Go Back   Steve's Digicams Forums > Digicam Help > Batteries or Power Packs

Reply
 
Thread Tools Search this Thread
Old Jan 10, 2003, 9:38 AM   #11
Senior Member
 
Join Date: Nov 2002
Posts: 117
Default

The original question was about slowing down self-discharge. While it's true that under use, batteries will perform much better in warm condition, when they are being stored they prefer cold conditions.

Below are the highlights from a post that I put in some other forums ealier today after someone asked about the same thing regarding battery packs:

Tests while storing batteries show that if you put a Ni-MH battery in 45ēC conditions for 1 month it looses 75% of its charge, at 20ēC it looses 25% of its charge, and at 0ēC it looses just 10%. This is due to the battery's internal resistance dropping at high temperatures.

However when using the batteries, they like a low internal resistance to give more usuable capacity, so here it is better to have them in the warm. This problem becomes more evident when high currents are used - like with in cameras.

The moral of the story is if you're obsessed with prolonging battery life, store them in the cold and use them in the warm.

Hope that makes sense the way I have written it.
Eddie J is offline   Reply With Quote
Old Feb 11, 2003, 2:08 PM   #12
Junior Member
 
Join Date: Feb 2003
Posts: 2
Default Its Chemistry Jim

Its all down to internal resistance.

Remember when a cell (they're not 'batteries' unless they are more than one cell like your car battery ) is constructed there is a positive plate and a negative plate. You discharge a cell by creating an electrical path between each plate. We normally only think of one path here - the one outside the cell, but as the elecrolyte in the cell will also be slightly conductive there is an internal path too. The internal resistance is one measure of how conductive this internal path is.

With dry (ok non rechargeable) cells the internal resistance is quite high, so the self discharge rate is very low. This is why you can store alkalines for a long while. They do self discharge but over a period of years.

Now when you discharge a dry cell the properties that make it store for a long while work against it, the internal resistance is high so an amount of the energy goes into heating the cell - as power is a function of current[squared] times resistance as you try to draw more and more current this becomes dramatically more obvious and there is an effective limit to how much current you can draw - and as any of you who've used alkalines as an emergency backup will know they get very hot under these conditions. Digicams with backlights can easily draw 1 to 2 amps - which is a frighteningly high draw for alkalines and they will get very hot and deliver less energy overall than you would expect (it goes into the heat)

With rechargeable cells the internal resistance is quite low. This means that the internal path is more favorable and so the self discharge rate will go up. In turn this means that your storage life is coming down - for NiCd in the region of months, for NiMh in the region of weeks.

Of course the great adavantage is when you come to discharge them, the lower internal resistance means less energy is lost to heating the cell and NiCd/NiMh doesn't flinch at delivering 10's of amps if needed. This is why you should allways carry spares in a case - a short circuit with your keys say will cause a current of 60 odd amps to flow and this will rapidly raise the temperature of the cells where they will either cause burns directly, or cause gassing in the cell leading to the emergency vent on the cell opening and very hot gas being discharged, or if that fails to cope the cell might burst. Believe me you want none of these things happening in your pocket or in the bag next to your nice camera.

The internal resistance of rechargeable cells is very dependant on the temperature of the electrolyte in the cell. The lower the temperature the higher it gets, and as we all experience the ability of the cell to deliver energy goes down - the other energy isn't lost - but in cold conditions you will find the cells warm up - the internal resistance has gone up, so more power is lost to heat in the cells.

Again this explains why if you have a fairly intense shooting session things seem better - the cells start to warm up and thier internal resistance drops and things get better. Shoot sporadically then the cells cool down and the resistance rises again.

So if you want to lower the self discharge rate of your cells, you need to lower thier temperature. You need to be reasonable about this as if you lower the temperature of the electrolyte to the point where a state change will happen you may do bad things to the cell. Think freezing water turning to ice. Although most electrolytes won't expend if they freeze (water is fairly unique in this respect - but it can happen in lead acid cells) and damage the cell its still a bad idea.

Exact limits will vary with the electrolyte chemistry and the mechanical construction of the cells - something only your cell maker can tell you. However if you think water you wont go far wrong, so keeping the cells on the door of the fridge (around 5 celsius) should bring a useful drop in self discharge rate. I'd double check with manufacturing data if freezing them (around -18 to -25 celsius) is a good idea.

So in summary think about the internal resistance - its high if the cell is cold, and low if the cell is warm. What helps the cell to deliver its energy efficiently also helps it self discharge faster - they are two sides of the same coin.

When you want to use the cells you want them warm, when you want to store the cells you want them cold.

So a good practical scheme is to store the cells at as low a temperature as is safe, warm them to body temperature before use.

If you are shooting in the cold keep the working set of cells warm in an interior pocket, but keep the spare cells in the bag and nice and cold. I've found it best to run 2 sets in the cold - one in the camera (cold - otherwise you can get condensation) and one in a chest pocket. When I want to shoot I load the warm cells into the camera, put the cold cells in the chest pocket to warm up again. Everytime you think the cells may be cold again swap them. Its a hassle but you will get better usable life out of the cells and get more pictures in your shooting session.

