The Best Value Wattmeter

[jackhandy]

Thanks Daveh - Obvious when you know how

 

[Deleted member 4366]

Yes, its like exams. The questions are always easy when you know the answers. you just need to do your homework to swot-up and learn what the answers are.

 

[awol]

Thanks for this info. Keeping the watt meter powered when the bike battery is removed is something I have been wanting to look at, now got a job for the weekend.

 

[Deleted member 4366]

there'e an even cheaper way of getting an ammeter on your bike. These might suit some builds. Note the DIY shunt from ordinary battery wire:
http://endless-sphere.com/forums/viewtopic.php?f=2&t=58285&p=869482#p869482

 

[tongxinpete]

Hi, I went down the road of an analogue ammeter & shunt as I have seen constantly varying values make digital meters refuse to settle on anything, obviously this has now been cracked. I bought my kit from a guy called 'asia engineer' on ebay. Another dodge is to get a 75A & 75mV shunt (or 50A & 50mV) then connect a budget multi meter set on mV across the shunt, the mv then reads out as amps (downside is that only whole amps are displayed)
It did work at the time but it was some years ago, an idea I piked up from seller solarbbq. Hope I have got this last bit right as my background is mech not electronic. The shunt could be picked up for about £3

 

[drsolly]

If you use my recipe, you get an ammeter and a voltmeter on your handlebar for £3.59; I found that making my own shunt was very easy, it's as simple as "cut about nine inches of wire".

 

[jackhandy]

Having butchered my 3-button wattmeter, used the shunt (a long-wire type) remotely, I get about 19.5a from my Ezee motor, which is close enough for me:

I've now ordered a GT Power one, so I don't need to keep it in my waterproof phone holder on the bars & I'm thinking to use the shunt from my defunct speedict, as it's a nice, neat little unit:
Has anyone tried this? & if so, does anyone know if the shunt has a useable resistance for the job?

I'll try it anyhow, when the meter arrives, but was just wondering... My multimeter is not accurate enough for the job.

Picture of the disassembled speedict, showing the shunt legend, which reads:-
BVS
R002
1%
TO11

AFAICT

 

[drsolly]

If you know what resistance you want, then you can easily make a shunt out of a bit of wire. For example, if you want a shunt that is .75 milliohms, then http://www.powerstream.com/Wire_Size.htm tells you that 14 AWG wire is 8.282 ohms/km = 8.282 milliohms per meter, so you want .09055 meters = 9.055 cm = 3.56 inches.

So, I made a shunt out of 14 AWG wire, which I use in conjunction with this £3.49 meter:

http://www.ebay.co.uk/itm/LED-Digital-Volt-Amp-2-in-1-Panel-Meter-Voltmeter-Ammeter-100V-100A-Blue-Red-/390738004493?pt=UK_BOI_Electrical_Test_Measurement_Equipment_ET&hash=item5af9ccca0d

So, two questions. How did I know I needed .75 milliohms, and how do I know I got it right?

I looked at shunts on Ebay,, and they all seem to be 75 mv at their rated current. Since my rated current is 100A, using ohms law, the resistance needs to be .75 milliohms. And I don't worry about putting 100 amps through my 14 AWG shunt, because there's a 40 amp fuse inline with this, which will blow long before I get to 100 amps. You could, of course, use thicker wire for your shunt, just use the AWG table to calculate it.

How do I know I got it right? I have an array of ten 12volt lamps. I put the output of a 4S lipo throught that, and measure the current with an ammeter that I think is good, and I measure the same current with me made-up shunt-and-meter. And if I get pretty much the same figure, I know I did it right.

How accurate is this?

I don't much care. If it's 10%, I'd be very happy, if it's 20% I'd be OK. Because when I look at the current going through my motor, it's very variable from second to second. If my meter tells me I'm pulling 20 amps and I'm actually pulling 16, then that's good enough for me.

Accuracy is more important on the voltmeter part, because that tells me how much battery I've used and how much I have left. And my home-made shunt doesn't affect the voltage reading. Plus, the voltage reading agrees with my multimeter.

