Thinking about an HP-41CL

[Dieter]

(07-28-2016 04:41 PM)Dave Frederickson Wrote:  A few last comments:
Here's the GP50NH dartasheet:

Fine, but since the GP50NH is no longer available most probably a different type of battery will be used. ;-)
FTR: the datasheet for the X-Cell N battery seems to have some errors.

(07-28-2016 04:41 PM)Dave Frederickson Wrote:  It should be noted that to fast charge the cells the BC-700 needs to have the charging current changed from the default of 200mA, otherwise the - delta T end-of-charge detection circuit may not work as intended.

"Fast charge" is not a term with fixed definition. The GP data sheet states 0,5...1 C for the small N cell, while other sources will define the same term as charging with a current of at least 1 C and higher, especially for larger cells with a different surface-volume ratio which influences the cell temperature and therefore the voltage drop at full charge.

The –dU detection of modern processor-controlled chargers usually detects voltage drops as low as 5 mV, so we can safely assume that a charge rate of 0,4 C should be fine. The GP datasheet even states this value as cut-off condition. The temperature detection essentially is an additional safeguard.

I am charging my 2000 mAh Eneloop AA cells with 450 mA, which is 0,22 C, and it works fine in a different processor-charger. Even thirty years ago the battery pack for my Metz flash was charged with 0,2...0,3 C, and a connected voltmeter cleary showed the characteristic peak on full charge with various types of NiCd cells.

So I do not think that the 200 mA setting should be changed. I think it's just right for this type of battery. That's why the BC700 is the recommended charger – it defaults to this current. I'd definitely avoid the next higher 500 mA setting for N-cells.

(07-28-2016 04:41 PM)Dave Frederickson Wrote:  A couple of questions. The BC-700 also detects end-of-charge by temperature, and the datasheet shows a DT/dt spec. Will the BC-700 end the charge cycle properly if the cells are charged at 200mA rather than fast-charging at 250 to 500mA?

This can only be answered if we know the voltage drop of the battery in question (and the ambient temperature) at 200 mA charging current. But I think we can safely assume that there will be no problem – see above. Again: the main cut-off condition is the voltage peak resp. voltage drop (–dU). But where do you see a graph with dT/dt specifications? I just read the manual that can be downloaded from the LaCrosse website.

(07-28-2016 04:41 PM)Dave Frederickson Wrote:  Yesterday I received some 2200mAH AA flat top cells for use in Spice calcs. (No, they won't make contact with the charger without a washer.) Initial research turned up a 2200mAH Sub-C cell with a normal charge current of 220mA. Without buying the higher current BC-1000, what's the best charge current?

My personal experience for the best compromise is something like 0,3 C or maybe a bit more (that's why I think 0,4 C for the N cells is perfect). This translates to about 700 mA for your 2200 mAh battery. But how do you want to squeeze a SC cell into a Spice, or even the BC700?

The BC1000's additional 1000 mA setting is a nice option for high-capacity AA cells (at least if their specs allows charging with this current, which should be the case for most decent cells of the last decade). On the other hand charging is just 30% faster than with 700 mA...

In general there is no "best" charging current if only the battery capacity is given. There are very different battery designs, some offer higher capacity, others are specially designed for rapid charge, etc.

(07-28-2016 04:41 PM)Dave Frederickson Wrote:  It would be nice if someone wrote an article about smart chargers and high-capacity cells.

What high-capacity cells do you refer to?

Dieter