Readily available quality BMS

[DanGT86]

Well then it is probably never going to work without the charger current reducing. Any amperage above the shunt's value is going to cause the voltage to rise. If you could sit there for 2 days and continue to plug and unplug the charger like a bms would everything would eventually come into balancing range. In this case the shunting would eventually allow the lower cells to catch up but it takes a really long time. Every on off cycle the low ones that are not shunting come up a little more. This higher average causes the charger to reduce current to a level the shunts can handle.

You should really find a way to monitor it. Perhaps a $30-50 watts up or turnigy watt meter would be a good investment. Handy tool to have around anyway.

If you use a cycle analyst you can also set it up to charge through the current measuring shunt so it will display charger current while charging.

I get how frustrating it can be. I fought my non BMS headways for over a year before a $50 BMS cured the whole thing. It was my first pack and it was really making me question my understanding of charging in general.

I wish there were affordble BMS units available with massive shunting values. This is why you see ratings on electric vehicles that show quick fast charge times to 80% but way more time to %100. You can smack them with all kinds of current in the middle of their volt range.

If you are really opposed to buying a propper BMS at the industry price, you could always do a series of single cell chargers at 1 per cell. This isnt cheap but would be faster than a shunting type because you are controlling what goes in not trying to get rid of extra energy like the shunting type.

There is also this type of charger. But again not cheap for your needs.
http://endless-sphere.com/forums/viewtopic.php?f=31&t=41107

 

[roddilkes]

Here is a bit of video showing how our balancers work on our own E-Kart.
Note the way the balancers are installed on the cells, which makes for a clean look and good function.
This is just how we do it in this application and the charge current is pretty much irrelevant. We use a 25A charger.
Once the cells are in balance, if the balancers have been correctly installed, they will stay that way.
Initially get them in balance by charging individually to 3.5V or by using a charger that has <500mA charge current.
We are living proof that it works.

https://youtu.be/EpGlLE_Bw9I

 

[johnnyz383]

Here is a bit of video showing how our balancers work on our own E-Kart.
Note the way the balancers are installed on the cells, which makes for a clean look and good function.
This is just how we do it in this application and the charge current is pretty much irrelevant. We use a 25A charger.
Once the cells are in balance, if the balancers have been correctly installed, they will stay that way.
Initially get them in balance by charging individually to 3.5V or by using a charger that has <500mA charge current.
We are living proof that it works.

https://youtu.be/EpGlLE_Bw9I

Unbelievable!!...Of course you have never had an issue because you said yourself in the video you have a BMS installed. You say you charge at 20 amps?...
"In addition we have a battery monitoring system....just to double check that every cell is in its SAFE range"...what a joke!.. So here is Rod telling everybody that the modules will keep the battery pack balanced but he is using a BMS board which without any doubt is doing that job NOT the balancers.

Rod told me when I went to purchase these that they would do the job. I told him I didnt have anything else to monitor or make sure the cells would go over voltage. Based on this, he said they would work. Pure rubbish. This video ONLY proves that the BMS is doing the job the balancers cannot. Rodney you have proven nothing and actually made your case worse. If the modules have no problem keeping the cells balanced why install a BMS? well maybe it because they are there to " just to double check that every cell is within it's safe range"..

He then goes on to say that the charger begins cutting back the amperage to the cells the closer they get to a full charge to 1/2 amp. Rod never told me when I purchased these things that I would need that kind of charger. As a matter of fact I was up front and told him what I had. He insisted they would still work. Demonstrate that these units will work independant of a BMS getting an initial charge of 20 amps. Actually discharge the headways using 150 amps untill you use lets say 900 Wh.(without a BMS). Record the voltages. Attach the modules and give it 20 amps of charge. Demonstrate that the cells will be kept in balance using your modules ONLY. When you can do that I will say they will work and that it must be something im missing, but until then you havent shown anything.

John

Since you have NOT proven these can work via this video I am still waiting for you to demonstrate how they can work the exact way and under the exact pretense under which I bought them.

John

 

[johnnyz]

Here is the exact charger I am using to charge the headways using the modules. I cannot get any information from the seller. Does anyone know if this particular charger actually begins cutting back the amperage the closer it gets to full charge?.

