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  #1  
Old 07-27-2012, 01:15 AM
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Default [EVDL] Measuring Pack Capacity

I have a question based off the current discussion about how much capacity
lithium cells lose and how fast.

Since I have a lithium pack, I could theoretically measure pack capacity on
a regular basis to see how it trends. Since the pack is two years old
already, it isn't a great situation (no original baseline), but it would
still be useful to plot the remaining life.

My question:

In order to do this so that everyone agrees the conditions are controlled
enough to be valuable, what should be the procedure? How much draw (1C,
0.1C, ?) What would be the best load? A resistive load? That seems the
easiest to control.

How far should the cells be taken down? I don't want to draw down the pack
so far that it becomes a damaging event, but it needs to be past the flat
portion of the discharge curve.

I have 45 100Ah ThunderSky cells in my pack, but I would expect the
procedure would be pretty similar for all large-format prismatic LiFePO4
cells.

Suggestions?

Mike

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  #2  
Old 07-27-2012, 08:35 AM
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Default Re: [EVDL] Measuring Pack Capacity

One of the easier ways might be to just pick a route that you can drive under
repeatable conditions, and that discharges your pack to about 30% SOC or so.
Do this maybe once per quarter to twice per year to accumulate data. You
need controlled conditions, so charge your pack to the same voltage each
time, minimize stop lights and other variables such as wind, drive the same
speed each time, etc. Also do it the same time of the year at the same
ambient and pack temperatures. At the end of the drive measure cell
voltages. If pack capacity decreases over time, you should see these
voltages showing a clear drift to lower values over the years with some
variability superimposed on it. Would be good to measure cell voltages at
the end of charge, before the drive, too, to ensure they are charging to
about the same voltages each test.



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  #3  
Old 07-27-2012, 08:35 AM
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Default Re: [EVDL] Measuring Pack Capacity

Option 1: You can get a meter from EVTV or elsewhere which will charge and
then drain a cell. You get to choose the high and low voltages. It will
tell you the capacity of the cell in amp hours. Once you find your weakest
cell you can measure it's capacity periodically.

Option 2: (assuming you have some sort of low voltage cell
protection/alarm/controller cutback) Charge the car, get in and drive
normal to the nearest highway and then drive a set speed until the car
cries weeee. Record the miles. Repeat each year.

Option 1 is a bit more scientific and accurate but the meter costs money.


[quote] Mike Nickerson <xxx@xxx.xxx>wrote:

> I have a question based off the current discussion about how much capacity
> lithium cells lose and how fast.
>
> Since I have a lithium pack, I could theoretically measure pack capacity on
> a regular basis to see how it trends. Since the pack is two years old
> already, it isn't a great situation (no original baseline), but it would
> still be useful to plot the remaining life.
>
> My question:
>
> In order to do this so that everyone agrees the conditions are controlled
> enough to be valuable, what should be the procedure? How much draw (1C,
> 0.1C, ?) What would be the best load? A resistive load? That seems the
> easiest to control.
>
> How far should the cells be taken down? I don't want to draw down the pack
> so far that it becomes a damaging event, but it needs to be past the flat
> portion of the discharge curve.
>
> I have 45 100Ah ThunderSky cells in my pack, but I would expect the
> procedure would be pretty similar for all large-format prismatic LiFePO4
> cells.
>
> Suggestions?
>
> Mike
>
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Marcus Reddish

*North Valley Systems LLC*
Stevensville, Montana
406-360-8628
northvalleyev.com
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  #4  
Old 07-27-2012, 11:05 AM
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Default Re: [EVDL] Measuring Pack Capacity

Hello Mike,

When I pick up my first EV back in April of 76, it was suggested to make a
EV Log Book that you record your daily travels. So that's what I did and
place it on a table next to the EV. Lay it out in a spreadsheet design
which contains:

Date - Miles - AH - SOC - Bat.Volts - Charge Cycles - Charge Time


Other Data: Ambient Temperature
Battery Temperature
Controller Temperature
Motor Temperature

Highest battery ampere
Highest motor ampere
Highest motor rpm

Battery Data: Type
Weight
No of Cells
Maximum Charge Voltage
Minimum Discharge Voltage

Road Conditions: Grades
Type of surface - rough to smooth
Wet
Ice
Depth of Snow
Wind

Mechanical Changes: Motor type
Transmission type
Transmission gear ratios
Overall gear ratios
Type of wheels
Type of tires
Weight changes

I now have been recording this data since April 76 and just now about 10
minutes ago recorded the Date - Miles travel - AH use - Remaining SOC% -
Battery Volts at rest - Charge Cycles - Charging Time for driving up and
down a hill for 2.2 miles that I have been doing for more than 20 years for
a EV with a 180V battery pack.