So while I'm riding along, my ammeter gives me a pretty good idea of my moment-by-moment power draw, and shows me the difference between cruising at 15 mph on level road, and heaving my way up a nasty incline on squishy ground. And the voltmeter tells me the state of charge on the battery; when it's down to about 28 volts (that's on an 8s) then it's nearly empty. I also have a beeping battery monitor on each cell, set to alarm at 3.2 volts, so I don't ruin a cell.

 

[Deleted member 4366]

If you search for a current measurement thread I did earlier. I gave a link to my ES thread where there's a spreadsheet that does all the conversion for you.

The GT Power shunt is .005 ohms, which is 5cm of 14g wire. I think the speedict one might be .010 Ohms.

There seems to be a limit to how thin the three wires from the shunt can be. If you go too thin, it doesn't work, but 1 amp wire is OK.

 

[drsolly]

According to the figures I was using here: http://www.powerstream.com/Wire_Size.htm

14awg is 8.282 ohms/kilometer, 8.282 milliohms per meter.
So .005 ohms (5 milliohms) would be 0.60 meters, which is 60 centimeters, not 5. Did you mean that the GT shunt is .0005 ohms? Or did I misplace a decimal point?

To save people searching for d8veh's spreadsheet, it's here:

http://endless-sphere.com/forums/download/file.php?id=123974

 

[Deleted member 4366]

One thing I found out is that the wires that run from the shunt to the meter mustn't be too thin. It needs to be as thick as that servo wire that you can see in the first pictures (about 1mm). If the wire is too thin, you don't have the right voltage difference at the meter, plus there's a danger of solder joints breaking.

 

[jonathan75]

What would be the cheapest wire to run from the controller bag to the handlebar where a wattmeter would be mounted,and back again? Could I use household electric cable?

 

[Deleted member 4366]

You only need one thick cable if you don't do the shunt mod. The wattmeter only measures on the negative wire. The positive is only used to power the meter, so on the input (source) side, you want one thin positive wire (24g) and a thick negative (14g). On the output side (load), you only need the thick negative.The thick positive from the battery can go directly to the controller.

Although the wattmeter has a thick positive or connections for it, it runs straight through, though they take a branch off it to power the meter

 

[jonathan75]

You only need one thick cable if you don't do the shunt mod. The wattmeter only measures on the negative wire. The positive is only used to power the meter, so on the input (source) side, you want one thin positive wire (24g) and a thick negative (14g). On the output side (load), you only need the thick negative.The thick positive from the battery can go directly to the controller.

Although the wattmeter has a thick positive or connections for it, it runs straight through, though they take a branch off it to power the meter

Thanks D8veh. That's a great help. Is the positive input wire, powered by splicing it into the thick wire coming from the battery ?

 

[Deleted member 4366]

Yes

 

[jonathan75]

Yes

Good stuff. Is there another way to power it, say from a 9v (or other type of) battery, so as to take advantage of the tiny power draw and keep the data onscreen when the main bike battery is switched off? I've got the blue one with the unlit screen. There are three power poles (?) for auxiliary power input but I can't figure out which is which.

 

[Deleted member 4366]

Yes. You can use a 9v battery with its negative tied to the thick black wire.

 

[jonathan75]

Yes. You can use a 9v battery with its negative tied to the thick black wire.

Thanks, do you mean spliced into the black load/output wire which also goes back to the controller?

Here's what I think you mean:

36v negative (black) to wattmeter source/input; then output runs to controller, but is also spliced into the 9v battery negative.

9v positive goes straight to wattmeter thick red input wire

36v positive (red) just goes straight to controller

wattmeter thick red output wire doesn't connect to anything.

Have I missed anything there? Are you sure the red wattmeter output doesn't go to the 9v battery negative?

 

[Deleted member 4366]

That's correct.
Just a thought, though. I guess that only the current will be displayed correctly, but the voltage will read 9v and the watts will be approximately 20% of the actual value.

 

[jonathan75]

That's correct.
Just a thought, though. I guess that only the current will be displayed correctly, but the voltage will read 9v and the watts will be approximately 20% of the actual value.

Thanks. Perhaps I'm better keeping it at 36v. My fear was the glowing blue ' on ' light on the 09 battery would attract vandals or mean the system was discharging or both -would you know is the discharge quite substantial for leaving the battery powered on over a day? (ku93 controller + 810). Probably I imagine I can at least hide the light with some tape and stuff.