Why is 1 battery lower than the others?

[bds70]

I have 4 diy built batteries. Batt 4 always seems to be a lower % SOC than the others.

All the settings are the same, and they are in parallel. Batt 4 is the first one closest to the lynx distributor. Both POS and NEG cables attach to Batt 4.

Would it help to attach the NEG cable to the Neg of Batt 1 being the furthest one.

See the attachment for clarity.

Cheers.

 

[WindWizard]

It would help to have some pictures of your wiring.

 

[bds70]

It would help to have some pictures of your wiring.

I will have to wait until tomorrow afternoon to take pictures. But basically there is solid 20x10mm copper bus bars, same length, between each battery, with a T fuse between each battery.

The final connection between Batt 4 and the distributor is 35mm² cable. The Neg is marginally longer than the Pos. The Pos runs through the isolating switch.

As soon as I can I will take photos and post.

 

[Brucey]

I will have to wait until tomorrow afternoon to take pictures. But basically there is solid 20x10mm copper bus bars, same length, between each battery, with a T fuse between each battery.

The final connection between Batt 4 and the distributor is 35mm² cable. The Neg is marginally longer than the Pos. The Pos runs through the isolating switch.

As soon as I can I will take photos and post.

Can you connect to the mid point of the busbars instead of one of the batteries?

What are the voltages of the batteries? might just be BMS soc being unreliable like they tend to do.

 

[wpns]

I have 4 diy built batteries. Batt 4 always seems to be a lower % SOC than the others.

All the settings are the same, and they are in parallel. Batt 4 is the first one closest to the lynx distributor. Both POS and NEG cables attach to Batt 4.

Would it help to attach the NEG cable to the Neg of Batt 1 being the furthest one.

See the attachment for clarity.

Cheers.

1). Yes, do the 'diagonal' thing and use the negative terminal of Battery 1.

2). I'd check the wiring between Batteries 1 and 2, they _should_ be at the same voltage if you have them in parallel.

3) Wait, T-Fuse between each battery? Yowza, please post pictures.

 

[BentleyJ]

Note that battery #1 has the lowest voltage by a fairly wide margin yet it shows the highest SOC. Also Battery #1 has the lowest discharge current at -1.7A while the other 3 are -2.2A to -2.4A. This is exactly the behavior one would expect. Lower voltage will drive less current therefore batteries in parallel are mostly self correcting. Lastly, the cells in each of the 4 batteries are balanced well, at least at those voltages. Cell voltage divergence, if any, will not become apparent until 3.4V and above.

Based on the information posted, I would conclude there may not be a problem at all. SoC values from the BMS are subject to all kinds of inaccuracies. There are MANY posts on the forum with that same complaint.

You need to charge the battery bank to 100% at a minimum of 13.8V until the current drops off to an amp or less for each of the 4 batteries. At this point all the BMS should read 100%. If they don't then something is not working correctly. Also, verify cell voltage balance while charging.

Once all BMS read 100%, then start a discharge cycle and note the current from each battery, should be approx. equal. Check for hot spots on the wiring connections.

 

[Skypower]

What surprises me is the voltage difference between batteries 1 and 3 with little current flowing. There’s 300 mv difference! This reading should be no more than +- 10 mv reading (.010). Assuming that the bms’s are JK’s you will need to calibrate voltage AND current. You will need an accurate voltmeter/clamp and preferably a good shunt. Disable the charge/discharge on the batteries that you aren’t working on or remove the fuses. I like to use a steady 10 amp charge. Adjust, save, observe and if good, isolate from the others and repeat.
Cycle to full. It may take a few cycles and maybe not. You can force it to 100/99% SOC by temporarily lowering your Cell protection values. For instance if your full charge is 13.8 (cell 3.45) in this order; move OVPR to 3.4 and cell OVP to 3.45. Once you have observed the bms(s) and the app reports 100% with yellow text “Battery fully charged” turn off charging and reset protection values, cell OVP then cell OVPR in that order and save. I like to use cell OVP at 3.55 and cell OVPR at 3.5 for normal use as I’ve discovered that it seems to help BMS remember the full level well below this value. It not as if the bms is likely to trigger the protect having the active balancer. You will need to be there when the event happens to disable the charge so the bms(s) aren’t constantly going in and out of protection(unnecessary wear) and make the above action. I see no problem doing capacity reset on all four at the same time. As others have mentioned check your cabling so that all batteries do their fair share.

