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diy solar

Heltec BMS' (up to 350A) with Active Balancing & Independent Active Balancers

Maybe check with a light bulb and see if it is just a tiny leakage current, or if it can actually pull current to the case. Some meters are very sensitive and a tiny bit of moisture can carry enough current to show full voltage. But if it won't light a small bulb, it is likely not a concern in actual use. Some meters even have a Low Z input mode to check if a voltage source can supply a few milliamps or if that will bleed it down to nothing.
Seems to be the case that it's a leakage current. It can't power a bluetooth development kit I hooked up which works fine if I connect it to the main terminals. I then just shorted the case to the main terminal negative (I know....) without issues.
 
Maybe check with a light bulb and see if it is just a tiny leakage current, or if it can actually pull current to the case. Some meters are very sensitive and a tiny bit of moisture can carry enough current to show full voltage. But if it won't light a small bulb, it is likely not a concern in actual use. Some meters even have a Low Z input mode to check if a voltage source can supply a few milliamps or if that will bleed it down to nothing.
I just did a quick experiment.

I started off with my Fluke 87 from the negative terminal to the case. It measures 3.2V
I added a 5.1kohm resistor in parallel as a load and it dropped to 2.4V. The resistor didn't heat up.
As an extra step, I replaced the resistor with a 10ohm power resistor and that dropped the voltage ti near zero.
I removed the resistor and the 3.2V returned.
It seems like there is a connection, but not a high current path. Im sure we can calculate the internal resistance, but I don't think it is necessary.

Just know that the EVE 280Ah cells could have some phantom loads if the outer skin gets worn.
 
2.4 volts at 5,100 ohms is just 0.00047 amps, less than half a milliamp. That is a small leakage current. If that was shorted across the cell, it would take over 200,000 hours to bleed off 100 amp hours of energy. I think we can safely assume that won't be a problem.
 
When I turn on the buck converter, the cell voltage readings all start to bounce a bit, but just the cell 12 dropped the 20 mv. I can't explain why just that one went more goofy than all of the 13 others.

I'm 99 % sure it's because of some conducted noise from the DC/DC converter (and probably not because of radiated noise because you said you have an Al sheet between the two) so some ferrite toroids on the battery side of the converter might help a lot, a big capacitor might too, both would be even better ;)
 
I have a few large caps laying around, and a few medium sized toroid cores as well. Next time I shut down the battery bank, I will try adding some filtering and see what happens. The noise in the readings is not a concern, but the fact that it activates the balancer and causes it to start pushing energy into cell 12 is just plain weird. I may have to dig out my scope and take a look at all the wires. Maybe I can see where the noise is getting in.
 
Well, I added a pi filter, a 1,200 uf low ESR cap, followed by a toroid wrapped with 14 turns of #8 awg wire (That was no fun) and then two more of the 1,200 uf caps after the toroid. I put it on the feed into the 4 breakers so it will filter the input to all the aux panel loads. 30 amps, hence the #8 wire. With no load on the 12 volt converter, it seems like it was fixed. But when I plugged in my box with USB charge ports (obviously another buck converter to 5 volts) it started acting up again. Still Cell 12 going low. It's just odd. The balancer wires do go near the main load cables, but I do not see any wat around that.

I was hoping to have these DC-DC converters power my gateway, switch, and PLC so they would be powered even if I shut down the XW inverter. At this point, I may end up with the Victron 48-800 inverter to power these small loads. I only need about 40 watts to run all of it, I hope the inverter is better than 80% efficient down there.
 
Chasing electrical noise is always a PITA...

What if you put filtering on the BMS balance leads? usually it's more effective to filter at the source but you never know...

Do you have an oscilloscope?

Edit: did you try to add ceramic caps in // to the electrolytic ones? even low ESR electrolytics aren't super good at very high frequencies.

Edit²: you have a scope (I should read more carefully...), ok well try to use it to see what level of noise and what frequency range we're talking about.
 
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Chasing electrical noise is always a PITA...

What if you put filtering on the BMS balance leads? usually it's more effective to filter at the source but you never know...

Do you have an oscilloscope?

Edit: did you try to add ceramic caps in // to the electrolytic ones? even low ESR electrolytics aren't super good at very high frequencies.

Edit²: you have a scope (I should read more carefully...), ok well try to use it to see what level of noise and what frequency range we're talking about.
Some caps to ground on the balance leads would send the noise to ground. You just need to see what frequency you are dealing with and select the value base on that.

I would still swap cell 12 with another just to rule out a weak cell.
 
