Voltage difference between modules

[Jonndeka]

Hi there,

I have 10 battery modules in a pack. Each module has 51 volts. I would like to know how much voltage difference are allowed between to modules? I mean more than how much voltage the BMS needs voltage balancing between these modules?

Best regards,
Reza

 

[eMark]

Are they from a used EV battery that is nearing End-Of-Life?

Would be helpful if you tell us more about the brand and age of the 10 battery modules (13s or 14s?). It sounds like you may have already attempted capacity (HPPC) test on one of the cells. Is this other post about the same battery modules ...

So far we had battery characterization test for a single cell. I would like to know how can we test the module (series and parallel multiple cells) using HPPC test.

https://www.researchgate.net/figure/Hybrid-Pulse-Power-Characterization-HPPC-test-profile_fig2_235602059
https://www.researchgate.net/publication/302964629_Lithium_Ion_Batteries-Development_of_Advanced_Electrical_Equivalent_Circuit_Models_for_Nickel_Manganese_Cobalt_Lithium-Ion
(Hybrid Pulse Power Characterization - HPPC)

It's probable a BMS is unable to balance these used (13s or 14s?) modules of varying capacity (HPPC tests) if nearing EOL.

 

[Jonndeka]

Hi.

These module are not new and now they are working. Type of cell is LFP. I just want to know how much voltage difference should be between these modules if each module is 51V in nominal capacity? Yes the configuration is 14S6P.

The second question is related to first question. But the module is brand new and I just want to test 1 module for battery characterization.

 

[eMark]

These module are not new and now they are working. Type of cell is LFP. I just want to know how much voltage difference should be between these modules if each module is 51V in nominal capacity? Yes the configuration is 14S6P.

Off-hand i'd say no more than 100mV ... based on those active equalizing balance boards that are capable of balancing the parallel groups (used battery) as close as 100mV. Can take several hours depending on variance to bring the 14-6p groups within 100mV of each other - https://www.amazon.com/Equalizer-Balancer-Transfer-Equalization-Capacitor/dp/B09W4H2VWD/ref=sr_1_1_sspa?crid=2JYQX8540UQHU&keywords=14s%2Bactive%2Bbalancer%2Bboard&qid=1676381448&s=hi&sprefix=14s%2Bactive%2Bbalancer%2Bboard%2Ctools%2C142&sr=1-1-spons&ufe=app_do%3Aamzn1.fos.006c50ae-5d4c-4777-9bc0-4513d670b6bc&spLa=ZW5jcnlwdGVkUXVhbGlmaWVyPUFROVI1OUtLWkY2TFImZW5jcnlwdGVkSWQ9QTA2MjU0OTQxSk44UUQzVUwyVEdNJmVuY3J5cHRlZEFkSWQ9QTAyMjQyNDRLSEFaWjRWM09ZMTgmd2lkZ2V0TmFtZT1zcF9hdGYmYWN0aW9uPWNsaWNrUmVkaXJlY3QmZG9Ob3RMb2dDbGljaz10cnVl&th=1 ...

• "The voltage gap of each battery must be within 0.5V. When the battery voltage difference does not reach more than 0.1V, the light on the balance board is triggered by mistake."

(mixed opinions on the effectiveness of those 'active' equalizing 5A Balance Boards)

If not i'd still add a 14S (JST-HX) sensing leads for monitoring voltage variance of the14-6p groups. That allows advantage of manually balancing the modules (if/when necessary) to get the most cycles possible out of those used modules (too much of a bother for most).

Better to have an intelligent so-called SMART (AI) BMS for monitoring and active balancing the 14 parallel groups.

The second question is related to first question. But the module is brand new and I just want to test 1 module for battery characterization.

Are you able to measure voltages of the 14S sensing leads to check the variance between the fourteen 6p groups? With a new module the variance between the 14s 6p groups could be as little as 5mV with a quality (AI) BMS.

