bestech bms how to adjust Lvc

[splitfire]

I got the d131s 21s lipo bms that i want to adjust the LVC. I just had my bms hooked up today. I was running it to know when will it cut off and it cut off at 77v isnt too early 3.7v each cell i know 3.7v is ideal but i want to run my bike a bit longer. According to bestech website this d131s lipo bms have adjustable LVC. Anyone of you guys knows how to adjust? I want to set it at 72v LVC so I will have 3.4v per cell.

 

[Ykick]

Adjustable means "when ordering" you can specify fixed value from factory. It is what it is now.

 

[splitfire]

Oh I c thanks Ykick....ill just gonna make a connection where I can bypass the bms just for emergency use and install a lipo reader so I can still monitor them.

 

[dnmun]

why not just fix your charger? there is nothing wrong with your BMS. i don't think you know what you are doing so you should try to read up on how a BMS works and how we tried to educate other people about the need to balance the battery cells before you assemble the battery the first time.

 

[splitfire]

Dont know where I can get a cheap 72v 5-10A. For now ill use my charger. Yup bms works great. I just dont like the LVC setting at 77v . So what will I do is when the LVC kicks in, I have my bypass connection when I need to run. It will still be balance for charging and for running it till it cuts off. Lipo monitor will do its job.

 

[dnmun]

i have no idea what you are talking about. the LVC is not 77V.

 

[agniusm]

When your BMS cuts off at 77V have you measured each cell? I think what is happening is one cell hitting lvc thats why bms cuts you off early. The way it suppose to work.

 

[Ykick]

When your BMS cuts off at 77V have you measured each cell? I think what is happening is one cell hitting lvc thats why bms cuts you off early. The way it suppose to work.

This is correct. BMS doesn't care about pack voltage with regard to LVC. It's looking for any cell voltage dropping below 2.7.3V which should then shut-down the -P (main power) in/out of the circuit.

Say for example, one of your cells died and shorted to 0V. If the BMS were relying on 77V pack voltage, 20 cells could be at 3.85V while the 0V dead/shorted cell is passing all the current through it's series string. This is why relying on global "controller" or CA LVC is so dangerous with RC Lipo.

If the BMS is wired up and working properly LVC will be based on individual cell voltage and not a give a crap about pack Voltage.

84mA balance current is practically a joke on the large capacity cells/groups we use. Balance manually if need be but by all means allow the BMS to protect LVC/HVC via the cell channels as it's intended.

 

[dnmun]

no, he has misrepresented what he told me his problem was. he told me it would only charge up to 77V so i am assuming it is just grossly outa balance and he does not understand what the LVC does. i think it is just stuck on one of the full cells at HVC.

 

[splitfire]

My charger works up to 86.4v. It used to charge at 87.6. Dont know the problem but that isnt a very big deal for me. When my battery died. I checked all my cells individually they are at 3.7v but there is one cell that has 3.3v. I will keep monitoring that cell every time I charge and probably buy lipo monitor so I dont need to open and close the side panel. Thanks guys you are all right...lvc doesnt care about the total voltage output. It cares about every single cell.

 

[LuboN]

There is high possibility of having cell with self discharging problem when using lipo. I have two in my pack and I cure it by charging those two cells individually via balancing leads from time to time. (I have no BMS and I balanced the packs only once before first use. Other cells stays in balance for months.) Balancing capabilities of Your BMS could be insufficient in such case. You should charge the battery to 4,2V/cell and then check voltage of the weak cell (if the cell gets to 4,2V with others, it has lower capacity and there is no help except replacing) and charge it separately to 4,2V. After discharging the pack, check cell voltages if they are even. If yes, the battery is ok.

 

[splitfire]

i got 4 cycle use with my new lipo packs. i run my bike from full charge 4.2v to 3.7 theres 1 cell that keeps dropping to 3.2 when others are 3.7 . but when i charge it they are all 4.18-4.2. should i replace my cell? LuboN

 

[dnmun]

balancing capacity of the D131 is about the highest available. it is 175mA.

did you do a standard discharge test on the pack you are working on? i doubt if the new lipo pack's pouches have more than .5% variation in capacity so if you find otherwise you should seriously examine your analytic technique.

can you verify that your voltmeter and watt meter or ammeter are close to accurate?

 

[Ykick]

You should determine if it's a genuine "leaker" cell or the BMS balance transistor shorted and constantly draining the suspect cell/channel.

For "leaker" testing - top charge every cell close to 4.2V then disconnect from everything and let it sit a while. After 24hrs if the suspect cell voltage has dropped considerably compared to the others, you probably have a "leaker" and it should be culled out ASAP.

If the cell appears to hold charge after sitting unattached to anything then the BMS is most likely culprit and you'll need to qualify the balance transistor operation/state.

