Noob Question re BMS

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bobbill   100 W

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Re: Noob Question re BMS

Post by bobbill » Jul 25 2021 10:01am

Thought I posted this...thanks.

Have one 10S 4P pack set, using PFs all within .1V and in groups...and BMS hook up

Now, charging as pack

Usin g very slick NESE modules, no-weld...which are parellel first series later and with balancing need to mark each module so balance wires match up correctly.

Thank to all who assisted this dim noob. We learned lots of stuff.

H

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E-HP   100 MW

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Re: Noob Question re BMS

Post by E-HP » Jul 25 2021 11:19am

Let's all hope that's the end of it :thumb:

eMark   10 kW

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Re: Noob Question re BMS

Post by eMark » Jul 25 2021 3:25pm

Chalo wrote:
Jul 23 2021 12:55pm
As he pointed out, balancing only occurs at the top charging voltage. With most BMSes I've used or replaced, the balancing current is very small-- no more than 30 milliamps and usually less. So it takes a long time and sometimes a lot of restarting the charger to even out relatively small imbalances.
Some BMS balancing starts as soon as cells reach 3.70v. However, the mV rate of resistance discharge is so little it's insignificant when the charge rate is 2 amps. As the battery pack ages the charger has to be left plugged in (on) for several hours a couple times a month after the chargers' green light comes on. Even so such attempts to balance the p-groups does not heal the under-performing cells, only delays the inevitable. Thus, why i believe bottom balancing the p-groups before bulk charging is the way to go as the pack ages instead of active top balancing. To each their own :idea:
Chalo wrote:
Jul 23 2021 12:55pm
One device that can be used with or without a BMS is an "active balancer", which evens out voltage imbalances whenever they occur, and which can use much larger currents to do so ... https://www.ebay.com/itm/184869799584
How can those little stand-alone boards actively balance the p-groups via resistance discharge balancing at 1.2 amps as some advertise ... https://www.ebay.com/itm/312774496524?_ ... %3A2047675 ... OR ... even higher at 5 amps ... https://www.ebay.com/itm/313099411423?_ ... %3A2563228 .
Chalo wrote:
Jul 23 2021 12:55pm
This kind of board has a single function, and can't protect your battery against overcharging, overdischarging, short circuit, or anything besides voltage imbalances.
So, with 1.2 amp or 5 amp active balancing what should be the charge amperage if active balancing is as high as 5 amp? What am i missing :confused:
Last edited by eMark on Jul 25 2021 3:34pm, edited 1 time in total.

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Chalo   100 GW

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Re: Noob Question re BMS

Post by Chalo » Jul 25 2021 3:33pm

eMark wrote:
Jul 25 2021 3:25pm
So, with 1.2 amp or 5 amp active balancing what should be the charge amperage if active balancing is at 5 amp? What am i missing :confused:
The board uses juice from the higher voltage cells to top up the lower voltage cells. 5A or 1.2A is a maximum rate, and of course will diminish greatly as the voltage gradient decreases. When the voltage differences reach .01V or whatever programmed value, the circuit stands by at very low current consumption. (25uA was one claimed value I saw.)

It's definitely not something you want to leave plugged in over the winter.
This is to express my gratitude to Justin of Grin Technologies for his extraordinary measures to save this forum for the benefit of all.

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Re: Noob Question re BMS

Post by eMark » Jul 25 2021 3:50pm

Chalo wrote:
Jul 25 2021 3:33pm
It's definitely not something you want to leave plugged in over the winter.
You mean like RVWinkle :wink:

I thought a standard (dumb) BMS resistance discharge was measured in mV (e.g. 20mV, 25mv, or 30mV). Should it instead be stated as mA (e.g. 30mA as you posted) or millivolts or makes no difference. Please explain - THANKS

Amazing that a stand-alone board can discharge a higher p-group at 5 amp rate ... while charging a lower p-group at a rate as high as a 5 amps. That tells me the pack is on its last leg if a 5 amp rate is enacted/required. What am i missing? Can that 5 amp "ACTIVE" equalizing board take place the same time a pack is being charged at say 2 amp ?
Last edited by eMark on Jul 25 2021 4:02pm, edited 1 time in total.

