You may have intended to be helpful, but if you do want to really help, you do need to supply full and accurate data, or leave the subject alone.
Many examples of incorrect or missleading data in your post, that the OP will certainly need to know.
You also did not know the OPs electrical knowledge level, so suggesting testing on 240 VAC was possibly dangerous....even an accidentally struck arc on an e-bike battery, could result in serious burns and or a fire..
I myself use a known accurate meter (of mine, I have several) to compare with an unknown one, simply because it easy, accurate enough for e-bike work, and can be done very quickly.
Regular checkups are also needed, here in Germany, once a year is recommended.
But the correct way is to go a laboratory and to use very accurate voltage, resistance and possibly capacitance sources, to test with!
But who has one of those in his back pocket at all times of the day and night. Not me!
The mulitmeter may need a new battery.
Certainly a possible cause, but with most modern (but not all) digital meters, they show when a battery is low in some manner.
Note:- Older analogue meters only need a battery for measuring resitances.
One can test it with any other dc or ac supply to see if the voltage reads accordingly.
AC & DC are two quite different ranges, with very different (internal to the meter!) circuit and possibly firmare requirements. I have seen and tested meters that on AC were relatively accurate, but on DC were way off. Or the other way round.....There is a lot of cheap cr*p out there, some of which is good enough for government work, but not all!
Therefore don't confuse the two voltage types.
For instance a kettle plug or similar should show up 240v.
Maybe you meant socket, certainly no voltage should appear on a plug! Actually, unless you are so close to the generating station for your mains, and can almost hold conversations with the workers without leaving your house, the voltage measured, even on an accurate meter, is unlikely to be 240VAC! Normally less! The farther away you are from the generator or a substation, the greater the losses in the cables. And as in all things, age also also plays a role with cabling under the street....
Also, e-bikes are always DC (except for the charger of course!) , so though its nice to have a working and accurate AC range(s), it won't help the OP much in reality....
If you use 18650 torch batteries, charge one and check the voltage.
Quite a good idea, provided you also know the voltage the charger goes up to when charging, that was used, and that the battery is in a good condition, and was recently charged and not used again!
Assuming that, he should then measure close to 4.2 volts DC on such a battery.
But I have noticed that some single 18650 cell chargers, they only charge to around 4.0 volts or less (if I bought one, I world return it and request my money back!), which will result in a possibly large error being shown on the much higher level of voltage battery, percentage wise.
It would be very coincidental that two chargers both put out 50v instead of 54.6v.
I myself have never had anything to do with 48 volt e-bike Batteries, and I wonder if there are possibly two "styles" of 48 volt batteries, (just guessing) with the number of "cell banks" inside being different?
For example, 12 banks of cells with each bank at 4.2 volts gives 50.4 volts DC, which would imply that the chargers are working correctly.
But 13 banks gives 54.6 Volts! Either might possibly be sold as a nominal 48 volt Battery.....But I was unable to ascertain if both "styles" are used on e-bikes. Does anyone here know for 100% certain?
I periodically check my charger outputs just as a matter of course.
As we all should, once the accuracy of the meter has been fully checked out of course.
I assume that you meant well with your post, but your level of knowledge precludes you doing this in a safe and accurate manner. Some extra study of the subject would appear to be needed IMHO, and if you are sufficiently interested, Libraries are generally a good source of knowledge for most things..
regards
Andy
