Capacity test inconsistency between VC8S and C3100

Batteries and Chargers Chargers
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I’ve had my trusty Opus C3100 for years, but just picked up a VC8S recently to get some more convenience. Lithium cells seem fine/consistency on both, but I’m getting vastly different NiMH capacity test results on the VC8S versus the C3100. For example the VC8S I started with a capacity test of an older 2450mAh cell and got 1191mAh. I put it in the C3100 and get 1927. Put it back in the VC8S and it’s reporting 1105mAh. Both are doing the charge-discharge-charge single cycles (not Opus refresh mode).

Both chargers appear to be charging up to about 1.5V, and discharging to about 0.85-0.9V (I’m not staring at the screens all day, so I don’t know exactly where they’re terminating).

I’d like to believe the C3100 is correct, but with most of these cells probably going to 10 years of age, I also wouldn’t be surprised if the capacity of these things has degraded well below 2000mAh either. Cycle counts are likely only a few dozen though, so the C3100 is more likely correct.

Lastly, I’m seeing this on all AAA and AA cells I have, even new Eneloop Pros. All slots, and even if I only do one cell.

I’m thinking the VC8S may be defective? Any ideas? It’s powered from a 100W USB-PD power supply, not that any of this should matter on the discharge cycle.

Thanks for any suggestions!

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The only way to truly compare is to charge them up with the same charger. Then do a discharge with each charger, that will give you a better idea of how they compare. At times I use up to 8 opus 3100, but I use a hobby charger and charger a bunch of cells in bulk at one time. All of my opus were not calibrated with the other, even within the same charger. In general Opus measure on the high side.

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Discharge cut off voltage is important. Also discharge current.

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I have the Opus. It does ‘tend’ to overstate the capacity. OTOH, you have a large discrepancy.
On the Opus you can use various charge/discharge currents. That will make a difference both in how it will discharge, and more importantly, how it will charge.

I assume The VC8S has something similar. If they are way different you can definitely get different results, especially with older cells. Older 2450 cells, even Eneloops, will probably have a serious increase in internal resistance, which will complicate charge discharge cycles.
Best to simply charge>stop. Then discharge (using the same settings in both chargers), and stop. Then compare results.

I’m messing with some REALLY old cells right now (15 years+).
They charge poorly (actually using a very old ‘dumb’ charger) and have very little tolerance for even modest discharge current (can’t do 0.5A). Doing ‘repair cycles’ in ANY charger for these would give seriously whonky results.
I only use them in very low discharge motion lights so I thrash them until they are REALLY dead. Just gotta check HOW dead once in awhile before recycling. These are about at the dreary end.

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flydiver

I’m messing with some REALLY old cells right now (15 years+).
They charge poorly (actually using a very old ‘dumb’ charger) and have very little tolerance for even modest discharge current (can’t do 0.5A). Doing ‘repair cycles’ in ANY charger for these would give seriously whonky results.
I only use them in very low discharge motion lights so I thrash them until they are REALLY dead. Just gotta check HOW dead once in awhile before recycling. These are about at the dreary end.
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If these cells take forever to finish charging and are very warm when at the almost finished state. They are junk. I would toss them unless you have a way to stop the charge earlier 4.10- 4.14 v

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Should have specified more clearly…NiMh cells. The OP was testing NiMh hi-cap.
I would not push lithium that hard.

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:+1:t2:

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I’ve been trying to catch the bottom end of the discharge to pull the cells off and measure with my 87V meter. I have noticed that the Xtar’s charged Voltage is pretty accurate, whereas the Opus says 4.6V resting, but 87V shows 4.8V, and when topping off, the Opus reads 4.8V, while the 87V says 1.52V. So there’s already an issue there with the Opus.

I need to catch them at the end of the discharge to yank the cell and measure them.

Right now I am testing some new Fujitsu 2450mAh cells. They consistently (for the same cell) read around 1900mAh on the Xtar, and around 2450 on the Opus. As a lot, they’re all within about 50mAh of one another, when tested on the same charger. I have been attaching labels to each battery each time I have another test completed, and now prefixing them with “C” and “V” for the C3100 and VC8S, making it a bit easier to see what measurement came from which device.

When I go back to the VC8S, I get the lower readings again. Refresh mode on the Opus isn’t increasing it further on either unit’s discharge test.

The Opus is set to 500mA, the VC8S discharge is 300mA. The Xtar should be reading higher capacity if anything.

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I have tested both chargers with a 300mA discharge (set on the Opus, fixed on the Xtar).

Opus 87V measured: 1.52V → 0.88V → 1.48V (“resting” - trickling)
Xtar 87V measured: 1.48V → 0.97V → 1.44V (resting on charger - weak trickling?)

The first number is the measured Voltage at charge termination, the second is what I measured before the discharge switched off, and the third number is what I measured after the charger stopped charging.

The resting voltages after discharge and about an hour off the charger is 1.13V from the Opus and 1.10V from the Xstar. After charge, off charger the cells show 1.41V from the Xtar and 1.44V from the Opus. The Opus appears to go into an obvious trickle charge (at least it holds a higher Voltage when a cell is left inserted), whereas the Xtar stops charging/Voltage begins to drop, although reviews suggest it trickles.

The Xtar is pretty accurate with its display - it consistently rounded up from actual measurement. If the charger shows 1.48V, the 87V would measure anywhere from 1.471-1.480V. On discharge, if the display read 1.00V, the 87V would show 1.00V until 0.991V, and when it hit a measured 0.990V, the charger would switch to displaying 0.99V. Always rounds up, to the next hundredth of a Volt.

The Opus on the other hand reads high on discharge, low on charge - about 0.03V-0.04V high during discharge, and 0.04V lower than measured on charge.

All that said, the Opus charges a bit higher, and discharges a bit lower than the Xtar, but they are pretty close, and the slight overdischarge is only a couple minutes apart from hitting true 0.9V (87V measured) and termination. The charge is more difficult to tell as the Opus holds a higher Voltage when trickle charging. After removing cells from both chargers, they settle in near one another when resting, as mentioned above.

Charging on the Opus, then putting the cell in the Xtar, the Xtar still reports lower capacity. The Xtar will still start its charge cycle for a few minutes before it starts the discharge, but with the pictured cells, they’re still reporting 20% lower than the Opus, and on older cells, 40% lower then the Opus. I have some brand new Eneloop AA 2100mAh cells arriving in a few days - I want to use them to compare the charger capacity tests with as fresh as cells as possible.

I didn’t want to go this in-depth, but I was really hoping to use the Xtar every couple years to do 8-cell capacity tests to see when I should recycle cells.

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On new hi-cap cells of that quality the difference in real capacity between 500mA and 300mA is negligible. The cell will sustain voltage until the cut-off, then stop. Really, there probably is little difference between 300mA and 1.0A.

Now…when the cells get older and have higher internal resistance the difference in ability to support current becomes highly magnified. Some of those old cells I’m testing I mentioned simply will NOT sustain 500mA at all. But they will report over 1000mA capacity if tested at 200mA current.
They also will NOT charge at a very high rate-they simply terminate. The really bad ones will only charge reliably on a time limited, VERY low current ‘dumb’ charger. That generally puts them into the recycle box.

As cells age charge and discharge become more erratic and ‘smart’ chargers also become more erratic in dealing with it. Some chargers do a better job than others.
Individual brands of cells also may be more ‘acceptable’ to smart chargers as they age. For instance, I’ve found EBL NiMh to NOT charge nearly as well when brand new, as some other brands that are much older. I have no idea why it works that way.