NiMH battery life
- Thread starter BrendanP
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mark.duyvesteyn
Member
NiMH batteries have a high a rate of self discharge, so you could probably half the above estimate.
The newer type of NiMH are much better, but still have bad shelf life compared with non-rechargeables.
have a look here
http://en.wikipedia.org/wiki/Nickel_metal_hydride_battery#Self-discharge
The newer type of NiMH are much better, but still have bad shelf life compared with non-rechargeables.
have a look here
http://en.wikipedia.org/wiki/Nickel_metal_hydride_battery#Self-discharge
I'll put this comment in to perspective I think ... These new NiMH's are way better than previous NiMH. It's a night and day comparison. They may not compare well to a purpose built low power lithium backup cell for shelf life but they really do do well for a rechargeable. And they are readily available at the local shop, unlike the latest LiIons.
Evan
westaust55
Moderator
NiMH batteries
Some further info (related to Sanyo NiMH batteries and likely others):
Batteries are typically given an Amp-hour (Ah) rating based on a particular discharge rate. If the actual discharge rate is lower then the availabel Ah capacity can be a little higher. COnversely as the discharge rating increases, the effective Ah capacity is reduced.
Some further info (related to Sanyo NiMH batteries and likely others):
You do need to look at the datasheet for the battery you are considering (you do not state the make or model).
Batteries are typically given an Amp-hour (Ah) rating based on a particular discharge rate. If the actual discharge rate is lower then the availabel Ah capacity can be a little higher. COnversely as the discharge rating increases, the effective Ah capacity is reduced.
All things being equal, yes, 370 hours. 3700maH is pretty good. Dippy will no doubt say "serve you right" for the following, but I recently bought two different brands of NiMH batteries off ebay, both rated at 2500mAh. One really was 2500mAh. And one was 800mAh.
If the capacity is real (and it probably is), then the self discharge etc may well be better too. NiMH technology is moving ahead at present. The only problem is how to catch the fakes.
One wonders whether this could be a useful picaxe project - an amp hour measuring system for batteries?
If the capacity is real (and it probably is), then the self discharge etc may well be better too. NiMH technology is moving ahead at present. The only problem is how to catch the fakes.
One wonders whether this could be a useful picaxe project - an amp hour measuring system for batteries?
thursdaybloom
New Member
Can I just add that if you drain a NiHM battery to empty it is basically dead and hard to resurrect. I'm of the belief that the mAh rating on a rechargable is it's max [since a higher number sound better right?] but if you were to force a battery to supply this max then you've just killed the battery
True in a battery pack - complete discharge is not wise. But it is ok if it is a single battery running a stepup converter and you completely flatten it. Eg solar garden lights. NiMH are happiest when completely flat. Just like lead acids are happiest when charged.
So if you want to really look after the batteries you stop discharge at 1.1V. If the device stops working suddenly, so be it.
Of course, consumer electronics don't work that way. Your cordless drill will keep going, and getting slower, and 3 of the cells in the pack will be reverse charged and will be dying rapidly and the next time you charge you find they have lost 10% of their capacity because you boiled off some of the electrolyte. Then after 10 charges you find the pack is dead. Then you buy another drill. And then the manufacturer is happy because they keep selling you consumer products.
Makes one think about a picaxe project to prevent reverse charging batteries...
3700mAH is sweet though. What brand are they, and what price?
So if you want to really look after the batteries you stop discharge at 1.1V. If the device stops working suddenly, so be it.
Of course, consumer electronics don't work that way. Your cordless drill will keep going, and getting slower, and 3 of the cells in the pack will be reverse charged and will be dying rapidly and the next time you charge you find they have lost 10% of their capacity because you boiled off some of the electrolyte. Then after 10 charges you find the pack is dead. Then you buy another drill. And then the manufacturer is happy because they keep selling you consumer products.
Makes one think about a picaxe project to prevent reverse charging batteries...
3700mAH is sweet though. What brand are they, and what price?
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Why not just do as WA suggests - read the DATA SHEET ???????
Then you can see specifications and DEFINITIONS so you can compare.
http://www.farnell.com/datasheets/101929.pdf
and
http://www.farnell.com/datasheets/59550.pdf
Is it REALLY SO difficult?
Sorry, I have little sympathy if something from an Ebay supplier turns out erroneous or rubbish. But I know you knew that I was going to say that haha.
"The only problem is how to catch the fakes." - don't buy from Ebay for a start
Buy from a reputable supplier who has been in existence longer than Ebay. I know you'll pay more but they will be genuine makes and if you can demonstrate that the product is rubbish then you will get a refund.
I had an incident when I stupidly bought some cheap Chinese CFLs from CPC. I got a refund. No fuss. Try doing that with the Beijing Anonymous Duff Products-for-Cheapskates Company Ltd via Ebay.
There must be an equation/algorithm for this...
Then you can see specifications and DEFINITIONS so you can compare.
http://www.farnell.com/datasheets/101929.pdf
and
http://www.farnell.com/datasheets/59550.pdf
Is it REALLY SO difficult?
