Some Modeling to go with Will's current sharing videos.

[FilterGuy]

The cable itself was actually very hot. You are correct, typically its the connection. Very common issue.

My lugs were nearly the same temperature as the conductor, from what I could feel.

But whether it was the connection or the conductor, the cables were well made and over sized, and there were still current sharing issues. I do not know how to calculate the interconnection losses between the lugs, but this is a factor as well. I would assume that with the crimper I used, the terminations were gas tight cold welds. But again, more factors going against you.

This is why bus bar w/ equal length conductors connecting the server racks is the best method.

I have used the diagonal configuration for ages in past packs, and I always had these issues.

Keep in mind that wires can get quite hot even if they are sized correctly.

 

[JJJJ]

I had raised the idea of heat shrink for bus bars on another thread but it was not well received. The concern was that the current carrying capacity might be reduced due to stored heat and reduced cooling.

Wish I could find out the heat generated on bus bars for different sizes and amps . I came up with the shrink wrap idea since this is often used in harsh d/c conditions in automotive applications.

 

[FilterGuy]

Wish I could find out the heat generated on bus bars for different sizes and amps . I came up with the shrink wrap idea since this is often used in harsh d/c conditions in automotive applications.

It is true that the heat shrink will 'insulate' and that will retain heat. However, to serve the intended purpose, the bus bars will need to have a significantly high current rating (low resistance). If the bars are generating very much heat, they are probably not sized large enough to keep the batteries well balanced.

 

[justgary]

In the specific case of his video with different sizes batteries, it would have helped if @Will Prowse had put the higher capacity batteries on the outside. E.g., the big packs where the bus connections are and the other packs in between. The bigger packs are better suited for the higher charge/discharge current, essentially balancing the combined battery by virtue of their lower internal resistance.

 

[JJJJ]

This is a long article which gets into the science

Understanding Bus Bar Ampacity Charts

A brief overview of ampacity charts for both copper and aluminum busbar and how to interpret the data within.

stormpowercomponents.com

Basically included in the article resistance in dc bus bars can be offset by a thicker bar. The increased frequencies of ac can contribute to higher temperatures.

 

[JJJJ]

FilterGuy posts an interesting idea with bus bars. Wonder what the current flow differentials might be with and without the heat shrink?

 

[nodoze]

Would a setup like this would a really good copper busbar be OK? :

Would all equal length wires to an oversized/maximized busbar be similar resistance-wise to Method 3 in this article? :

SmartGauge Electronics - Interconnecting multiple batteries to form one larger bank

 

[justgary]

Would a setup like this would a really good copper busbar be OK? :

Would all equal length wires to an oversized/maximized busbar be similar resistance-wise to Method 3 in this article? :

SmartGauge Electronics - Interconnecting multiple batteries to form one larger bank

Interesting how the video shows the bottom battery at lower current both times he measured, and he glosses over it as, "maybe one or two percent." Well, one amp out of twenty is five percent, e.g. (19-20)/20*100 = 5%.

Yes, his bus bar is working, but in his installation it should probably be a thicker bar. He could prove that to himself by measuring the current at different points in the bar using his clamp meter.

To answer your question, yes, an oversized bus bar is similar to #3, but more importantly it makes the individual branches of #3 look electrically very short. So short as to be negligible if you get the bar big enough, which is the entire point.

 

[nodoze]

Interesting how the video shows the bottom battery at lower current both times he measured, and he glosses over it as, "maybe one or two percent." Well, one amp out of twenty is five percent, e.g. (19-20)/20*100 = 5%.

Yes, his bus bar is working, but in his installation it should probably be a thicker bar. He could prove that to himself by measuring the current at different points in the bar using his clamp meter.

To answer your question, yes, an oversized bus bar is similar to #3, but more importantly it makes the individual branches of #3 look electrically very short. So short as to be negligible if you get the bar big enough, which is the entire point.

Thanks. Makes sense. The bar looks fairly thin, not very wide, and silver to me so I was thinking more copper (pure?) and both thicker & wider would be better (toward the high end of those charts linked earlier in the thread).

 

[FilterGuy]

and silver

They are probably tinned

 

[Hedges]

The downside of this idea is that you have to remove all the busbars if you need to swap out one of the batteries.