I will dig out my scope and take some measurements. I only used the Low ESR electrolytics so far. These are very high grade, they came out of a variable speed motor drive, so they were intended to filter high frequency switching DC loads. With only 3,600 uf total, I was not expecting much of a spark, but wow, 50 volts made a nice snap. I will use my pre charge resistor next time I connect them. Glad it didn't pop the 30 amp MRBF fuse feeding it.

I am certain there is nothing wrong with Cell 12. The XW inverter/charger runs up to 80 amps charging or discharging and no cell deviates more than 10 millivolts under those loads, and with the little buck converter off, Cell 12 is smack in the middle of the cell voltage range. And even when the 12 volt buck converter is running and the cell voltage readings are going stupid, my Fluke meter shows all of the cells stable as a rock and within 10 mv from highest to lowest. And the way the cells are in the Chevy Bolt bricks, I can't really swap around cells. Cell 12 is actually 6 cells, consisting of 2 groups of 3, in 2 different sections of the top brick. Cells 1 through 10 are each 3 cells in the bottom brick, paralleled with the matching 3 cells in the middle brick. The Cells 11 through 14 are the top brick. It was originally an 8S3P setup. I cut the middle buss bar, and paralleled the back 4 groups with the front 4 groups to make it 4S6P. The balance leads of the 2 separate 3P strings are all fused at 7 amps and tied between them with #16 wire to the BMS #20 balance leads. The small wire resistance between the strings parallel balances the 2 groups and reports the average between them to the BMS. I was very careful to keep all the resistances matched. When the balancer is moving 2 amps, I see 1 amp to each group on the balance leads. The two main current paths are the factory Chevy Bolt Buss bars, and #2 cables, fused at 125 amps to the main buss bar. So swapping cells around is not really an option.

I guess I could add 14 capacitors across each cell at the balancer end of the leads, but that will be a bit of a pain. About the only way I could see doing it wuld be to cut all the balance wires and splice in th caps about an inch from the connector. The balance wires are about 0.13 ohms from each cell, so even a 1 uf cap should filter a fair bit of RFI. Probable use a good film or ceramic type, but with 14 of them in a row, it will be an interesting experiment. Maybe I can slip a ferrite bead on each lead as well. 15 of those. Since it is measuring the voltage of each cell, I would just use low voltage caps wired the same as each cell at the balance connector.

First I tried an audio range digital scope and it was basically useless. So I pulled out my old BK Precision crt scope. The caps on the input side are certainly doing their job. I can't lock any voltage waveform on the input side of the buck converter, or on the lead to the batteries. I expected to see some 120 HZ as the XW inverter is pumping out 1,600 watts into my house right now. On the output side of the buck converter, WOW, it has all kinds of hash. I could see the 12 volt DC but the screen had a glow above and below it. I put it in AC coupled so I could center the trace without the 12 volt offset. Turned the gain up just a little and it was able to sync a waveform. To see the wave, I had to dial up my sweep rate to the highest setting, on a 15 Mhz scope. 1 us / div. The repetition rate of the main pulse is about 154,000 hz (6.5 us period). Then it has a ringing well over 2 Mhz after each pulse. The peak to peak voltage at the main switching pulse is OVER 12 volts. In DC mod, it went from below 6 volts to over 18 volts. The ring after the pulse quickly drops to about 1 volts, then tapers to just about nothing right as it hits the next switch pulse. Here is the best pic I could get on my phone.

IMG_8588.JPG

The phone image does not really do it justice.
Here are pics of the toroid and the filter installed.

IMG_8582.JPG IMG_8585.JPG

I am hoping some caps on the output of the buck converter might be enough to stop the balancer from going goofy.
 
Wow, that's some nasty spikes. I wonder if radiated noise might not be part of the problem after all; with that strong noise not sure a simple Al sheet is enough.

Ok so it's based on a 150 kHz frequency. That's not super high but still pretty high, I don't think electrolytic caps are good enough here. VFDs typically operate in the 2 - 20 kHz range, so far lower than 150 kHz, that's why they can get away with electrolytic caps.
 
Hi Folks.

I have recently come across this Company (not a new company) which produced BMS' and Active Balancers. I found them via my ongoing search for an Active Balancing System and watched a couple of vids on the products and felt it is worthy of mention in the forum.
They have High Amp mosFET Based BMS (up to 350A), some with Active Balancing built-in.

