Building first small battery pack, ESR question

[JonCombat]

Hi, I am building my first small 12V maybe 20-30A pack. I have an xtar vc8 plus charger on order that can determine capacity and ESR. When building a series/parallel pack what is the ideal method of grouping low vs high ESR batteries? I am talking about batteries ESR that are in spec but may vary. ESR will impact discharge rate, temperature... Would the ideal configuration be to keep the total series ESR low, and put them in parallel with other similar series ESR values in parallel? Example: 3s 2p pack. Is this worth the effort? I believe it will keep the discharge rates similar. These numbers were made up for this example.

mOhm- series cells	mOhm series cells
22	20
15	16
17	18
Total	Total
54	54

 

[Korishan]

Are you using LiFePO4 cells, or some other chemistry?

Why 12V, 3s?

 

[floydR]

ESR (Equivalent Series Resistance) is the internal resistance present in any cell that
limits the amount of peak current it can deliver. In short IR

I have an xtar vc8 plus charger on order that can determine capacity and ESR

The IR reported by the xtar is calculated using the dc method iirc, it may not be as accurate as an IR meter such as the YR1030+,1035+, SM8812A, RC3563 meters. Wolf did a comparison of IR metersincluding 4 slot Chargers/dischargers I believe the xtar vc8 was tested

​

Later floyd

 

[Wolf]

Hi, I am building my first small 12V maybe 20-30A pack. I have an xtar vc8 plus charger on order that can determine capacity and ESR.

Wolf did a comparison of IR metersincluding 4 slot Chargers/dischargers I believe the xtar vc8 was tested

Unfortunately I have not tested the XTAR VC8 but I did test the XTAR DRAGON VP4 PLUS.

The XTAR dragon has a seperate mode to test IR with a special set of probes and was relatively accurate just clumsy to use and really not designed for mass harvesting. Also at $84 it was kinda expensive for a mediocre Charger/Analyzer that will charge 4 cells at 1A but only discharge at 0.5A. It also has no cooling fans.

Your XTAR VC8 is just another( please don't take this the wrong way) mediocre Charger/Analyzer . You get what you pay for and in this case it is a Charger/Analyzer that thinks for you for $40. It will determine by the "IR" (Internal Resistance) of the cell at what amperage to charge, which is not adjustable as far as I know, and will only discharge at 300mA. Just think discharging a 3000mAh cell will take 10hrs...... No thank you.

Once you get good at harvesting you will want to have a bit more control over your testing procedure. The OPUS (with the fan mod) is probably the least expensive "proper" Charger/Analyzer you can get at $60. You get full 1A C/D/C and pretty accurate results. IR cannot be trusted on any slide style Charger/Analyzer no matter how expensive. I have The SKYRC MC3000 and its IR readings are also not quite up to snuff although better than most and it should be at $110.

When building a series/parallel pack what is the ideal method of grouping low vs high ESR batteries? I am talking about batteries ESR that are in spec but may vary

What do you consider low vs high ESR/IR? From now on AC IR
Also what chemistry are you using? ICR, INR or hybrid such as NCR, CGR, etc.
In your made up example you show cells in a range of 22mΩ to 15mΩ a 7mΩ difference between high and low. Depending on how many cells you have in parallel that can be a significant issue. In your example if using 3 cells I wouldn't have more than a couple of (2) mΩ difference between cells and certainly all cells within ≈100mAh of capacity difference. Once you expand your parallel pack cell numbers to 10 or more those values can be "fudged" a bit without too much of a problem. Although I would still prefer to see all cells in all packs as close as possible to each other in both IR and mAh as this makes for an optimally balanced battery and will require very little maintenance.
Also for successful harvesting and determining at first touch if a cell will be good or not, as @floydR mentioned get an YR1035+ or my favorite the RC3563.
I have them all..............
Wolf

 

[JonCombat]

Are you using LiFePO4 cells, or some other chemistry?

Why 12V, 3s?

just experimenting with my first battery pack Li-ion. The end goal is to make a powerwall, baby steps!

 

[JonCombat]

Unfortunately I have not tested the XTAR VC8 but I did test the XTAR DRAGON VP4 PLUS.

The XTAR dragon has a seperate mode to test IR with a special set of probes and was relatively accurate just clumsy to use and really not designed for mass harvesting. Also at $84 it was kinda expensive for a mediocre Charger/Analyzer that will charge 4 cells at 1A but only discharge at 0.5A. It also has no cooling fans.
View attachment 26762
Your XTAR VC8 is just another( please don't take this the wrong way) mediocre Charger/Analyzer . You get what you pay for and in this case it is a Charger/Analyzer that thinks for you for $40. It will determine by the "IR" (Internal Resistance) of the cell at what amperage to charge, which is not adjustable as far as I know, and will only discharge at 300mA. Just think discharging a 3000mAh cell will take 10hrs...... No thank you.
View attachment 26766
Once you get good at harvesting you will want to have a bit more control over your testing procedure. The OPUS (with the fan mod) is probably the least expensive "proper" Charger/Analyzer you can get at $60. You get full 1A C/D/C and pretty accurate results. IR cannot be trusted on any slide style Charger/Analyzer no matter how expensive. I have The SKYRC MC3000 and its IR readings are also not quite up to snuff although better than most and it should be at $110.
View attachment 26763View attachment 26764

What do you consider low vs high ESR/IR? From now on AC IR
Also what chemistry are you using? ICR, INR or hybrid such as NCR, CGR, etc.
In your made up example you show cells in a range of 22mΩ to 15mΩ a 7mΩ difference between high and low. Depending on how many cells you have in parallel that can be a significant issue. In your example if using 3 cells I wouldn't have more than a couple of (2) mΩ difference between cells and certainly all cells within ≈100mAh of capacity difference. Once you expand your parallel pack cell numbers to 10 or more those values can be "fudged" a bit without too much of a problem. Although I would still prefer to see all cells in all packs as close as possible to each other in both IR and mAh as this makes for an optimally balanced battery and will require very little maintenance.
Also for successful harvesting and determining at first touch if a cell will be good or not, as @floydR mentioned get an YR1035+ or my favorite the RC3563.
I have them all..............
Wolf
View attachment 26765View attachment 26767View attachment 26768

The batteries are relatively new Li-ion 3.7 2500ma+ from 3p/2p packs that contain a small bms and temp sensor. My company uses them in our products

 

[Wolf]

The batteries are relatively new Li-ion 3.7 2500ma+ from 3p/2p packs that contain a small bms and temp sensor. My company uses them in our products

Nice so your AC IR between different cells shouldn't be too far off from each other. Still with the XTAR VC8 C/D/C cycle just the discharge at 300mA will be 8+ hrs. bad enough on an OPUS at 1A a complete C/D/C from an empty cell is still 7.5+ hrs.