Comprehending the Battery Specifications from ADVISOR data -reg.

[Ajay Babu]

Dear sir

           Once again, I have come up with a fundamental doubt. Please consider the following two sets of input data for the ESS in PARALLEL_default_in (Parallel hybrid):

1) Internal resistance model (Lithium ion battery)

Number of modules = 13

Nominal voltage = 139 V

Weight 15 kg

ess_cap_scale = 1

2) RC Model (Lithium ion battery)

Number of modules = 39

Nominal voltage = 140 V

Weight 15 kg

ess_cap_scale = 1

Suppose I use the above battery input parameters for analysis. I wish to know the battery specifications (mainly the rated Ah) for both cases. Since the nominal voltage and the weight for both cases are same, can I consider them as representing the same battery. Please advise.

Thanking you.

Best regards

Ajay Babu

[Michael P O'Keefe]

Hi Ajay,

Depending on the battery model you select, you are creating the mathematical equivalent of either an open circuit + resistor model (Rint) or resistance capacitance network (RC). So your question of whether the two batteries are equivalent depends on whether the parameters to the Rint model or RC model are equivalent. You should be able to tell by opening the data file and looking at the values. You can determine Amp-Hours by dealing directly with the equivalent circuits -- unfortunately, that information is not directly available in ADVISOR.

You might peruse some of these reports if you need more clarification:

http://www.nrel.gov/transportation/energystorage/pdfs/evs17paper2.pdf

http://www.nrel.gov/transportation/energystorage/pdfs/evs17poster.pdf

Hope that helps,

Michael

[Ajay Babu]

Dear sir,

        Thanks a lot for the reports sent by you. I went through the reports sent by you. Also, I went through the m-files (ESS_LI7_temp.m and ESS_LI7_rc_temp.m) in ADVISOR that present the data used for the internal resistance and RC battery models. I shall list the facts that I understood. Please correct me, if I am wrong:

1) Internal resistance model - Based on the tests conducted on 6 Ah Saft Lithium Ion battery. Each battery module comprises of 3 cells in series, each rated for 3.56 V, 6 Ah. Thus one battery module is rated for 3.564 V * 3 = 10.69 V (and 6 Ah). So, if the autosize results give 13 number of modules to be used, it means 13* 10.69 = 139 V. Hence, the entire battery pack is rated for 139 V, 6 Ah. But the file "ESS_LI7_temp.m" says that ess_max_ah_cap gives max. capacity at C/3 rate, indexed by ess_tmp. Does it mean that the Ah rating is to be taken as 3 times (i.e. 6*3 =18 Ah, with deviations as indexed by temperature)  or that the maximum discharge current shall be the one-third of the rated value of the battery capacity (ie. 6/3 =2 A with deviations as indexed by temperature)?  

In simple term is the battery pack rated for 6 Ah or (6*3) =18 Ah. Or, is there some other way to confirm the battery capacity?

2) RC model (ESS_LI7_rc_temp.m)- Based on the tests conducted on 7 Ah Saft Lithium Ion battery. Here, each module comprises of a single cell rated for 3.59 V. Thus 39 modules means 140 V. Here again the same doubt with respect to the battery capacity exists.

     Kindly help me to understand the way battery capacity is modelled in ADVISOR.

[Michael P O'Keefe]

Hi Ajay,

I'll admit that batteries are not my strong area but the reason that the discussion of battery capacity is so tricky is that the capacity of a battery changes with how you use it. You may find this document useful:

http://web.mit.edu/evt/summary_battery_specifications.pdf

A C-rate is a measure of the rate at which a battery is discharged relative to its

maximum capacity. A 1C rate means that the discharge current will discharge the entire

battery in 1 hour. For a battery with a capacity of 100 Amp-hrs, this equates to a discharge

current of 100 Amps. A 5C rate for this battery would be 500 Amps, and a C/2 rate would

be 50 Amps

With regard to the ADVISOR variables, the information in the file header is for information purposes. It does appear that max Amp-hour capacity is at a C/3 rate. Hopefully the report above will clarify what C/3 rate means. My understanding is that a C/3 rate would be a discharge at 1/3 of the rated capacity. The actual capacity of the battery is going to change depending on how it is discharged (and also on temperature if you model temperature dependence).

The documentation on the Rint model is here:

http://adv-vehicle-sim.sourceforge.net/ess_rint.html

As they mention, "The sum of all discharging current and all charging current (scaled by an estimate of average coulombic efficiency) determines an effective total Ah change in the battery since the beginning of the simulation." So the Amp-hour capacity of the battery is the C/3 rate you specify but depending on how you use the battery, you might get different results. The mathematical model is Amp-hour counting but your amps get hit with coulombic efficiency depending on SOC and discharge rate. Equation 4 of the report sort of tells what's going on in the relation for SOC.

Does that help at all? Battery capacity is modeled as an equivalent circuit coupled to a charge reservoir (your max Ah capacity). The values of the equivalent circuit are functions of SOC (state of charge -- i.e., the remaining charge), temperature, coulombic efficiency, etc. 

Thanks,

Michael O'Keefe

[Michael P O'Keefe]

By the way, here's another report that may also be helpful:

http://server3.eca.ir/isi/forum/Battery_performance_models.pdf

Cheers,

Michael

[Ajay Babu]

Dear sir

           Thank you so much for all the reports and the information shared by you.

          In fact all these doubts arose because of some results that I obtained by working with ADVISOR. The autosize functionality in the software gave the following features for a parallel hybrid:

1.       

1.         Engine: Spark ignition type with maximum capacity of 124 kW.

2.         Motor: Three-phase induction motor with maximum capacity of 35 kW.

3.         Battery: 139 V lithium ion battery with 13 lithium ion battery modules, each of which has three 3.56 V, 6 Ah cells. 

      My doubt was: How can 139 V, 6 Ah battery pack support a 35 kW motor?

That is why I doubted the battery capacity that I obtained from ADVISOR. But now I believe, that the results are indeed feasible. 35 kW is the maximum motor capacity, that may occur for a short duration (during instances of acceleration or slope climbing). This means the 6 Ah battery can discharge at 16.67 C rate. From the reports sent by you, I understand that the battery in a HEV can discharge at a rate as high as 40C. The battery cycle life can be taken care by maintaining a small depth of discharge.

Is there any mistake in what I understood? Thank you so much for your support.

Best regards

Ajay Babu

[Michael P O'Keefe]

Ah, I think I see your problem. When you change the number of modules (ess_module_num) you are placing modules electrically in series (a module being what you define in the data file) with each other which increases the peak voltage but not the amp-hour capacity (check me on that but I believe it's correct). If you want to change the Ah capacity as well, try the ess_cap_scale which is like putting modules in parallel.

Cheers,

Michael

[Ajay Babu]

Dear sir

            Yes... but can I infer that the autosize functionality in ADVISOR optimizes the battery size by changing ess_module_num and not the value of ess_cap_scale (default value = 1). ess_cap_scale has to be changed manually, if required. I hope I am not wrong.

             

Best regards

Ajay Babu

[Michael P O'Keefe]

Yes, it only changes the ess_module_num (as shown in the GUI) -- you can confirm this yourself by querying the variables from the GUI or from the matlab workspace.

Cheers,

Michael

[Ajay Babu]

Dear sir,

            Thank you so much for your help and guidance.

Best regards

Ajay Babu