Sorry, I have little sympathy if something from an Ebay supplier turns out erroneous or rubbish. But I know you knew that I was going to say that haha.
"The only problem is how to catch the fakes." - don't buy from Ebay for a start
Buy from a reputable supplier who has been in existence longer than Ebay. I know you'll pay more but they will be genuine makes and if you can demonstrate that the product is rubbish then you will get a refund.
I had an incident when I stupidly bought some cheap Chinese CFLs from CPC. I got a refund. No fuss. Try doing that with the Beijing Anonymous Duff Products-for-Cheapskates Company Ltd via Ebay.
There must be an equation/algorithm for this...
Dippy, I just threw out a line and... Sorry, couldn't resist.
That first one is 2700mAH with a nice pic of a battery with 2850 written on it.
But that would be correct if there is a discharge curve.
I am interested in 3700mAH - that is getting up to LiPO capacities. I remember my first rechargeables I ever bought. 400mAH. And they leaked after 5 charges. And they were from a reputable supplier (ebay didn't exist back then).
I did some $/benefit calcs on my cheapie 800mAH batteries though. They pay for themselves after two charges. So they are still better value than non-rechargeables. Refilled the kid's torches again tonight. And will do that again in two night's time as the kids seem to spend a lot of time reading under the bedclothes. So I'm happy with my purchase!
That first one is 2700mAH with a nice pic of a battery with 2850 written on it.
But that would be correct if there is a discharge curve.
I am interested in 3700mAH - that is getting up to LiPO capacities. I remember my first rechargeables I ever bought. 400mAH. And they leaked after 5 charges.
And they were from a reputable supplier (ebay didn't exist back then). I did some $/benefit calcs on my cheapie 800mAH batteries though. They pay for themselves after two charges. So they are still better value than non-rechargeables. Refilled the kid's torches again tonight. And will do that again in two night's time as the kids seem to spend a lot of time reading under the bedclothes. So I'm happy with my purchase!
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Good point Drac, but complete flattening can be iffy surely?
When cells are connected in series, the cell with the lowest capacity will reach a lower point of discharge than the others. The more cells that are connected in series, the greater the possibility of a cell being fully discharged and driven into overdischarge and polarity reversal.
Device designers can help prevent overdischarge by designing a cutoff voltage for device operation of 1.0 volt per cell.
DURACELL claim that their rechargeable batteries have capacities that are “matched” to each other. So that series packs can more consistent - ignoring manufacture failure of course.
PS. Yes, I know I get baited by Ebay purchases , ditto Alibaba. But the risk is too much for my liking. I'll leave the politics and hypocricy for another time....
When cells are connected in series, the cell with the lowest capacity will reach a lower point of discharge than the others. The more cells that are connected in series, the greater the possibility of a cell being fully discharged and driven into overdischarge and polarity reversal.
Device designers can help prevent overdischarge by designing a cutoff voltage for device operation of 1.0 volt per cell.
DURACELL claim that their rechargeable batteries have capacities that are “matched” to each other. So that series packs can more consistent - ignoring manufacture failure of course.
PS. Yes, I know I get baited by Ebay purchases , ditto Alibaba. But the risk is too much for my liking. I'll leave the politics and hypocricy for another time....
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Sorry, miscommunication there on my part. Complete flattening of a series of batteries (two or more) is always very risky. But complete flattening of just one battery is fine, because it can never be reverse charged. The worst that will happen is that it will go to zero volts. But using single cells and stepup converters is a bit of a specialised field. Though some of the newer MP3 players will run on a single cell. They will stop working at about 0.6V so the battery never gets near zero volts anyway.
In a pack of many cells with high discharge currents, there might be minus 1.5V on a flat cell. Very bad. You can hear the gas escaping via the safety valve.
The more cells, the more chance of reverse charge. Eg a device running on two batteries probably will not work at all when one goes flat. But a device running on 10 batteries in series will probably keep running when several are flat and being reverse charged. And the user probably won't know or care.
I'm sure there must be a picaxe project here somewhere. Eg, measure the volts across groups of 3 batteries in a pack, and provide an alert when the group of three is undervoltage. One 08M per group of 3 batteries.
In a pack of many cells with high discharge currents, there might be minus 1.5V on a flat cell. Very bad. You can hear the gas escaping via the safety valve.
The more cells, the more chance of reverse charge. Eg a device running on two batteries probably will not work at all when one goes flat. But a device running on 10 batteries in series will probably keep running when several are flat and being reverse charged. And the user probably won't know or care.
I'm sure there must be a picaxe project here somewhere. Eg, measure the volts across groups of 3 batteries in a pack, and provide an alert when the group of three is undervoltage. One 08M per group of 3 batteries.
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But regardless, it is NOT good to have a single cell in a pack going into polarity reversal.
Your comment about gassing/venting is important. Extended overdischarge will lead to elevated cell pressure.
