BMS cable lengths - same length?

[BjornM]

In my opinion you have one of the best bms’s on the market. With a reasonably well matched set of cells that 2amp balance should be able to do the job but it could take a while the first time charging. You may need to greatly reduce the charge current once it starts to disconnect to give the balancer the time it needs to work. The default voltage when balancing starts is usually 3.4 volts, so you need to get it there for the magic to happen or temporarily reduce that setting if you’re having problems getting there because of a high cell voltage protection (3.65). If that’s triggered,the charging will disconnect but balancing will continue as long as you are also within the balance voltage range. The app will tell you if it’s balancing if by looking at the balance current, on some JK’s the “Balance On” just means it’s enabled, not active. There will be a yellow notification letting you know about the disconnect. I try to avoid the disconnect by lowering the charge and giving it time. I sometimes manually augment the process but that’s a whole other story. Yes, hwse is on the right track.

Thanks, good stuff to know. Makes sense. I have a current limiting function in my charger.

 

[BjornM]

As far as how I top balance, you have it right except that after you get the pack balanced at 13.6v it will not take days, it will take minutes to get to each new high point. By the time you get all cells to 3.45v, you are at 98%-99% SOC and pretty well balanced. It is getting to 13.6v that can take a long time. The reason I choose 12.6v is because it is low enough to let the charger run but limit the charge current without tripping any of the cells for over voltage (OVP). If you have really badly out of balance cells, and the BMS triggers an OVP, lower the CV setpoint so that it will charge without tripping.

I assume the 12.6V is a typo, and that you mean 13.6V. So when I have charged the cells to 13.6V, I wait out the BMS and keep an eye on the max delta figure (max diff in cell voltage) to drop below a reasonable threshold. Could take several hours. After that I charge again to 13.7V, and repeat. Then to 13.8V, and so on. I keep doing this until I hit normal top balancing voltage, which is in the 3.55 - 3.65 V range, and then I'm done and can return to the normal charging profile.

I don't have a access to a high quality charger, except the one I'm using for my solar power system. So I'm planning to do this procedure while at the same time using the bank for my home. Since there is limited daylight hours, I suspect I'll only be able to do one charge per day. First day to 13.6V. Second day to 13.7V and so on.

 

[BjornM]

I take exception with your next statement. I believe that it is very important to top balance. I just choose to let the KJ do it rather than to manually do it in parallel with a benchtop power supply. If your pack is not top balanced, you are losing capacity. I also have two 460Ah batteries in sealed 8D cases. They unfortunately have JDB BMS's which makes balancing very difficult but not impossible. I was able to reduce the imbalance on the worst of them from 300mv to 50mv which increased the tested capacity from 445Ah to 480Ah.

This part I don't understand. In my mind, the only advantage of top balancing is to get the optimum pack capacity from day one instead of later. Looking two months down the road, what if any is the difference in capacity between a pack that was top balanced, and one that wasn't, if they both use an active balancing BMS? What does the top balancing do that the BMS can't correct later?

 

[BjornM]

Your last statement about charging to 100% is incorrect. It is not necessary or even desirable to charge LFP to 100% every time. That is a carryover from AGM which are damaged by not charging to 100% and being stored at a partial SOC. LFP are very happy at partial SOC and not harmed by it. In fact, for storage you want to lower them to about 50% - 60% SOC. In my boat and motorhome, I charge conservatively with bulk/absorb at 13.8v for 30 minutes. I then float at 13.3 if I am using the batteries and at 13.2v if they are in storage. That means that they are down about 20%-40% when in storage. Before I head out on a trip, i cycle the charger which fully charges it back up to 13.8v to top it off.

Sorry, I must have been unclear here. I was only referring to the top balancing phase. After a few days of charging to 100% this procedure is dropped, and the normal charging profile is used instead. I plan to do a top cutoff at 90% SOC and bottom at 10% SOC, as is specified on the EVE datasheet.

 

[hwse]

You’re blind to what’s happening inside a Battle Born. Is ignorance really bliss? No app, no display = less customer complaints ? Same for Big Battery.

My thoughts exactly. Some folks want to be blissfully ignorant about what is happening and just want to plug and play. Me, I have a burning need to know.

 

[hwse]

This part I don't understand. In my mind, the only advantage of top balancing is to get the optimum pack capacity from day one instead of later. Looking two months down the road, what if any is the difference in capacity between a pack that was top balanced, and one that wasn't, if they both use an active balancing BMS? What does the top balancing do that the BMS can't correct later?

It is a semantics thing. Connenting cells in parallel and charging them slowly until they are all at 3.65v is not top balance. It is one of the many methods to top balance. Top balance is a state of being where all of the cells reach the exact same voltage at or near the high limit of 3.60 to 3.65v. The other option it bottom balance where all cells hit the same voltage at the low limit of 2.50 to 2.55v. You can only assess the balance of LFP cells at the max or min voltage because at all other voltages in between, the curve is so flat as to be meaningless. On my Basen battery, it showed a voltage delta of 1mv when I got it and it stayed there until the pack voltage got to 13.4 and between there and 13.6v, it spiked to almost 300mv.
What the JK does when you connect it to cells and set it to charging is Top Balance the pack. It is just a much easier way to do it than connecting a bunch of cells in parallel and then charging them with a tiny benchtop power supply for weeks to do the same thing.

