BMS only showing 5v out

Joined
Jun 13, 2017
Messages
17
Hi all.

I've just built a new battery and for some reason the BMS that came with the pre-built old battery is now only showing 5v on the output.

I've measured between each of the balance leads and each cell is bang on 4.0v and are connected correctly to the BMS.

The pack is reading 52v.

Do I need to wake up the BMS somehow perhaps?

Cheers!

Hunt
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The 5V out probably indicates the BMS has shutdown for one of the various safety limits.

Is it a common-port (charge / discharge on same connector) BMS? If not, does it have the correct voltage on the charge port?

If all the cells are presently within limits, read at the solder joints on the other side of the PCB for the BMS balance/sense wire connector to ensure the BMS is actually connected to the cells correctly, (vs measuring on the connector contacts or wires which only tells you if those are ok to the cells) then it might be possible to just reset the BMS.

Some of them have a pair of pads to short for this, some just need a charger connected to the charge port (or common charge/discharge port), of sufficient voltage to be above what the pack-level "LVC" voltage would be (or perhaps at the voltage the pack-level HVC would be).
 
So the BMS has a B- and P- and the 3rd terminal is for the Charge-

Voltage between Charge- and pack+ is around 5V
Voltage between B- and pack+ is 52.6V
Votlage between P- and the pack is 5V

When I connect the charger I get:
P- to Pack+ is 26V
Charge- to Pack+ is 54V

Something weird is going on.

It 'was' working when I disconnected it from the old battery.
 
redbeardbeer said:
Voltage between Charge- and pack+ is around 5V
Voltage between B- and pack+ is 52.6V
Votlage between P- and the pack is 5V
Well, that does indicate the BMS has turned off the ports, for whatever reason.

BTW, if you have 4.0v on every cell, a 13s battery does add up to 52v that you read across (presumably) the actual main battery (cell) wires.

But if you read 52.6v somewhere, that should be impossible with only 52v--there's nowhere for the 0.6v to come from. :?


When I connect the charger I get:
P- to Pack+ is 26V
Charge- to Pack+ is 54V
What is B- to Pack+ in that event?

If it stays at 52v, it means the BMS is not enabling the charge port either.

If it goes to 54V (presumably the charger's actual voltage output), then it is enabling the charge port at least while the charger is connected.



If the BMS has actually failed:

A BMS is an electronic device, so it is possible for it to be damaged by ESD (static electricity), and that can happen during any handling of it (including the factory and any sellers between there and you). This happens more often on small-voltage inputs (like BMS sense inputs, or on IC chips on the board).

It is also possible that depending on it's design, an incorrect sequence of connection or disconnection of it's leads could cause voltages across components that they can't tolerate, leading to their damage or failure, which can cause the BMS to not respond correctly to within-limits readings (because it can't read the cells correctly, or doesn't respond correctly to them, etc).


Unfortunately neither problem is usually repairable as parts to do so aren't typically available. (or else the markings on them were removed so you can't tell which ones to replace them with).
 
amberwolf said:
redbeardbeer said:
Voltage between Charge- and pack+ is around 5V
Voltage between B- and pack+ is 52.6V
Votlage between P- and the pack is 5V
Well, that does indicate the BMS has turned off the ports, for whatever reason.

BTW, if you have 4.0v on every cell, a 13s battery does add up to 52v that you read across (presumably) the actual main battery (cell) wires.

But if you read 52.6v somewhere, that should be impossible with only 52v--there's nowhere for the 0.6v to come from. :?


When I connect the charger I get:
P- to Pack+ is 26V
Charge- to Pack+ is 54V
What is B- to Pack+ in that event?

If it stays at 52v, it means the BMS is not enabling the charge port either.

If it goes to 54V (presumably the charger's actual voltage output), then it is enabling the charge port at least while the charger is connected.



