Charge current for battery pack

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Apr 17, 2022
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Hello, I have a question about charging current. Hopefully this is my last question regarding batteries(!)

So I have two 7s4p batteries that I will be charging separately, and I'm trying to determine what current I should charge at. I have a hobby charger (icharger x8). All my cells are Samsung 30q, and the standard charge current is 1.5A, so that's what I'd like to charge at.

My simple question is, do I charge at 42A (7x4x1.5) ?
 
Actually I just realised that the max charge current for that charger is 30A, so I'm pretty sure my calculations are incorrect.
 
I have been looking more in to this and have came to the following conclusion:

4p = 12000mha (3000 * 4 Samsung 30q batteries are 3000mha)
charge rate = 1.5A, or 0.5c, = 6A (1.5 x 4p)
7S = ~26v, so 26V * 6A = 156W

Therefore as long as my PSU can output 156W then I'll be fine. Eg, I can use a 12v PSU that can output at least 13A, or a 24v PSU that can output at least 6.5A.

Charging times should be about ~2 hours @ 0.5c for the whole pack @ 156W.

Does that sound about right?
 
0.5C is a good rate. Any higher than that and the batteries will be heating significantly. Lower rates are better for battery life.
 
(it's 3000mAh not 3000mha)
justaperson76 said:
charge rate = 1.5A, or 0.5c, = 6A (1.5 x 4p)
0.5C = 1.5A charge rate for 30Q. I prefer to charge my 30Q battery pack at either 0.50C (1.5A) or at 0.67C (2.0A) instead of 1C rate (3.0A) ... depends on which charger i'm using.
justaperson76 said:
7S = ~26v, so 26V * 6A = 156W
26V is only 3.71V/p-group. You want to charge to at least 85% (28.35V or 4.05V/p-group) which would be a cell capacity of 2550mAh (assuming full capacity is 3000mAh). This is based on Controller Cut-Off of 3.20V being 0000mAh usable capacity (based on cycle life).

With 4.20V being 3000mAh FULL capacity (4.20V=100%, 4.10V=90%, 4.00V=80% .. 3.70V-50% .. 3.50V=30%, 3.40V=20%, 3.30V=10% ). Again based on achieving manufacturers suggested number of charge/discharge cycles. In other words best to only charge to 4.10V (28.70V/7s)) and discharge no lower than 3.20V (22.40V/7s).

Each of your 7s strings is 4p or 12.0Ah (4 x 3000mAh = 12Ah total) when new and fully charged. If only charged to 28.7V (90%) the capacity is 2700mAh x 4 = 10.8mAh (assuming FULL capacity is 3000mAh).

As your 30Q pack ages with say 200 charge/discharge cycles the capacity even if charged to Full (4.20V/p-group) the actual p-group capacity may only be at 2700mAh (2700mAh x 4 = 10.8Ah). In other words Full is no longer FULL. Increasing the number of series strings only increases the voltage it does not increase the capacity of battery.

7S x 4.2V = 29.4V ... if PSU has a trim pot adjustment and you only want to charge each series string p-group to 4.1v then 7S x 4.1V = 28.7V. Would suggest you only charge to 28.7V for increased cycle life. Also cut-off voltage at 22.4V (3.2V/p-group).

29.4V x 12Ah = 352.8W
28.7V x 12Ah = 344.4W
justaperson76 said:
Charging times should be about ~2 hours @ 0.5c
As the pack ages with more and more cycle charges/discharges with a decrease in cell capacity ... the charging time may still be close to the same due to an increase in IR (cell Internal Resistance).
 
Thanks. This all makes sense. You are right, I should have used the full capacity charge rather than nominal. I'll be charging to around 4v or 4.1v max to prolong life, so total of 28.7v.

eMark said:
29.4V x 12Ah = 352.8W
28.7V x 12Ah = 344.4W

Seeing I am only charging at 0.5c, then I think that means I will only need a PSU that does half the above quoted figures.
 
justaperson76 said:
This all makes sense. You are right, I should have used the full capacity charge rather than nominal. I'll be charging to around 4v or 4.1v max to prolong life, so total of 28.7v.
29.4V x 12Ah = 352.8Wh (FULL charge to 4.2V)
28.7V x 12Ah = 344.4Wh (90% charge to 4.1V)
justaperson76 said:
Seeing I am only charging at 0.5c, then I think that means I will only need a PSU that does half the above quoted figures.
A charge rate of 0.5C (1.5A for 30Q 3000mAh) or 1.0C (3.0A for 30Q 3000mAh) to 90% (4.1V) still requires 28.7V (7s). The difference in charge rate is not a change in voltage, but rather the time it takes to charge. Charging 90% means the cell capacity is also reduced ...

