La Crosse BC700/ Eneloop charging confusion

[TK41]

I took out some batteries from my flashlight (4 AA eneloops), put them in my charger and let them go to work. 3 of them stopped charging after about ~700 mah went into them. But 1 of them kept charging until 1,488 mah! I had to take it out because I was going to sleep. I put it back in the next morning and now this battery says "full."

I am confused by two things:

1) Why did 1 battery need more than 800 mah more charge than the other 3 if they were all fully charged prior to being put in the same flashlight? (in other words, they all started at the same charge, went into the same light, why did one need more charging?)

2) Why did this 1 battery say "full" charge in my BC700 this morning when I prematurely took it off the charge the night before?


Thanks for any help.

 

[grant_lee]

Hi,

Obviously the charger didn't detect the end of charge and didn't stop on time. On next charges, watch this slot as it may be faulty.

 

[moldyoldy]

What was the charge rate? How old were the cells? How warm was the one cell with the listed 1488mah charge when you removed it from charge?

Assuming that an Eneloop cell was not old/failing:

- The seemingly excess charge into the one cell potentially indicates a missed charge termination. That is common with newer cells, and especially on a 200ma charge rate in the BC700/900 series for just about any cell. The BC700 defaults to a 200ma charge rate. a 500ma charge rate is much more desirable. The 700ma charge rate will create quite a bit of heat at the end of the charge.

The "Full" indication when that ?overcharged? cell was reinserted was to be expected if the cell voltage was in the ~1.47v area or higher. Most chargers will not charge a cell with high voltage - correct behaviour. Otherwise under normal conditions, the charger is looking for a -dv/dT change before ending the charge.

I have had several of the BC700/900 series for years as well as Maha and other chargers. A missed charge termination can happen with any of them with newer cells, or abused cells.

 

[Mr Happy]

Remember that AA eneloops have a capacity of about 2000 mAh. So we would expect the charge put into a near empty battery to be more like 2000 mAh than 700 mAh. Therefore it may be that three of the batteries terminated prematurely and are not full, whereas the fourth battery is closer to being fully charged. A simple test is to put the three less charged batteries back in the charger at a charge rate of 700 mA and switch the display to show the battery voltage during charging. If the voltage is somewhere in the range of 1.38 V to 1.43 V the batteries are not charged yet and need much more charging. If the voltage is more like 1.48 V to 1.55 V then the batteries are fully charged and the charger should in that case show full within 10-15 minutes.

 

[TK41]

Thank you for all of the help everybody! Yes, these are fairly unused cells. I bought them a year ago but have only charged them 2 times before this. Indeed, the charger was charging them at 200 mA. I will charge at 500 mA from now on and will try the test Mr. Happy suggested to see if they are truly charged.

 

[Russel]

[...]I will charge at 500 mA from now on and will try the test Mr. Happy suggested to see if they are truly charged.

That would be better then 200mA, but I would suggest that you follow Mr. Happy's suggestion and charge at 700mA. That is what I've always used when I charge Eneloop AA cells with a BC-700.

 

[markr6]

+1 @ 700mA

 

[TK41]

Okay, I did the 700 mA method Mr Happy suggested and yes, the cells all needed some more charging. They were at ~1.43v. After about 10 minutes, they all read FULL, and they were all ~1.5v.

But...here's where I get lost. A few hours later, I pop the cells back into the charger, put it back in 700mA mode, and now the cells are at ~1.45v. And, one of the cells actually charged a bit more, I believe around 40 mAh worth.

What's going on there?

 

[Mr Happy]

Okay, I did the 700 mA method Mr Happy suggested and yes, the cells all needed some more charging. They were at ~1.43v. After about 10 minutes, they all read FULL, and they were all ~1.5v.

But...here's where I get lost. A few hours later, I pop the cells back into the charger, put it back in 700mA mode, and now the cells are at ~1.45v. And, one of the cells actually charged a bit more, I believe around 40 mAh worth.

What's going on there?

This is quite normal. The last few percent of capacity in a NiMH cell is very "leaky" and bleeds off by itself after a few hours and days. This is observed as a gradual decline in cell voltage as the cell sits in storage. The voltage drops rapidly in the first few hours, a little slower over the next few days, and slower still over the next few weeks and months. If you leave an Eneloop in storage for long enough the voltage will eventually drop all the way down to 1.35 V or less.

 

[VidPro]

. . . and on some chargers , depending on the termination method , re-charging ni-mh batteries when they are already charged is not a good idea.
On chargers like this they sorta overcharge a battery to determine that it is done charging (terminate), then add in topping also which could overcharge it slowly more.
Some chargers might "know" (actually guess) that the battery is already mostly charged, and just finish, some will be pretty mean.

If you know that a battery has been fully charged, dont put it back on to charge again.
unless:
1) There is something you are testing.
2) You know the charger will recognise it as charged cell (battery charger combo)
3) you have an idea about what the charger is going to do to the cell, and want that to happen.
4) so much time has passed that, it is no longer charged.

I could say , discharge it some first, but depending on the charger, it doesnt change what happens much at all. Just using it, then charge again when needed. After you get done testing stuff of course.