 2 year mark since the batteries were manufactured.


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## EVfun (Mar 14, 2010)

Not this time. I would expect the voltage would have been almost the same as the previous discharge because the cut off voltage range under load was almost the same (3.000 to 3.082 on the previous discharge test, 3.000 to 3.084 on the last discharge test.) 

On the previous test the cell voltage after the discharge test and then 16 hours with no load was between 3.174 and 3.213. So the pack minimum seems to be in a range between 0.02 resting and 0.08 while under a 11 amp load. 

The pack has seemed quite happy for the last couple of days. I think that has more to do with actually charging the pack, I had been driving around with the pack never over about 48 amp hours (with a 60 amp hour pack.) It is easy to short cycle because of the lack of cell drift with Lithium. Clearly, stopping the short charge with the pack hits 110 volts (3.44 vpc) while charging at 12 amps (0.2C) was quite a bit short (20% or so) of full. Stopping at 3.50 volts with the charge rate down to about 0.06C is only about 2% to 2.5% short of full. That is perhaps 1.5 miles shy of the maximum range.


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## EVfun (Mar 14, 2010)

EVfun said:


> The pack has seemed quite happy for the last couple of days. I think that has more to do with actually charging the pack, I had been driving around with the pack never over about 48 amp hours (with a 60 amp hour pack.) It is easy to short cycle because of the lack of cell drift with Lithium. Clearly, stopping the short charge with the pack hits 110 volts (3.44 vpc) while charging at 12 amps (0.2C) was quite a bit short (20% or so) of full. Stopping at 3.50 volts with the charge rate down to about 0.06C is only about 2% to 2.5% short of full. That is perhaps 1.5 miles shy of the maximum range.


I'm going to update this ancient thread because I have a *few* more years of data (and I added some cells.) Looking back, I remember the switch from an early fast cut-off to a to a slower lower voltage cut off because I used it for a year (perhaps two.) Then I found a couple of cells where finishing closer to 4 volts than 3.5 volts so I rebalanced the pack again switched to charging to 3.43 vpc (now with 39 cells) and holding at that voltage for around 45 minutes to let the current taper. That system has carried me well all the way through 2017 with the cells staying together. I have stronger and weaker cells, with a 7 year old pack, but none vary much and they don't seem to be the same as when it was assembled. It has been maintenance free for 4 or 5 years. The pack is still good, but in the fall of its life-cycle. Using my conservative discharge cut-off of 3.000 volts for the lowest cell with a discharge rate of about 15 amps (for 60 amp hour cells) the pack is now down to 48 amp hours. I've never noticed in driving because the pack is still close together and I don't push the discharge limit. The internal resistance is still low, the pack still has no problem putting out 6C when warm and 5C if cool Seattle winters. 

I will get some data from my last cycle test posted this weekend. If anyone has specific questions I am open to try and answer.


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## e^2 (Jul 22, 2017)

Good to hear it is still doing well. I have about 29 miles on my lipo pack with no BMS and am still within the 0.01V balance I set them at. We will see how it goes.


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## Duncan (Dec 8, 2008)

Hi I now have about 14 months on my volt pack
not sure how many miles or charges but several track events as well as road use 

I just checked all my cells - all within a 0.01v range


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## EVfun (Mar 14, 2010)

This is an information dump from the initial checking and then charging of my old pack. 

I generally use the car for about 8 months of the year and part it for the other 4. In those winter 4 months I get a couple drive in per month, but I generally don't fully charge the pack, trying to store it around 1/2 to 2/3 charged. When I pulled the buggy body off, so I could measure all 39 cells, it had been about a week since my previous drive. The cells where in an unknown state of charge, neither full nor empty.

The cell voltages:
2 at 3.305 volts
17 at 3.306 volts
18 at 3.307 volts
2 at 3.308 volts


When the set voltage is reached the timer on the charger starts. I wanted to get a voltage reading quickly so I reduced the resolution of the DMM.

The cell voltages:
all cells between 3.42 and 3.44 volts except one at 3.47 volts


I know the charger timer is 52 minutes so at 50 minutes I went around and checked the pack again. By this time the voltage is moving slowly so I can jot down notes while checking.

The cell voltages:
3 at 3.41 volts
33 between 3.42 and 3.44 volts
1 at 3.47 volts
1 at 3.50 volts
1 at 3.51 volts


At this point I realized I hadn't put the shunt in the pack to measure the charge current at the end of a regular charge cycle, so I didn't get a current number for that. Before doing a load test I give the pack an overnight rest with no load and then run the charge cycle again. It goes into the finish charge stage within 2 minutes and the current tapers down further. 

At the end of the second finish here are the numbers:
The current was 2.1 amps at the end.
Based on the rapid taper down in current I estimate 3.0 amp hours where added in 54 minutes.
32 cells between 3.42 and 3.44 volts
1 at 3.45 volts
1 at 3.46 volts
2 at 3.47 volts
1 at 3.48 volts
1 at 3.50 volts
1 at 3.52 volts


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## EVfun (Mar 14, 2010)

This is an information dump from the discharge test of my old pack.

The cells where allowed to rest for 3.5 hours after the charge was complete. The cell voltage check before the start of the test showed all cell voltages where between 3.353 and 3.359 volts. 

The average discharge current was 12.5 amps. Because my load is a ceramic heater the initial current starts at about 6 amps and rises over a couple of minutes to 13.2 amps before falling to 12.1 amps. All of this takes place in the first 12 minutes of the discharge test. The discharge current rises to 12.2 amps one hour into the test. It continues rising throughout the test. At the end of the test the current is 12.7 amps just before the load is removed. The discharge lasted 3 hours and 54 minutes. 2 hours into the test all the cells where between 3.20 volts and 3.22 volts. At the end of the test all the cells where between 3.00 volts and 3.09 volts. By my calculations the pack delivered 48.7 amp hours of capacity until the first cell dropped to 3 volts with a discharge load of about 0.2C. 

I left the pack overnight without any load to see how the cells recovered. The next morning, 12 hours after the discharge test, all the cells where between 3.14 volts and 3.21 volts (only 1 cell at 3.14 and 1 cell at 3.15.) 

I know most people test to a lower cutoff voltage so decided to run an second stage discharge test to 2.80 volts for the lowest cell. This was a quick test, lasting only 15 minutes and delivering an estimated additional 3.1 amp hours. The cell voltages at the end ranged from 1 at 2.80 volts with the other 38 between 2.82 and 2.98 volts. 

After completing the discharge test the charger was set to 8.8 amps and the pack allowed to charge for 4 hours. At this point the pack is resting with the no load as I plan to do some work to the buggy.


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