Another noob question - self discharge.

[fireguy]

Hopefully there aren't forum rules against how many noob questions can be posted in one week, and hopefully I'm not annoying everyone with my silly questions. :poke:

I have some 2700 mAh generic NiMH batteries that I bought about 8 months ago. They seem to charge fine and give me reasonable life from the cells, but I haven't had any way to actually measure this until recently.

I know that the cells self-discharge and the cheap ones may be more likely to discharge more because of a lack of quality control, poor chemicals, etc.

However, I have a wireless weather station that uses 3 AA's inside and 4 AA's outside. I put these cheap NiMH batteries in the station in May and it's still going strong. After 9 months, wouldn't these self-discharge to the point where they would need to be recharged before being used? I'm wondering if having a constant small drain on the battieries (such as running the LCD display on the inside station or transmitting the wind/temperature from the outside station) might cause the self-discharge to be smaller?

I really don't know what I'm talking about here, so please excuse my ignorance. I don't know if my thought is plausible or not. Any thoughts?

John

 

[Hoggy]

I'm wondering if having a constant small drain on the battieries (such as running the LCD display on the inside station or transmitting the wind/temperature from the outside station) might cause the self-discharge to be smaller?

You know - I've been DYING to ask that very question for AGES now! :twothumbs I've been reading the batteries section for over a year and still don't recall seeing it asked or answered. I am of the belief that it might due my use of an outdoor weather sensor as well.

Although I replaced them with alkaline because I got sick of seeing the 'false' battery-low indicator from the outside sensor.. Not to mention running it in this kind of cold (-9.6 F now).

Looking forward to hear some battery guru's more educated answer!
:popcorn:

 

[Fallingwater]

Your station most likely draws so very little power that you don't notice the self-discharge. In the months the station has worked with your NiMH cells, it's very likely that only a very small percentage of the power they've lost has actually gone to the station; most has been lost as self-discharge.

For such applications, primary cells are usually the best choice. If used in cold weather, lithium AAs would probably be even better (but considerably more expensive, of course).

 

[was.lost.but.now.found]

I'm wondering if having a constant small drain on the battieries (such as running the LCD display on the inside station or transmitting the wind/temperature from the outside station) might cause the self-discharge to be smaller?

It's an excellent hypothesis. I recall seeing an illustration of self discharge on batteryuniversity.com, it was a picture of a battery with a small spigot on the side slowly loosing water (charge). If one were to place another spigot with equal or greater "flow" coming out of another location of the battery, would that not reduce the loss of "water" flowing out the other faucet (due to laws of physics which someone smarter than me can better explain:))? I think it would be very difficult to test though.

An added theory might also be that while high heat increases self discharge, low temps might reduce it. I'm a noob also so someone feel free to correct me if this is false.

 

[Resqueline]

My first reply to this was lost due to an error, but I'll try again, a little shorter though..
Reputed brands probably have a more consistent self-discharge characteristics than cheap cells do. Good cells should still retain some charge even after 1-2 years of storage at moderate temperatures.
You can identify high-discharge cells in a batch in that they will have a few 10s mV lower resting voltage after charging than their good "siblings" have.
All batteries will have a considerably lower self-discharge if stored in a fridge (and a lot lower in a deep-freezer) than at room-temperature.
I don't believe self-discharge is lowered by an accompanying "outside" discharge. Rather it is simply "masked" by it.
NiMh cells now have more capacity than alkalines so I'm not very surprised they can last this long in these low-power applications, but they'll "never" return their investment being used like that.

 

[Fallingwater]

All batteries will have a considerably lower self-discharge if stored in a fridge (and a lot lower in a deep-freezer) than at room-temperature.

I was under the impression that storing cells of any kind in the freezer can damage them.

I've found that tiny NiMH cells, such as the ones used in dynamo lights and such, have surprisingly low self-discharge.

