Hello Peter,
Welcome to CPF.
In order to successfully charge a cell, there needs to be a method to terminate the charge. A common method of charge termination involves looking at a drop in voltage as the cell passes through full charge.
The higher the charge rate, the larger the termination signal. Conversely, the lower the charge rate, the smaller the charge termination signal.
As cells age (and also with brand new cells) the charge termination signal can trigger early, or not show up at all at lower charge rates.
With all of this to consider, it is usually best from a charge termination perspective to charge at a rate that completes the charge in 1 - 2 hours. This is referred to as a 0.5C - 1.0C charge, with C = capacity of the cell.
Eneloop cells have a capacity of around 2000 mAh, so a 0.5C charge would involve charging at 1000 mA. A 1.0C charge would be charging at 2000 mA.
When you charge at a rate below 0.5C, you run a risk of missing the charge termination signal. You have experienced this. The issue is that overcharging wears out the cell. Most cells can stand an overcharge at a very low charge rate for a limited amount of time. The standard for determining capacity of the cells involves charging at 0.1C for 16 hours. This involves some overcharge, but the rate is low enough and the time is limited to the point where it does not damage the cell. On the other hand if you leave a cell charging at 0.1C constantly, it will be used up in about a year.
Many people observe that their new cells that are in good condition will terminate properly at lower charge rates, but over time they stop looking at what the charger is doing and after the cell ages missed termination is common. To guard against this, chargers have a timer that limits the charge time. Depending on what the timer is set to, this limits the amount of overcharge the cell can receive.
Low self discharge cells may be more prone to overcharge damage than normal NiMh cells. This hasn't been looked at in depth, but the few cases of Eneloop cell failure have involved leaving the cells on a charger for an extended period of time. I have also seen this with an Eneloop battery pack that was being topped off to balance the cells in the pack. A cell actually started to vent even though the charge current was low.
We tell people to attend to their charging operation. Keep an eye on things and know when to step in and stop a charge when termination is missed. Your charger will have less problems determining the termination signal if you charge in the 0.5 - 1.0C range. With the BC-700 and Eneloop cells you don't have the capability to charge at 1000 mA, so you will just have to keep an eye on things. Set a timer. If the charge hasn't terminated when the timer goes off, manually stop the charge.
You can play with this by setting the charge rate to 0.1C. With Eneloop cells that would be 200 mA. Put the cells in and set a timer to 16 hours maximum. I am not sure what the upper limit for the BC-700 is, but you can get an idea of how well things are going by looking at the mAh put into each cell. Some cells should terminate properly, others won't. While this also happens at higher charge rates, it is much less frequent. Looking at the amount of charge put into a cell will give you an idea if the termination was proper. You may have to do a discharge followed by a charge to find a target number, but then you can compare how much you are trying to put back in. There is some variation in this depending upon charge rate, but it should give you and idea of what is going on.
Tom
