One of the most important tools in an e-biker's toolbox is a multimeter (aka Digital Volt Meter or DVM). It is sometimes needed to be used to measure voltages with a high degree of accuracy, for example looking at the output of a charger to tell if it is set up to the correct voltage or not. Unfortunately, unless you spend a lot of money, the chances are you will have a low cost Chinese made device with an accuracy at best of around +/- 1% and sometimes this can be as bad as +/- 2%. From experience, I've found that a lot of DVMs are not too bad on the 20 V scale, but on the 200 V scale things can be quite poor.
1% doesn't sound like much, so what's the big deal? Well +/- 1% of 42V can be a difference of as much as 0.42 V higher or lower than the indicated voltage. So if your meter gives you a reading of 42 V from your charger, the actual value might be as low as 41.58 V or as high as 42.42 V. As most battery BMS modules are set to balance at 4.18 V/cell and above, then at the bottom end, 41.5 V, the pack would never balance. At the top end of the error range it would be over charging the cells and potentially causing damage. Reduced cycle life is likely. If the error is as much as 2%, the situation can be far worse. Some meters I've had have been out by more than a volt when trying to measure 42 V.
I am lucky in that I have access to meters with a high level of accuracy, so I can check things and make sure my chargers are set up properly and I can check the accuracy of my cheaper meters. I am also interested in charging to 4.1 V/cell to increase pack cycle life. This requires voltage monitoring with a reasonably accurate DVM, otherwise you are lost.
So how can the average person with a little technical knowledge check that their meter is giving them a sensible reading?
There are IC chips you can buy that are used as voltage references for different kinds of circuits. You can get them in different voltages and also levels of accuracy. Typically, the most accurate versions seem to be designed to work at 5V. The idea is to get one of these ICs, wire it up to a power supply and connect it to your DVM to see how accurate it is.
I chose the Analogue Devices IC - MAX6126B50+. It has an accuracy of at least 0.02%:
I got a couple from RS:
It's a fairly simple circuit, all it needs are a few capacitors for stability and filtering:
For super accurate measurements, you can even set up a 4 wire measurement to compensate for lead resistance in your DVM connecting wires:
But I thought I'd just start with the basic circuit, with GND / GNDS and OUTF /OUTS connected directly together as in the top circuit
The ICs arrived and then I opened the package. I knew they would be small, normally used in surface mount PCBs, but I was not expecting this small!
I mounted the chip on a piece of acrylic using super glue. I soldered in the caps on the "top" side of the board, and used the clipped off legs of the caps to connect the IC leads to the Vero board. This was fine for the GND / GNDS and OUTF /OUTS connections as it was soldering two sets of pins together. The really tricky part was soldering to Vin and NR. I pulled a piece of multi strand wire apart, so I could source wire thin enough and soldered these single strands to the right points:
It was not easy and as you can see from the solder, it took a few attempts - plus I got Vin and NR the wrong way round the first time...always check your work!
The top side:
I used 1 uF caps for stability and a 10 uF cap on the Vin side.
I connected it to a bench top PSU running at 7.5 V , but you could use anything that gives 5.2 V up to about 12 V.
Then the moment of truth, connecting it to a meter on the 200 V range:
What a relief, 5 V followed by 3 zeros. On the 5 V range, I got 4.9994 V. So that could be the limit of the calibration on my old Fluke, or it could be the limit of the accuracy of the voltage reference.
So what about a cheap multimeter?
On the 20 V range:
And on the 200 V range:
So 0.1 V out on the 200 V range, compared to an accurate DVM. 5 V is at the bottom end of the 200 V range. Ideally it would be nice to have a voltage reference at a higher voltage, as an error of 0.1 V may not be a fixed systematic error across the whole 200 V range. However, I have measured various sources with this meter and the Fluke and it does seem to be consistently 0.1 V out. So this DVM is not too bad and I know the error to expect and I can compensate for that. Other meters I've checked have been far, far worse.