If the BMS has actually failed:

A BMS is an electronic device, so it is possible for it to be damaged by ESD (static electricity), and that can happen during any handling of it (including the factory and any sellers between there and you). This happens more often on small-voltage inputs (like BMS sense inputs, or on IC chips on the board).

It is also possible that depending on it's design, an incorrect sequence of connection or disconnection of it's leads could cause voltages across components that they can't tolerate, leading to their damage or failure, which can cause the BMS to not respond correctly to within-limits readings (because it can't read the cells correctly, or doesn't respond correctly to them, etc).


Unfortunately neither problem is usually repairable as parts to do so aren't typically available. (or else the markings on them were removed so you can't tell which ones to replace them with).

Sorry my measuring tool is not exactly accurate - each cell is reading ~4V - could be 4.04V which would be 52.6V pack voltage.

With charger connected (charger output is a 13S - so 54.6V) the pack voltage does not change - 52.6V

With the charger connected the output voltage from the BMS is 26V

Seems like the BMS is not turning on, disabling the charge and discharge ports.

I've tried the trick like with the JK BMS's where you put a 9v between the P- and B- on the BMS - this appears to turn the BMS on as long as the 9v is connected but as soon as I disconnect it, the BMS shuts down.
 
redbeardbeer said:
Sorry my measuring tool is not exactly accurate - each cell is reading ~4V - could be 4.04V which would be 52.6V pack voltage.

Its' ok, as long as you can see there is a relative difference between what cells read, charger reads, and BMS reads, it still shows there's probably an issue with the BMS itself vs the cells or charger.


Seems like the BMS is not turning on, disabling the charge and discharge ports.

I've tried the trick like with the JK BMS's where you put a 9v between the P- and B- on the BMS - this appears to turn the BMS on as long as the 9v is connected but as soon as I disconnect it, the BMS shuts down.

It sounds like the BMS may have failed, assuming that youv'e verified the cell voltages at the solder pads on the BMS board itself are correct..

Before giving up, though, can you post pics of the back side PCB, so we can clearly see any markings, pads, etc., in case there's something for a reset there? Also pics of the edges of the board that can be seen under the overhanging metal plate, if possible. (pics taken in direct sunlight are usually easier to see (cameras autofocus better) than those using a flash or indoor lighting).


Another thought: I once had a BMS that would not enable charge until I drained the cells down a fair bit from near full (I don't remember how far, but it should be in one of my threads, for a Luna 18650 pack if you're curious). AFAICR it did still have the discharge port enabled, but that may not matter. I think I just hooked i tup to a controller and motor and ran it for a while, but it might've been a stove-element load. Been too many years and packs between then and now. :pancake: :lowbatt:
 
Thanks for that. It's evening here in NZ so without sunlight these are the best I could do.

I've verified the pads for all 14 pins are reading correctly.

There is a looped wire seen in one of the side pics and circled on the back of the board pic. I've cut and rejoined those and that made no change. No idea what that's for.

Cheers!
Hunt
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redbeardbeer said:
I've measured between each of the balance leads and each cell is bang on 4.0v and are connected correctly to the BMS.

The pack is reading 52v.

Were the cells all charged to 4V before assembling the pack, or charged after assembly?
 
Have you walked the voltage probes up the balance connector to verify that the voltages increase sequentially. More than a few of us have put a balance wire out of order. One time, I put them all in backward,
 
redbeardbeer said:
There is a looped wire seen in one of the side pics and circled on the back of the board pic. I've cut and rejoined those and that made no change. No idea what that's for.
Usually it would be for a thermal sensor (actually a thermal breaker) that would be embedded in the pack of cells. It usually looks like a white ceramic block, sometimes a metal one, and has two wires from it to the BMS board.

If that sensor is not on the wires, and that's what they are for, the BMS will remain inactive because it thinks the pack is overheated and the sensor switch has opened up.
 
Can it be the solder flux, it looks like there is some on the bottom of the bms, top-left. I didn't connect more than two BMS but i know here it change resistor values:)
 
Hi all.

