Battery new self-build bms issue

[djah]

Looking for some help again please.
I finally got the courage to build my own battery. It all went well and has been a great learning experience. I wired in my second bms this evening as I thought I blew my first one. I followed the suppliers wiring diagram but I’m getting almost full pack voltage(44.1V) when the switch is in the off position at the discharge port. I get full pack voltage when the switch is on. Full pack voltage is 45.8V as it has not been charged yet. Have I got two faulty bms or have I wired it incorrectly?
I will attach a photo if it helps. Thanks.

 

[Woosh]

if I understand correctly, you have no load at the discharge port.
I don't think you've blown anything yet.
Connect a load, something like an incandescent 40W lightbulb to the output port for testing.

 

[djah]

if I understand correctly, you have no load at the discharge port.
I don't think you've blown anything yet.
Connect a load, something like an incandescent 40W lightbulb to the output port for testing.

Thanks for the reply Woosh.
No I don’t have any load at the discharge port.
I have another battery pack that I bought and when the power switch is off on that one there is no voltage between + and - at the discharge port.

 

[Woosh]

if you can't connect any load, press and hold the test button on the battery cover.
The LEDs don't burn much electricity but should be enough to clear the ghost voltage you read at the discharge port.
I have one of these rheostats for testing batteries:
https://www.amazon.co.uk/Electronics-Salon-Wirewound-Potentiometer-Rheostat-Resistor/dp/B01489T6H6/ref=asc_df_B01489T6H6/

 

[djah]

Just for full disclosure. There are a pair of white wires which are for the on off switch which I connected correctly. This switch also has another two wires( black and red) which are for an internal light to indicate when the switch is on. I connected the red and black from the bms to these. The ones from the bms are supposed to be connected to the “power indicator panel”. Is this where I went wrong I wonder?

 

[Woosh]

I don't know. I am not familiar with the model of your BMS. There are usually a pair of wires for the thermistor and a pair of wires for the soft switch.

 

[djah]

I don't know. I am not familiar with the model of your BMS. There are usually a pair of wires for the thermistor and a pair of wires for the soft switch.

Ok Woosh. Thanks for taking the time.

 

[Woosh]

does your power indicator panel work?
the BMS I use has soft on/off switch and test functions on the same button.

 

[Nealh]

One thing I have noticed is you have soldered the BMS sense wire (B-) small black directly back to the BMS B-, usually all instructions state that it is soldered directly to 0v of the #1 cell group. Whether this has a bearing on the reading I don't know as never connected to BMS B- in this way.
Whether the BMS B- port see's the whole V- reading in series I don't know.

 

[vfr400]

Some BMSs have leaky MOSFETs. When switched off, charge can leak through the MOSFETs so you can measure voltage on the terminals. Around 18V on a 36V battery is common, but can go as high as 40V. It's a voltage that can be measured with a voltmeter, but you can't do anything with it. If you try and draw any current, it'll immediately drop to zero. It's nothing to worry about.

 

[djah]

One thing I have noticed is you have soldered the BMS sense wire (B-) small black directly back to the BMS B-, usually all instructions state that it is soldered directly to 0v of the #1 cell group. Whether this has a bearing on the reading I don't know as never connected to BMS B- in this way.
Whether the BMS B- port see's the whole V- reading in series I don't know.

Thanks Nealh.
I swapped it now just to check but made no difference. It’s basically a different end of the same wire.

 

[djah]

Some BMSs have leaky MOSFETs. When switched off, charge can leak through the MOSFETs so you can measure voltage on the terminals. Around 18V on a 36V battery is common, but can go as high as 40V. It's a voltage that can be measured with a voltmeter, but you can't do anything with it. If you try and draw any current, it'll immediately drop to zero. It's nothing to worry about.

Thank you vfr400. That would be a huge relief if that’s the case. I haven’t put the bike together yet but the best way to test it, I suppose, is to test it.
I did notice that when I tested the led indicators, as Woosh suggested, the voltage dropped to 1V but rose again when the test button was released. This was with the on/off switch on. The led test button doesn’t bring on the led indicators when the on/off switch is in the off position.
Thanks for your help guys.

 

[harrys]

I had a 13S BMS last year that would not shut off. It would read about 39V across the output port. The controller load did not drain it. In fact, the LED light in the pack stayed on when it was turned off. I recall it could supply about 1 ma when I put a resistve load on it. I had waited too long to install it, so I couldn't return it.

It's a defect, Pure and simple. That didn't give me good feelings about the BMS being able to do its other functions. so I replaced it with a different manufacturer (Annpower.) It shuts off. It delivers 30A. I'm happy.

 

[djah]

Just to come back with my findings. The voltage disappears when a load is put across the discharge contacts. I went ahead and temporarily set everything up and it worked fine. Charging the battery will be my next step and hopefully will be ok.
I would like to thank everyone for their input and suggestions. I am humbled by the help that I get time and again on this forum.