Checking Multimeter Calibration at Home
- Thread starter WheezyRider
- Start date
why do you need a 0.01% accuracy for your bike? You only need a relative, not absolute, accuracy for most application. If you measure for example the output voltage of your battery in an hour or two, the error bias is the same, you compare the reading with previous readings, the difference does not depend on the absolute accuracy. The margin of error should be within +/- least significant digit on voltage or frequency measurements, +/- 5 for amps.
With a true RMS, intergrating multitester, you already remove the bulk of the causes for errors like supply voltage, ambient temperature in the first place.
No, you do need an absolute, not relative value. You have only a 200 mV window between not balancing (less than 41.8 V) and cell overcharging (over 42 V). My feeling is that you want an accuracy of at least 40 mV to be confident you are in the right range. So then if your DVM measures a reading of 42 V, at the low end it could be 41.96 V - ie balancing ok but not over charging, or at the high end 42.04 V so a bit over, but enough probably to be taken up by shunts and other losses in the BMS (eg MOSFET Rds on resistances etc).
the balancing function does not have to be triggered at precisely 41.8V. A lot of battery control ICs check the voltages of individual banks all the time. When any bank shows a difference in voltage with its adjacent bank more than a set value (eg 35mV), the balancing function is triggered until it is brought to less than 15mV. That's why when a battery is out of balance, its days are numbered.
There are some fancy ones around, some where you can even customise your own voltage for balancing with an app etc, but most don't. Look at the specs of your average BMS. Balance starts at 4.18 V. Why don't you pull a pack apart, deliberately discharge one bank slightly, charge the pack to less than 4.18 V/cell and see if it balances? Of course, you will need a calibrated DVM to see that you are actually charging at less than 41.8 v
Plus, if you have a DVM which is one volt out on the 200V range and you measure 42 V from your charger, your battery could be getting 43 V, reducing it's life and increasing the risk of fires etc - and you would be none the wiser. So you do need an absolute not a relative measurement.
Measure your charger's output - mine's a little high according to my even cheaper non-calibrated XL830L DVM, and I read somewhere this is by design to compensate for losses along the way to the battery. Voltage also appears to fluctuate slightly. I've started checking charger voltage before every charge, in case of increase (@Nealh reported this happening with one of his chargers), because my charger aint young - bought end of December 2019.
https://www.ebay.co.uk/itm/386024965215
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I did that sort of thing 7-8 years ago. Back in those days, welding sometimes failed. Really there is no need to worry about modern batteries made with Panasonic or Samsung cells. You can leave the battery in storage the whole year and see nothing special. The packs are not serviceable nowadays for at least 5-6 years now, except for changing the BMS. If a welding spot fails, you have to bin the pack. I still have a box full of BMSes that we don't use. The sole weak spot is the output port. Customers damage the leaf springs or the plastic cracks around the charging port.
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But how do you know that it's high if the DVM isn't calibrated? The charger may be fine and your meter could be out.
The MAX6126B50+ is out of stock at the moment, also out of stock at Mouser and Digikey, not available on ebay. I'll check rs-online.com again in a week or two.
If your wanting to read a voltage of 42V accuratly, how do you calibrate for that voltage, a 5522A at a rental of circa £1500 per week is a tad expensive.
https://eu.flukecal.com/blog/how-calibrate-digital-multimeter
https://eu.flukecal.com/blog/how-calibrate-digital-multimeter
Replace the DVM's potentiometer with a digital resistor with 128 positions and I²C?
Thanks, I may well buy that to avoid a useful but very fiddly looking soldering job.
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You have to think about the economics of spending a load of money on chargers and test equipment to get a theoretical few days extra battery life, which has never been proven it practice.
These also look interesting:
https://voltagestandard.com/01%25-voltage-references
https://voltagestandard.com/001%25-10v-reference
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True but it's also an investment in safety necessitated by the BMS's balancing threshold being unknown or too high. If you can build a battery with an external balancing port life gets a lot easier.
What do they do in China? Are there unbranded charging stations, do they use generic aftermarket chargers safely? They've way more experience with lithium batteries. @saneagle @Woosh @flecc (and @ everyone of course)
it's only an issue if you need to charge at well under 4.2V per cell. Let's say you use a 42V charger with the output specified as 42V +/-0.1V. The amount of extra charge charge between 41.9V and 42V is very small. Saneagle used his programmable PSU to illustrate that point.
I've been on this forum for nearly 13 years. Nobody's battery ever caught fire because their BMS was a bit off or because their charger had a bit of ripple. It's great that people are trying to get the details on what's going on, but you have to be careful about drawing conclusions from single factors. As an example, somebody provided evidence that 80% charging extends battery life, so people go out and spend vast amounts of cash on special chargers and monitoring equipment and probably end up ruining their batteries by it. So far, nobody has proven that an ebike battery lasts longer by 80% charging. maybe many tried, but when their battery failed prematurely, they didn't post about it because they were too embarassed.
80%: what would be proof, if not evidence? Here they report test results from BMZ, and my adjustable supply cost £35 much the same as a charger.
pushevs.com
100% agree that it's great that people are trying to get the details on what's going on, that could be linked to why we haven't seen battery fires on here.
Battery charging: Full versus Partial
Batteries are part of our lives. They are present in everyday devices such as electric cars, smartphones, tablets or laptops. For this reason it’s important to know how to treat them right. We already know that TMS (Thermal Management Systems) are important to keep batteries at recommended...
pushevs.com
100% agree that it's great that people are trying to get the details on what's going on, that could be linked to why we haven't seen battery fires on here.