12 years ago, the AH/Mile started out at 2.6 AH and in 8.6 years of running,
the AH/Mile increase to 3.9 AH with a longer charge time which I made a new
battery pack change.

My new pack also started out at about 4 AH/mile and after three years it is
now up to about 4.3 AH/mile before I did some Weight Reduction and
mechanical changes.

By using a EV log data, you can calculated the estimate range and life of a
battery pack. Mechanical changes can also improve the life and range of a
battery pack. I decrease the weight of the wheels and tires by 50% per wheel
and increase the overall gear ratio which increase my range by 25%!!! which
resulted in a 2.9 AH/mile about 30 minutes ago.

Roland









On Fri, Jul 27, 2012 at 1:11 AM, Mike Nickerson
[quote]<xxx@xxx.xxx>wrote:

I have a question based off the current discussion about how much capacity
lithium cells lose and how fast.

Since I have a lithium pack, I could theoretically measure pack capacity on
a regular basis to see how it trends. Since the pack is two years old
already, it isn't a great situation (no original baseline), but it would
still be useful to plot the remaining life.

My question:

In order to do this so that everyone agrees the conditions are controlled
enough to be valuable, what should be the procedure? How much draw (1C,
0.1C, ?) What would be the best load? A resistive load? That seems the
easiest to control.

How far should the cells be taken down? I don't want to draw down the pack
so far that it becomes a damaging event, but it needs to be past the flat
portion of the discharge curve.

I have 45 100Ah ThunderSky cells in my pack, but I would expect the
procedure would be pretty similar for all large-format prismatic LiFePO4
cells.

Suggestions?

Mike




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  #5  
Old 07-27-2012, 01:35 PM
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Default Re: [EVDL] Measuring Pack Capacity

[quote]Mike Nickerson wrote:

> My question:
>
> In order to do this so that everyone agrees the conditions are controlled
> enough to be valuable, what should be the procedure? How much draw (1C,
> 0.1C, ?) What would be the best load? A resistive load? That seems the
> easiest to control.
>
> How far should the cells be taken down? I don't want to draw down the
> pack so far that it becomes a damaging event, but it needs to be past the
> flat portion of the discharge curve.
>
> I have 45 100Ah ThunderSky cells in my pack, but I would expect the
> procedure would be pretty similar for all large-format prismatic LiFePO4
> cells.
>
> Suggestions?

My opinion is that the discharge load needs to be the same each time the test is performed, but the actual value of the load is less important. I usually test batteries at the accepted standard discharge rate for the particular application (e.g. 75A for 6V golf batteries, 56A for 8V, etc.), or at a rate for which the manufacturer rates the capacity (e.g. 75A, or the 5hr rate, etc.).

It is common for large-format lithium cells such as yours to be rated at 0.3CA charge and discharge, so performing the test at 0.3CA (33.3A) would allow you to compare your results directly to the manufacturer's rating. Since the capacity of lithium cells is less affected by discharge rate than that of lead-acid, even if your discharge load differed slightly from 0.3CA the result shouldn't be greatly affected.

Ideally, you would control the cell temperature so that it is the same each time you test the cells, however, without taking the cells out of the car and placing them in a controlled environment each time this is impractical. A reasonable approximation is probably to try to perform the test about the same time each year, so the temperatures will be similar, and to note the test conditions so that whatever temperature variation there is from one test to the next is known.

I prefer to use an electronic load bank (e.g. <https://www.valuetronics.com/detail/Used-dynaload-dlvp50-120-1500.cfm> is a small (0-50V, 0-120A, 0-1500W) one that might work well for this) so that the load current can be maintained precisely despite the voltage variation as the cell discharges. However, since the lithium cell voltage varies relatively little over most of the discharge, you might be able to get acceptable results with a passive load. I prefer to use light bulbs for this sort of thing, since the load current will be even less affected by voltage variations than with an ordinary resistive load.

I haven't bought any for a while, but Home Depot used to carry 12V 50W (and 100W?) screw-base bulbs that would fit ordinary $0.99 household light sockets. RE/AE suppliers are another source for these bulbs. At 12V, the 50W bulbs draw 4A each; the 100W draw 8A. At the 2.5-4V of your test, the current will be a bit less, but you just wire several light sockets in parallel and add bulbs as required to achieve the desired load. Remember that you are dealing with significantly more current than usually associated with light sockets and wire accordingly. When I built my light bulb load bank, I wired the light sockets by mounting them to a pair of parallel 1/8" x 1" copper busbars.

