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".

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.http://img.tapatalk.com/d/14/04/06/9u7egyne.jpg

Picture of the disassembled speedict, showing the shunt legend, which reads:-

BVS

R002

1%

TO11

 

AFAICT

Edited April 6, 201412 yr by jackhandy

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.

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.

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

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.

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?

Edited July 6, 201510 yr by 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

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 ?

Yes

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.

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

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?

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.

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.

Are you using the KU93 with a LED panel? If so, you can pickup the 36v from the red wire that goes into it. If no panel, you put a switch on the ignition wire and power the wattmeter from the battery side of the switch. With the control panel or ignition switch switched off, no power goes through the controller, so there is no consumption. The only consumption from the battery will be the blue LED and the wattmeter. The GT wattmeters have a backlight, which takes a small amount of power. The other types without the backlight consume negligible power. Stick a bit of black tape over your battery switch to stop it glowing.

Are you using the KU93 with a LED panel? If so, you can pickup the 36v from the red wire that goes into it. If no panel, you put a switch on the ignition wire and power the wattmeter from the battery side of the switch. With the control panel or ignition switch switched off, no power goes through the controller, so there is no consumption. The only consumption from the battery will be the blue LED and the wattmeter. The GT wattmeters have a backlight, which takes a small amount of power. The other types without the backlight consume negligible power. Stick a bit of black tape over your battery switch to stop it glowing.

 

Fantastic. Yes I do have an LED panel. However the red wire leading to it is incredibly thin, suprising to think there's 36v going through that! Still it's good to not have to splice into the thick battery positive, always slightly disconcerting.

I've read that the wire can cause resistance of 1 to 1.25v per metre - that's 2-2.5 volts if I attach the wattmeter to my handlebars, instead of putting it in the controller bag with a very short wire. That sounds quite significant, maybe a loss of 5-6%+ of power and thus efficiency, so shorter rides to empty. Is this true?

 

If it's true I don't know why anyone bothers mounting wattmeters or cycle analysts on the handlebars.

The loss of power doesn't account for much. I think I calculated about 4 watts for two wires going up to the wttmeter and back. With one wire, it'll be about 2w. Use wire no thinner than 14g. 12g would have less loss, but is more ugly.

 

This is why I do the remote shunt mod. There's no losses at all then.

Yes I'm trying to make a virtue of well-ordered ugliness on the bike at the moment, thankfully the frame is black-coloured. Thanks D8veh.

The wattmeter is working well inside the controller bag at the moment pending being mounted on the handlebars - but is there

 

1) a cheap way of having one sit next to the battery while it's being charged, i.e. that isn't attached to the bike frame, so I can see the volt level while sitting in a cafe or library with the battery charging next to me?

 

2) something cheap which will also automatically cut off at e.g. 41v so as not to max-charge?

 

3) lastly, when balancing the battery periodically (monthly?), for how many hours ought one to run the charger after the battery is fully charged?

 

Thanks

You can buy the cheapest wattmeter for about £8. You can cut your charger lead and join the two wires to each side, or you can solder the same charge socket as what's on your battery to one side of the wattmeter and the same jack that's on your charger to the other side, so that you can connect it in or out when charging. It'll show voltage, charge current and amp-hours gone in to yoyr battery.

 

Why do you want to cut-off at 41v? You'll compromise the balancing if you don't fully charge it to 42v. I guess you've been reading theoretical stuff on ES. Some of this theory is for lithium batteties that were made years ago. Neatly all modern cells have ingredient X in them that combats all the shortcomings. Nobody knows what ingredient X is. They keep it a secret, but each brand has its own one.

 

Your battery is deigned to be charged to 42v and discharged to 31v. If it would be better to do it a different way, those values would be set in the charger and BMS. Take my advice: Use your battery like everybody else and it'll last a long time. By the time it's knackered, there will be much better ones anyway, so it'll give you a chance to improve something.

 

Charging your battery to 41v is a bit like in the old days when it cost extra for velour covered seats in your car. The people that paid extra for them wanted to keep them nice, so they put polythene or pvc covers on them.

I think the main thing which got me was the chart on life cycle and charge level on the Cycle Satiator page. Yes probably was influenced a lot by reading ES.

 

But I appreciate what you're saying about it lasting ages anyway and the older tech. 500 cycles estimated for 42v charging is a lot - I just get a bit depressed at the speed with which my 14.5ah sags on a run. I think it doesn't like hills as much as I'd like, it's a 270+rpm SB CST HT.

 

Do you know how long it's right for the charger to be left on while on the battery, after the light has gone green, to balance the cells? And how frequently?

 

Going to put a 48v on it as second battery. Thought would try to mount 48v upside down under bottom tube, but I fear it might get jiggled around too much on the mount, for its health (?)

 

I intend to use steel straps slid through small slits drilled through the frame mount slider piece, tightened onto the bike bottom tube with some material inbetween to stop the slider from buckling under the pressure. Possibly drill in a single rivnut or small stud to the frame to stop the whole mounting sliding down the tube. Would have to lose a cog or two from the front derailleur but that's alright.

 

Thanks

You can leave the charger on as long as you like. I've left mine on for days sometimes. If your battery measures more than 41.5v after charging, it's pretty well balanced, so no point in leaving it on longer.

You can buy the cheapest wattmeter for about £8. You can cut your charger lead and join the two wires to each side, or you can solder the same charge socket as what's on your battery to one side of the wattmeter and the same jack that's on your charger to the other side, so that you can connect it in or out when charging. It'll show voltage, charge current and amp-hours gone in to yoyr battery.

 

Do these 5.5mm things look ok? They share the same connector as my charger/battery but I don't know if they're 36v/48v 2amp capable. Some on eBay say 1 amp only. The ad says that the wire is '22AWG 80 degree 300V cable'.

 

Thanks