 

[coppertrail]

Freezer storage of NiMH cells is not recommended. I store my NiMH and Li-Ion cells in sealed containers in the fridge and have had great results.

 

[Lynx_Arc]

It is possible your outdoor sensor works down to nearly 0.9v per cell and has a very low current use. most batteries will self discharge slower and slower the closer they get to .9v. I would check the batteries of your sensor to see what voltage they are running at as it may be possible they coud even damage rechargables.

 

[fireguy]

It is possible your outdoor sensor works down to nearly 0.9v per cell and has a very low current use. most batteries will self discharge slower and slower the closer they get to .9v. I would check the batteries of your sensor to see what voltage they are running at as it may be possible they coud even damage rechargables.

I never thought of that. Maybe I'll pull 'em out tomorrow and replace them, then see what voltage level they actually have come down to. Thanks for the heads up!

 

[Lynx_Arc]

if they are still above 1.1v I would just recharge them and keep using them as nimh are probably better suited for the cold than alkalines.

 

[Hoggy]

if they are still above 1.1v I would just recharge them and keep using them as nimh are probably better suited for the cold than alkalines.

Really? I know Lithium would be the best if temps go below -20F or so, but I thought Alkalines would be better than NiMH in the above -20F range..??

One of the main reason I replaced the NiMH in both outdoor sensors is because of the 'false' low-battery indicators due to the devices not seeming to understand the NiMH 1.2v norm. This seemed to really shine when I tried putting some Rayovac Hybrids in it this winter. After fully charging right after opening the package, the low-battery indicator came on within a week or two.

The other reason is that after using a couple of old alkalines(ones that were sitting around for several years), it lasted for well over a year and a half to 2 years! Another is the fact that in order to replace the batteries, one needs to remove 4 screws every time they need replacing. ;-)

SO - all-in-all, it's possible that the self-discharge was masked when I tried the normal NiMH's quite a while ago now.. (The base unit seems to be ok with NiMH's though.)
(For posterity - Radio Shack # 63-1030)

-----------

HOWEVER --

It still doesn't quench my geeky desire to know if a very low drain (I'm thinking testing both constant current as well as pulsed current here) will counteract the normal self-discharge of the battery itself. Obviously, I'm talking about drains that are lower than the normal self discharge rate.

I would try testing this myself, but I just don't have the equipment - nor enough knowledge to do so. Well - it's theoretically possible I might have enough knowledge - after trying to understand this forum for so long.. :D But then there is that whole pesky equipment issue. :poke:

 

[Kremer]

I got tired of that same false battery indicator and occasional shutdown in low temps a couple years ago, it happened with both Alk and NiMH. I put a set of L91's in the outside unit two years ago and it is still going strong. No problem even in the -10F and lower temps last week.

Outside weather stations/sensors in cold weather are one of the the perfect applications for Lithium primaries.

 

[Lynx_Arc]

alkalines definately have more problems with the cold than nimh. I put lithiums in mine because it is oversensitive to low voltages also and I had rayovac hybrids that after 3 months seemed to stop working right Lithiums start at 0.2-0.3v higher than alkaline and another 0.2v higher than nimh after a few days resting off a charger. They are more costly but after having alkalines leak in things I decided I didn't want to ruin the only sensor I had for 2 indoor thermometers since one of the sensors vanished without a trace (think someone took it).

 

[Hoggy]

hmmm.. Seems like I should consider putting some lithiums in there then. Thing is, I have a crapload of AA alkalines from original remote packaging as well as other original packages (like from flashlights. :-). I may try to use those first..
Would it really be ok to use those L91's(Energizer e2's as I understand) without blowing the circuitry? (I heard they start at 1.7v.)

The other thing I'm wondering is - in an application such as an outdoor weather sensor - is how long they'll last. I've read reports of lithiums lasting 10 years unused - so does that mean they should last about 9 years or so in an outdoor weather sensor?? THAT would be awesome! :grin2:

Thanks.