Managing NiMH AA batteries with a Fenix TK20

Okay . . . I've decided to save my AA lithium batteries for my my EOTech, and since my Fenix TK20 is going to be my primary utility light, I'm going to use rechargeable AA in it.

Here's my question (and be patient if this has been addressed a billion times here; I try to search as much as possible, but I usually get overwhelmed by the amount and depth of the available information):

If I don't discharge the NiMH batteries all the way, will I reduce their useful service life?

My intent is to swap batteries as soon as I can notice a decrease in the amount of light I see from the TK20. Naturally the batteries will have a significant amount of usable charge left.

Is it okay to just throw them back in the recharger? Or do I need to discharge them fully in order for them to consistently take a full charge for as many cycles as possible?

I'm using an Energizer 15-minute charger if that matters. Does my charger discharge the batteries before starting to charge them as part of its function?

THANK YOU for your patience!

The quick answer is charge often. There is no such thing as "memory effect" in nimh batteries and in fact you cause them damage by over discharging them.

But, on the other hand, your 15 minute energizer charger is very hard on batteries, both because of the charge rate, and the fact that I am 99% sure it is a timer based charger and would charge them the same if they are drained or nearly full.

So, the right answer is to get another charger. And, I am guessing you are using Energizer batteries as well? If so, get another charger and batteries both. Eneloops are very well thought of here, or the Duracell precharged. However, if you are willing to charge more often and the lower capacity of the precharged batteries doesn' appeal to you then you could get some higher capacity cells. Others can do a better job than me of directing you towards 2700 mah cells or higher, I exclusively use LSD.

Thanks for the advice. This is my first foray into rechargeables that has lasted past the first try.

You are correct that most of my batteries are Energizer 2500 mA-h.

I now have four Titanium 2700 mA-h that I snagged from BatteryJunction recently.

Do you (and anyone else) have a charger recommendation?

What is "LSD"?

and . . . THANK YOU!

LSD = Low Self Discharge
Not really relevant for you.
NiMH batteries loose their charge while in storage. Typically 1/10 of the charge is lost each month (storage at room temperature). LSD cells (or 'precharged cells') do hold up much better. Disadvantage is, that the max. capacity of these cells are typically lower (2000 mAh).

Charger: There is an older thread that gives you an idea what you need to consider in a good charger: https://www.candlepowerforums.com/threads/70935

Hello Vegas,

The 15 minute chargers by both Energizer and Duracell are very sophisticated chargers. They are not simple timed chargers.

The charge stops after about 15 minutes, or when the cell is fully charged.

These chargers give you around 100 - 150 cycles from your cells, so that is less than the 400 - 500 cycles you expect from 1 hour charging. In that way they are harder on your batteries.

However, there are times when having cells ready to go quickly is worth the loss in cycle life.

Also, keep in mind that even at 100 cycles, a rechargeable battery is still a good value.

Tom

Hello Rino,

The Maha C9000 is a good charger to check out.

The Energizer 2500 mAh cells have not faired very well. They seem to quickly develop high rates of self discharge. They work fine right off the charger, but if you let the sit for a day or two, then end up completely empty.

The same goes for most of the other brands of 2500 mAh cells.

LSD stands for low self discharge rate. You can search on Eneloop to find a lot of information on these cells. Basically, they have very little self discharge and are great if you charge your cells up and let them sit for a while before using them.

Tom

Thanks for all of the feedback, guys. It is most appreciated.

I'm also going to do some research at batteryuniversity when I have time so I can be a bit more fluent when I ask my questions!

The LSD thing is interesting and something I have not considered. I was focused solely on the total amount of charge the cells can hold without regard to how much they'd lose over time. I clearly need to rethink and I will give the Sanyo Eneloops close scrutiny. The Eneloops I see for sale are 2000 mA-hr, whereas the other Sanyos are rated for 2700 mA-hr. Should I assume that the 2700 mA-hr Sanyos will "self discharge" faster than the Eneloops, or do they share similar technology?

Will I see a diminished output of light in my TK20 from the Eneloops? Obviously it won't be as bright as long, but will the maximum be about the same when they are freshly charged?

THANK YOU!

drmaxx said:

NiMH batteries loose their charge while in storage. Typically 1/10 of the charge is lost each month (storage at room temperature).

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I would say more like at least 15% pcm

I find most of mine are pretty useless after 3 months, non use in anything requiring high current, ie camera or torch.

I think for daily use light - highest possible mAh rating is preferable as long as you deplete/recharge 2700 batteries often. For lower usage lights/backups - Eneloop can't be beat.
BTW, it has been found that Eneloop are very durable and have less voltage sag under high load than other rechargeable batteries. If your light has very high amp draw (5 amp??? whatever) Eneloop may provide a comparable performance.

Somebody found that break-even point for Eneloop vs 2700 is about 60 days. It was derived by 1 person on this forum. I am doing my own self discharge test on Eneloop 2000 mAh vs Sanyo 2700 mAh to verify this info.
All batteries are new and right after Maha 9000 breakin cycle.

They will be discharged at 500mA on Orbit PocketLader to find capacity.
Edit - this is for my primary light -TK20

abvidledUK said:

I would say more like at least 15% pcm

I find most of mine are pretty useless after 3 months, non use in anything requiring high current, ie camera or torch.

Click to expand...

Here's a site in German - translated with babelfish:
http://babelfish.yahoo.com/translat...kutest/index.html&lp=de_en&btnTrUrl=Translate

Go down and look at the chapter self discharge (for new batteries). Some measurements with room temperature and fridge.

drmaxx said:

Go down and look at the chapter self discharge (for new batteries). Some measurements with room temperature and fridge.

Click to expand...

Self discharge for new batteries doesn't always tell the whole story. After a bit of use and abuse the self discharge performance can get much worse than "as new" performance. I really have not seen any regular NiMH cells that managed to hold a useful charge beyond two or three months of storage.

I've had very good luck with the Sanyo Eneloop AA's and the Rayovac Hybrid AA's. The Eneloops can usually be found at Circuit City, the Rayovac Hybrids at Wal-Mart. Both provide very good shelf life after charge. In fact, after 6 months sitting in my digital camera, they show 80% charge.