Cheap ammeter/voltmeter

[drsolly]

I got one of these:

LED Digital Volt Amp 2 in 1 Panel Meter Voltmeter Ammeter 100V 100A Blue/Red
http://www.ebay.co.uk/itm/161224223929?ssPageName=STRK:MEWNX:IT&_trksid=p3984.m1439.l2649
Costs about $5

It arrived a couple of days ago, and I was working with it today.

It will go up to 100 volts, and 100 amps; it needs a shunt. You can buy them (I ordered one), but it hasn't arrived yet, and, impatient to try out my new thing, I decided to make one. That turns out to be rather easy.

The shunt needs to be such that at 100amps, 75mv appears across the shunt. Ohms law says that means .75 milliohms. AWG 10 is 1 milliohms per foot, so I want 8 inches.

But how accurate is this? So I used my array of 50w, 12v lamps; when I power it from a 4S 20C, I get about 17 amps. I put that in series with a Hobbyking ammeter to determine the number of amps, then put my ammeter in series with that, and I tried various lengths of 10AWG until both meters read the same number (8 inches was about right). I wouldn't say that this is going to be tremendously accurate, but I'm just trying to keep an occasional eye on how much current the bike is using, I dont really mind if it's plus or minus an amp..

The meter wants a power supply of between 4.5 and 30v, so I decided not to take it from my main battery, which is slightly more than 30v (but would be a lot more if I decided to run it 12S, which I have tried, and which is pretty frisky. So I put a couple of AA batteries in a box with a little switch, and that sits with the main batteries. There's four wires that run from there to the front.

1) Common negative
2) Power for meter
3) +ve side of home-made shunt
4) +ve of battery

These four wire carry almost no current; I looked around my workshop for a four-conductor cable, and I'm using a couple of yards of ethernet (8 conductor) cable, and a plug-in connector so I can take the battery-and-shunt assembly off (that's built in to my saddlebag wattmeter/fuse/switch/main-battery-connector).

I'm just waiting now for a delivery of EC5 connectors so that I can solder up the final version of this, but it is working nicely.

 

[d8veh]

Thanks for that. It's so simple, I don't know why this has only just come to light. If you don't mind, I'll add a link to this frommy warttmeter thread on Pedelecs.

 

[dnmun]

i like that idea of using the conductor as the shunt. the super cheap $13 wattmeters use 3 little 3mR resistors stacked up to make 1 mohm equivalent so the meter is restricted to 100A because it gets so hot from the little resistors coping with the current.

if you follow this idea then you could remove the shunt from the cheap wattmeter and take the kelvin leads that are at each end of the shunt inside the watt meter, and run the kelvin lead wires out to the spot on the main conductor where you wanted to measure the current.

for me i put the wattmeter in the B- lead from the battery so it stays on after the BMS hits the LVC. so now instead i can just use a properly sized conductor and solder the kelvin leads to it, and then run the kelvin leads up to where i mount the wattmeter so the wattmeter can be out where it can be observed real time. i am gonna try this on my new batteries i am building.

the neat thing about the cheap wattmeters is that they have adjusting trimpots so i should be able to use another ammeter inline to accurately measure the current and then adjust the cheap wattmeter scale to match the true current reading. maybe use 12G wire so it could be relatively short and get the full 1mR for a shunt. temperature coefficient might make some difference though.

 

[Subsa]

I am using 2 of these - http://www.ebay.co.uk/itm/Three-wire-precision-DC-digital-voltmeter-head-digital-voltmeter-DC0V-32-/321337188179?pt=UK_BOI_Electrical_Test_Measurement_Equipment_ET&var=&hash=item4ad1305f53
no need aux power supply and very good accuracy

 

[drsolly]

The key idea, I think, was that I can make my own shunt at practically zero cost, and calibrate by using another ammeter. Yes, 12AWG would work, but don't try to put much more than 50 amps through it. A shunt is (in this case) anything with a resistance of 0.75 milliohms. At 12 AWG I'd guess around five inches.

What I liked, was not having to open up a device and desolder things, because I know I'm likely to mess things up, because I'm not very good at desoldering.

 

[drsolly]

I am using 2 of these - http://www.ebay.co.uk/itm/Three-wire-precision-DC-digital-voltmeter-head-digital-voltmeter-DC0V-32-/321337188179?pt=UK_BOI_Electrical_Test_Measurement_Equipment_ET&var=&hash=item4ad1305f53
no need aux power supply and very good accuracy

The meter you cited only goes up to 32V. You can also get meters that go up to 100v without needing an aux power supply, but what I like about the one I referenced, is that it will display current as well as voltage, and does 0-100v, and 0-100 amps, which is going to be enough for any bike I ever cobble together.

 

[dnmun]

i wanna have Ah recorded so i use wattmeters. that is why i like the cheap wattmeters except the linear voltage regulator is always burning up the little 2kR surface mount resistor when it is exposed to the full 60V of the 48V lifepo4 battery. i have gone in and hacked it to replace the 2k surface mount with a 3k 1/8 watt resistor that i can fit in but for a 24S lifepo4 pack i cannot use the full voltage since it is limited to 60V.

so i take the voltage tap off the 12S cell and just double the number to give me a good estimate of the full pack voltage.

but i really like this idea because it will allow me to remote the wattmeter away from the B- terminal of the battery where it is hard to see and put it up where i can put it together with the other watt meters for the other batteries i have in parallel along with the switch to turn off the BMS and a switch to turn off the power to the wattmeter which i deliver at the side terminals for 12V power.

so then i can reset the wattmeter by cycling power when the battery is fully charged and it will stay on even after the battery is fully discharged and cuts out for LVC so i know what is going on with the batteries.

this is for a ZENN car and i have three battery packs to supplement the 72V 100Ah SLA, 24S 50Ah ping pouches 24S 40Ah of headways, and 21S 39Ah of lipo. each on their own BMS and wattmeter and all charged to the same voltage, except when balancing. then i have to turn off the lifepo4 packs and allow the lipo to charge to 1.8 volts higher to force it to balance under the BMS.

so i can use a wire on the B- lead from the battery for each pack as the shunt and not have the hassle of making a remoted shunt. just run the 1/2 pack voltage to the wattmeter and the two kelvin leads from the battery, and they can all be tiny, 26AWG, so i like it, a lot.

 

[fechter]

I've used the wiring as the shunt before and it seems to work OK. One drawback is the copper resistance changes quite a bit with temperature, so if the shunt wire gets warm, the readings will be a little higher than actual. As long a the wire doesn't really get hot, the accuracy will stay reasonably good.

 

[dnmun]

19.19cm of 12 gauge wire is better than three R003 surface mount 2512 resistors. wish i had thought of this, 7.55" but i know these guys who made this romex were most probably cheating when they called it 12AWG.

knowing our culture they maybe were cheating 10% so i am gonna add 10% to the expected length. my cheating back. now if the new BMS would just get here.

 

[drsolly]

It doesn't matter if the wire isn't exactly on the gauge. I calibrated by runnung it in series with an already-calibrated ammeter, and adjusted the length of the wire shunt until they both read the same.

 

[dnmun]

yep, i know, just for estimating it.

i am actually gonna use doubled up 12AWG so i can extend it for 15-16" and it allows me to put the P- spot of the BMS close to the positive end of the battery now so the two power leads can come out close to each other.