difference in charger voltage readings

I recently did a test on some batteries and posted a thread here. I was using an Opus 3400 and a Lacrosse 700. The voltage on batteries read differently from one to the other. Checking 10 AAA nigh batteries, the Opus consistently read 0.04v lower than the Lacrosse. I plan on double checking the voltage with a DMM when I get home to see which one is out of spec, but I think it is the Opus. Any ideas? Is this a common problem?

I did some testing on my Opus 3100. It seems that there are two separate voltage measurements. One that is used by the charger, when charging, but not displayed. And one that is displayed to you.There may be a difference between these two measurements. Actually they are more or less bound to be a little different. Two measurments are never exactly identical, there always is some error, even if it is smaller than what can be displayed.

So the voltage shown may very well be off a little, but that does not have to influence the function. Or the voltage displayed may be spot on, but the function is bad. Measure with a DMM to better see the actual voltage applied to the batteries by the charger. Make sure you use a calibrated DMM...

Also I assume that contact resistance may be the cause of some difference. In other words, the strength of the springs and the shape and condition of the contacts may give a difference in the measurements.

If I had to choose, I would choose a charger that charges with a slightly lower voltage than the optimum, to save the batteries. For instance li-ion to 4.19 volts instead of 4.20 volts.

Ok, got home and tested about 20 fully charged batteries between the two chargers and 2 DMM’s. Both DMM’s and the Lacrosse read the same voltage. The Opus was consistently 0.04V less on every single battery. The batteries weren’t charging as they read FULL. The resistance between the contacts on the Opus was greater than the Lacrosse. I don’t know if that has anything to do with it.

If the Opus uses one voltage to charge and displays another, shouldn’t they be the same when reading full? On my battery tests I did, they both discharged the batteries down to the same listed voltage, but the Opus charged them back up to a lower voltage than the Lacrosse. That means that it is if the Lacrosse charged to say 1.42v and the Opus to 1.38, that’s a 2.8% difference in voltage. Are the mAh 2.8% off too?

Stupid question: do you just put the DMM leads directly to the ends of the battery? I always thought that would short the battery

No, it won’t short the battery. All it does is complete a circuit. Your flashlight does the same thing. Only difference is you have a driver/led between the leads instead of a multimeter. That and the led uses some of the power to generate heat and light.

My Opus 3100 Ver. 2.1 ends charge at 4.195v according to my DMM a 0.005v difference is fine with me as long as it doesn’t overcharge the cells

I have the Opus 3100 V2.1 and have a difference of 0,01V between my Opus and my Multimeter (Metrel MD9015).

Like n10sivern wrote, you doesn't short a battery, if you have appliance (for example a LED, DMM or something else) in the circuit.

That would be nice, and for some chargers that is most likely the case. But it may be that the display says 4.16 V while the actual voltage over the battery is 4.20 V. Or some other values.

I suggest that you check this using a good DMM. Don't trust what the charger says, without testing. Check the voltage over the battery while it is charging, and compare with the display. If you think the contact resistance is too high, then you can attempt to correct that using a little soldering tin and a sandpaper to the contacts.

But if you check using a DMM, then it comes down to how much you trust your DMM...

There is another issue with the OPUS v2.1. I believe they are purposely set to charge 0.05V too low as a margin of safety. The designer pointed us to J1 on the board and a way to increase charging voltage by that same 0.05V
post #325 in this thread

Typical digital multimeters have an input resistance of 1 or 10 MOhm, which, if I’m doing my math right, is going to result in a current of 3.7 or 0.37 microAmps when testing a 3.7v battery.

Lemme get this straight, you are worried about a 0.04v difference in measuring NiMh?

  1. Unloaded voltage measurement in NiMh is semi-useless. Unless you know HOW each device is measuring you don’t really know for sure what it should be reading.
  2. You leave a freshly charged NiMh alone for 24 hours and it will probably lose 0.5-1.0v.
  3. That amount is less than the probable error in measuring NiMh with devices that of caliber and price range. They may measure in 0.0x but they aren’t accurate to that level.
  4. Functionally it doesn’t matter at all. You put it in a light, turn it on, blink, and 0.04 volts is gone.

I am not certain but I believe that was reported to Opus and that was corrected afterwards?
Maybe the OP got one of those charges?
Perhaps he should open it and look at the J1 solder and see if its open or closed?

Flydiver I think you are missing an important detail in the op message. He says that the Lacrosse readings match the DMM.
So the Opus is not charging the batteries to 1.50V and in return reports wrong capacities which for an analyser is not acceptable imo.

OP how are the voltage readings while discharging? Are they also off by 0.04V? If it is and you say the charger stops the discharge at 0.9V then it means that the Opus is actually stopping the discharge below 0.9V. Is this not a good thing for a NIMH battery?

Don’t cross up NiMh and LiOn posts, please. It’s happening all over this thread and they aren’t the same animal.
Does anyone know for SURE the 0.05v fix is for NiMh? Somehow I doubt it. If it is I’d take some convincing to think it does any actual good.

HammerJoe - I have not missed his point. My point is dithering about 0.04v in charging NiMh is not a very useful pursuit.

The OP says he did this:
[On my battery tests I did, they both discharged the batteries down to the same listed voltage, but the Opus charged them back up to a lower voltage than the Lacrosse.]

The OP does a discharge test, gives us voltage, which is not a terribly useful value for NiMh, but does NOT give us the discharged capacity? Capacity is where the real action is (and output but that’s not what is being discussed).
IMO he should cycle a battery in the LaCrosse, then cycle the same one in the same manner in the Opus. Trouble there is with NiMh you are likely to do no better than 2-5% with the same battery in the same charger on multiple successive cycles. NONE of my chargers do and I have some decent ones, including an Opus.
NONE of my chargers push NiMh to 1.50v either, and I’d be pissed if they did. THAT is hard on the battery and does not improve the functional capacity a whit unless you use all the capacity immediately. The excess is just warm excited electrons, not genuine capacity.

