how to measure Battery voltage with a meter

[Nine6Teen]

When measuring the voltage of a 48 Volt battery, the meter reads 13.3 volts. This particular battery has five (5) electrical contacts, from top to bottom, numbering them 1,2,3,4,5 the measurements are as follows
1-5 13.3 volts
2-4 13.3 volts
3 appears to be not connected

When mounted on the bike, the display reads 51 volts, as the battery is nearly full.

thoughts ? How do you read Battery voltage with a meter ?

 

[Chuy]

It must depend on the battery type. Mine has 2 circular pins, 1 neg, 1 pos. My pack has an on/off switch and it has to be on to read the voltage correctly. In off position it reads, I think, 6 volts.

 

[biknut]

1-2 are common to each other, and 4-5 are common. 1-2 are positive, and 4-5 are negative

 

[Unicorn]

When measuring the voltage of a 48 Volt battery...

Is there a reason you don't trust the voltage display on the bike?

Stay safe.

 

[Nine6Teen]

duh , sorry guys, rookie mistake. I had the switch to the battery switch in the OFF position when making the measurement. When I put the switch to the ON position, the measurement is proper.
Which produces another question, with the switch in the OFF position, why is there any voltage reading at all. Shouldn't it be zero ?

 

[Atavar]

It is because the resistance of the volt meter is very high, many Megohms.
When you shut off your battery what you actually shut off is a MOSFet in the BMS.
The output resistance of the turned off MOSFet is apparently about three times the input resistance of the meter.
When you connect them together they create a 1:3 voltage divider.
If you measured the output of the disabled battery with a lower impedance (kilohms) analog volt meter the reading would appear to be zero.

 

[Atavar]

Maybe this picture will help.
R1 can represent the resistance of the MOSFet, R2 could represent the resistance of the meter.