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Chalo   100 GW

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Re: Noob Question re BMS

Post by Chalo » Jul 25 2021 4:02pm

eMark wrote:
Jul 25 2021 3:50pm
I thought a standard (dumb) BMS resistance discharge was measured in mV (e.g. 20mV, 25mv, or 30mV). Should it instead be stated as mA (e.g. 30mA as you posted) or millivolts or makes no difference. Please explain
Pretty sure it's mA, limited by the heat dissipation of the small surface mount resistors used to drain the high cells.
Amazing that a stand-alone board can discharge a higher p-group at 5 amp while charging a lower p-group at a rate as high as a 5 amps. That tells me the pack is on its last leg if a 5 amp rate is enacted/required. What am i missing? Can that 5 amp "ACTIVE" equalizing board take place the same time a pack is being charged at say 2 amp ?
I don't know, because I haven't done a circuit analysis of one of these boards. But since it balances groups while the battery is charging, discharging, or neither, I'm guessing it doesn't interact with the charger at all. Both charging and discharging happen outside of and separately from the balance board.

I doubt that these things use anything approaching their rated balancing current unless you place exhausted cells in series with full cells.
This is to express my gratitude to Justin of Grin Technologies for his extraordinary measures to save this forum for the benefit of all.

eMark   10 kW

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Re: Noob Question re BMS

Post by eMark » Jul 25 2021 4:47pm

Chalo wrote:
Jul 25 2021 4:02pm
Both charging and discharging happen outside of and separately from the balance board.
Can you imagine a disfunctional battery pack that is being charged at a 2 amp rate; while at the same time the disfunctional p-groups are being actively equalized at a 2 amp rate :wink:
Chalo wrote:
Jul 25 2021 4:02pm
I doubt that these things use anything approaching their rated balancing current unless you place exhausted cells in series with full cells.
Exactly my thought ... so why one would even need an active equalizing balance board rated at 5 amp escapes me ... unless the p-groups need to be balanced in the twink of an eye :wink:
Chalo wrote:
Jul 25 2021 4:02pm
I doubt that these things use anything approaching their rated balancing current unless you place exhausted cells in series with full cells.
Surely you jest :wink:

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bobbill   100 W

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Re: Noob Question re BMS

Post by bobbill » Jul 26 2021 8:52am

My-O-My, the BMS physics is so mysterious...

I developed the notion that in a balance need, the cells in B simply balanced themselves, (high cells rushing current to bring low cells to match--to a point,whereon, the BMS acts to shut or continue). Which is also why I set the modules without BMS first and kept the like charged cells to groups.

I missed something...again.

john61ct   100 GW

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Re: Noob Question re BMS

Post by john61ct » Jul 26 2021 9:04am

Passive, "resistive-bleeding" based balancing is just one method,

not used by these dedicated balancers.

Some do **not** slow down as the delta gap narrows, and are not designed to use in conjunction with the charging process

and work to balance at any voltage / chosen point of SoC including bottom or midpoint.

Any balancing current above 1A will be fast enough for small cells, say under 50Ah capacity.

The point indeed is for them to get the job done in a session lasting only a few minutes

with a healthy bank this might only be needed annually

as it wears, maybe quarterly or once a month

assuming the normal cycling is avoiding the shoulders both top and bottom.

Of course for larger cells, normal cycling charge rates may be hundreds of amps

so 5A becomes a tiny trickle rate in comparison.
Last edited by john61ct on Jul 26 2021 9:04am, edited 1 time in total.

john61ct   100 GW

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Re: Noob Question re BMS

Post by john61ct » Jul 26 2021 9:17am


bobbill wrote: the notion that in a balance need, the cells in B simply balanced themselves, (high cells rushing current to bring low cells to match--to a point,whereon, the BMS acts to shut or continue). Which is also why I set the modules without BMS first and kept the like charged cells to groups.

I missed something...again.
Putting all your cells in parallel at 1S voltage for say 3-4 days, does indeed do that, but only possible before the pack is assembled

and make sure they are all within 0.1V delta before you start. The goal is to get each within say 0.01V aka 10mV.

Read that again carefully, and make sure you understand every part of it before moving on, or ask specific questions

I think that is what you are missing in your learning process, thinking you have understood before you actually do.

For example, "at 1S voltage" give me the numeric range you will read off your DMM of that group, containing maybe hundreds of cells.

In that configuration, there is no BMS involved, nothing a BMS could do.

. . .

Once you have cell / groups wired into a series string, say 10S, then no current ever flows between them, without the help of a circuit designed to do balancing.

If that is (part of) a very cheap BMS, the balancing current rate will be very slow, it only works with current flowing after the charge cycle was complete

and most likely the BMS has no way of telling you balancing is complete (delta under say 0.03V 30 mV), the process may literally take days

much less giving you the actual voltages of each cell / group.

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bobbill   100 W

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Re: Noob Question re BMS

Post by bobbill » Jul 26 2021 9:26am

Thank-you much...have learned more about BMS and balancing here than anywhere.

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