Heltec BMS & Active Balancers:

A testing / review by one of our members (SolarEngineering) who is checking out Active Balancers
Since these are so cheap, what do you think about using one or two of these in parallel (in my case just the balancers as I'm going to use the SMBS0) to get cells to an initial top balance in a reasonable time compared to passive balancers, therefore avoiding the dangers that some have faced with using bench top power supplies.
 
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I am not using the Heltec Balancer as I went with the QNBBM-8S ($145 USD ea + S&H&Duties/Taxes) for my LFP packs, even though I started this thread when I discovered them and came to know that these are indeed GOOD Balancers (there are some real craptastic ones out there being flogged). I would NOT have posted the info if I did not know they are acceptable & good.

No Balancer or external device is a substitute for proper battery cells preparation. If everyone bought Matched, Batched & Binned cells (double the cost of them) then you could get away with such a shortcut. The COMMODITY Cells we are getting like the EVE-280LF for <$75 per cell must be prepped. I suppose if you wanted to you could put a Heavy Active Balancer (capable of >5A) and continually charged for a few days the Active Balancer would do the heavy lifting and balance out the cells but it work very hard to do so and the end results may not be perfect... they never are in any case...

LINK QNBBM-8S for reference info.

BTW: Chargery is working on developing the next generation of BMS with Active Balancing + more goodies. Possibly available by end of 202 or early 2021.
 
I am using a Ji Kong aka Heltec BMS with the 2 amp active balancing and it has been working great. Only 4 months in, so I can't so how long it will last, but so far no issues at all.

The way the Heltec balancer works is that it pulls current from the highest voltage cell. Only one cell at a time. But if there are a few that are high, it will switch between them and pull from a few before it switches to push mode where it pushes the current into the lowest voltage cell. And if there are 2 or 3 that are low but close to each other, it will switch between those, but for the most part, I see it pushing into 1 cell on each cycle. It is very effective and accurate, but it is not super fast. Even at 2 amps, it is only doing one cell at a time. If you have 5 cells that are all high, it is only pulling 2 amps from each high cell for only 1/10 of the time. That makes the net balance current just 200 milliamps out of the 5 cells. That is close to the newer high power Daly passive balance units. But the flip side is, this is able to take that power and push it to low cells. The passive balancers just turn it into heat. In theory, the active balancing is more efficient, but not much faster. If you only have 1 or 2 high and low cells, then it is pretty quick, but I don't think speed is really that important as long as it keeps the cells safe.

I totally agree that a high power balancer is no replacement for good cells that are well matched. For most solar storage systems, a good top balance is going to help a lot. When we have sun to charge, we want to get all of the cells full at the same time. Even if the cells do not match, a good top balance will go a long way to making the system behave.

Lets say you have an 8S LFP pack. One of the cells is only 80 AH while the rest are a real 100 AH. If you top balance, all of the cells are at the top knee at full capacity. As the inverter pulls them down, the one low capacity cell will drop faster and reach a lower state of charge. But as long as the weak cell is still in a safe range, this is not too big of a problem. You basically have to treat the entire pack as just 80 AH because of the one weak cell. This is true no matter how good your balancer might be. A 5 amp active balancer might gain you a tiny bit, but don't bet on it. When the sun comes out the next day and starts charging, the weak cell started lower, but since it is a lower capacity, charging at the same current, it will come up faster as well. And in the end, all of the cells will get up to the full charge knee at about the same time. And efficiency difference is likely to be pretty small, and that is what the balancer should deal with. If a cell comes up too fast, the balancer can bleed off some charge current to let the others catch up. As long as the absorption charge voltage is a little below the protection voltage setting, the charge current will fall off and let the balancer do it's job and re top balance the cells each time they come up to full charge. An active balancer will use the bled off energy to also help pull up the lagging cells, but we are talking a small difference.

The better the cells match, the less work a balancer will need to do. The BMS's more important job is monitoring the cells and making sure they are all staying in the safe range. For 6 years, it may not need to do any balancing, but it can if needed, but when one cell starts to fail, the BMS will see it and shut things down before it can cause a big problem. A cell that becomes weak will climb in voltage much faster than the other cells. It will become over charged and could fail in a bad way. A proper BMS will shut it off and give you a chance to fix the problem before a cell bursts. Even though LFP may not catch fire, they can still bulge and burst and spew liquid and smoke. And if you use Li NMC cells like I am, yes, they could turn into a torch aimed at the next cell over. With a good BMS and a cabinet protecting the cells from shorts and damage, I am not concerned about them catching fire, but without a BMS, they can easily become a time bomb. With the BMS, people are parking cars full of them in their garage. My pack is about 30% of what is in a Chevy Bolt. The car only uses a passive balancing BMS.