The advantage of using an electronic load bank is that you can set it to a desired current and measure Ah simply by how long the test runs. With a resistive (or light bulb) load, you will need to measure the load current before each round of tests to verify (and adjust it, as required), and ideally you would measure the current at regular intervals throughout the test to account for the variations in load as the voltage drops so that your Ah total can be more accurate.

If I were putting a test system like this together, and didn't want to invest too much money in test gear that would probably sit idle otherwise, I'd look at one of the inexpensive USB DAQ type devices to use as the data logging and control for the test setup. Typically these can directly measure 0-5V or 0-10V, which is fine for the cell voltage. I would use a 100mV 50A shunt on another channel to monitor current, and configure that channel for the smallest sensing range available (e.g. 0-1V). Choose a DAQ with some digital I/O lines so that you can use one line as an output to control a relay between the cell and the load and another to control a relay between the cell and charger/power supply (or the AC power to the charger, etc.).

On my light-bulb based load tester/cycler, I used an E-Meter with RS232 option as the voltage and current measuring DAQ and a simple QBASIC program running on the PC to log the data and control a pair of Ford-style starter solenoids with continuous duty coils (commonly available for RV battery isolator, etc. use) via a pair of parallel port I/O lines to connect/disconnect the load bank and charger (power supply).

Depending on the features of the software that comes with the USB DAQ, you might find it necessary to put a small control program together as well to allow the load and charger to be connected or disconnected at the appropriate times.

Any capacity test such as this has the potential to damage the cell if the load is not removed at the proper time, so don't rely solely on the SW (or manual intervention) to ensure this! I would include a HW circuit to ensure that the load will be disconnected if the cell voltage falls below some absolute minimum level (e.g. 2V). Ordinarily the SW would disconnect the load at 2.5-2.8V, but if something goes wrong, the HW failsafe will prevent the cell from being run completely dead.

So, you will need a charger/power supply that you can set to a desired full charge level. If you want to compare your capacity values directly to the manufacturer spec, then you should use the same charge voltage as the manufacturer specs. What is most important is that each cell is charged to the same voltage, and in the same way before each test. Easiest is probably to cut the charger off as soon as the cell reaches the target voltage, however, this will not result in a fully charged cell so your measured capacities will appear a bit lower than the cell is truly capable of. Also, as the cells age, their internal resistance will increase, and so they will be undercharged to a greater extent as they age, resulting in greater apparent loss in capacity over time. Better would be to let the cell charge to the target voltage and then continue charging at that voltage until the current drops below some minimum level. For your 100Ah cells, I would charge until the current fa!
lls to 1-2A, but as long as you use the same threshold for all tests there shouldn't be a problem setting it higher than this if you are concerned about overcharging the cells.

I would prefer a power supply to a charger for this, as I know for sure what the power supply will do ;^>
The more current the power supply can deliver, the quicker the tests can be completed, however, the more the supply will cost.

For direct comparison to the manufacturer specs, you should use the same charge voltage and end-of-discharge voltage as they spec. For (older) Thundersky LFP cells example specs would be to charge to 4.25V, then discharge to 2.5V at 0.3CA.

My procedure would be to charge the cell using the power supply to whatever full charge criteria I've defined for the testing, allow the cell to rest for a fixed period (e.g. 30min), then discharge to the end-of-discharge voltage, allow to rest for a fixed period (e.g. 30min), then recharge to the full criteria using the power supply.

Log voltage and current at regular intervals (e.g. 10s) throughout. Ideally, log temperature as well (e.g. tape a thermistor or other sensor to the side of the cell, or bolt it to one of the cell terminals).

Compute Ah and Wh removed during discharge and returned during the subsequent recharge. (Ideally, Ah removed and returned should be very nearly equal, so comparing them offers a sanity check of the result. Tracking Wh removed will allow you to see if the cell degrades more in terms of energy (Wh) than Ah over time; it is Wh that moves you down the road, so it is important to know if the Wh decreases more significantly than the Ah. Comparing the Wh removed vs returned allows us to see how the energy efficiency of the cells change over time.)

If you are comfortable with constructing electronic circuits, then you could build this Lee Hart-design battery/cell cycler device that is intended to be used with a USB DAQ to allow capacity testing of cells/batteries:

<http://www3.telus.net/nook/balancerland/cycler/index.htm>

Cheers,

Roger.



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  #6  
Old 07-27-2012, 04:10 PM
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Default Re: [EVDL] Measuring Pack Capacity

[quote] tomw wrote:
> One of the easier ways might be to just pick a route that you can drive under
> repeatable conditions, and that discharges your pack to about 30% SOC or so.
> Do this maybe once per quarter to twice per year to accumulate data. You
> need controlled conditions, so charge your pack to the same voltage each
> time, minimize stop lights and other variables such as wind, drive the same
> speed each time, etc. Also do it the same time of the year at the same
> ambient and pack temperatures. At the end of the drive measure cell
> voltages. If pack capacity decreases over time, you should see these
> voltages showing a clear drift to lower values over the years with some
> variability superimposed on it. Would be good to measure cell voltages at
> the end of charge, before the drive, too, to ensure they are charging to
> about the same voltages each test.

