BC-900 accuracy on capacity readings?

[geek4christ]

I recently received my BC-900 and proceeded to put 4 cells in it on refresh mode. These are old (maybe 4 years or so) Energizer 1850mAh cells that have been severely neglected since I bought them.

The refresh cycle has finished on all 4, and I ended up with capacities of 1776mAh (~96%), 1893mAh (~102%), 1943mAh (~105%), and 1834mAh (~99%).

My question is, did I actually end up with overcharged cells or is the BC-900 just not very accurate in its readings?

I have the charger plugged into a surge protector with noise-reduction, and I have a very small fan blowing on it. I wanted to try charging with the fan because I don't want the cells hitting the high-temp cutoff. I want a solid current going into them until -dV is obtained.

Having said that, I'm not really sure what the charging voltage profile of cells that are this old and neglected would look like, so I'm wondering if -dV occurred in overcharge.

Any thoughts or recommendations are much appreciated. And, as always, please be gentle on the newb

 

[shadowjk]

What charge/discharge rate did you use?

 

[geek4christ]

What charge/discharge rate did you use?

500mA discharge, 1000mA charge

 

[SilverFox]

Hello Forrest,

When you test cells "hot off the charger," you will get larger numbers than if you charge the cell up, let it rest and cool off, then run a discharge.

The BC-900 runs the discharge immediately after completing the charge. Thus, you will get slightly higher numbers from it.

Tom

 

[linterno]

Yes, I guess there is something wrong with BC-900. It has charged almost new (three times charged) 1000 mAh AAA batteries up to 1800+ mAh (probably what is wrong are the batteries). I guess is waits for a voltage drop to stop charging and if it is not detected the charged just keeps charging the batteries.

I have also noticed that sometimes is just get stuck charging batteries, I mean, sometimes I have seen for more than one hour the same charging capacity (current stored in batteries. Sorry if it is not clear but english is not my primary language and sometime I have difficulties with it) while charging. I guess the charger is not detecting any voltage increase to consider "charging" not decrease to consider "done" and just keeps showingthe same current until the last voltage increase.

There is something with this charger that I am not sure if it is bad or not: It stops discharging batteries when those reach 0.9 volts. As I understand NiMh batteries should not be discharged under 1.0 volt.

 

[geek4christ]

Hello Forrest,

When you test cells "hot off the charger," you will get larger numbers than if you charge the cell up, let it rest and cool off, then run a discharge.

The BC-900 runs the discharge immediately after completing the charge. Thus, you will get slightly higher numbers from it.

Tom

Ah, thanks Tom.

So another question came to mind after I had already started this thread. How much variance is there in cell capacity from the manufacturer? I'm assuming tolerances aren't all that strict, so is it also reasonable to expect that some cells may just have more or less capacity?

 

[SilverFox]

Hello Linterno,

The end of charge termination is dependent on the rate you use for charging. The BC-900 uses several methods of terminating the charge. At higher rates, it terminates on -dV. If this is missed, it terminates on maximum voltage. If this is missed, it terminates on maximum capacity put into the cell.

The maximum capacity for AA cells is around 3000 mAh. I am not sure what it is for AAA cells.

When using several cells together in a battery pack, you should limit your discharge voltage to 1.0 volts per cell. However, with a single cell it is OK to go down to 0.9 volts.

Tom

 

[SilverFox]

Hello Forrest,

It actually depends on the manufacturer of the battery. The Eneloop cells seem to come in very close to each others, but there can be wider variations.

When matching cells you generally try to match them within 2% or less.

Tom

 

[geek4christ]

Thanks again, Tom. I really appreciate the time you spend answering questions here. I've benefited from many of your posts in many threads.

 

[geek4christ]

Thanks again, Tom. I really appreciate the time you spend answering questions here. I've benefited from many of your posts on many threads.

 

[Greta]

Bump

 

[Darkpower]

Yes, I guess there is something wrong with BC-900. It has charged almost new (three times charged) 1000 mAh AAA batteries up to 1800+ mAh (probably what is wrong are the batteries). I guess is waits for a voltage drop to stop charging and if it is not detected the charged just keeps charging the batteries.

I have also noticed that sometimes is just get stuck charging batteries, I mean, sometimes I have seen for more than one hour the same charging capacity (current stored in batteries. Sorry if it is not clear but english is not my primary language and sometime I have difficulties with it) while charging. I guess the charger is not detecting any voltage increase to consider "charging" not decrease to consider "done" and just keeps showingthe same current until the last voltage increase.

There is something with this charger that I am not sure if it is bad or not: It stops discharging batteries when those reach 0.9 volts. As I understand NiMh batteries should not be discharged under 1.0 volt.

I have a BC-900 and the "Charge Test" will give you the test results on the 2nd discharge cycle of that mode.

On that particular charger, when you select "Charge test":
1) it will discharge the battery to 0.9v
2) then charge it up from that known empty state (i.e. 0.9 volts),
3) then discharge it and report the capacity and hold the display with that information,
4) Finally then charge it again so you have a freshly charged cell, but the display will not update on that final charge dumped into the cell. But instead will simply hold the info from the 2nd discharge cycle.

The process can take a full 24 hours depending on the current rate you select.

When you select Charge only, (default) the charge cycle is generally about 15% to 60% (depending on the current charge slected) above the rated capacity because there are heating losses that get dissipated. The higher the current rate, the greater the losses is what I believe happens.

Don't interpret the charge dumped into the cells as the capacity because its much higher then the capacity, its the charge taken out that counts and the only accurate reading of the cell capacity is when you select "Charge Test".

 

[linterno]

Thank you Tom and Darkpower for your replies.

I usually charge my batteries using 100mA discharge and 200mA charge. Faster rates would terminate faster but my batteries would last less. I have several spare batteries and never need a fast charge.

Sometimes I discharge 2000+ mAh batteries at 250mA and charge them at 500mA. AAA batteries 1000mAh or less are always discharged at 100mA and charged at 200mA.

Juan C.