1amp +/- Charging Diff Between 2 Parallel 8s Packs ~ Should I Tweek It

MrM1

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Just finished building up two 8s 230ah packs (here starting at #28) each with its own 100amp Overkill Solar BMS. The packs are joined in parallel at big busbars. I went to great pains to make sure all connection cables were the same length.

Everything is Flowing pretty close charging and discharging. When charging using a steady-state charge of 90 amps (45amps to each 8s pack) , but there is about a 0.5 to 1 amp charge difference between the two 8s packs.

My question is, should I tinker with cable length to try and get less resistance on the pack that is taking less amps?

Or does less than 1 amp charge difference matter that much for parallel charging?

If I were to tinker with the cable length, how much shorter should I make the cable that is not charging as well as the other pack?

Is 1 inch too much with #4 welding cable. Or would 1" pretty much not do much?
 
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FilterGuy

What, me worry?
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Just finished building up two 8s 230ah packs (here starting at #28) each with its own 100amp Overkill Solar BMS. The packs are joined in parallel at big busbars. I went to great pains to make sure all connection cables were the same length.

Everything is Flowing pretty close charging and discharging. When charging using a steady-state charge of 90 amps (45amps to each 8s pack) , but there is about a 0.5 to 1 amp charge difference between the two 8s packs.

My question is, should I tinker with cable length to try and get less resistance on the pack that is taking less amps?

Or does less than 1 amp charge difference matter that much for parallel charging?

If I were to tinker with the cable length, how much shorter should I make the cable that is not charging as well as the other pack?

Is 1 inch too much with #4 welding cable. Or would 1" pretty much not do much?
So.... it appears that one battery is getting worked ~2% harder than the other. If you are driving the batteries hard with full cycling every day, you might see one set wear out sooner than the other in several years, but even then, you might just find that as the one wears out and contributes less, it might all balance out.

As far as trying to balance it, you might find it kind hard to do. Without trying to do any math, my guess in the difference is more due to the resistance of your crimps and connections than anything else. Next in line would be the difference in the two batteries and last in line would be the resistance in the wiring itself. It would not surprise me if you tried to make a change in cable length and the difference actually got worse.

Before you try changing cable length, take all the connections apart (including the cell bus bars) on the lower current one, lightly sand all the connecting surfaces with 600 grit or finer and put them back together. You might find it will balance things out a bit.

If you do try to add cable length..... save the old one so you can get back to where you are now if you need to.
 

MrM1

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Yeah when I put the cells together on Monday I used a fine wire brass brush, scotch Brite, then alcohol. I then immediately put a lite coat of No-ox-id on the terminals. But maybe I introduced micro scratches with the brush that did more harm than good.
 

FilterGuy

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Yeah when I put the cells together on Monday I used a fine wire brass brush, scotch Brite, then alcohol. I then immediately put a lite coat of No-ox-id on the terminals. But maybe I introduced micro scratches with the brush that did more harm than good.
You are probably pretty close to as good as you are going to get it..... Let us know if you try to improve it and how well it works out.
 

MrM1

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Ok thx.
Also, if I do mess with cables should it be the positive, negative or both ?

I guess I could just swap the cables from the packs and see if the current levels change on each pack.
 

Mcgivor

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Measure the voltage drop across the cables of each battery independently with a fixed load, that will determine if the cables are within close proximity to one another, measuring resistance without load is not dynamic, current draw will reveal any high resistance terminations.
 

MrM1

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Measure the voltage drop across the cables of each battery independently with a fixed load, that will determine if the cables are within close proximity to one another, measuring resistance without load is not dynamic, current draw will reveal any high resistance terminations.
@Mcgivor is this you? From the solar-electric.com forum? Hey! Yeah I sent you and email of my builder on Monday. You were giving me pointers on lfp and charging last march. Took me this long to get cells
 

RCinFLA

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0.5 to 1 amp sharing difference on 90 amps is excellent. Don't expect them to stay that good over time.
 

Mcgivor

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@Mcgivor is this you? From the solar-electric.com forum? Hey! Yeah I sent you and email of my builder on Monday. You were giving me pointers on lfp and charging last march. Took me this long to get cells
Yes, I haven't been on NAWS forum in a while, seemed to become stagnated on lead acid with LFP being referred to as not suitable for DIY, here its the other way around and since I would never consider LA again this environment is more suitable.
 

Bud Martin

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MOSFETs and the current sensor in the BMS also in line with the flow of the current too.
 

Steve_S

Offgrid Cabineer, N.E. Ontario, Canada
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1A difference is NOTHING to worry about. I run 5 Packs in Parallel. 3x280Ah & 2x175AH. ALL Cables are Equal Length 4/0 (I have a 12' Cable Run from Pack to Inverter) Length is compensated as Each Pack has its own Shunt and DCC (Solid State Contactor).

During Charge it is NOT unusual to see the packs taking a slightly different amperage, of course with different sized packs (175 vs 280) that is also a factor. But sticking to just the 280's, I can sometimes see as much as 3A difference going into the 280's (typically when charging above 70A) and the higher the Amperage (I can push 200A) the more that will be noted, BHUT they do level up as the charge process continues. By the time the System sees 14A (EndAmps / TailCurrent) being taken across the bank, it then switches to FLOAT Mode and the active balancers bring everything to within 10mv.