 

[hwse]

Sorry, I must have been unclear here. I was only referring to the top balancing phase. After a few days of charging to 100% this procedure is dropped, and the normal charging profile is used instead. I plan to do a top cutoff at 90% SOC and bottom at 10% SOC, as is specified on the EVE datasheet.

90% SOC is pretty low for OVP and is hard to control because that is in the flat part of the curve. On my packs, I set OVP to 3.60v but set the charge controllers to 13.8v which is 3.45v. I use a 0.01C tail current for absorb which is 5.6A and it only lasts a few minutes because my cells are very well top balanced. That way I am charging to just inside the knee region where delta V starts to show so that I get the active balancing to make any small adjustments. Then I drop to 13.2v which drops it back down to 80% to 90% SOC.

 

[hwse]

Thanks, good stuff to know. Makes sense. I have a current limiting function in my charger.

In my opinion, the method of lowering the CV voltage to slow down charging as the JK is balancing is more effective than current limiting. When I first built my 560Ah pack, I used the current limiting technique because I did it at home and had no charger that I could set to less than 14.0v. I was using my automatic car battery charger that charges to 14.0v at either 15A or 2A of current. It worked but only because the output was at 2A which matched the ability of the JK to shuffle current. On my battery, I had three cells that were very close and one that was at a significantly lower SOC. If it would have been the other way around or worse yet, two high and two low, 2A would have been too much.

It is much better to use your better-quality smart charger and let it send as much current as possible without driving the voltage over the OVP threshold because it can slow charging down to any current needed to prevent overdriving the cells.

 

[hwse]

I assume the 12.6V is a typo, and that you mean 13.6V. So when I have charged the cells to 13.6V, I wait out the BMS and keep an eye on the max delta figure (max diff in cell voltage) to drop below a reasonable threshold. Could take several hours. After that I charge again to 13.7V, and repeat. Then to 13.8V, and so on. I keep doing this until I hit normal top balancing voltage, which is in the 3.55 - 3.65 V range, and then I'm done and can return to the normal charging profile.

I don't have a access to a high quality charger, except the one I'm using for my solar power system. So I'm planning to do this procedure while at the same time using the bank for my home. Since there is limited daylight hours, I suspect I'll only be able to do one charge per day. First day to 13.6V. Second day to 13.7V and so on.

D'oh! Brain fart, not typo. I went back and fixed it.
You don't need to watch it. In fact, watching it will just cause you frustration. You know the old adage. "A watched pot never boils." Until your cells get close to balancing at the 13.6v step, nothing will appear to be happening. It could take hours if you are very, very lucky like I was and your cells are actuall pretty close out of the box. If you are not so lucky, it could take many hours or even days to get to 13.6v with delta of 10mv.

Once you get to 12.6v the rest of the steps will go quickly but you can ignore the pack and just let it do its thing. The final steps from 14.2 to 14.5 will go very quickly and it is very fun to watch the balancing in action. Each step will take less time than the previous one.

By "high quality charger" I would mean one that you can set the absorption voltage to whatever voltage you choose. For my shore charger I use two of the Victron AP22 30-1 chargers and use the "expert mode" which allows me to set absorption to ±0.01v. The two chargers synchronize over the Bluetooth VE-net so I do my increasing steps at 60A, and the chargers regulate the output from 60a to 0A so that it does not go over. Very nice and relatively inexpensive chargers. As a bonus, they are almost completely silent when running at max current. If I put my ear right next to it, i can hear a very slight humm.

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[BjornM]

It is a semantics thing. Connenting cells in parallel and charging them slowly until they are all at 3.65v is not top balance. It is one of the many methods to top balance. Top balance is a state of being where all of the cells reach the exact same voltage at or near the high limit of 3.60 to 3.65v. The other option it bottom balance where all cells hit the same voltage at the low limit of 2.50 to 2.55v. You can only assess the balance of LFP cells at the max or min voltage because at all other voltages in between, the curve is so flat as to be meaningless. On my Basen battery, it showed a voltage delta of 1mv when I got it and it stayed there until the pack voltage got to 13.4 and between there and 13.6v, it spiked to almost 300mv.
What the JK does when you connect it to cells and set it to charging is Top Balance the pack. It is just a much easier way to do it than connecting a bunch of cells in parallel and then charging them with a tiny benchtop power supply for weeks to do the same thing.

Thanks. Yes I understand the part about top balancing, that there are different ways, and one of the ways is to use the JK BMS on a fully assembled pack. What I don't understand is why top balancing is necessary at all, instead of just starting to use the pack at the normal charge voltage and let the BMS do its magic. When you advocate for top balancing before you start using the system, you make it sound like this is a must, or the pack will never be ok. I do understand why top balancing is a MUST if you have no BMS at all. But with a BMS, and especially with an active BMS like the JK, it doesn't make sense to me. Unless you want optimum capacity from day one. Which is not at all important to me.

