New to BMS

Hey all. So as I posted in another thread, Im pretty new to this stuff, with a high level understanding, but missing some specifics. Please correct me on my understanding here:

With a BMS for a li-ion pack (lets say 3S8P), I understand the BMS will regulate the charging for each parallel group. It will discharge and recharge groups until all are around the same value. So lets say group 1 hits 4.2V, but group 2 is at 4.0 and group 3 is 4.1. It will discharge group 1 down to group 2 level, then start charging it all up again. It will continue this loop until all are around 4.2?

For cells in the parallel groups with differing internal resistance, they should never diverge due to the nature of the parallel connection, yes? The higher voltage cells will naturally discharge into the lower voltage cells until they are all balanced?

If both of my understandings above are correct, is there anyway to identify a bad or severely lagging cell in a large pack...lets say 6S10P....when using reused 18650, how do I monitor health for individual cells when there are so many?

mobinick said:

With a BMS for a li-ion pack (lets say 3S8P), I understand the BMS will regulate the charging for each parallel group. It will discharge and recharge groups until all are around the same value. So lets say group 1 hits 4.2V, but group 2 is at 4.0 and group 3 is 4.1. It will discharge group 1 down to group 2 level, then start charging it all up again. It will continue this loop until all are around 4.2?

Click to expand...

A BMS primarily is concerned disconnecting the battery (from charge or discharge)when a cell's voltage get's too hi or too low.
In addition, a BMS might disconnect the battery (from charge or discharge) based on temperature - too hi or too low.
In addition, a BMS might offer a 'balance feature'.....
A balance feature in most BMSs is simply to burn power thru a resistor on a cell to lower it's voltage - to be same as other voltages.

There is such a thing as moving power from 1 cell to another - but this is very specialized and not standard to BMSs.

BMSs do not charge and discharge the way you describe above... they simply let 'a charge thru'. As a cell get's higher than others, and it has a balance feature, it will burn power from the cell to lower the voltage - but its not 'smart' the way you are thinking and does not coordinate with charging. The burn power to lower the voltage occurs regardless of overall charge or discharge to the battery.

mobinick said:

For cells in the parallel groups with differing internal resistance, they should never diverge due to the nature of the parallel connection, yes? The higher voltage cells will naturally discharge into the lower voltage cells until they are all balanced?

Click to expand...

Correct.

mobinick said:

If both of my understandings above are correct, is there anyway to identify a bad or severely lagging cell in a large pack...lets say 6S10P....when using reused 18650, how do I monitor health for individual cells when there are so many?

Click to expand...

You can identify a 'bad pack' in a batteryfrom the fact that it constantly drains faster than the others in series. A healthy battery - with packs in series that is balanced - will not need much balancing as it charges/discharges. So if you find that a particular pack is constantly out of balance - it should be fixed. If it's a pack with many individual cells - you take the pack apart and retest the individual cells till you find the bad ones and replace them.

So a 3S BMS can independently charge each of the three packs? When I was looking at how it was wired, I was having a hard time conceptualizing how it would charge each pack independently, but it looked like it could discharge independently.

Is there a particular brand of BMS that you trust? I saw some on Amazon, but you never know. I have experience with a test set of large (space application) NiH cells exploding due to a glinch in the charging system. Literally sent the cell through the wall of the lab and into another room. Like a grenade went off. Hope to avoid something like that at home haha.

Like mentioned a bms is like a relay (electronic) that is between the battery and a charger or items discharging it. If the battery goes out of balance or voltage gets too high or to low it disconnect the battery from charge/discharge.

The BMS doesn't regulate charging it only lets current in/out of the battery. Even when it's balancing full current is still going into the battery. The BMS will try to bleed the high cell at 60ma, if the current is still too high and the 60ma doesn't reduce the voltage of the high cell the bms will stop the charge. If the bms stops the charge too soon because of an out of balance condition, you have an undercharge battery.

The best way to charge is to use a balance charger (they can bleed the high cells at up to 1 amp) , you can fast charge the pack without problems. Myself I prefer to use buck converter to charge my lithium packs, they do a good job. I also use an overvoltage protection relay(5 dollars), that is like a deadman switch if the pack voltage goes too high. You can never trust a bms to work everytime. You need to have a back up. Also it wouldn't hurt to put an audio alarm to let you know voltage is too high or low. The tenergy lipo chaecker (12 dollars) sound an alarm if the cell voltage goes above 4.22 volts or below 3 volts. With lithium you need to monitor very closely. All the bms you buy, you need to test, I bought some that charge my cells above 4.25 volts, always test them.

If you got a pack that has balance problems, thats a sign that you have a bad cell in the pack. All you need is one bad cell to cause you balance issues. But to fix you need to take the whole pack apart, then to find the bad cell is easy, once all cells are disconnected wait several hours, the cell that has a lower voltage will be the bad one.

