DVM accuracy.

[Benjahmin]

So, I've just received my re-celled battery back from Insat. (8 days from pick up to delivery - wonderful). Given it it's first full charge and thought this would be an opprtunity to check my dvm.
This battery, and an older soft shell one, is Boston Power cells.
I use a dvm to check battery voltages before connecting them in parallel. So the absolute voltage is not as important as ascertaining that they are within less tha 0.5v of each other. However it is useful to measure full charge voltage as an indicator of health.
Assuming that the new battery charges to a full 42v, it's annoying that one of my meters reads 41.5v and the other 41.6v. They also give these readings on the older battery (which I had started to assume was indicating loss of balance or failure to reach full charge). So it would seem that both meters are approx 0.5v adrift. They are low end, uncalibrated meters. Does anyone know how/if they can be adjusted?
My only calibrated meter is a 17th edition multi test meter that doesn't have a DC range, so can't compare.

 

[danielrlee]

Open them up and have a look for a voltage adjustment potentiometer.

 

[vfr400]

Before adjusting anything, you need to check their accuracy. Don't forget that some BMSs drain down the cells to 4.15. Did you check the voltage coming out of your charger?

 

[RossG]

I'd say both of your multimeters are within manufacturers tolerance limits anyway. You could mess around with adjusting them to read the same but then they might both be off.
If you placed one in a fridge for an hour or so and the other in a warm airing cupboard then took readings I'd guess both would be way out, or there again both might be spot on. The kind of test equip you can buy over the counter good though it is can't be compared to lab gear and in reality a tenth of a volt is negligible.

 

[danielrlee]

If you don't own an accurate power supply or multimeter that you can trust, a voltage reference board such as this can be invaluable:

https://www.ebay.co.uk/itm/AD584-High-Precision-Voltage-Reference-Module-4-Channel-2-5V-7-5V-5V-10V/141888417230?hash=item2109356dce:g:J20AAOSwXshWqF~m#rwid

 

[RossG]

The problem with those reference boards using AD584 devices is that in many cases they use fake components, also that particular setup has no built-in temperature compensation.
It's ok I suppose but I'd go for a different chip if I wanted a reference source.

 

[soundwave]

i just use a fully charged 18650 cell @4.2v

 

[Nealh]

I use a few sources I have and they all read within 0.01v, so accept all are fairly accurate when used on a new or newish 18650 fully charged.

 

[Benjahmin]

Yeh, must check the charger.
I just wasn't sure if what was being measured was actually 41.5, or if it is actually 42 and the meter was reading low.

 

[vfr400]

That's right, if you get 42v on the charger and 41.5v on the battery, your meter is probably accurate. If you get 41.5v on both, either your charger is set low or your meter is reading low, but you won't know which.

 

[RossG]

The reality is a top quality accurate lab test meter could cost 5 grand or more (really) and that could buy you a very good e-bike.
Again I have to emphasize you have to allow for product variation, no two items will perform exactly the same.
I have three multimeters and one of those is very high end, that said it almost certainly would give me different readings to that of laboratory equipment. Yer pays yer money as they say, and good test gear cost big bucks.

 

[danielrlee]

The problem with those reference boards using AD584 devices is that in many cases they use fake components, also that particular setup has no built-in temperature compensation.
It's ok I suppose but I'd go for a different chip if I wanted a reference source.

I've used a couple of these and they've both output reference voltages within 0.002V of their advertised voltage, confirmed with a calibrated meter and another trusted voltage reference. Out of interest, what would you use as a preferred voltage reference source?

 

[Sturmey]

Hi . I have 2 different (not very expensive) digital multimeters with handbooks and the accuracy spec on the voltage scale in one case is quoted as ((+/- 0.5% ) + (+/- 2 digits)) and the other is quoted as ((+/- 0.5% ) + (+/- 1 digits)). This could amount to an inaccuracy of 0.3 - 0.4 volt when reading 42 volts with the normal 200v scale. (about 1%),

 

[RossG]

If I were still in the electronics business I'd buy something like a Keysight B2962A bench power supply and get it spot on, but for home/hobbyist use sure the simple ref. source is perfectly ok. You have to bear in mind these Chinese sellers often sell test, ref and control equipment built using fake components, it's fairly well known in the world of electronics.
Of course that doesn't mean everything produced there is fake, but there's no doubt some of it is and if you get stung you can't really go to trading standards and start screeching.
I tend to buy electronic items of this kind from uk suppliers, ok so it still comes from China but the seller is based here and subject to our laws so you have something to fall back on.

 

[Benjahmin]

I've measured the charger voltage now, it's also coming up as 41.5v. So probably supporting the view that my dvm is reading 0.5v low. Also showing that batteries are charging to the full charger voltage.
May dismantle the dvm and see if there's a pot to adjust.