Since I am only running my cells from 50% to 90% state of charge, I never actually did a full top balance. I had planned on it, but when I did the first charge up to 90%, the active balancing pulled all of the cells to within 3 millivolts. All of the cells track even through the "S" curve that happens at about 3.76 volts per cell. So they balanced up great and the active balancing basically has been sitting idle for 2 months. I never expected these surplus cells to be this well matched, but I am glad they are.

When I build another pack (I do plan on adding more capacity), I will just let it charge up at a reduced current with the active balancing turned on and let it get the cells all charged up and balanced at the top. I will then manually check them at the cell terminals and ensure they did balance at the top. If any cells are still out, I will set the current even lower to give the balancer more time to pull them in. I would only resort to separate cell or parallel manual top balancing if the BMS has to shut it down for going too far out of balance with a cell hitting high voltage cut off. If I was starting with more "questionable" cells I might be a bit more cautious, but when they arrive all between 40% and 60% charge, and they track together as they charge, I am not worried about them. My initial charge was at just 10 amps with a 2 amp balancer, so it could handle a 10% error in capacity and make up for it. And if it was worse than that, the BMS could still shut it down. The best part about the Heltec (JK) balancer/BMS is that it can keep pulling the high cells down and pushing the low cells up even when it turns off charging for one cell getting too high. With no charge current going in, it very quickly starts bringing the cell voltages together.

Yes, LFP cells have a VERY flat voltage curve and measuring the voltage is not a good indicator of their state of charge, but when all of the cells are the same type and at the same temperature, having them at the same terminal voltage will still put them very close to the same state of charge. The best state of charge balance will still occur at the top knee, but having the active balancer pull the voltages together even in the middle of the flat discharge curve is not hurting the pack at all. And if the curve is flat enough, you can easily set the balance threshold so that it won't balance as long as they stay close. I have mine set to 0.006 volts, but you could open it up to 0.02 and it probably would not do anything until you charge up near full and a few cells start hitting the knee before the others. It will pull current to slow those high cells and help pull up the ones that are lagging. This is never a bad thing.
 
Hi Folks.

I have recently come across this Company (not a new company) which produced BMS' and Active Balancers. I found them via my ongoing search for an Active Balancing System and watched a couple of vids on the products and felt it is worthy of mention in the forum.
They have High Amp mosFET Based BMS (up to 350A), some with Active Balancing built-in.

Heltec BMS & Active Balancers:

A testing / review by one of our members (SolarEngineering) who is checking out Active Balancers
interesting article, do you know if it has a low voltage cut? do you think it could stay connected all the time?

Thanks!
 
The device in that video is only a balancer. It has no protection for high or low cell voltage. You could add it to a BMS to help balance the cells and the BMS will handle the protection shut off.
 
The device in that video is only a balancer. It has no protection for high or low cell voltage. You could add it to a BMS to help balance the cells and the BMS will handle the protection shut off.
I was referring to if the balancer itself cuts off balancing due to low voltage, so that they do not empty in case of leaving it on.
I have already seen the specifications and if it has some disconnection parameters, as stated in the specifications if it stops balancing below 2.9v

1604864426013.png
 
I’m looking at getting an 8S Heltec BMS and from perusing this thread, it seems like user experience on the board is generally positive (unlikely to be a rip-off / scam).

Any owner’s care to chime in and agree?

I need at least 250A and may go as high as 300 or even 350A depending on delta cost).

Was planning to just get the basic model with 85mA of passive balancing since my 280Ah cells are well-matched, but if experience here with the ‘active bslance’ capability is that it cannot be worse and can be better (ie: if one or two cells degrade faster than the others), I’d consider going that route.

Any feedback before I pull the trigger appreciated...
 
I’m looking at getting an 8S Heltec BMS and from perusing this thread, it seems like user experience on the board is generally positive (unlikely to be a rip-off / scam).

Any owner’s care to chime in and agree?

I need at least 250A and may go as high as 300 or even 350A depending on delta cost).

Was planning to just get the basic model with 85mA of passive balancing since my 280Ah cells are well-matched, but if experience here with the ‘active bslance’ capability is that it cannot be worse and can be better (ie: if one or two cells degrade faster than the others), I’d consider going that route.

Any feedback before I pull the trigger appreciated...
I don't know if the active balance model will work for 8S and lifepo4
You should find out for sure before you order.
 
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