It sounds like you are making the argument that SOC can be divined from
cell voltage?

--
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http://counter.li.org Linux registered user #228836 since 1995
Debian3.1/GNU/Linux system uptime 10 days 6 hours 19 minutes

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  #7  
Old 07-28-2012, 03:07 AM
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Default Re: [EVDL] Measuring Pack Capacity

For bench-testing the capacity of individual 12v batteries, I use a "Watt's
Up" meter. These sell in the $50-60 range.

They can handle up to 60 volts. The manufacturer says they can measure up
to 100a, but since the supplied source and drain pigtails are only #14 wire,
I'm skeptical. I don't exceed 20 amps.

At any rate, I connect the meter between a fully charged battery and a 12v
inverter. The inverter powers a 250w incandescent lamp and an old plug-in
alarm clock. I set the clock for 12:00:00 and let 'er rip. The inverter
shuts down when the battery falls to 10.5 volts - essentially flat - and the
clock tells me how long it was able to supply 250 watts. From that I can
calculate the battery's capacity in watt-hours.

The only flaw is that it's entirely manual. I think it's Lee who uses an
old Rudman Regulator to automate a capacity testing jig that repeately
cycles the battery and logs the results.

For 24v lithium batteries, I use a similar rig with a 24v inverter instead.
The inverter shutdown is ~21v, but the 24v LiFePO4 batteries I'm tinkering
with have a BMS with an undervoltage shutdown.

David Roden - Akron, Ohio, USA
EVDL Administrator

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  #8  
Old 07-28-2012, 08:15 AM
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Default Re: [EVDL] Measuring Pack Capacity

"It sounds like you are making the argument that SOC can be divined from
cell voltage?"
Yes I am. If Mike plans to track them for enough years that he sees a
10-20% change in capacity, then the discharge to 30% will gradually turn
into discharge to 22% SOC (10% capacity loss) or 12.5% SOC (20% capacity
loss), and you will definitely see significant changes in cell voltages over
this range of SOC.



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  #9  
Old 07-28-2012, 07:06 PM
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Default Re: [EVDL] Measuring Pack Capacity

I like toms approach, but to keep things predictable between tests, why not
find someone with a dyno ?
[quote] "tomw" <xxx@xxx.xxx> wrote:

> One of the easier ways might be to just pick a route that you can drive
> under
> repeatable conditions, and that discharges your pack to about 30% SOC or
> so.
> Do this maybe once per quarter to twice per year to accumulate data. You
> need controlled conditions, so charge your pack to the same voltage each
> time, minimize stop lights and other variables such as wind, drive the same
> speed each time, etc. Also do it the same time of the year at the same
> ambient and pack temperatures. At the end of the drive measure cell
> voltages. If pack capacity decreases over time, you should see these
> voltages showing a clear drift to lower values over the years with some
> variability superimposed on it. Would be good to measure cell voltages at
> the end of charge, before the drive, too, to ensure they are charging to
> about the same voltages each test.
>
>
>
> --
> View this message in context:
> http://electric-vehicle-discussion-list.413529.n4.nabble.com/Measuring-Pack-Capacity-tp4656800p4656812.html
> Sent from the Electric Vehicle Discussion List mailing list archive at
> Nabble.com.
>
> _______________________________________________
> | Moratorium on drag racing discussion is in effect.
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  #10  
Old 07-29-2012, 04:15 PM
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Default Re: [EVDL] Measuring Pack Capacity

Mike Nickerson wrote
>
> I could theoretically measure pack capacity... what should be the
> procedure?
>

Your digital BMS will that for you. You do have a BMS, right?

An analog BMS (such as the MiniBMS, EV Power, Pacific EV) is nice and easy.
But a digital BMS costs only marginally more yet it has all sort of
additional features, such as measuring your pack's effective capacity on an
ongoing basis.

The Elektromotus, Lithiumate Pro, Lithiumate Lite, Orion BMS, Lithium
Balance or REAP BMSs will simply show you your pack's effective capacity on
their Graphics User Interface application.

The Anhui, Claiton power, Electric Blue, EVPST, GWL Power BMSs will show it
to you on the included display.

The REC, Rozwiazania and Tritium BMSs will report it on a data bus.

Source: http://liionbms.com/php/bms-selector.php



-----
Davide Andrea
Elithion
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