NiMH Battery Question

[Kevin Rooney]

I am needing to replace some older NiMH AA batteries (Kodak) that I use in my GPS. They simply aren't able to hold a charge long enough to trust them in the field, so will retire them to powering my mouse, etc.

Walmart has a Kodak brand set of 4 for about $8 that are rated around 2000mAh, and Staples has another brand rated around 2700mAh at about $15, nearly twice as much.

My old Kodaks are 1850mAh, and I didn't have any complaints about them lasting a day in my older Garmin Vista. In winter I'd need a backup set to run the GPS the balance of the day, and if about treeline and cold/windy, a second backup set to swap out - the warmth of my pocket would revive it.

My question - without making this a major project - is it worth paying about double for the Staples selection, or just get the cheaper Kodaks at Walmart?

Thanks.

Kevin

 

[DougPaul]

First of all, there are now two classes of NiMH AA (and AAA) cells: those that are optimized for capacity (mAh) and those that are optimized for charge retention (shelf life).

* Capacity optimized cells:
- now go up to 2700-2800 mAh.
- some are electronically delicate (ie can be damaged by high drain usage)
- There is a good review of capacity optomized NiMH AA cells at http://www.imaging-resource.com/ACCS/BATTS/BATTS.HTM. Needs updating, but it is a good place to start.

* Charge retention optimized cells
- Shelf lives of a year or more
- Several brands, Sanyo eneloop seems to be one of the leaders.
- capacities ~2000 mAh
- pretty new to the market

So choose which type you want--capacity optimized cells if you want the higher capacity and can top off/recharge not long before use (eg a week or less) or charge retention optimized cells if you want them to last a long time without recharging.

http://www.thomas-distributing.com/index.htm is a good place to research NiMH cells and chargers. (Their prices are generally pretty good, but I have occasionally found a better price with a search engine.) You can also get better prices if you get cells with a slightly less than the largest available capacity. You should be able to get good quality cells at ~$2.50/cell or less.

standard caveat: I have no connection with the above company, but have been a customer.

Doug

 

[Kevin Rooney]

Thanks for the update on the evolving types of rechargeable batteries. In the class of "charge retention (shelf life)" type batteries I personally wouldn't use rechargeables. Rather, I carry a 4-pack of AA lithiums.

The chart at Digital Imaging is interesting. For my purposes, it looks like price is the determining factor.

Thomas Distributing looks like a good source, but if you only need a 4-pack, the shipping charges offset the savings.

 

[DougPaul]

Thanks for the update on the evolving types of rechargeable batteries. In the class of "charge retention (shelf life)" type batteries I personally wouldn't use rechargeables. Rather, I carry a 4-pack of AA lithiums.

The only real difference between the "charge retention" NiMHs and the older ones (of the same capacity) is that they have a longer shelf life. Say a GPS consumes about 1 pair of batts per day--the economic advantage of NiMH over primary cells (alkaline or lithium) is still there. In practice, the "charge retention" NiMHs don't need to be topped off shortly before using.

The chart at Digital Imaging is interesting. For my purposes, it looks like price is the determining factor.

The chart was last updated in 2005. Just choose some cells from a line with a good past record. And if you buy less-than-highest-capacity cells, you can keep the cost down.

Thomas Distributing looks like a good source, but if you only need a 4-pack, the shipping charges offset the savings.

I generally buy larger batches. And don't forget to add sales tax into any local purchases. (Thomas Distributing is in IL.) Don't forget that once you have chosen a cell, you may be able to find better prices with a search engine.

Doug

 

[MichaelJ]

damaged by high drain usage

Is that referring to, say, a camera flash unit? Do any other devices come to mind, because besides my flash, my other AA usage is headlamp and GPS, which I don't think of as high drain.

 

[DougPaul]

Is that referring to, say, a camera flash unit? Do any other devices come to mind, because besides my flash, my other AA usage is headlamp and GPS, which I don't think of as high drain.

One of the highest capacity cells reported in http://www.imaging-resource.com/ACCS/BATTS/BATTS.HTM suffered from extreme sensitivity to drains of ~.5A or so. Most NiMH cells should have no problem in the above applications. The study used a 5 ohm load on 4 cells (1A load) to simulate the load of a digicam.

I use NiMH AA and AAA cells in headlamps, bikelights, and a digital camera. The headlamps have a continuous draw of up to 750mA (AA cells) and I have not observed any problems. I have tried using AA cells to drive a 10W (1.7A) bikelight, but it seemed like a little much. The only time I have had problems with cell damage was when I was using a fast (~1hr) charger* which overheated the cells during charging.

The nominal lifetime of an NiMH cell is about 500 cycles. If it only gives 200 cycles in heavy useage, it is still a win over primary cells.

* Ray-o-Vac PS-4 set to NiMH (1hr charge rate), NiCad setting (2 hr charge rate) is ok for NiMH cells.

Doug

 

[Rick]

I read somewhere (Zbattery.com, perhaps) that if you have an inexpensive charger (I have a Walmart Energizer charger that came with some NiMH 1800's), that if I upgraded to 2700's, the charger would only charge them to a capacity consistent with 1800's.
I have long since recycled the old batteris and bought Sony 2700's which I have used for about 2 years now and it seems like they last much longer than the old 1800's, when charged on the old charger. Or course, I did upgrade from an Etrex to a GPS 76 CSx last year.
I'd appreciate any thoughts from you, DougPaul, (but dumb them down a little so I understand the terms!!!)

 

[DougPaul]

I read somewhere (Zbattery.com, perhaps) that if you have an inexpensive charger (I have a Walmart Energizer charger that came with some NiMH 1800's), that if I upgraded to 2700's, the charger would only charge them to a capacity consistent with 1800's.
I have long since recycled the old batteris and bought Sony 2700's which I have used for about 2 years now and it seems like they last much longer than the old 1800's, when charged on the old charger. Or course, I did upgrade from an Etrex to a GPS 76 CSx last year.
I'd appreciate any thoughts from you, DougPaul, (but dumb them down a little so I understand the terms!!!)

A decent smart charger will automatically determine when to stop charging (essentially) any cell in a reasonable range of capacities. It does this by sensing the state of the battery, not a specific amount of charge. So if it is a decent smart charger it will work properly with the larger batteries.

If you have a dumb charger such as one that works by time alone, throw it out immediately and get a decent one. (It will simply destroy the batteries.) I have made recommendations on decent smart chargers in previous threads.

Doug

 

[BCG]

"My question - without making this a major project - is it worth paying about double for the Staples selection, or just get the cheaper Kodaks at Walmart?"

I have a set of the Kodak batteries from Walmart that you mentioned and they work very well. Much longer life than my older Evereadys. A single set lasted about 16 hours this past weekend in my Vista C and still had 2 bars left.

 

[Kevin Rooney]

I wanted to follow-up on my post ""My question - without making this a major project - is it worth paying about double for the Staples selection, or just get the cheaper Kodaks at Walmart?"

Like BCG, I bought a replacement set of 4 Kodak batteries at Walmart for about $7.50, and have been thru a couple of charge cycles with them. Huge improvement over the old ones which are nearing their useful life of 4-500 recharge cycles. Plus, the new ones have a much higher mAh rating, so that should provide longer battery life per charge.

The new batteries are a good investment.