With LiOn, being a bit more fussy is reasonable. With NiMh>>if it charges it up, battery gets a little warm, has a voltage a bit over 1.4x when fresh, and works in your devices, then the charger has done it’s job.
If the battery gets HOT, or the charger consistently terminates early with a noticeable undercharge, then you have something to gripe about.

is .04 (four hundredths) of a volt really that much off?

I could understand if it was 4 tenths off…then that would be a cause for concern

OK, I didn’t get a chance to do all of the measurements I wanted at work tonight because it was busy. I put 2 AAA eneloops and 2 AAA Keyko batteries in each charger. I set both to the charge at 500ma and ran another discharge test. Unfortunately I didn’t get a chance to get back to the charger until it was done, so these are the measurements at the end. Both were tested on 2 DMM’s (a Sperry DM-4100A and a Mastech MS8268). The readings on the DMM’s were less than a hundredth of each other on every battery check. The measurements were averaged for the voltage shown below.

Eneloop #1 863mah at 1.47v, DMM’s measured at 1.480
Eneloop #2 858mah at 1.46v, DMM’s measured at 1.471
Keyko #1 850mah at 1.44v, DMM’s measured at 1.449
Keyko #2 808mah at 1.44v, DMM’s measured at 1.443

Eneloop #3 805mah at 1.47v, DMM’s measured at 1.497
Eneloop #4 792mah at 1.47v, DMM’s measured at 1.495
Keyko #3 785mah at 1.42v, DMM’s measured at 1.451
Keyko #4 756mah at 1.43v, DMM’s measured at 1.467

I need to check the Opus on a Lithium vs the DMM’s. That’ll happen this week.

Yes, I’m questioning the validity of the readings on the Opus and I may be making a mountain out of a mole hill. In my mind, if one value is wrong, then everything else could potentially be wrong. It appears to be actually charging them to the right level, but it is presenting a different measurement. I’ll have to check with Lithiums, but with the NiMH there is a 2-4% difference in readings between the charger and the DMM. Does this difference in readings present itself with lithiums as well? If it shows my Lithiums at 4.20v is it actually 2-4% higher which would be 4.28-4.36v? To be honest, I haven’t check the Opus that closely when charging lithiums. It charges to a displayed 4.17-4.2v. This was my first time charging NiMh on the Opus and all this may just be limited to the NiMh. That’s why I asked here….to see if somebody said yeah it has an issue with NiMh but the Li-ion is spot on or nope never had that problem.

Regarding nimh. Another big difference between chargers is how they detect that the battery is full. Typically this is not just by max voltage, but instead by monitoring the voltage and detecting a slight "bump" in the voltage plot over time. -dV/dt. The voltage fall a little as the chemistry change in the full battery. Then there is a question of how fast or well this bump is detected. That may depend on the current used (if the current is too low, there may not be a bump at all) and the charge of the battery when it is inserted in the charger. Most chargers have at least a few of three possible fail saves if no bump is detected: Max voltage. time and temperature rise.

So the end voltage may vary quite a bit when charging nimh, depending on how full charge is detected or if a fail save are triggered.

For li-ion there is no such bump. There just max voltage is used. And the current is typically reduced when voltage is very low or close to max.


You keep doing these useless voltage measurements and NOT giving capacity. I don’t care if you are measuring it with a precision Fluke, it simply is not a big deal. There is no useful difference in the functional voltage of ANY of those cells. They are ALL charged just fine. :party:

AND……0.04v DOESN’T MATTER A DAMN, for NiMh. For that matter it doesn’t matter THAT much for LiOn. 0.0Xv is BEYOND the accuracy specifications for this level of device, for ALL the chargers you are using.
Measuring NiMh to 0.00Xv is positively fruitless. Go measure those same cells with the DVM today. ALL of them will have LOST more than 0.04v.

I have no trouble at all charging any NiMh or LiOn on my OPUS.

Hate to shout but you seem to be slow in figuring this out. :wink: :evil:

Yell all you want to. I understand the difference between mah and voltage. My concern about the voltage is over primarily with charging or discharging the batteries. If it isn’t reading the voltage correctly then it could be over charging or discharging which can damage the cell. If you read my last post, I need to check it with my Li-ion batteries. It is reading the voltage 2-4% lower than DMM on NiMh. It’s charging them to a good voltage but showing less. If it is doing the same thing to the Li-ions, then it is overcharging them because it shows 4.2v on many of my lithiums when it is done charging. If it is actually 4% higher then that would be to around 4.36v which is not good for the cell.

As far as your comments about me not mentioning the battery capacity

mAh (milliampere-hours) - a rating of battery capacity in thousandths of an ampere in an hour.

The numbers I posted clearly state the mah from the discharge test, in addition to the termination charge voltage and the measured voltage on my DMM. For that matter, the charger should be calculating the mah by discharging the cell at a steady rate to a number deemed the end of the usable voltage (say 0.9v). If it is not reading the voltage correctly then how can it calculate the mah correctly?

NiMh is like a sponge. How much ‘electricity’ can it soak up. It’s a pretty sloppy charge method, though delta V termination is a pretty fussy job. It’s actually harder than LiOn termination.
LiOn is like a closed vessel. You need to be more exact about it to not break it.

Since the 2 chemistries use totally different charge methods I doubt you’ll find the same issue with that circuit. But that remains to be seen. FWIW all the problems I’ve read about on the Opus are slight undercharge (an advantage as far as I’m concerned), not over charge.