Gummi, on Jan 15 2006, 11:02 AM, said:
So far every NiMH brand I've tried all loose there ability to hold a charge when not used for over a month. I've only tried energizer, quest and rayovac rechargeables and have 3 slow charge units, two are different energizer models and one by quest.
I've gone through so many 4 packs of rechargeables I find it better to get a 24 pack of cheapos for 3-5 dollars. If lithiums where the price of energizer or duracel alkalines I'd go with them.
I have to agree with kerecsen
A properly designed high quality charger is imperative to obtain long cell life. In addition, cell sets need to be rotated so they are used in service on a regular basis verses just being backup sets. They also need to be kept in sets if your charger charges cells in series (if you need to add 2 or more cells before the charging light goes on, they are being charged in series).
The bottom line is, if you are having problems with high quality NiMH cells, then you either have:
. Defective cells
. Bad quality cells (brand)
. A defective or a badly designed charger/s
. A charger designed for another chemistry such as a Ni-Cad charger
. Are not following proper charging procedures
The rest of this post is not directed to a particular user:
I use MaHA MH-C401FS chargers
(5 of them currently). They have independent charging circuits for each cell and utilize the 'Negative Delta V' charging algorithm, although I still keep the cells in sets from the standpoint of brands and purchase date. They also offer a fast and slow charging mode. I have found the fast charging mode of this charger to be the best of any unit I have tried including some of the Lenmar chargers (who also make good chargers).
I also have several Panasonic BQ-390 travel chargers that come in a kit from Costco. It comes with the charger, 6 2300ma NiMH AA cells and 2 ?????ma NiMH AAA cells for well under $20.00. I have found the 2300ma Panasonic cells provided with this kit to be every bit as good as the Sanyo 2500ma cells, which are/were my current favorites (any of you R/C enthusiasts out there know Sanyo cells). While the BQ-390 chargers seem to work well, I only use them when traveling when the MaHa chargers are not available to me, or as charge boosters. BTW, a 'charge booster' is a charger you use to start the initial charge process on a set of cells when the main chargers are busy charging other cells. When the main chargers are done charging their cells, the cells from the 'charge boosters' are then transfered to the main chargers to complete the charging process.
When charging NiMH cells, the last part of the charging process is one of the most critical stages to make sure the cells are fully charged and yet not overcharged. Transferring cells from basic chargers used as 'charge boosters' to chargers with known good final charging procedures half way through the charging process insures the final charging stages are performed correctly, and yet the total charge time in the main chargers is reduced since the cells already contains a partial charge.
As far as battery life for Alkaline -v- NiMH cells are concerned, it depends on the current draw. (All of the following data is based upon published manufacturer specs.)
AA alkaline batteries from the big three are rated at a maximum of ~2800mah. This rating is based upon a 20-30ma constant current draw to a cell voltage of ~.8vdc at which time the cell is considered to be depleted. As the constant current draw increases, the mah rating decreases. At 100ma draw the rating at .8 vdc will be closer to just under 2500mah, and at 250ma it drops to ~1900ma. At 500ma it's only about half at 1400mah. If it looks bad now, it gets worse.
These ratings are based upon an operational voltage of .8vdc. While there are quite a few clocks, radios, and other low draw devices that will 'still' operate at these voltages, most digital electronics are not happy. So how bad is it really? Well, at ~250ma current draw at a voltage rating of 1.1vdc (a ~26% drop from 1.5vdc), the rating is ~400mah. That sucks, but that's how alkaline batteries discharge for the most part.
On the other hand we have NiMH cells. Like most rechargeable cells they give their all and then fall flat on their face. While this will result in far superior performance in high current devices, it makes it very hard to produce accurate battery meters for such devices. This is because, regardless of the current draw, the voltage will remain somewhat the same until the pack is fully depleted. Since most battery meters are mostly voltage centric, this results in a less than accurate reading. This is why 'smart' battery packs were developed.
If we look at most portable GPS units, we see battery life ratings from ~10-30hrs, while 15-20hrs seems to be the most common, let's be overly optimistic and select 20hrs of operation for all conditions. At an operational voltage of 1vdc, this equates to ~100ma draw. This isn't that much, but I have a feeling it goes up dramatically if the backlight is used much of the time. Let's say it increases to 150ma.
OK, so let's equate this to Alkaline and NiMH batteries in such a GPS unit. First we must assume that the NiMH cells were recently charged and have not lost a significant amount of their energy. Freshly charged NiMH cells will be assumed.
Alkaline (2800mah - 100ma discharge rate to 1vdc) - discharge to 1.2vdc:
80 ma - ~20hrs
100ma - ~15hrs
150ma - ~10hrs
NiMH (2300mah - 460ma discharge rate to 1vdc) - discharge to 1.2vdc:
80 ma - ~??hrs
100ma - ~18hrs
150ma - ~12hrs
I have no idea what minimum voltage various GPS units will operate at, but I took a stab at 1.2vdc as being a fair amount since it is a reference marker on the graphs and made computations easier
So, properly charged NiMH cells seem to offer every bit as much operation time as alkalines. Obviously keep some alkalines for backup is a good idea due to their operational shelf life.