 

[Jonndeka]

Thanks eMark,

It was good information. So what I understood, maximum voltage difference between cells should be 5mV and if I want to consider voltage difference between modules, the maximum value should be (5mV*14 = 70mV). Is it correct?

I still didn't measure the cell voltage of new brand. But I'm going to measure it and compare with the used module.

Thanks again!

 

[eMark]

... maximum voltage difference between cells should be 5mV

5-10mV is optimum with 25-30mV more practical with new 14s6p with quality BMS. If possible try to keep the variance between the 14 parallel 6p groups within 100mV of each other on a used 14s6p battery (IMO). Hopefully, others will offer their advice from their LFP battery experiences.

if I want to consider voltage difference between modules, the maximum value should be (5mV*14 = 70mV). Is it correct?

NO ... the voltage variance between the fourteen 6p groups on a new brand name lithium battery with brand name A-grade cells could be as close as 5mV ranging from say 50.998V to 51.003V (as received brand new at its nominal voltage). Perhaps wishful thinking?

I still didn't measure the cell voltage of new brand. But I'm going to measure it and compare with the used module.

Ideally the fourteen (6P) groups should be within 25-30mV on a new top-grade 1st class lithium battery (e.g. 14s6p) as received (IMO). At the most i'd say no more variance among the 14 (6p) groups than 0.050V and closer to 25-30mV. You'll need a DMM that reads millivolts ... https://www.amazon.com/ANENG-AN870-Digital-Multimeter-19999/dp/B0828PR3HN/ref=sr_1_1?crid=9M9J6VQLH2HL&keywords=gazechimp+AN870+19999+Counts+True-RMS&qid=1676386110&sprefix=gazechimp+an870+19999+counts+true-rms%2Caps%2C223&sr=8-1&ufe=app_do%3Aamzn1.fos.006c50ae-5d4c-4777-9bc0-4513d670b6bc

 

[Jonndeka]

What I understood based on your comments and research papers, the answer can be the following:

• For cells connected in series within a module, it is generally recommended to keep the voltage difference between cells below 5% to avoid overcharging or over-discharging of any individual cell.
• For modules connected in series within a pack, it is generally recommended to keep the voltage difference between modules below 1-2% to maintain proper balance and prevent uneven aging of the modules.

In the case of a battery pack with 7 series-connected modules, each with a nominal voltage of 50 V, the total nominal voltage of the pack would be 350 V. Thus, a voltage difference of 1-2% would translate to a maximum of 0.5-1 V difference between any two modules in the pack.

I would like to know do you agree with it or not?

 

[eMark]

For cells connected in series within a module, it is generally recommended to keep the voltage difference between cells below 5% to avoid overcharging or over-discharging of any individual cell.
For modules connected in series within a pack, it is generally recommended to keep the voltage difference between modules below 1-2% to maintain proper balance and prevent uneven aging of the modules.

Nominal LFP cell voltage is figured my some at 3.20V (17% SOC)? As previously posted in another thread the nominal voltage of a new lithium cell is usually at 50% SOC. 7S x 3.26V would be 22.82V ... 1-2% of 3.26V = 33-66mV ... 1-2% of 22.82V = 0.228-0.456V.

Here's a comparable nominal voltage chart showing 50% SOC for 18650/21700 ebike battery packs of different voltages ...

Before connecting a used 7s6p module with a new 7s6p module the 14 6p-groups of both LPFs should be balanced at least within 0.050V - 0.100V in both batteries whether at 20%, 50% or Full (resting) SOC. If you can't adequately balance any "Gross Imbalance" of the 14 parallel 6p groups of your used 14s module DO NOT connect it to a new 7S6P module.

NOTE: With new XenForo Platform there is yet no way for me to preview before posting. The two charts

may not post with XenForo's new formatting language. It will take awhile getting use to XenForo.

 

[Jonndeka]

Many thanks for your good explanation. If the cell is NMC we will have different value for voltage difference. right?
I calculated the voltage difference for NMC cell in my previous post. I'm investigating voltage difference fir both LFP and NMC cells.