Let's say we have Pack A that was top balanced and Pack B that was never balanced in any way. They both use the same JK BMS. We start using those packs, and then we check in on them two months later. What advantage in capacity will Pack A have at this point? I'd say none, they will have exactly the same capacity. Is this correct?

 

[BjornM]

90% SOC is pretty low for OVP and is hard to control because that is in the flat part of the curve. On my packs, I set OVP to 3.60v but set the charge controllers to 13.8v which is 3.45v. I use a 0.01C tail current for absorb which is 5.6A and it only lasts a few minutes because my cells are very well top balanced. That way I am charging to just inside the knee region where delta V starts to show so that I get the active balancing to make any small adjustments. Then I drop to 13.2v which drops it back down to 80% to 90% SOC.

Yea it looks like the curve just starts to take off around 90% so maybe slightly higher is better. I just tried to follow the EVE datasheet where they write "Recommended scope of SOC 10%～90%".

Good to know your settings, I will have something similar. I think I'll use a bit lower than 0.01C tail current because I only charge at 0.1C at most, usually lower.

 

[MisterSandals]

What I don't understand is why top balancing is necessary at all, instead of just starting to use the pack at the normal charge voltage and let the BMS do its magic.

When all the cells charge and discharge similarly, then its just fine to just put the pack to use. If there is a small cell deviation at upper and lower charge states and the BMS can keep the cells from hitting over or under volt, then thats just fine too.

The problem that is most common is that in the process of charging up to 14.4V for example, one of the cells will run away and trip cell over volt protection. It is in this scenario that top balancing has the most benefit in getting all the cells in the same state when fully charged. But if the cells have different capacities, which is the primary reason a cell(s) will run away at high and low charge states, there is little that can be done other than lowering the charge voltage to the point before any cells hit the protection limits.

 

[BjornM]

D'oh! Brain fart, not typo. I went back and fixed it.
You don't need to watch it. In fact, watching it will just cause you frustration. You know the old adage. "A watched pot never boils." Until your cells get close to balancing at the 13.6v step, nothing will appear to be happening. It could take hours if you are very, very lucky like I was and your cells are actuall pretty close out of the box. If you are not so lucky, it could take many hours or even days to get to 13.6v with delta of 10mv.

Once you get to 12.6v the rest of the steps will go quickly but you can ignore the pack and just let it do its thing. The final steps from 14.2 to 14.5 will go very quickly and it is very fun to watch the balancing in action. Each step will take less time than the previous one.

By "high quality charger" I would mean one that you can set the absorption voltage to whatever voltage you choose. For my shore charger I use two of the Victron AP22 30-1 chargers and use the "expert mode" which allows me to set absorption to ±0.01v. The two chargers synchronize over the Bluetooth VE-net so I do my increasing steps at 60A, and the chargers regulate the output from 60a to 0A so that it does not go over. Very nice and relatively inexpensive chargers. As a bonus, they are almost completely silent when running at max current. If I put my ear right next to it, i can hear a very slight humm.

Victron BPC123047102 Blue Smart IP22 Charger 12/30(1) 120V NEMA 5-15

Buy Victron BPC123047102 Blue Smart IP22 Charger 12/30(1) 120V NEMA 5-15 with an everyday low price and fast shipping! JB Tools

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Thanks, good info here, and easy to understand your process. I look forward to see this in action. Unfortunately I must wait two months for the shipping from China to arrive first...

 

[BjornM]

When all the cells charge and discharge similarly, then its just fine to just put the pack to use. If there is a small cell deviation at upper and lower charge states and the BMS can keep the cells from hitting over or under volt, then thats just fine too.

The problem that is most common is that in the process of charging up to 14.4V for example, one of the cells will run away and trip cell over volt protection. It is in this scenario that top balancing has the most benefit in getting all the cells in the same state when fully charged. But if the cells have different capacities, which is the primary reason a cell(s) will run away at high and low charge states, there is little that can be done other than lowering the charge voltage to the point before any cells hit the protection limits.

Thanks, now it all makes sense to me.

 

[hwse]

Thanks. Yes I understand the part about top balancing, that there are different ways, and one of the ways is to use the JK BMS on a fully assembled pack. What I don't understand is why top balancing is necessary at all, instead of just starting to use the pack at the normal charge voltage and let the BMS do its magic. When you advocate for top balancing before you start using the system, you make it sound like this is a must, or the pack will never be ok. I do understand why top balancing is a MUST if you have no BMS at all. But with a BMS, and especially with an active BMS like the JK, it doesn't make sense to me. Unless you want optimum capacity from day one. Which is not at all important to me.

Let's say we have Pack A that was top balanced and Pack B that was never balanced in any way. They both use the same JK BMS. We start using those packs, and then we check in on them two months later. What advantage in capacity will Pack A have at this point? I'd say none, they will have exactly the same capacity. Is this correct?

With the JK you do not need to do any "top balancing". As I said before top balance is not an action it is the state of the cells being balanced (at the same voltage) at the top end. It is important for the cells to be top balanced, and it does not matter how you get it there. My instructions are just one way to get the cells balanced without sending any of them over 3.65v. I try to never make the BMS shut off charging. It is there just to protect the cells if I screw up.