 

[time2roll]

At that current level it would seem the voltage calibration is off a bit.
Have you verified the voltage with a separate meter?

 

[GXMnow]

Based on the information posted, I would conclude there may not be a problem at all. SoC values from the BMS are subject to all kinds of inaccuracies. There are MANY posts on the forum with that same complaint.

What surprises me is the voltage difference between batteries 1 and 3 with little current flowing. There’s 300 mv difference! This reading should be no more than +- 10 mv reading (.010). Assuming that the bms’s are JK’s you will need to calibrate voltage AND current.

I agree with both of these.

I have a JK-BMS and I see this issue all the time. The reported state of charge on the JK BMS is just plain useless. It only resets to 100% if you run the battery up until a cell hits the over voltage protection. Then it tries to track the current from discharge and charge to count the energy used. It drifts off fairly quick. Mine will sometimes show down to 0% even with the battery voltage above nominal per cell. So I just ignore it now, it is not worth fighting.

I would double check all of the electrical connections to make sure there is no issues. Run the system at a high current and check if any connections are getting warm. If all the connections are good, and the 4 batteries are sharing the current within about 15%, then I would not worry about it. Parallel batteries will share the current based on their true capacity. A weaker battery will initially discharge a little faster, and once it's internal voltage falls a bit more than the others, it's current will drop until the voltage falls at the same rate as the other batteries. A 110 amp hour pack will end up pulling 10% more current than a 100 amp hour pack. My 2 banks should both be the same 360 amp hours, but the newer bank consistently pulls about 5% more current in both charge and discharge. That is just due to the cells having a little more capacity. Even different wire lengths are not that big of a deal. The batteries will find their balance.

And I agree, the voltage calibration is likely off on at least the one reading 300 mv low. Use a good quality meter and measure the voltage at each BMS and see how close that reads to the BMS reading. The voltage is easy to calibrate. Just tell the BMS what the true reading is and it will apply a correction multiplier.

I do not recommend trying to calibrate the current. You will most likely just make it worse.
To do it right, you need a very steady high current charge power source. You want to do it at as high of a current as possible as any error will be multiplied as the current increases. And use a shunt, not a clamp meter as DC clamp current meters are not all that accurate. A good shunt is far better. Once you have a known solid current supply, you can then tell the BMS what that current is and it will again apply a correction value. But if the current changes at all from your measurement to when you set the value, this will cause an error. And I also found out the JK BMS only samples the current, it does not average over time. This makes it so you need a very clean true DC current source. My Schneider inverter puts out pulsing DC at 120 Hz. It is just a full wave rectified sine wave of current. And the JK BMS could take it's sample anywhere along the wave. So the reading I see on mine varies from 50% to 115% of the true average DC current. This issue alone can also cause the state of charge to drift off. Over time, it does average out a bit, but it can never fully hit a perfect average of the current waveform. When my system is charging just from the Victron DC charge controller, the current reading is far more stable. But it still is constantly changing some due to the amount of sunlight on the panels and the MPPT tracking causing small variations in output current. So that is also not a perfect current calibration source.

 

[Skypower]

I agree with both of these.

I have a JK-BMS and I see this issue all the time. The reported state of charge on the JK BMS is just plain useless. It only resets to 100% if you run the battery up until a cell hits the over voltage protection. Then it tries to track the current from discharge and charge to count the energy used. It drifts off fairly quick. Mine will sometimes show down to 0% even with the battery voltage above nominal per cell. So I just ignore it now, it is not worth fighting.

