How to measure output current of battery

[anees ur rahman]

Hello!
Nec has 69.8(A) for max current calculation but there is no part that say battery output current
so i need to take max out current from battery datasheet?
and ampacity will be 1.25 * max current?
or is there code for it ?

 

[paulengr]

Just like other voltages. You size to the load. If it’s say a 15 A residential circuit, that’s what you size to. If it’s a known load such as a 10 HP motor, that’s the load. Breaker/fuse is sized to load and so is cable (overcurrent protects cable). Size the source to equal or exceed the combined loads. See it all works together. The feeder between the source and overcurrent protection is protected by the first breaker/fuse connected to it. A short can happen in between but there is nothing you can do about it in a battery.

A transformer or generator is the same way. There is a short circuit current that vastly exceeds the ampacity limited only by the source itself and maybe primary side protection with transformers (don’t bet on it!). Batteries have series resistance. Need to protect that cable from damage. But excessive load just trips the breaker/fuse.

This “universal” aspect only matters as we go up in frequency where the skin effect applies. On 400 Hz systems the cable has to be derated because the ampacity is lower than at 60 Hz because the current mostly rides along the surface so the cable effective cross sectional area is reduced. It would be cheaper to run hollow tubes (wave guides).

 

[tom baker]

Hello!
Nec has 69.8(A) for max current calculation but there is no part that say battery output current
so i need to take max out current from battery datasheet?
and ampacity will be 1.25 * max current?
or is there code for it ?

Where in the NEC does the 69.8 Amp come from?
Batteries have high short circuit current available. You will need to fuse it at a much lower level and there are special fuses for batteries
Code for what? Typically we use overcurrent protection for the wire size.

 

[jaggedben]

Where in the NEC does the 69.8 Amp come from?
...

I think he meant to refer to article 690.8(A). Which however does not apply to batteries.

 

[paulengr]

If the answer you seek is measuring DC currents it is typically done two ways. Portable meters use a Hall effect sensor. Permanent mounted sensors are typically shunts. So a typical shunt spec might be 1 mV per A so 1 V is 1000 A. Very easy to use.

 

[GoldDigger]

If the answer you seek is measuring DC currents it is typically done two ways. Portable meters use a Hall effect sensor. Permanent mounted sensors are typically shunts. So a typical shunt spec might be 1 mV per A so 1 V is 1000 A. Very easy to use.

I know you were just grabbing numbers, rather than citing a practical example, but I have to point out that a shunt that reads 1 Volt at 1000 Amps will be dissipating 1000 Watts of power.
For those practical reasons shunt specs in the microvolt per amp range are common, and so your meter needs to be accurate at low voltages.
That aside, the shunt also needs to be chosen to have an operating voltage that is low compared to the battery bank voltage.

Sent from my Pixel 4a using Tapatalk

 

[paulengr]

I know you were just grabbing numbers, rather than citing a practical example, but I have to point out that a shunt that reads 1 Volt at 1000 Amps will be dissipating 1000 Watts of power.
For those practical reasons shunt specs in the microvolt per amp range are common, and so your meter needs to be accurate at low voltages.
That aside, the shunt also needs to be chosen to have an operating voltage that is low compared to the battery bank voltage.

Sent from my Pixel 4a using Tapatalk

1 mV/A is typical for tens to maybe a couple hundred Amps. Yes you’d use a smaller shunt on higher currents if those were typical. Usually heat is not much of a factor in drives if you set up the shunt to produce 1 V at stall. Lower and you run into ADC resolution problems. Higher and heat dissipation is a factor. With say a GE MD 824 stall current is 6000 A but it normally operates in the hundreds of amps. So with that one a 100 micro volt/A shunt would be best. It will be 600 W at stall but normally well below that.

DC stuff tends to be high current, low voltage. Never mind heavy since frames are part of the magnetic circuits in motors.