Thanks

john

 

[Alan B]

Hi folks. Just another Electrical Engineer wandering by here and seeing all the fuss.

First of all, these simple shunting balancers can work. But one of two things must be true for them to work, and if not, they will fail to do the job.

1) One way they can work reliably is for the charger to be less than the shunting current of the balancers. That will always work. This works even if cells are out of balance. The charger may not shut off if its output voltage is too high (as it is here), but at least the cells will not overcharge.

2) The other way they can work is when the cells are very close to balanced, and the charger voltage matches the balancers total shunting voltage. If a charger with output current larger than the balance current is used and the cells are very close to balance then the gentle balancing of the shunt right at the end of charge can be sufficient. This is how the folks having success with higher current chargers are having success. The problem with this is there's no "failsafe" here. If the cells get too far out of balance it will fail. If the charger's output voltage is too high it will also fail. It can be slightly higher than the sum of balancers shunt voltage, but only if the current it delivers at that voltage is less than the shunting current.

The best way to achieve a proper starting condition and precisely balance the cells for this is to:

1) charge each cell individually to the same "full" voltage just a little under the balancer current settings, say 3.50V.

2) put the pack together with the balancers and charge with a low current charger (less than 500mA) to the full pack voltage of 28*3.55V which is 99.4V.

The charger you have for 28S charges to 3.65V/cell. The balancers are rated for 3.55V, so this is not a good match. The charger may put out full current until it reaches 102.2V, or it may start reducing current a little before that, but that depends on the design and is not something that you want to depend on.

One possible solution for this too-high charger voltage is to use a light bulb in series between the charger and the batteries during charging to limit the current and absorb some energy. A 6V or 12V automotive bulb that can handle 5 amps may work, it would require some experimentation.

Another solution is to turn your charger down slightly in voltage. It is a little too high for these balancers. Even if the cells are perfectly balanced the charger can trigger every balancer and keep them fully on. This is not what you want.

Or get an adjustable charger so you have control over the voltage and current. Or get balancers that are adjustable to match your charger. Or get a different BMS that can cut off the charger when the cells are full or are balancing.

Good luck with your project!

 

[johnnyz]

Hi folks. Just another Electrical Engineer wandering by here and seeing all the fuss.

First of all, these simple shunting balancers can work. But one of two things must be true for them to work, and if not, they will fail to do the job.

1) One way they can work reliably is for the charger to be less than the shunting current of the balancers. That will always work. This works even if cells are out of balance. The charger may not shut off if its output voltage is too high (as it is here), but at least the cells will not overcharge.

2) The other way they can work is when the cells are very close to balanced, and the charger voltage matches the balancers total shunting voltage. If a charger with output current larger than the balance current is used and the cells are very close to balance then the gentle balancing of the shunt right at the end of charge can be sufficient. This is how the folks having success with higher current chargers are having success. The problem with this is there's no "failsafe" here. If the cells get too far out of balance it will fail. If the charger's output voltage is too high it will also fail. It can be slightly higher than the sum of balancers shunt voltage, but only if the current it delivers at that voltage is less than the shunting current.

The best way to achieve a proper starting condition and precisely balance the cells for this is to:

1) charge each cell individually to the same "full" voltage just a little under the balancer current settings, say 3.50V.

2) put the pack together with the balancers and charge with a low current charger (less than 500mA) to the full pack voltage of 28*3.55V which is 99.4V.

The charger you have for 28S charges to 3.65V/cell. The balancers are rated for 3.55V, so this is not a good match. The charger may put out full current until it reaches 102.2V, or it may start reducing current a little before that, but that depends on the design and is not something that you want to depend on.

One possible solution for this too-high charger voltage is to use a light bulb in series between the charger and the batteries during charging to limit the current and absorb some energy. A 6V or 12V automotive bulb that can handle 5 amps may work, it would require some experimentation.

Another solution is to turn your charger down slightly in voltage. It is a little too high for these balancers. Even if the cells are perfectly balanced the charger can trigger every balancer and keep them fully on. This is not what you want.