Every single cell has it's own "Personality" with regards to taking & dumping charge. When properly Matched & Batched the cells will generally behave identically throughout their entire Working Voltage Range (2.900-3.450). When using Bulk cells they will deviate (in Millivolts) BUT their Internal Resistance will also vary and affect the overall intake of power, this will result in the "pack" taking differing Amps (relative to the combined cells).

Where does 14A come from for EndAmps/Tail Current. This is based on 0.05C-Rate which is 14A for a 280AH Pack. The Highest AH Pack is within a Bank the one used to make that calculation. What this does simply, is that while charging in Bulk/Boost (Constant Amperage) mode (terms vary by product) that can take the packs up to 90% using High Amps (as Available) but NOT ever exceeding 0.5C-Rate. As the charge progresses and the TARGET Voltage is Attained, then the Amperage will start to decrease as the cells become saturated and the EndAmps stops the Hi Amp charging and flips to Float (Constant Voltage, Variable Amp) to finish off down to trickle and fully saturated.

The Gotcha to confuse... a little anyways.
With multiple packs in a bank you will see some weird things (at first anyways) and wonder WTH is going on... BTW: Everyone goes through that and boy it creates many queries. Regardless if Fully & Properly Matched & batched or not, every single battery pack has it's own "personality" as it is the collection of cells that creates the pack, Bulk Cells of course have Far More personality (as in quirks with runner cells etc). But once you have the whole thing setup ad tested to be working well, observe the packs during charge & discharge in both Heavy & Light modes and it will become quite apparent what the behaviours are. As long as all the packs can be fully charged to your Target Voltage and allowed to "settle" there until they balance out with float, your Golden.

Target Voltage ?
Nominal Voltage is 3.200
LFP Functional Range is 2.500-3.650
LFP Working Range is 2.950-3.450 (A+) to 3.000-3.400 (Bulk A- and lower)
There is Less than 5% total capacity below 2.900 and above 3.450 and those are in the Cliffs/Knees
* Fully Matched Cells always rate above spec, a 280AH cell will be at least 284AH up to 292AH, Bulk cells rarely ever reach 280AH, they typically top at 275AH and sometimes lower
The "Target Voltage" is what you set YOUR 100% at, which most people choose to set at 3.450, and so they never drop below 2.750Vpc. Of course allowing for Voltage Sag due to load, most will set a 20 Second Delay for Low Volt Cutoff (because a surge pull can drop the voltage momentarily below threshold)

I have done Hard Thrash Tests with pulling 225A and pushing 150A Charge using the full bank and down to One Pack standing. This was Hard Abuse tests as I live Rural/Remote (can be landlocked for a week) and had to ensure the integrity of my build in Worst Case Scenarios. We ALL know that Shit Happens, expected or not and Always at the worst possible time, in my case that could be dire. The behaviours I mentioned above were all seen during the Thrash Testing (really highlighted it) but during normal daily use it is quite minimal.

Do remember to keep your Battery wires together to reduce field noise (DC can be nasty). Make sure your wire lengths are the same (+ & -) from Battery Packs to common DC Bus Bars and again from the Common DC Bus to the devices (SCC/Inverter/Charger). Remember to adjust for any Shunt or Contactor/Relay being in place.

PS: Something Most never mention...
Every BMS senses the millivolts and are quite delicate in that respect. IF your Batt Wires are "noisy" because they are improperly routed and there is a field between them, every BMS will be affected by that resulting in fluctuating readings and "flutter". Lots of AIR has been wasted on this point. BTW: Loose Connections not only generate heat and other issues, they also throw a LOT of noise into the circuit because of the "constant short" that results from loose connections.

Hope it Helps, Good Luck.
 

MrM1

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Wow !! @Steve_S that is some amazing feedback. Couple of questions if you have time.

1. So u have 5 packs of different Ah all sharing the same charge current?
- and so 1-3 amp as the current goes up is not a concern ? (yes when I have very low charge current it is pretty much the same on both packs. I only see the 1amp diff at the top of my charge current curve)
- are they also sharing just one SCC?

2. Since you mention 3.450vpc as a top limit is that your hi setting (in most chargers, where bulk switches to absorb)?
- and more specifically, when Your SCC switches to absorb, where do you set your absorb timer? How Long?
- do you stay in absorb to the 14 amp end amp setting ?
- or are u quickly ending absorb and just going to float?
- if Float, is your float vpc setting the same as your bulk setting (I assume 3.45,vpc or whatever u'ved deemed 100%.)

3. Wherever do you set your Rebulk in relation to your float voltage?

The "Target Voltage" is what you set YOUR 100% at, which most people choose to set at 3.450, and so they never drop below 2.750Vpc.

4. I'm not understanding "2.750vpc"? Guessing that is a typo?
 

Steve_S

Offgrid Cabineer, N.E. Ontario, Canada
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Vpc = Volts Per Cell.

1) Yes, All 5 packs are fed by Midnite Solar Classic-200 & Samlex EVO 4024 Inverter/Charger (set for 80A Charge Rate).
Typically, the 175's may get 10A each while the 280's will pull 15A and of course that floats all over pending on sun/clouds etc...
As the packs fill that Amps Taken will shift too during the process. This id NORMAL.

Look at my signature "About My System" as most info you want is there already. Charge Profiles, equipment, components etc...
 
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