Rear connections would have made this possible.
Ideally set up so they can be unbolted and swapped live, with shields preventing shorts by wrench or lugs.

We're doing something vaguely related at work, for a rack of power supplies.

High current connector. Data sheet shows 200A to 500A models. 0.1 milliohm

2204080-1 TE Connectivity AMP Connectors | Connectors, Interconnects | DigiKey

Order today, ships today. 2204080-1 – Terminal Busbar Clip Connector Solder - from TE Connectivity AMP Connectors. Pricing and Availability on millions of electronic components from Digi-Key Electronics.

www.digikey.com

 

[Johnson]

I thought that was one of Will's best video's, I never expected how much difference it would make.

To get the batteries even better in sync we might even have to try different cable lengths to connect to the bus-bar. I wonder if it would be a good idea if each battery pack could reduce its output so they all supply a percentage of what is required. But is that going to far?

 

[FilterGuy]

Rear connections would have made this possible.

I like that idea!!

I love it when the discussion comes up with a good idea and then further discussion improves on it!!

 

[Hedges]

To get the batteries even better in sync we might even have to try different cable lengths to connect to the bus-bar. I wonder if it would be a good idea if each battery pack could reduce its output so they all supply a percentage of what is required. But is that going to far?

Some DC power supplies can be paralleled. Usually that means one sits in CV mode while another is in CC at times.

A "400V" battery made from 48V batteries and boost circuit could, with communications, share current and synchronize SoC.

A battery bank with BMS can't do that, not unless some SMPS is build in, e.g. a buck converter designed for CV/CC operation. It just contains cells and FETs.

If you need a significant distance between batteries and inverter, putting a busbar near inverter and running longer wires from individual batteries may help with balancing, using resistance of the wires. That could help reduce impact of differing internal resistance, at least somewhat.

 

[FilterGuy]

But is that going to far?

Yup.... I wonder about this too. In the DIY world we tend to fixate on doing crazy-ass stuff to minimize a possible problem....but in the bigger picture it may not be a big enough problem to even worry about. @Will Prowse likes to point out that for solar, the ultimate demise of the battery is likely to be calendar aging and he has a very good point. All these things we do to try to get the batteries to last longer may not make one iota of difference in the long run.

 

[Johnson]

Yup.... I wonder about this too. In the DIY world we tend to fixate on doing crazy-ass stuff to minimize a possible problem....but in the bigger picture it may not be a big enough problem to even worry about. @Will Prowse likes to point out that for solar, the ultimate demise of the battery is likely to be calendar aging and he has a very good point. All these things we do to try to get the batteries to last longer may not make one iota of difference in the long run.

Some DC power supplies can be paralleled. Usually that means one sits in CV mode while another is in CC at times.

A "400V" battery made from 48V batteries and boost circuit could, with communications, share current and synchronize SoC.

A battery bank with BMS can't do that, not unless some SMPS is build in, e.g. a buck converter designed for CV/CC operation. It just contains cells and FETs.

If you need a significant distance between batteries and inverter, putting a busbar near inverter and running longer wires from individual batteries may help with balancing, using resistance of the wires. That could help reduce impact of differing internal resistance, at least somewhat.

My main concern is that when batteries are full/empty that the rest will see a higher current, until there is only one left. I assume there will be current protection in the BMS, depending on the BMS and not have to rely on manually having to reset breakers. But communication and some management would be nice to have especially for server rack batteries.

Anyway, following this thread with interests.

 

[justgary]

All these things we do to try to get the batteries to last longer may not make one iota of difference in the long run.

My primary battle is with heat, so I don't think I can talk about calendar aging yet. My installation is essentially a solar trailer that I bought from a friend who finally was able to run power lines to his farm. He had done a few things wrong, so he discounted it enough to get me interested.

The primary problem is that the trailer is not insulated, so the batteries are subjected to quite a bit of heat for a large portion of the year. I'm not going to buy new batteries until I solve the heat problem. Of course, solving the heat problem will probably need some power. I'm still scratching on the back of an envelope to figure out the best way to solve all of the problems at once.