 

[friendly1uk]

Your charger is no good.
You say you have 86.4v and 21 cells so you have 4.11v per cell
You say you had 87.6 which over 21 cells is 4.17v per cell.
You say you have some cells at 4.2 with others at 4.18. This is mathematically impossible.
The bms your using won't balance a cell unless it goes over 4.2v but you charger can't get all the cells to 4.2v so you can't balance your pack.

You need 88.2v or this bms won't work.
If you used 88v then you could have 20 cells at 4.2 and one at 4v exactly, so it would be your weakest link
With 87v you could have 20 at 4.2 and one at 3v about to hit lvc hot off the charger.

You need 88.2v minimum and should not pass 89.88v as that would put all cells at hvc if the pack was perfectly balanced. I suggest 88.3v

 

[dnmun]

i think his charger can go to full voltage but he is hitting the HVC because the pack started outa balance but no way to know. the 4 cycles is a clue. but it does require 88.2V or a little more to allow all the cells to reach full charge so the charger voltage should be set to allow the 175mA of balancing current at 88.2V to force the pack to balance.


2yk: he does not have to wait for the cell to drain down in voltage to determine if the shunt transistor has shorted, which i doubt, by measuring the voltage across the shunt resistor. if the transistor is shorted then there will be current flowing through the shunt resistor and he could measure the voltage drop across the shunt resistor.

on the D131 the power board prevents directly probing the resistor but there may be through holes that connect to the backside so he could measure the voltage drop there.

 

[friendly1uk]

I think we need to verify what is coming out the charger. He could well be talking about his packs finishing voltage, which is drifting down as it's going further out of balance at a quite rapid rate. Though I don't see 4.28 hvc levels being recorded, I can't doubt it as the numbers don't add up anyway.

We need a charger voltage. A multimeter on the charger while it is not connected to anything. If this is under 88.2v then that needs correcting before the bms can work. I want this doing twice, with two multimeter's, as there is to many discrepancies.

 

[Allex]

Can someone explain what exactly this means, marked with bold

Maximal continuous charging current 20A
Maximal continuous discharging current 80A

Over charge detection voltage 4.28V±0.025V
Over charge detection delay time 0.5~2S
Over charge release voltage 4.08V±0.05V

Over discharge detection voltage 2.80V±0.05V
Over discharge detection delay time 50~200mS
Over discharge release voltage 2.800V±0.1V How does release voltage work with discharge detection voltage, should it be same, or higher than detection?

Over current detection voltage 0.1V~0.2V
Over current detection current 240±40A Does this mean that it can sustain a burst of <240A before tripping? And if so for how long can you sustain those Amps?
Detection delay time 5~20mS

 

[izeman]

With an overcharge release voltage of 4.08 all cells need to be down below 4.08 before the charging will continue. Same goes for over discharge but the other way.

 

[Allex]

Ok so it might be wise to stick with same values for Over discharge 2,8-2,8 or maybe 2,8 for detect and 3V for release.
One question remains, about the Over current detection voltage/Over current detection current.

 

[izeman]

The release voltage should differ somehow from the upper/lower max. Otherwise they would be senseless.
But you have very limited values/combination to choose from.

 

[izeman]

Over current voltage - no idea

 

[Ykick]

Bestechpower standard values work fine. They always share same ODDV-ODRV for any BMS that I’ve used.

Here’s a celllog graph depicting 8S, 2.8V/cell ODDV operation.

As you can see, once that cell hits 2.8V, the load disconnects and the cell voltages immediately pop back up over 3V. I don’t think different ODDV-ODRV values would make any practical difference.

In my experience you’re much better off accepting default values. Anything else will only confuse the situation and delay production of the BMS/PCM you want.

 

[Allex]

So next question would be Maximal continuous discharging current
Say we take this one: http://www.bestechpower.com/481v13spcmbmspcbforli-ionli-polymerbatterypack/PCB-D276.html

They only mention continuous without peak values, while many other do also mention Peak, similar to: http://lunacycle.com/batteries/connectors/48v-13s-50amp-bms-mighty-mini/
This one states to be 50A continuous and 70A burst.

First of all, how can high quality bestech BMS with a heatsink on 18 genuine IRB fets only go for 55A and Lunacycle, with what looks to have 12FET go for 50Amps cont. and 70A burst.

I tried to ask bestech if they could give me burst values but the engineers did not understand why I would need that. When I then asked them if I can hold a current of 80A for 3 minutes they said it was fine. So what do you guys think?
Does bestech underestimate their performance and other BMS manufacturers are overly optimistic?

 

[Ykick]

I’ve only tested what you’re asking on small 15A continuous rated Bestech BMS. It handled 25A for a few moments (not minutes) but immediately cut-off when asked to deliver 30A.

Bestech would appear to be conservative about continuous ratings but it likely varies from model to model.

If claimed 55A continuous, it can likely handle momentary 70A’ish range loads but I wouldn’t count on that much current for several minutes on a regular basis.