Tuesday, June 8, 2010

How to Bodge Ammeters

One of the big problems you have when putting together a battery based solar system is measuring where the juice is going.  You need to know whether it's going in the battery or to the inverter and how fast it's going.

For this you need low loss ammeters.  You need ammeters that can measure maybe 100-200A without causing much voltage loss.  You can buy shunts that you can connect in series with cables to read the current but this means more joins in the cable and each join adds resistance that causes loss.  Shunts are also mostly of the type that create a Voltage of 50mV per 500 Amps passing through them which is quite a small signal when most of the time you'll be measuring a few 10's of Amps.  They're also quite expensive...

Enter the In-Line Bodger's Shunt... :D

Solar battery systems by necessity need to be connected together by big fat cables to carry the heavy current without losing Volts ('coz we're working at maybe only 12V or 24V).  Any bit of copper cable (no matter how thick) will have a resistance and so make a voltage across it's length when a current flows.  Even the cheapest digital multimeter will read down to 0.1mV (they all have a 200mV range).  So all you need to do to measure the current in a cable is attach some sensor wires to the cable at a distance apart that causes some useful Voltage to be developed relative to the current.

I have two types of in line shunt in my system.  One uses a long spacing (about 50cm on a 10mmsq cable) to give a reading of 1mV per Amp.  On my big group of four 6V batteries, I made one of the huge 35mmsq link cables that join the blocks a bit longer than the others - just enough to make 0.1mV per Amp when I soldered the sensor wires at the lug ends. So that DMM reads 0.1 for 1A and 1.0 for 10A - you just have to ignore the decimal point on the display.
This one's a shorty as it's on a bit of 6mmsq cable.
It should go without saying (but I'll say it anyway) that you should only do this on low voltage DC power cables.  Don't try it on mains AC lines, as a) it won't work and b) you'll probably get electrocuted and die.

All you need to do is connect another DMM with a 10A current range in series with the cable in question and arrange for a few Amps to pass through it.  Then solder a sensor wire to the fat cable and using a pin, poke the cable at different places until you find the distance that gives you 0.1mV per Amp reading on the new meter.  Then you can either cut the insulation away at that point and solder the other sensor wire in there or sometimes on big 35mmsq cables you can just solder the sensor wire to the head of a drawing pin and just stick the pin into the cable at the spot.  Tape it up with electrical tape and run the sensor wires to where you want to see the meter.  It can be some distance away (like a couple of meters) as the sensor wires carry no current so won't lose any volts on the way (the DMM input is usually about 10MOhms so the fraction of one Ohm on the sensor wire makes no difference).

Here you can see my three bodged ammeters.  The top and left ones read net battery current in or out of the two main banks at 0.1mV per A (so showing 3A charge each) and the third one reads the current from the smaller Morningstar MPPT15 charge controller at 1mV per A (so showing 2.0A charge).  The big Morningstar TriStar has a built in meter so I didn't need to bodge a fourth meter. The meters are arranged so that positive readings mean charge current and negative readings means discharge current.
Ok, these shunts aren't going to be super accurate and the short (or fat) ones will only read to a resolution of 1A but mostly that's all you need to know - whether the battery is charging or discharging and by roughly how much (some Amps or lots of Amps).  

Maybe most importantly, it can tell you when there are no Amps flowing as that tells you two things: a) something is busted or b) the battery has finished absorbing power and is full.  In a system like mine with several batteries, it also tells me how well (or otherwise) the batteries are sharing the load or charge... More on this in the next post!

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