If I did this again, I might look for a 10 V reference chip with a similar level of accuracy. I would certainly try to find one with a standard dual in line package to make soldering easier!
Using this simple circuit, you can get an idea as to the accuracy of your meter, without spending a lot of money having it professionally tested.
1% doesn't sound like much, so what's the big deal? Well +/- 1% of 42V can be a difference of as much as 0.42 V higher or lower than the indicated voltage. So if your meter gives you a reading of 42 V from your charger, the actual value might be as low as 41.58 V or as high as 42.42 V. As most battery BMS modules are set to balance at 4.18 V/cell and above, then at the bottom end, 41.5 V, the pack would never balance. At the top end of the error range it would be over charging the cells and potentially causing damage. Reduced cycle life is likely. If the error is as much as 2%, the situation can be far worse. Some meters I've had have been out by more than a volt when trying to measure 42 V.
I am lucky in that I have access to meters with a high level of accuracy, so I can check things and make sure my chargers are set up properly and I can check the accuracy of my cheaper meters. I am also interested in charging to 4.1 V/cell to increase pack cycle life. This requires voltage monitoring with a reasonably accurate DVM, otherwise you are lost.
So how can the average person with a little technical knowledge check that their meter is giving them a sensible reading?
There are IC chips you can buy that are used as voltage references for different kinds of circuits. You can get them in different voltages and also levels of accuracy. Typically, the most accurate versions seem to be designed to work at 5V. The idea is to get one of these ICs, wire it up to a power supply and connect it to your DVM to see how accurate it is.
I chose the Analogue Devices IC - MAX6126B50+. It has an accuracy of at least 0.02%:
I got a couple from RS:
It's a fairly simple circuit, all it needs are a few capacitors for stability and filtering:
For super accurate measurements, you can even set up a 4 wire measurement to compensate for lead resistance in your DVM connecting wires:
But I thought I'd just start with the basic circuit, with GND / GNDS and OUTF /OUTS connected directly together as in the top circuit
The ICs arrived and then I opened the package. I knew they would be small, normally used in surface mount PCBs, but I was not expecting this small!
I mounted the chip on a piece of acrylic using super glue. I soldered in the caps on the "top" side of the board, and used the clipped off legs of the caps to connect the IC leads to the Vero board. This was fine for the GND / GNDS and OUTF /OUTS connections as it was soldering two sets of pins together. The really tricky part was soldering to Vin and NR. I pulled a piece of multi strand wire apart, so I could source wire thin enough and soldered these single strands to the right points:
It was not easy and as you can see from the solder, it took a few attempts - plus I got Vin and NR the wrong way round the first time...always check your work!
The top side:
I used 1 uF caps for stability and a 10 uF cap on the Vin side.
I connected it to a bench top PSU running at 7.5 V , but you could use anything that gives 5.2 V up to about 12 V.
Then the moment of truth, connecting it to a meter on the 200 V range:
What a relief, 5 V followed by 3 zeros. On the 5 V range, I got 4.9994 V. So that could be the limit of the calibration on my old Fluke, or it could be the limit of the accuracy of the voltage reference.
So what about a cheap multimeter?
On the 20 V range:
And on the 200 V range:
So 0.1 V out on the 200 V range, compared to an accurate DVM. 5 V is at the bottom end of the 200 V range. Ideally it would be nice to have a voltage reference at a higher voltage, as an error of 0.1 V may not be a fixed systematic error across the whole 200 V range. However, I have measured various sources with this meter and the Fluke and it does seem to be consistently 0.1 V out. So this DVM is not too bad and I know the error to expect and I can compensate for that. Other meters I've checked have been far, far worse.
If I did this again, I might look for a 10 V reference chip with a similar level of accuracy. I would certainly try to find one with a standard dual in line package to make soldering easier!
Using this simple circuit, you can get an idea as to the accuracy of your meter, without spending a lot of money having it professionally tested.
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