Shunts

Re: Shunts

Well, what is significant is at 12 volts and 500 amps is that the shunt generates 50 watts of wasted heat, vs 25 watts for a 50 mV shunt (out of about 6,000 watts total)...

So, in real life, the extra 50 mV probably does not make a huge difference on the shunt--and you could always use a 2x larger shunt at 100 mV and just simply use it at 1x current and 50 mV max... If the price is right (the 100 mV shunt would probably be bigger and, normally more costly, because of the extra heat that would need to be dissipated).

-Bill

PS: I should add the extra 50 mV drop probably would not affect the load--however, if you had a shunt in the charging circuit--the extra 50 mV would terminate your charging early (by at least 5% capacity)... Hence, why good meters have separate sense leads from the charge controller to accurately measure the voltage at the battery, and not based on the wiring and shunt losses).

Re: Shunts

Bascially, you place the shunt in the "wire" you want to measure the current in...

You can measure current from your panels, to your loads, and in/out of your battery. They all have their uses.

A reason for placing a common shunt on a battery is that you can connect it with a "smart battery monitor"... With a battery monitor, you now have the ability to accurately measure the "state of battery charge" directly. Batteries are pretty much electron storage devices. Charging, you move the electrons one direction (to 100% capacity) and discharging you move them the other (towards 0% capacity).

The advantage is that a battery is fairly close to 100% efficient if you are counting electrons (amps*time). So you can have a pretty simple totalizing meter (amps*time=Amp*Hours) and simply add up all of the charging current and subtract all of the discharging current = what is left/available in the battery.

Check out Wind-Sun's battery monitor page or other vendors for various brands.

The problem with measuring voltage to determine charge state, is it is dependent on temperature, battery "Mode" (charging, no-current, and discharging). So, just reading a voltage does not tell you much with out knowing several other variables (temperature and current flow).

The other "accurate" way to measure state of charge is with a hydrometer--but that has its own issues (temperature, mixing of electrolyte, contamination, loss of fluid over time, sealed batteries, etc.).

Placing the shunt in-line with the battery (and, ideally, connecting it to a battery monitor), and the other end connecting loads, charging sources, etc. would be best for monitoring the state/health of the battery. You can place other shunts in load circuits and charging circuits if you want to monitor those more closely.

Obviously, keeping the voltage drop low is the best (reducing losses). However, you also have accuracy to worry about. You want to worry about the meter reading the shunt... Look at the sensitivity of the meter measuring the current.

Just making a number up... Say the meter reads 0-100mV in 1mV increments... So, from the spec., we cannot feed it more than 100mV or it will "peg" the meter. Accuracy wise, a 500 amp 100mV shunt would read 5 amps as its minimum current. And a 50mV shunt would read 10 amps as its minimum current. So, you can see in this case a 100mV shunt would have twice the resolution of a 50mV shunt in this system.

And, if you start with a small system--you have the ability to only measure 0, 10, 20.. amps or 0, 5, 10, 20... amps. A too large of shunt would not allow your battery monitor to accurately totalize the storage capacity of the battery--you could have a 9-19 amp load and never see it (for example).

A 100 amp/100mV shunt would allow your system to accurately read down to 1 amp (in my example above) ... Much more accurate (5-10x more accurate) for a "smaller" system.

So, calculate the maximum current you want to measure (so that full current reads ~50-100mV--depending on meter and shunt) and read the requirements of the meter you are going to connect to the shunt.

So, I would recommend not to over size the shunt for future expansion (unless it is going to happen shortly).

And for a small system, the amount of power wasted by either 500 amp shunt is probably pretty trivial (assuming you are not anywhere near 500 amps)... I would tend to "error" more towards accuracy (100mV shunt, lower current rating)--in most cases.

-Bill

PS: I should add that it is "easy" to measure small voltages (such as a few amps going through a 500 amp shunt), but the errors become larger and larger relative to the "signal" (dirty contacts, alternative current sources like dirty/moist insulators, galvanic corrosion, thermocouple effect, radio noise, etc.).