Multi-bay chargers, calibration and precision of each bay

[Darkpower]

I'm relatively new here and quite impressed with the level of knowledge by many here. One of the issues that has been raised many times is the differential discharge found in batteries discharging in series, in flashlights, lanterns, and other portable appliances. One of the parameters that I haven't seen raised as of yet is whether battery cells being charged in multi-bay chargers get the same precise charge dosage when in the same charger with multiple bays.

In many of the discussions it has been raised that battery electrochemistry vary from cell to cell in a number of things such that no two cells are identical. Instead we find a range of +/- 3% (I'm just throwing a number out here) capacity in a given brand name even if it is a given batch of batteries charged and discharged together. When these cells are put into series, one cell inevitably discharges faster, even if these cells are paired up by voltage and or capacity and kept together.

Anyhow, its come to my mind lately especially in the multi-bay chargers like the Maha C808 or the Maha 9000, or the Lacrosse BC900 that each channel has its own regulation circuit, and status display. This means each channel has a controlling device such as a MOSFET or bipolar transistor controlled by a digital micro-controller. From my engineering experience, I know that transistors and transmission gates themselves can also have variations in them such as Rds-on (the transconductance between the source and drain). The result is that inevitably, some charging bays may charge at a higher or lower current, (or voltage) and as well as the A/D circuits for display and control may have variations and calibration issues.

I was curious if anyone has gone as far as to actually measure the precision of the output of these bays. The reason I raise this, is that differences in the output of the charging bay, as well as the variation in calibration of the control circuit, could be a contributing factor to differences in a final charge across say…4 cells or 8 cells all done at the same time.

After giving much thought to this, I realized the only way to precisely charge four cells identically would be in a parallel jig using a Hobbyist type charger that many people use here.

Has anyone ever calibrated or examined the calibration of these consumer type multi-bay chargers. Do the manufacturers publish calibration specs? Does anyone have any user-calibrated charger? Or have I rambled too long? :ironic:

 

[Mr Happy]

Typically if a charger continues with a timed top-off charge after the main fast charge has terminated, this will tend to even out any differences in the accuracy of when the fast charge was terminated.

People have noticed calibration differences in between the channels in a multi-channel charger, but normally such differences should be within engineering tolerance and measurement accuracy. If the differences seem large, then Maha, for example, have been very good about offering a replacement under warranty.

 

[SilverFox]

Hello Darkpower,

I have actually ran comparison tests on a couple of the C9000 units, the BC-900, the Vanson Speedy Box, and the C808M. All of them have very little variation from slot to slot.

If you have a set of NiMh cells that you are using in series, the best way to balance them is to give them a 0.1C charge for 16 hours, from a discharged state. The next best way to balance them is to individually charge them. Parallel charging of NiMh or NiCd cells does not work, and is not advised.

Tom

 

[Darkpower]

Typically if a charger continues with a timed top-off charge after the main fast charge has terminated, this will tend to even out any differences in the accuracy of when the fast charge was terminated.

People have noticed calibration differences in between the channels in a multi-channel charger, but normally such differences should be within engineering tolerance and measurement accuracy. If the differences seem large, then Maha, for example, have been very good about offering a replacement under warranty.

I would say that the variations are generally +/- 0.03 volts on a set of batteries out of the same package and charged and discharged together repeatedly. I would agree that kind of variation is like what you said within engineering tolerances. Its just that I haven't seen any spec sheet for any of these chargers that give the manufacturer's acceptable tolerance range for their devices including the accuracy of the digital readout of their digital intrumentation. In addition I was curious if there were trim pots to calibrate these chargers if a specific channel runs a little high.

Hello Darkpower,

I have actually ran comparison tests on a couple of the C9000 units, the BC-900, the Vanson Speedy Box, and the C808M. All of them have very little variation from slot to slot.

If you have a set of NiMh cells that you are using in series, the best way to balance them is to give them a 0.1C charge for 16 hours, from a discharged state. The next best way to balance them is to individually charge them. Parallel charging of NiMh or NiCd cells does not work, and is not advised.

Tom

Thanks Tom, but what about those jigs that I think I have seen in a thread or two here where people have rigged up a battery(s)l holder for a Shultze (or perhaps some other hobby charger) charger. The impression that I had was that they were able to do parallel charging, since the output cables of those chargers are designed for serial or parrallel battery packs.

 

[SilverFox]

Hello Darkpower,

Parallel charging works great for Li-Ion cells, but not for NiMh.

As a matter of fact, I had LuxLuthor rig me up a connector that I can use to balance and charge the 3 18650 cells that are used in my Wolf Eyes M300 light. Charging Li-Ion cells in parallel makes sure that then end up balanced.

If you have some time and want to run some tests, you can take a fully charged NiMh cell and parallel it with a fully discharged NiMh cell. After around 24 hours, you may have transfered around 10% of the capacity from the full cell to the empty cell.

Keep in mind that the voltages will equal out, but if you do a discharge for capacity you will find that the full cell is unable to charge and equalize the empty cell.

So, parallel Li-Ion cells to balance them, but this does not work for NiMh cells.

Tom