Or get an adjustable charger so you have control over the voltage and current. Or get balancers that are adjustable to match your charger. Or get a different BMS that can cut off the charger when the cells are full or are balancing.

Good luck with your project!

Thanks for the reply

This all makes sense but given that I already set out my parameters with Rodney before purchasing the modules, and the fact that he appears to be an absentee participant in this discussion save a little video this isnt as relevant as it might have otherwise been...

The fact that 2 modules have now failed, makes the whole experience a negative one. Those modules did not experience any voltage higher than 3.9 volts as I monitored them all while charging. Not enough information when asked before purchasing so that I could make an informed decision and this is the result. Had he told me that the modules wouldnt work with my charger then I obviously would not have purchased them.

I will be wiring the cells up to accept charging via an RC 10 C charger. This has the benefit of making sure each of the 10 cells get exactly the 3.65 volts, not any higher like the parameters set out with these modules (3.8 volts). The negative is that only 10 cells can be charged at a time. The cells are all good cells and even after not all getting an even charge they stay relatively balanced.

John

 

[Alan B]

I have had good results with the Cellpro 10S charger for 10 cell groups, I think the manufacturer has moved on to a slightly more capable model now, the 10XP. That charger uses a typical bleed system but compensates for wire resistance and is very accurate. For faster charging without bleeding the BC168 charger has six independent 1 cell chargers in it so each cell charges fully without bleeding, and it will charge through the balance lines so you don't need separate charge leads as most other chargers require, so it saves on pack wiring and reduces complexity. This has become my go-to charger for balancing charging, and all my packs are wired for 5 or 6S cell groupings to facilitate this. I've also used the Hyperion 1420 which can do up to 14 cells, but I think that unit is no longer available. I used it to do two 6S banks in series at the same time. Again it is the bleed type balancing. The bleeding balancing takes a long time, the charge has to be slowed way down to allow that to work, whereas the per cell charging can go full bore on one cell while another is done and not charging at all. The BC168 is rated for 8 amps but the wiring is fairly small gauge so I restrict it to 4 amps which may seem slow but since it doesn't drop to 500mA for balancing it actually gets done more quickly than the larger bleed balance RC chargers.

Checking the balance of the pack is facilitated by using a Battery Medic which can read up to 6 cells at once, so with 3 readings I can check my 18S packs while the 10 and 12S packs only require 2 readings. Balance charging is needed initially when building the pack, but bulk charging and checking after that. Frequent checking and periodic rebalancing is done when the balance gets out by 20mV or so, generally this is only needed once a year or so. Even if a BMS is in place it is good to periodically check the balance as BMS failures are often silent. A weekly check is probably sufficient, or anytime there has been an event such as a deeper than usual discharge or a crash or other mechanically significant event. During the initial couple of weeks I check balance twice a day as infant mortality is possible. After a couple of weeks of daily riding the checking can be relaxed, but anytime anything suspicious happens I reverify the balance right away.

One day I heard a pop that sounded like a pouch failing from pressure, I immediately stopped and rechecked the balance (I carry the Battery Medic in the trunk) and found no issues. I continued to monitor it for a few days but a review of the GoPro indicated the sound had come from the environment and not the pack, and the balance never faltered.

Pouch batteries give us some clues by swelling and popping, cylindrical and prismatic batteries have built in vents that provide no indication of a problem, so monitoring the balance and pack voltage vs capacity are about the only ways to know when the pack is suffering. Anytime the low voltage cutout of the system is reached it is good to check the balance. It may not be perfect at LVC but if one or more cell groups are much lower than the rest then they are not matched to the pack and will possibly be damaged. If only one or a few cell groups is dropping low ahead of the rest the LVC value for the pack will need to be increased as it is normally based on all cells dropping together. This is generally a good indication that the pack should be rebuilt, repaired or retired as the risk of failure on an unequal cell group capacity pack is significantly increased, and the complete cycling of a few cells will shorten their life compared to the rest of the pack, making the situation worsen over time.

Good luck with your pack and be prepared to cull a few cells if needed to get a good matched and balanced healthy pack. It will last longer and provide better range and power.