The bms I prefer to use for a large pack is the chargery bms8, especially with solar. It has a lcd screen so you can always check the status of the pack. It has temp sensors, 1.2 amp balancing, the voltage where it activates is programmable, even has audio/visual alarms, it uses mechanical relays to stop the charging. I paid 92 dollars for the chargery but it was worth it. It never let me down, but I still have an overvoltage protection relay as a backup. 5 dollars is cheap to save you from overcharging your battery.

When reusing 18650 I highly recommend an IR tester. I had problems before when cells checked good on capacity tests, self discharge test but when I put in packs the pack eventually developed balance problems. I recently bought an IR tester to check my cells even further. The majority of the cells I tested where in the 40 to 60 milliohms but 2 or 3 cells checked in the 100 to 200 milliohms. All the cells checked good in all the tests I gave them and I would normally use them in a pack, but now I don't use the high IR cells. I paid 57 dollars for the IR tester, I like that it gives me quick results. With this tester as soon as you get the cells, you can test them, those with high IR, you don't have to waste time capacity testing them.

IR tester




tenergy lipo checker audio/visual alarm

I have a charger/tester that displays the internal resistance. Does an IR above 100 really dissuade your from putting in a pack? Is there a limit to how many per battery pack could have out of family internal resistance.

The IR tester I use, test IR differently then a charger. My opus charger also test the IR but the readings are all over the place that I don't trust it. With this charger (YR-1030) if I check the battery 3 times the results will always be about the same. With the opus most of my cells test at 60 to over 100 milliohm but I see some at 400 and 500, if I test a cell 3 times, the opus gives me 3 different readings 25 to 50 milliohm apart. I never use the opus or any piece of equipment that doesn't produce repeatable readings on the same cell.

The battery charger will give you higher IR readings, so cells testing over 100 doesn't mean the battery is bad. You just have to trust that it is giving you good accurate readings. If the majority of the cells read close to each other and one is higher then the rest, I would not use the high one in a battery pack.

The YR-1030 tests the cells using the same method that the cell manufacturers use. Some of the best cells I have test in the 15 milliohm range, which is almost the same as the actual specs for those cells. Some cells test too high and I also found cells that give me readings that are too low or different from the other readings. Any cell out of the ordinary I put aside. You want cells that are close to each other in milliohms.

Before I never bothered testing for IR, but after having numerous packs with balance problems now its a big priority for me check everytime. It's too much hassle to take a pack apart to repair it. I want my cells to be as perfect as possible before I put into a pack. When your pack is in good balance, that gives your battery a better charge everytime since the bms won't stop the charge too early.

So I will have the 3S25A max BMS on a 3S15P pack that will only be used for constant current (low amount of transient or peaks).

I have .15x8 mm nickel strip for parallel and seriesconnections (and 30 AWG fuse wire for the cells). I want to pull no more than 20 A from the pack.I know how to connect parallel groups in series so as to not overcharge the nickel (1 strip per ~4 A), but what about the connections to the BMS? I see all these packs where a single strip ofnickel is used to connect to each BMS terminal. I would think that if a load pulled 20A from my pack, then the single nickel strip on the B+ and the B- connection would far exceed the nickel capability....what am I missing? Do I need to just stack up 5 strips for those connections? Or use 12 AWG tinned copper wire?

mobinick said:

So I will have the 3S25A max BMS on a 3S15P pack that will only be used for constant current (low amount of transient or peaks).

I have .15x8 mm nickel strip for parallel and seriesconnections (and 30 AWG fuse wire for the cells). I want to pull no more than 20 A from the pack.I know how to connect parallel groups in series so as to not overcharge the nickel (1 strip per ~4 A), but what about the connections to the BMS? I see all these packs where a single strip ofnickel is used to connect to each BMS terminal. I would think that if a load pulled 20A from my pack, then the single nickel strip on the B+ and the B- connection would far exceed the nickel capability....what am I missing? Do I need to just stack up 5 strips for those connections? Or use 12 AWG tinned copper wire?

Click to expand...

The 'balance'/voltage measuring wires on the BMS are not what carry the load of either discharging or charging of the battery pack. They will not see 20A. The connections for the discharge and charging of your battery pack are in addition to the connection of the, most likely, 4 balance wires that the BMS will have.

edit - Having said that, the image you have shown of how they have connected that particular BMS will not handle 20A. The connection from the terminal ends of the packs should be in addition to the balance wires ... I'm sure it will function correctly but I don't believe it will handle 20A. You would need to, as you have suggested, beef up the terminal connections.

i prefere Daly BMS the guy on YouTube (will prowse) Always use them with good results!!