DC Current measurement

[bosparra]

I am in the process of designing an intelligent battery monitor. Its for an RV where a freezer will be running off a 12V lead acid deep cycle battery. Just monitoring the voltage is not enough, since it will report a run down battery after the fact. I want to monitor the current draw of the load, to calculate an estimated time before the battery runs down.

My question is this: using a shunt resistor is not practical because the resistor will have to be huge. A 1ohm shunt resistor measuring 4 amp on a 12V battery will already have to handle 48W! Then we haven't even started measuring charging currents of at least 6A. What other way is there to measure up to 20A-30A in this environment, without using a shunt resistor?

I have looked at hall effect sensors, but is there another cheaper and more practical way of measuring DC current accurately?

 

[engr.waqas]

A specialized resistor called"shunt resistor" is used for this purpose.Basically that is not normal carbon resistor but a piece of copper wire which can handle high current.
see this link
**broken link removed**

hope i helped you

 

[bosparra]

Ok, that makes more sense. So, using that example, a 0.0005ohm resistor will dissipate 0.008W, when measuring 4A? I can live with that.

Thanks for the quick reply.

 

[aj9999]

Try MPJA.com have used their shunt resistors before and they are alot cheaper

 

[umesh49]

You can use op-amp and trasistor based current mirrors.Do some google you may find lot of circuitry.

Umesh.