Thanks for the replies. I have tested each of the balance leads at the pad on the other side of the board - all connections and voltages are good and correct.

I've cleaned the flux on the back of the board, tried a combination of connecting the balance cable and the B- in various orders, tried adding a load to the output, nothing has worked.

Still showing 5V on the output.

Interestingly the BMS has a slight electrical hum/buzz when the balance plug is connected. It does not get warm and doesn't appear to be drawing any measurable current.

Am leaning towards a dead BMS and with a replacement being $40 locally, it might be more cost effective to just replace it rather than waste any more of everyones time.

Bit of a shame as I'd like to know why it failed.

Cheers!
Hunt
 
redbeardbeer said:
Interestingly the BMS has a slight electrical hum/buzz when the balance plug is connected. It does not get warm and doesn't appear to be drawing any measurable current.
Buzz? That's very strange. It shouldn't have anything on it that could do that. (to buzz, it would have to create an alternating current flow of some kind, or switching a DC voltage off and on, etc).

If it is an active-balancing charge-transfer device with an SMPS-style inductive switching unit to do the transfers, it would at least have the possibility of creating a noise, but it should be very very low amplitude and very very high frequency, so you shouldn't be able to feel it at all. I also don't see any place on it for the hardware for this to be mounted.

If it has an audio alarm device (piezo, etc) on it, then it might make a buzz to alert the user to a problem, but you should be able to hear that, too (or else it wouldn't be very useful), and i don't see a place such a buzzer could be.
 
amberwolf said:
redbeardbeer said:
Interestingly the BMS has a slight electrical hum/buzz when the balance plug is connected. It does not get warm and doesn't appear to be drawing any measurable current.
Buzz? That's very strange. It shouldn't have anything on it that could do that. (to buzz, it would have to create an alternating current flow of some kind, or switching a DC voltage off and on, etc).

If it is an active-balancing charge-transfer device with an SMPS-style inductive switching unit to do the transfers, it would at least have the possibility of creating a noise, but it should be very very low amplitude and very very high frequency, so you shouldn't be able to feel it at all. I also don't see any place on it for the hardware for this to be mounted.

If it has an audio alarm device (piezo, etc) on it, then it might make a buzz to alert the user to a problem, but you should be able to hear that, too (or else it wouldn't be very useful), and i don't see a place such a buzzer could be.

The buzz/hum is not from a piezo or buzzer. It's super faint - you can hardly hear it. It's the same sort of noise you can often hear from electronic devices - very high pitched. CRT tv's used to make a similar noise sometimes.
 
Those other devices that make that kind of sound have circuits that create either pulsed DC or alternating current waveforms, and the wasted energy from that process is what makes the sound/vibration. CRT TVs' high-pitched noise was generally from the high-voltage flyback circuit generating the voltage to charge the CRT to "drive" the electron guns. There were also oscilators to drive the horizontal and vertical magnetic coils to shape the beam.

Many chargers or devices with voltage converters in them have oscillators to drive the converters, those often drive coils (which vibrate a bit like a speaker does and make noise you can hear if it's within the human hearing range, under the right circumstances).

For the BMS to do that it would also have to have an oscillating circuit. The only one it should (for any common BMS design) have is an active balancer, but this should not be processing enough power to create an audible sound or tangible vibration, especially if there is no detectable current draw. If it's making this noise because of a defect or failure then the power drain in it might be the reason the BMS isn't turning the output or input on.

I've poked around a few times to see if I could find that specific BMS anywhere with info that tells anything useful about it, but I can't find a similar enough version. :( So I don't know what is making the noise in it, or if the noise is normal or a symptom of failure.


Note that if it does have an active balancer, it shouldn't be running if all the cells are equal, anyway. :) (the oscillator might still be on, but it wouldn't be processing current so it wouldn't have a load---but to save power epecially in a shutdown state the BMS should have turned off the oscillator since those use up power just by operating).
 
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