As a final note unless you have a very good charger its probably not a good idea to leave the cells on charge - overcharging can lead to some of the effects often mis-labelled as 'memory' effect. Check the charger documents and don't leave them on any longer than it says for your type of cells. Cheap slow chargers just limit the current and rely on you to take the cells out after the time the manual. Cheap fast chargers just run a timer and charge at a high current and then drop to the lower current, and again its up to you to take them off in time. Good chargers monitor the individual state of each cell, and will usually have indicators on them to indicate the state of each cell. Overcharging the cells in the belief that you will get more capacity or in trying to account for self discharge is likely to do more damage than anything else you can do - normal life is 500-1000 charge/discharge cycles under ideal conditions, overcharging can shorten this to 50.

Gosh that was a long post - hope it helps - feel free to ask any other questions and I'll do my best!
CharlieO is offline   Reply With Quote
Old Feb 12, 2003, 3:54 PM   #13
Senior Member
 
Join Date: Aug 2002
Posts: 2,162
Default

Quote:
When you want to use the cells you want them warm, when you want to store the cells you want them cold.
Spot on! my feelings exactly! works with alkalines, my wife goes barmy everytime she opens the fridge - it's full of inkjet refills and batts!
voxmagna is offline   Reply With Quote
Old Mar 7, 2003, 6:50 AM   #14
Junior Member
 
Join Date: Mar 2003
Posts: 14
Default

Quote:
Originally Posted by steve
Nothing takes the zap out of a rechargeable battery faster than cold temperatures. Just ask anyone who lives up north and uses their camera outdoors. Refrigerating NiMH would not be productive.

-Steve
yea.. the temps have been low.. the lowest so far was 3F batterys did not last as long as in the house.
digitalcamuser is offline   Reply With Quote
Old Mar 12, 2003, 8:36 AM   #15
Junior Member
 
Join Date: Feb 2003
Posts: 16
Default

I hope this isn't redundant, but I just found this on the MAHA sponsored NiMH FAQ website:

"At normal room temperature, NiMH batteries can generally retain 70% of their charge after 30 days. Normal self discharge of NiMH batteries is about 1 percent per day at normal room temperature. Of course, environmental factors and higher temperatures will play an important role in the above said value and will cause NiMH batteries to discharge at a slightly higer rate. Lower temperatures ( 40° to 60° F ) on the other hand will cause NiMH Batteries to hold their charge longer."

You can see it for yourself at:
http://faq.alltekpower.com/faq/index.asp?a=4&q=17

This suggests that refrigerating them would help, at least in a frig set to 40°. It doesn't say about lower temps.
Daddyfish is offline   Reply With Quote
Old Mar 12, 2003, 12:48 PM   #16
Senior Member
 
Join Date: Jun 2002
Posts: 1,050
Default You're all completely wrong.

Gotcha :lol: :lol: :lol: :lol: :lol: :lol:

CharlieO, I've been using NiCds etc for model car racing for many years and using car/caravan bats to in turn to charge the model car's bats so I know a thing or two.

That was a spot-on post.
steve6 is offline   Reply With Quote
Old Mar 12, 2003, 12:50 PM   #17
Senior Member
 
Join Date: Jun 2002
Posts: 1,050
Default

The heading trick doesn't work does it - never mind.
steve6 is offline   Reply With Quote
Old Mar 12, 2003, 3:22 PM   #18
Senior Member
 
Join Date: Aug 2002
Posts: 2,162
Default

I'd go with CharlieO.!
voxmagna is offline   Reply With Quote
Old Mar 12, 2003, 9:45 PM   #19
Senior Member
 
Join Date: Jan 2003
Posts: 134
Default

I believe people are getting mixed up with nickel halide and alkaline batteries since it maybe beneficial to store alkaline batteries in a fridge. some batteries such as lead acid batteries will have 1/2 their amp power at low temps.
kschewe is offline   Reply With Quote
Old Mar 13, 2003, 3:45 PM   #20
Senior Member
 
Join Date: Aug 2002
Posts: 2,162
Default

No, the point being made is lower temperatures are good for long term storage because of lower self discharge, BUT cold batteries are worst for delivering energy, so after you warm 'em up, you've got more in them after a couple of weeks in the fridge, than if you left them out in the sunshine. Of course most charge before use, so the only benefit of 'fridging' is if you're away in deep s**t without a charger, or if leaving NiMh's to go absolutely flat is bad for them. Personally, I favour the intelligent charger with trickle after charge and batts permanently on trickle charge. The self discharge rate can then be compensated by the trickle level.

I think that's the gist. Basic chemistry is reactions are slower when chemicals are cold (so that's your self discharge) but reactions are faster when warm (so that's your power state)
voxmagna is offline   Reply With Quote
 
Reply


Thread Tools Search this Thread
Search this Thread:

Advanced Search

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off
Trackbacks are Off
Pingbacks are Off
Refbacks are Off



All times are GMT -5. The time now is 4:56 AM.