I would double check all of the electrical connections to make sure there is no issues. Run the system at a high current and check if any connections are getting warm. If all the connections are good, and the 4 batteries are sharing the current within about 15%, then I would not worry about it. Parallel batteries will share the current based on their true capacity. A weaker battery will initially discharge a little faster, and once it's internal voltage falls a bit more than the others, it's current will drop until the voltage falls at the same rate as the other batteries. A 110 amp hour pack will end up pulling 10% more current than a 100 amp hour pack. My 2 banks should both be the same 360 amp hours, but the newer bank consistently pulls about 5% more current in both charge and discharge. That is just due to the cells having a little more capacity. Even different wire lengths are not that big of a deal. The batteries will find their balance.

And I agree, the voltage calibration is likely off on at least the one reading 300 mv low. Use a good quality meter and measure the voltage at each BMS and see how close that reads to the BMS reading. The voltage is easy to calibrate. Just tell the BMS what the true reading is and it will apply a correction multiplier.

I do not recommend trying to calibrate the current. You will most likely just make it worse.
To do it right, you need a very steady high current charge power source. You want to do it at as high of a current as possible as any error will be multiplied as the current increases. And use a shunt, not a clamp meter as DC clamp current meters are not all that accurate. A good shunt is far better. Once you have a known solid current supply, you can then tell the BMS what that current is and it will again apply a correction value. But if the current changes at all from your measurement to when you set the value, this will cause an error. And I also found out the JK BMS only samples the current, it does not average over time. This makes it so you need a very clean true DC current source. My Schneider inverter puts out pulsing DC at 120 Hz. It is just a full wave rectified sine wave of current. And the JK BMS could take it's sample anywhere along the wave. So the reading I see on mine varies from 50% to 115% of the true average DC current. This issue alone can also cause the state of charge to drift off. Over time, it does average out a bit, but it can never fully hit a perfect average of the current waveform. When my system is charging just from the Victron DC charge controller, the current reading is far more stable. But it still is constantly changing some due to the amount of sunlight on the panels and the MPPT tracking causing small variations in output current. So that is also not a perfect current calibration source.

True that many chargers have a lot of noise and it can definitely interfere with current calibration. For sure I wouldn’t be using a Schumacher automotive charger. I have used a higher charged battery as the power supply and paralleled it, then when the current between the two gets to the range I want, I made a settings. Clamp meters can be wild but so can shunts. Shunts are usually better but it’s nice verify with a second device. Clamps can sometimes be more accurate with more turns and divide the result by turns.
Bottom line it’s best if you know your tools quality. From what I’ve seen half of the JK’s are too far off from factory to expect good SOC so for me it’s hard to make it worse.
Many users don’t need to know state of charge so yeah, voltage controls some functions of the bms not SOC. I do use state of charge for solar assistant to control charging from grid and yes a victron shunt can do the same except then I can’t see the batteries remotely.
The OP probably should correct the voltage on battery 1 to average the others at a minimum. He wouldn’t even need a tool, just a steady charge, set and compare, repeat as needed. Other than checking connections, the rest are optional as needed.

 

[WindWizard]

I would make sure that all of your connections are clean and that they are torqued to the proper spec. The wire lugs should be directly connected to the battery posts without a washer in between.

It concerns me that the cell voltages have such a large variance ie 3.3 vs 2.6 volts. With the batteries in parallel, then this should not be the case. I have 128 cells and they are all within 10 mv of each other. OP cells are around 60-70 mv approximately.

 

[rogerheflin]

I have 4 diy built batteries. Batt 4 always seems to be a lower % SOC than the others.

All the settings are the same, and they are in parallel. Batt 4 is the first one closest to the lynx distributor. Both POS and NEG cables attach to Batt 4.

Would it help to attach the NEG cable to the Neg of Batt 1 being the furthest one.

See the attachment for clarity.

Cheers.

You need to have red connected to ie RED->bat4->3->2->1 But Black-bat1-bat2-bat3->bat4 such that the red + black cable length(and resistance) will be similar for all batteries. This resistance/cable length issue is only a big issue at higher currents so would not seem to be the cause of your issue.

If all of the batteries are connected to the same red/black correctly then the voltages should be almost the same, since the voltages are different there would seem to be some sort of connection issue to the battery with low voltage-battery1, or battery1's BMS is completely screwed up on its voltage calibration/reporting. Use a multimeter and verify the voltage on each battery and confirm if the bms is wrong or the voltage is really wrong.