Right now I'm leaning toward ripping everything out of the trailer, then digging a pit about four feet down and putting a small insulated shed over it and reinstalling the equipment in there. I can then sell the trailer if I want to. I really like the trailer, though, so I may try to insulate it and draw cool air from my two conduits (120' and 80') that are 20" underground. The simple button is to draw in the cool air from the conduit and pipe it through an insulated box around the batteries. The next easiest button is a micro A/C or small freezer to cool the insulated box around the batteries. The joy of solving these things is why I DIY!

I didn't mean to hijack the thread, but I just wanted to point out that we don't all get to keep our systems in a nice air conditioned space. The odds of seeing calendar aging issues goes way down with wild temperature swings.

 

[curiouscarbon]

for solar, the ultimate demise of the battery is likely to be calendar aging and he has a very good point. All these things we do to try to get the batteries to last longer may not make one iota of difference in the long run.

keeping the battery cell material between 10 and 25 degrees celsius

according to the research i've read, doing that will mitigate calendar aging more than allowing higher temperatures e.g. 30,40 C

almost all the research about calendar aging i've seen clearly describes a temperature dependence in this effect

• cold climates mainly require Heating
• moderate climates mainly require Heating and/or Circulation Cooling
• varied climate requires Heating, Active Cooling and/or Circulation Cooling

as you say, with DIY, sometimes the fun of it can supersede the utility of a given project. it's just fun to yak shave sometimes.

I've seen lots of active heating projects on this forum, and some ventilation cooling, but I don't remember seeing any active cooling ones. Some keep their batteries in an air conditioned space, which is effective the same thing, but in this case I'm talking about a thermal regulation system specific to the battery pack that is capable of active heating and active cooling.

most won't do this, complex, difficult to engineer etc... and for good reasons. i'm taking a crack at it and look forward to do more experiments and share as progress goes half of the merit of effort is the fun of learning for me. but also i do believe that there is a specific temperature dependence on cell calendar aging, and believe there is an empirical case to be made for the expected utility of active thermal management ?

 

[FilterGuy]

Right now I'm leaning toward ripping everything out of the trailer, then digging a pit about four feet down and putting a small insulated shed over it and reinstalling the equipment in there. I can then sell the trailer if I want to. I really like the trailer, though, so I may try to insulate it and draw cool air from my two conduits (120' and 80') that are 20" underground. The simple button is to draw in the cool air from the conduit and pipe it through an insulated box around the batteries. The next easiest button is a micro A/C or small freezer to cool the insulated box around the batteries. The joy of solving these things is why I DIY!

I have a few thoughts/cautions on earth tube work..... but that is probably best left for a different thread. If you decide to start one, let me know.

 

[robby]

I think a lot of guys are underestimating the resistance added by the Mosfets in the BMS when looking at the current imbalance.
I have three eFlex batteries that use a Big Relay for switching on and off the terminal to battery contact during charging and discharging.
I have all equal length wires going to Busbars and then to the Inverter and the one EG4LL battery that I have produces half the current during charging and discharging. The eFlexs are almost in perfect sink during every cycle while the EG4 lags by a lot. This is not just at the high points of the cycling but at any given point it will be from 30% to 50% lower current Charge/Discharge from the EG4.
I cannot imagine this is due to the ESR being that different between packs. It has got to be Mosfet vs Mechanical relay resistance.
Will is using mix brands of batteries and sizes, so it makes it kind of harder to interpret what is going on.

 

[curiouscarbon]

I think a lot of guys are underestimating the resistance added by the Mosfets in the BMS when looking at the current imbalance.
I have three eFlex batteries that use a Big Relay for switching on and off the terminal to battery contact during charging and discharging.
I have all equal length wires going to Busbars and then to the Inverter and the one EG4LL battery that I have produces half the current during charging and discharging. The eFlexs are almost in perfect sink during every cycle while the EG4 lags by a lot. This is not just at the high points of the cycling but at any given point it will be from 30% to 50% lower current output/Input from the EG4.
I cannot imagine this is due to the ESR being that different between packs. It has got to be Mosfet vs Mechanical relay resistance.
Will is using mix brands of batteries and sizes, so it makes it kind of harder to see whats going on.

some addition info about that available here, for a specific BMS that is different than the one being tested in the video. mainly hoping to add some concrete reference numbers to get the quantitative ball rolling, so to speak

JBD/Overkill vs JK/Heltec BMS

Has anyone done a comparison between the JBD and JK BMS's? They look very similar to me. The JBD seems to use the Xioxiang app, and the JK uses thr ENJpower app. The apps are also VERY similar. I know Overkill is JBD, ans Heltec is JK based. Is the same manufacturer making both? I am not as...

diysolarforum.com

Go by Rds_ON.