Charging to 4.10V the cell capacity is reduced to say 2700mAh ... 4p x 2.7Ah = 10.8Ah
28.7V x 10.8Ah = 309.96Wh (90% charge to 4.10V) ... Full charge = 352.8Wh

https://battlebornbatteries.com/watt-hours-to-amp-hours/#:~:text=Here's%20the%20equation%3A%20
watt-hours-to-amp-hours-equations-1024x640.jpg


On another thread i believe it was fechter that suggested discharging p-groups to 3.00V which is AOK. The reason i charge my 30Q pack only 90% and Controller Cut-Off at 3.20V (p-group voltage) is because Samsung 30Q cycle life may only be good for say 250 c/d cycles when charging battery pack to 4.20V (FULL) and discharging to 3.00V (p-group voltages).

Here is another way of figuring discharge voltage and capacity rates. Whatever the Controller Cut-Off it's considered 0.00V and 0000mAh for the sake of the following comparisons. There is very little useful Ah capacity below p-group voltage of 3.00V ...

4.20V - 3.00V = 1.2V divided by 10 = 0.12V
4.20V, 4.08V, 3.96V, 3.84V, 3.72V, 3.60V, 3.48V, 3.36V, 3.24V, 3.12V, 3.00V
100%...90%....80%...70%....60%....50%....40%....30%....20%....10%....0%

3000mAh (3.0Ah) divided by 10 = 300mAh
3.0Ah, 2.7Ah, 2.4Ah, 2.1Ah, 1.8Ah, 1.5Ah, 1.2Ah, 0.9Ah, 0.6Ah, 0.3Ah, 0.0Ah
 
Thanks for the info. My charge/discharge will be conservative. I will only be operating between 3.1v to 4.0v for now until I get a rough idea of how long this thing will last.

As for charge rates, I will only be charging slow (well, recommended rates at least, 0.5c). The charger I have (x8 icharger) has an input voltage between 9-49v. Trying to match a PSU, I'm pretty sure anything over 200w will be fine at 12v providing it outputs more than 6A. Charging at 0.5c, for 4p @ 3000mAh = 6A, multiplied by 28 (4v x 7s) = 168W, allowing for any deficiencies (and increase of capacity to 4.1v if I choose to later).
 
justaperson76 said:
I will only be operating between 3.1v to 4.0v for now until I get a rough idea of how long this thing will last.
7s x 4.0V = 28V ... Keep an eye on voltage and stop iCharger when it gets to 28.0V (28.25V to 28.35V about 85% charge on your 7s4p 30Q pack). Or if possible set to charge only to 4.0V/cell.

4.20V - 3.10V = 1.1V divided by 10 = 0.11V
4.20V, 4.09V, 3.98V, 3.87V, 3.76V, 3.65V, 3.54V, 3.43V, 3.32V, 3.21V, 3.10V
100%...90%....80%...70%....60%....50%....40%....30%....20%....10%....0%
(probably stop before reaching cut-off, with bounce back voltage about 24.5V)

3000mAh (3.0Ah) divided by 10 = 300mAh
3.0Ah, 2.7Ah, 2.4Ah, 2.1Ah, 1.8Ah, 1.5Ah, 1.2Ah, 0.9Ah, 0.6Ah, 0.3Ah, 0.0Ah

justaperson76 said:
As for charge rates, I will only be charging slow (well, recommended rates at least, 0.5c). The charger I have (x8 icharger) has an input voltage between 9-49v. Trying to match a PSU, I'm pretty sure anything over 200w will be fine at 12v providing it outputs more than 6A. Charging at 0.5c, for 4p @ 3000mAh = 6A, multiplied by 28 (4v x 7s) = 168W, allowing for any deficiencies (and increase of capacity to 4.1v if I choose to later).
Charge rate is based on cell capacity ... 0.5C x 3000mAh = 1.5A charge rate. A charge rate of 6.0A is 2C. I wouldn't charge over 2A rate (0.67C) for cycle life longevity.

4p x 2.4Ah = 9.6Ah ... 28.00V x 9.60Ah = 268.80Wh (80% charge)
4p x 2.7Ah = 10.8Ah . 28.63V x 10.8Ah = 309.20Wh (90% charge)
 
999zip999 said:
Nice read. Aren't you guys having fun.
I charge my 10s3p 30Q experimental pack at 1.5amp (as if it were a 10s1p), and sometimes charge at 2amps.

