Miboxer C8 leaving Ni-MH batteries half empty

So i received a C8 to complement my trusty OPUS BT-C3100 with extra charging bays. I have tried the charger with white IKEA LADDA 2450 mAh AA batteries (same as eneloop pro), with automatic mode the charger selects 700mA or 800mA and it charges something around 1500mAh into an empty (around 1 volt) battery while opus would always put something around the nominal capacity into the battery at 1A current. By manually selecting 1A current with C8 it goes slightly closer to 2000mAh but still nowhere near nominal capacity. Is the charger simply bad, or did i receive a defective unit? Li-ion charging works fine, although the automatic mode usually selects lower current than necessary (under 1A for 3000+ mAh 18650’s that are in decent condition). Does anyone here have similar experiences?

What’s the voltage of the cells as they come off the charger?

You can’t tell capacity accurately just by what the charger puts into the cells unless you attempt to do it exactly (as best as possible) as the factory datasheets lay it out. This is generally a slow controlled discharge to the stated voltage, followed by a brief rest period, then a controlled charge at whatever rate specified using the same charging method to the termination and top off points. So if you yourself pick 1.0V and then charge at a different rate, you’re just not going to know true capacity…needs to be the right discharge/charge procedure to be able to come close. With some chargers you can’t select the same current rates as what they may have used in the factory datasheet, so just try to come as close as you can. Or, don’t worry about it and just use normally…you can pick your charge rates based on speed or based on trying to maximize cycle/capacity over the life of the cells.

Once you know your cells you can try to monitor what the charger says it put in, so you get an idea of how much they may be losing over time, but just keep in mind the factory methods of determining capacity are rather specific (and also not necessarily how you generally use cells or would want to use cells in real life).

They usually come out at around 1,37 volts. At this time of year those batteries are mostly used in those cheap led string lights that are powered by 3xAA cells, the cells are usually discharged to around 1V. If i put one of the cells to my BT-C3100 it will put around 2500mAh into the battery and the other two go into C8, the two other cells will get around 1500mAh put into them in automatic mode, or a 200-400mAh more at 1A charging current. I have had those cells for around 3 years now and at least according to BT-C3100 there has not been a noticeable loss of capacity over time.

After the cells are charged on the Miboxer let them rest about an hour then charge them on the Opus and see how much you can still put in them— If it’s more than 100mah they are not getting a full charge on the Miboxer

I find that when I have NiMh batteries that haven’t been charged in awhile that I have to recondition them before they’ll take a full charge. That means 1) charge them until the charger says they’re full, 2) discharge them completely using a charger or running them in a flashlight, and 3) charge them again. Doing it several times is better than just once.

By way of example I had some Eneloops that were sitting in a drawer for several years. When I recharged them they came off the charger as full even though they were at 1.37v on my DMM. I discharged them and then charged them up again. This time they came off the charger at 1.49v and a discharge test had them at approx 1800mAh.

That’s weird. Chops has a good idea…as long as they aren’t feeling extra warm/hot when they come off the MiBoxer. It could be that they’ve just built up resistance and are wearing out. If the Opus can put more charge after the MiBoxer then hopefully you can talk to someone about a replacement (usually the retailer/seller you bought it from….not sure what MiBoxer’s status is right now as a manufacturer but I was thinking they might be going away).

A refresh cycle or two like Sig mentioned does help sometimes. I think for me it hasn’t done that much good but usually I’ve only felt the need to try with cells that were definitely getting long in the tooth or with ones that I stupidly allowed to self-discharge and sit for way too long without attention.

I actually forgot to mention that i have already tried putting the cells into the BT-C3100 after C8 “finished” charging in less than 2 hours at 1A. Depending on how early C8 has finished charging, BT-C3100 may put up to 1500mAh into the cells. The cells have not been sitting for too long, they are constantly in rotation at least in my battery toothbrush, and couple of AA flashlights and they have only tens of use cycles, not hundreds. The cells are in a dispenser that always gives out the one that has been sitting it longest: https://www.prusaprinters.org/prints/64323-fifo-aa-battery-dispenser

It’s a bad idea trying to charge half to fully charged NiMH again. A good charger might detect that a battery was already full after some minutes, but other chargers might overcharge them. The typical -dV/dt termination works only for high enough charging current and batteries that will charge for several minutes without being full.

You can’t say how full a NiMH is just by looking at the voltage, only if it’s empty. Check the capacity by discharging them in a charger with that function.

I don’t think it’s necessarily a bad idea, but one’s general level of caution/attention should go up a notch when doing this….just check ’em and monitor them frequently to be sure they aren’t getting hot, since you know there’s a question hanging in the air.

But with the new information it sounds like maybe the MiBoxer has taken a dump (assuming contacts are clean and power source is stable, etc). Maybe time to inquire about a replacement.

I still have MiBoxer C8-12. I gave up using it because it’s not charging my 14500 li-ions. It just don’t see them. I’ve tried with a few types, nothing. All of them work excellent with other chargers, so maybe there’s something wrong with the AA size at MiBoxer’s?

I wonder if they’re missing the protruding part of the positive side of the charger —- sometimes with AA batteries I have to raise them a little to make contact

The Miboxer C8 has a resistance reading. What does the Miboxer charger read for the batteries?

If the resistance is reading very high (maybe something more than 100mOhms [Miboxer C8 reading]), then it could be the batteries are not making very good contact with the charger slots, and this may possibly affect the charging process.

Try reseating the batteries and check if you can get a more consistent lower resistance reading of the battery (meaning it’s getting a better contact possible with the charging slots).

Well, it’s not neither seldom nor awkward that a charger reads resistance. So does my few other chargers.
The truth is other chargers (reading resistance) deal with my 14500s without a hassle. While the Miboxer reads nothing because it does not recognize anything was put into the slot. Note that my 14500s are Efest and Shockli, so they aren’t poor.