 

[Quattrohead]

Check the balance cables on battery #1, they are more varied than the other 3 batteries.

 

[Bananaman321]

 

[GXMnow]

It concerns me that the cell voltages have such a large variance ie 3.3 vs 2.6 volts. With the batteries in parallel, then this should not be the case.

I think this will be fixed once the JK BMS voltage readings are calibrated. The cells within each battery are well balanced. The cell differences match the whole battery voltage differences.

When you adjust the voltage calibration on the JK BMS it is actually adjusting the individual cell voltages. The total battery voltage is the cell voltages added together. I saw this when I had a balance lead fail and the total battery voltage reported was down by 2 cells, even though the battery measured correct at the battery terminals. The one open balance lead caused 2 cells to read as zero volts. Of course, that caused the discharge mosfets to go open so I could not pull any current from the battery.

The diagonal wiring for a group of batteries does help balance the current, but it is not all that critical unless you are running high currents. As this is a 12 volt system, yes the currents will be fairly high, so it is a good idea. I would run all 4 of the batteries to a short and thick bus bar. Have two batteries on each side of the bar and the load connected to the middle. This also reduces the total resistance as the bar carrying current is now half as long with 2 in parallel. Then equal length cables from the bus bars to the batteries, and the current will match up to the capacity of the batteries as I said before.

My two battery banks are wired to a DC breaker panel where each battery has it's own 125 amp breaker. The outputs of the breakers are on equal length wires to a bus block where the cable then goes to the XW-Pro inverter and another cable goes to the 50 amp breaker from the Victron charge controller. My newer battery has cables about 20 inches longer, but that battery still pulls a little more current as the cells are better. The difference in internal cell resistance is more than the difference in the cable length. Of course, my system is at 24 volts, so the differences come out as a much smaller percentage. At 12 volts, every millivolt and milliohm can make more of a difference.

 

[gotbeans]

Those are 4 different BMS on 4 different batteries?
The calibration for the voltages is right there up top of the config page. They aren't fully accurate, measure each battery with a multimeter.

JKBMS will move around a bit over time, and where you calibrate it (say at 100%) then more toward 0% will be less accurate, so calibrating it around 60% may get you the "most accurate all around" numbers.

 

[bds70]

Can you connect to the mid point of the busbars instead of one of the batteries?

What are the voltages of the batteries? might just be BMS soc being unreliable like they tend to do.

I will check on both points later and feed back.

 

[bds70]

It would help to have some pictures of your wiring.

Here's the photos, best I can do. These are in my bus in the external lockers. Not a lot of room. The cable from No 3 Neg to No 4 neg is the same length ( give or take a couple of mil) as the bus bar from 3 Pos to 4 POS. Ran out of copper.

 

[bds70]

1). Yes, do the 'diagonal' thing and use the negative terminal of Battery 1.

2). I'd check the wiring between Batteries 1 and 2, they _should_ be at the same voltage if you have them in parallel.

3) Wait, T-Fuse between each battery? Yowza, please post pictures.

Yes it was strongly suggested that each battery needs its own protection in the event of a dead short. That's not very likely but better safe than sorry. You can see in the pictures how I have done it, copper bus bar off POS post of B1, T fuse, copper to B2 and onward T fuse between 2 & 3 & 4 to isolation switch.

 

[wpns]

Yes it was strongly suggested that each battery needs its own protection in the event of a dead short. That's not very likely but better safe than sorry. You can see in the pictures how I have done it, copper bus bar off POS post of B1, T fuse, copper to B2 and onward T fuse between 2 & 3 & 4 to isolation switch.

Umm, wow. What value fuses? The individual fuses should ho between the batteries and a common busbar, not between the batteries.

 

[gotbeans]

each battery is usually recommended, but not each cell

 

[bds70]

Umm, wow. What value fuses? The individual fuses should ho between the batteries and a common busbar, not between the batteries.

What? Your saying the fuses should come off each battery post to a common bus bar is that correct?