I have a UPP 10s5p 12.5Ah battery with 2500mAh cells. Would charging it at 0.5C be a charge rate of 6.25amps? However, it came with a 2amp charger which is AOK by me. Is charging it at 2amps only a charge rate of 0.16C?

What i'm getting at is instead of charging his 7s4p 12Ah 30Q pack at 6amp (0.5C), if it were me i'd charge it at no more than a 3amp charge rate (0.25C).

Do you base your charge rate off of the discharge capacity (7s4p 12Ah 30Q pack)? Would you charge at 6.0amps figuring it's a charge rate of 0.5C? Or would you prefer charging it at 3 amps (0.25C) ?
 
Read the datasheet for the cells and go for the standard charge rate or less.

Need to check the bms, charger connectors and wiring is up to high current if you’re going higher than a few amps.

Anything less than about 1A per parallel cell is going to be too slow with little benefit.
 
eMark said:
Charge rate is based on cell capacity ... 0.5C x 3000mAh = 1.5A charge rate. A charge rate of 6.0A is 2C. I wouldn't charge over 2A rate (0.67C) for cycle life longevity.

It would be 2C if my pack is 1P but it's 4P remember, so 0.5C. I won't be going any higher than that.
 
justaperson76 said:
It would be 2C if my pack is 1P but it's 4P remember, so 0.5C. I won't be going any higher than that.
Because 30Q has had problems with high self-discharge in the past decided in 2020 to charge my experimental 10s3p 30Q-141 pack at only a 1.5amp rate (sometimes 2.0 amp) to 90% (41.0V) with discharge rate averaging around 6-7amps with average DOD around 20% (34.0V).

However, still had to replace 12 of the 30 "141" cells over the past two years. So, what was hoped to be TLC apparently made little, if no, difference when it comes to 30Q "141" inherent high self-discharge tendency (QC problem?). The replacement 30Q "6 KH1T" cells are doing better (so far). Hopefully, Samsung's newer 30Q-6T cells are no longer plagued with high self-discharge ... at least to the extent as were 30Q: "136", "138" and "141" cells (IMO).

Another reason for using such a low charge rate was that i apparently misinterpreted the following introductory quote from docwares "Ageing Tests" begun in 11/2019 ...
docware said:
Frustrated and tired by never ending unfruitful debate about cycle life of various Li-ion cells, I finaly decided to start some long term cycling to see the real results. Parameters was chosen in effort to imitate real utilization, but on the other hand to get some results in acceptable time. Important was to not use parameters like 100 % DOD, 0,5 C charging, 1 C discharging, as these are in my opinion too severe and unrealistic in the real life.
Perhaps i misunderstood the last sentence as docware has used a charge rate of 0.5C in his ageing tests and in some tests a DOD to 2.50V (100%?). FWIW, by my figuring, a 90% charge to 4.10V/cell is based on the cut-off (DOD) voltage to 3.20V/cell (not 3.10V or 3.00V or 2.50V).

Because i'm retired and as part of my experimental 10s3p 30Q testing the longer charge time at 1.5amp (0.167C) instead of 4.5amp (0.5C) was not a hindrance to me. In hindsite i must have misinterpreted docware's comment ...
Important was to not use parameters like 100 % DOD, 0,5 C charging, 1 C discharging, as these are in my opinion too severe and unrealistic in the real life.

It's still my belief (whether right or wrong) that you will prolong the cycle life of your 7s4p pack with a 3amp charge rate (0.25C) instead of a 6amp charge rate (0.5C). Also average DOD 23.8V (3.40V/cell) with 20% (600mAh) of usable pack capacity remaining (3.20V/cell).

Hopefully, you won't experience any of the 30Q-6T cells suffering from high self-discharge, or at most only 10% of the cells. Your Samsung 7s4p 30Q battery pack with the newer 30Q-6T cells should provide at least 250 trouble free cycles (if not abused).
 
Charging at 0.25C definitely won't cause any harm. Honestly I'm really in no rush to charge, it's fine if it charges the whole day really as I'll pre-charge the day before use, while I am home.

Yeah hopefully I don't get any bad cells :) I've let them sit for many days and they've all held their voltage, virtually. Not that that's the only way to determine if they're good, but they're new, and that's good enough for me.
 
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