| Products > Test Equipment |
| Battery life of multimeters |
| (1/3) > >> |
| theHWcave:
Not sure if there is already something on this. I did search but the forum is huge. If this is an old hat, apologies. This was prompted by a couple of recent reviews I did on KAIWEETS multimeters and also by Joe's current review of the Keysight U1282a in his multimeter robustness testing series https://www.eevblog.com/forum/testgear/hear-kitty-kitty-kitty-nope-not-that-kind-of-cat/ in particular about the automated way of testing battery life by reducing voltage until a drop in current shows that the meter has shutdown. My points are: * One should not assume that a multimeter only draws DC current. I found a couple that have DC-2-DC converters inside without much input filtering that superimpose a significant AC current on top of the DC. * If you do check for an AC current component, use a meter with high bandwidth because the switching frequency can easily exceed the 1 kHz AC limit of many standard multimeters. * When the “low-bat” indicator comes on, the batteries should be considered finished. Relying on the sudden current drop when a meter finally shuts down doesn’t mean it can still produce accurate measurements in the range from when low-bat comes on to when it shuts down. For low battery tests, the meter should be set to measure some known constant value and the test must check that the measurement is still accurate. BTW, I do have a lot of data on KAIWEETS multimeters because they keep sending me meters for review on my YouTube channel. That is very brave from them because I do check them quite thoroughly and exposed weaknesses in each of them. But they say they use it to improve their products which is a good thing. To show what I mean: Case1: The KAIWEETS ST120 draws nearly 3 times more AC current than DC from its battery The ST120 battery current tested with a Brymen 869s for example is around 8 mA DC from 2 AA cells. Moderately high for such a meter but understandable because it has a permanently on backlight. But there is an additional AC current of more than 20 mA on top. Together that is more than 22mA, eating batteries a lot quicker than what a simple DC measurement lead you to expect. That current is at a frequency of about 2.3 kHz which means that many cheaper multimeters like the OWON XDM1041 can’t measure it properly. In the picture, the OWON shows only about 14mA AC Case2: KAIWEETS KM601 loses accuracy when run at below low-bat voltages That meter shows the other problem. It used 3 AAA so its nominal voltage is 4.5V and low bat comes on at about 3.56V. In this test it displays a constant 5V voltage. The first picture shows it just slightly below low-bat with the correct (for this class of meter) readout. The second picture shows it at 2.85V battery voltage just before it dies. The readout is has dropped by 10mV. For this meter the low-bat indicator really means you should stop using it and replace the batteries. |
| Brumby:
The low battery accuracy issue is something I have understood for a long time - but I haven't gone into the nitty-gritty of meter power supply. I know a DC-DC converter can be used to power the backlight but it is news to me if it is being used to power the metering circuitry. Thank you for an interesting detail! Today I learned something. :-+ |
| theHWcave:
I promised @joeqsmith to test the Fluke 101 and Brymen BM869s battery current using my method On the Fluke101 There is no AC component just a nearly steady and amazingly low 0.74mA DC (with the Fluke 101 DC volts mode) and when low-bat came on at 2.23V the meter continued to display 5V DC correctly until it shut down at 2V On the BM869s There is no AC component, but when testing the behaviour below low-bat, the BM869s behaved somewhat unexpectedly. I recorded the BM869s main (DC V) and secondary (AC V) using its USB interface. For this test the BM869s was displaying 5V which it shows more accurately as 5.0009 in DC V mode but in dual DC and AC V mode which I used here, the main display showed 5.0023V DC and the secondary display showed 0 V AC. In the graph, BM869s main display is the blue trace and the secondary the green one. The battery current is the red trace. The low-bat indicator came on at about 6.43V. The meter kept going showing the same voltages on its main and secondary displays until 5.78V when “1nerr” was displayed with loud and continuous beeping. This beeping caused the current to increase. As the battery voltage continued to drop the “1nerr” display caused a gap on the blue and green trace. At 4.76V battery voltage the beeping and “1nerr” display stopped and the meter showed a readout but only 4.8174V DC. As the voltage dropped further the V DC on the main display started to increase and so did the AC voltage on the secondary display. With falling battery voltage, the readouts on the main display went crazy shown 831V at one time and -103 at another. The secondary AC readout had lots of OL but at one time was reading close to 5000V! At 2.29V battery voltage the IR USB interface stopped working. Unfortunately that blocked my logging program and so the rest of the test was not recorded. I am not sure when the automated test terminated the BM869s test in this case. Joe? |
| joeqsmith:
Hello Mr Cave. To calculate the nominal battery life, I use the ACV mode with nothing attached to the meter. I use the ACV as I felt it would provide a more realistic idea of what to expect. To calculate the minimum battery life, I use what ever is the highest current when selecting the various functions (less beeper), again without anything attached to the meter. To calculate the cuttoff, I use the current that I measured with the meter in ACV mode and lower the voltage until I reach half that value. In some cases, depending on the meter's power supply design we may not see that knee in the current. The BM869s is like that and will ride the battery till it is dead flat. The low battery indicator may be on and the meter may be throwing up bad data but the user gets to decide if they can continue. Because I do not use the cutoff voltage and apply no other signals to the meter when determining the battery life, I can get some idea how the meters all compare. The data for the meters I have tested is available on-line. I believe all of the meter listed with their battery life are still functional. If you feel one of these meters differs from the measurements I made, let me know. I can certainly repeat the test using my source meter along with another method and we can compare the results. I dare say, I pretty much open to anyone repeating any of the tests I have ran and comparing results. If we are not getting roughly the same results, I think we need to understand why. *** Should add that the Backlight current was with the meters set to their ACV modes as well, nothing attached and the backlight active. |
| Kleinstein:
A DCDC converter may draw some pulsed current and this can be decomposed to an DC part and AC part. However the AC part is not loading the battery very much. So the battery life would still mainly be determined by the DC part ( = average current drawn). The pulsed current however makes it more sensitive to the batteries output resistance. With modern alkaline cells and the still relatively moderate current this should not be a big deal. It would be with old style dry cells, but these are useless in essentially any application, except those who really need at least 1.5 V per cell. Another point with SMPS is that the current drawn likely will go up when the voltage goes down. So the average current drawn may be some 10% higher than measured with full battereis. For the meters with a higher current consumption it could interesting if the low bat warning come before 1.2 V per cell. For alkaline cells there is not much left between 1.25 V and 1.15 V, but for rechargible cells this can make a big difference. For a low power meter recharbible cells are less relevant. |
| Navigation |
| Message Index |
| Next page |