I don't see how that is any different to the arrangement I have. Each battery is protected against short current in either direction. The value of the 3 T fuses on the bar is 250Amp each and the final one is 400amp.

 

[wpns]

What? Your saying the fuses should come off each battery post to a common bus bar is that correct?

Yes, that's what I'm saying.

I don't see how that is any different to the arrangement I have. Each battery is protected against short current in either direction. The value of the 3 T fuses on the bar is 250Amp each and the final one is 400amp.

So if one battery shorts out it could have 500A dumped into it.

Since each fuse has some resistance, it's going to be impossible to balance the battery-to-inverter resistances out with this arrangement.

 

[GXMnow]

All of the negative battery outputs, after each BMS should go together to a common bus bar to the charge controller and loads.

Each battery positive should go through one fuse to a common bus bar, then your shut off switch, then the charge controller and loads.

This way if any single battery has a problem, it's fuse can pop and leave the other 3 batteries all connected and working for you.

The way it looks in your setup, one battery is only going through the 400 amps fuse. That will be a fairly low resistance. The second battery is feeding through the same 400 amps fuse, AND another 250 amps fuse. This adds a little more resistance. The third battery goes through the 40 amps fuse, the 250 amps fuse and another 250 amps fuse, for a total of 3 fuses in series, adding even more resistance. And the 4th battery is going through the 400 amp fuse again, both of the previous 250 amp fuses, AND another 250 amp fuse for a total of 4 fuses in series on the last battery.

A fuse is essentially a short piece of thin wire which will burn through at a given current. While it is a short distance, it is a thin conductor which has resistance. Having the one common 400 amp fuse and the disconnect switch after the main bus bar is a good idea. That will protect the main cables at the 400 amp limit.

But then each battery should only go through one 250 amps fuse and all combine to a common bus bar. If one battery does short, it will pop only it's fuse and the rest can stay working. With your series setup, a single battery in the middle shorting would have to burn out two 250 amp fuses. The battery on one side is feeding it through a 250 amps fuse, and the 2 batteries on the other side are also feeding it through another 250 amp fuse. So you could have up to 500 amps going into a bad battery before the fuses fail. This is bad.

 

[bds70]

All of the negative battery outputs, after each BMS should go together to a common bus bar to the charge controller and loads.

Each battery positive should go through one fuse to a common bus bar, then your shut off switch, then the charge controller and loads.

This way if any single battery has a problem, it's fuse can pop and leave the other 3 batteries all connected and working for you.

The way it looks in your setup, one battery is only going through the 400 amps fuse. That will be a fairly low resistance. The second battery is feeding through the same 400 amps fuse, AND another 250 amps fuse. This adds a little more resistance. The third battery goes through the 40 amps fuse, the 250 amps fuse and another 250 amps fuse, for a total of 3 fuses in series, adding even more resistance. And the 4th battery is going through the 400 amp fuse again, both of the previous 250 amp fuses, AND another 250 amp fuse for a total of 4 fuses in series on the last battery.

A fuse is essentially a short piece of thin wire which will burn through at a given current. While it is a short distance, it is a thin conductor which has resistance. Having the one common 400 amp fuse and the disconnect switch after the main bus bar is a good idea. That will protect the main cables at the 400 amp limit.

But then each battery should only go through one 250 amps fuse and all combine to a common bus bar. If one battery does short, it will pop only it's fuse and the rest can stay working. With your series setup, a single battery in the middle shorting would have to burn out two 250 amp fuses. The battery on one side is feeding it through a 250 amps fuse, and the 2 batteries on the other side are also feeding it through another 250 amp fuse. So you could have up to 500 amps going into a bad battery before the fuses fail. This is bad.

I think I understand. That's a bit disconcerting. The copper bar is freaking expensive, but I guess not as expensive as new fuses and battery.

The other major problem is room. As you can see there isn't much available space, which is why the 3 smaller T fuses are not in specific holders (plus the $100 each for the holders)

The T fuses could be bolted direct to the BMS POS posts is that correct?

The Neg bus bar can again be bolted direct to the BMS neg post?

I have lots of 35mm² cable, would that be as good as the 20 X 10mm copper?