Five back to back pair on top, five back to back pair on bottom side. 3.2 milliohms x 2 for each back to back = 6.4 milliohms @ 25C.

Ten back to back pairs in parallel = 0.64 milliohm series resistance at 25C.

Heating watts @ 200A = 200^2 x 0.64 milliohms = 25.6 watts. Won't take 200 amps very long before thermal shutdown.

For continuous current rating, no more than 10 watts heating with the limited heat sinking provided, = sqrt (10w / 0.64 millohms) = 125 amps.

Since Rds_ON goes up as MOSFET's get hot, 100 amp max continuous is more realistic. 50% derating, about par for Chinese BMS's.

on the order of 0.64 Ohms or nearly one ohm, in that example. absolutely makes sense that variance between BMS units would result in a difference in charge/discharge current as a result of the MOSFET resistances. e.g. one BMS is 0.60 ohm and another BMS is 0.70 ohm, etc..

 

[curiouscarbon]

For the JBD BMS, here is information about the MOSFET and Resistance thereof, from Steve himself!

Finally found a LiFePO4 BMS with Low-temp Charging Protection

yes, that’s why I posted my configuration of the cells into a single 12v battery. Also, I was referring to your setting in the “Capacity configuration“ portion of the settings tab of the BMS. You posted this as your settings . . . and my question was intended to differentiate my battery set...

diysolarforum.com

each individual MOSFET is rated for about 1.3 milliOhm, and there are apparently in 15 parallel, two series

so each "lane" should be (1.3*2) or 2.6 milliOhm and 15 of those "lanes" in parallel for the JBD BMS rated for 100/120A continuous

mosfet datasheet : https://datasheet.lcsc.com/szlcsc/HY4903B6_C133393.pdf

each one is rated for 1116 A pulsed drain current, limited by temperature, and 314 A continuous drain current (at 25C). of course it will overheat quickly with most flat heat spreaders. but it's good to know. 15 of those in parallel sounds like a pretty great tradeoff for the vulnerability of MOSFET in general!

 

[curiouscarbon]

working this out on my own propellorhead pace :D

@robby made me more curious about How Much Variance of Resistance From BMS to BMS.

i attached tr1035+ meter to two JBD BMS here. ensured that the mosfet are closed (on) while the measurement was taken. its from C- to P- terminals



0.98 milliOhm and 0.48 milliOhm were the results for two 4S JBD 120A BMS. i could have a systematic or methodological error, or maybe they are really just different. hope this helps.

JBD BMS #1	JBD BMS #2
0.00098 Ohm	0.00048 Ohm
0.98 milliOhm	0.48 milliOhm

 

[curiouscarbon]

this makes me realize that if i wanted to eliminate this effect entirely, buying extra BMS and selling the ones that are very extreme relative to average resistance, would be one way. tedious. inter-pack active balancing perhaps achieve similar.

reminds me of how some cell vendors will match/batch the product based on internal resistance.

how funny would it be if, when buying multiple BMS, they were also matched to be similar internal resistance?

value? just musing thanks everyone!

 

[FilterGuy]

working this out on my own propellorhead pace :D

@robby made me more curious about How Much Variance of Resistance From BMS to BMS.

i attached tr1035+ meter to two JBD BMS here. ensured that the mosfet are closed (on) while the measurement was taken. its from C- to P- terminals

View attachment 83265View attachment 83266

0.98 milliOhm and 0.48 milliOhm were the results for two 4S JBD 120A BMS. i could have a systematic or methodological error, or maybe they are really just different. hope this helps.

JBD BMS #1	JBD BMS #2
0.00098 Ohm	0.00048 Ohm
0.98 milliOhm	0.48 milliOhm

That is a pretty big difference!!!!

However, when dealing with such small resistance, I wonder how accurate you can be. Just a fleck of dirt under the alligator clip could make that much difference.