Ensuring you have matched (and perhaps non-Ebay) batteries and limiting the cutoff voltage is obviously the sensible way to go to reduce this possibility. I wouldn't dream of mixing Proper Makes with Ebay Specials.
Anyway, this is digressing from the capacity issues. The two Data Sheets I linked show capacities as defined by discharge to 1.0V for a single NiMh cell at various xC rates.
Real life and variable dicharge rates will complicate your calcs, but at least when capacities are defined with the same parameters it gives you a good chance of comparisons.
Maybe the Ebay specs were for discharge down to -1.0V
Your comment about gassing/venting is important. Extended overdischarge will lead to elevated cell pressure.
Ensuring you have matched (and perhaps non-Ebay) batteries and limiting the cutoff voltage is obviously the sensible way to go to reduce this possibility. I wouldn't dream of mixing Proper Makes with Ebay Specials.
Anyway, this is digressing from the capacity issues. The two Data Sheets I linked show capacities as defined by discharge to 1.0V for a single NiMh cell at various xC rates.
Real life and variable dicharge rates will complicate your calcs, but at least when capacities are defined with the same parameters it gives you a good chance of comparisons.
Maybe the Ebay specs were for discharge down to -1.0V
I've still got my first ever NiCd from back in 1971, a 500mA.h Sanyo AA,costing the equiv. of US$20 in todays money, & amazingly it still "performs" after 37 years.
The traditional way to verify NiCd/NiMH capacity is to hook them to a suitable resistive load (if 2500mAh maybe thus draining at C/10 = ¼A), in parallel with a simple analog battery clock. Start the clock at 12:00 & note at what time it later stops. Easy!
If you're unhappy about draining cells completely,use a lamp load with a LDR to hold off a sounder/relay etc until the lamp fades out- NiCd/NiMh have a "sudden death" end of delivery characteristic of course. Banks of low voltage Xmas tree lamps make cheap loads,as the admirable move to more reliable & efficient LED types means such classic lamps are bargain bin specials. I picked up a series string of 100 (c/w 230 V mains plug pak) for ~US$5 last year - each runs brightly on 2 x AA.
Several recharges & repeat runs are of course worthwhile. Stan
The traditional way to verify NiCd/NiMH capacity is to hook them to a suitable resistive load (if 2500mAh maybe thus draining at C/10 = ¼A), in parallel with a simple analog battery clock. Start the clock at 12:00 & note at what time it later stops. Easy!
If you're unhappy about draining cells completely,use a lamp load with a LDR to hold off a sounder/relay etc until the lamp fades out- NiCd/NiMh have a "sudden death" end of delivery characteristic of course. Banks of low voltage Xmas tree lamps make cheap loads,as the admirable move to more reliable & efficient LED types means such classic lamps are bargain bin specials. I picked up a series string of 100 (c/w 230 V mains plug pak) for ~US$5 last year - each runs brightly on 2 x AA.
Several recharges & repeat runs are of course worthwhile. Stan
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Thanks for everyones input.
Im using Gold Peak NiMH's. I bought a hundred through their distributor in Adelaide, it worked out at around 5$ AU each.
The MAX 713 data sheet mentioned the GP batteries. I thought I might be able to buy direct from the manufacture in say 1000 battery lots, but they passed me on to their Au agent. I hate paying retail.
GP are not well known/used in Australia, more common in the US it seems.
Im using Gold Peak NiMH's. I bought a hundred through their distributor in Adelaide, it worked out at around 5$ AU each.
The MAX 713 data sheet mentioned the GP batteries. I thought I might be able to buy direct from the manufacture in say 1000 battery lots, but they passed me on to their Au agent. I hate paying retail.
GP are not well known/used in Australia, more common in the US it seems.
westaust55
Moderator
In looking at the GP NiMH typical/standard data,
1. at a temperature of 25degC, after 150 days, the capacity is down to 60% of rated capacity.
2. To achieve the nominal rating of the battery the current drain is stated as 0.2C (=C/5) at 25degC (this is the 5-hour rate which is fairly typical of the discharge rate used for most battery types for rating purpose.
So 3700mAh /5 hr = 740mA draw will give a duration of 5 hours.
1. at a temperature of 25degC, after 150 days, the capacity is down to 60% of rated capacity.
2. To achieve the nominal rating of the battery the current drain is stated as 0.2C (=C/5) at 25degC (this is the 5-hour rate which is fairly typical of the discharge rate used for most battery types for rating purpose.
So 3700mAh /5 hr = 740mA draw will give a duration of 5 hours.
I don't think Brendan has said what the cell size is. It certainly isn't an AA as there is no AA's at that capacity. And the bulk price on AA's should be at least half what he has just paid anyway.
I'd say he has bought from one of the following - http://www.gpbatteries.com/html/techinfo/racing_car.asp
Here is some catalogue ones I found - http://au.farnell.com/9772847/electrical-circuit-protection/product.us0?sku=gp-batteries-gp370schr&_requestid=175609
and http://au.farnell.com/174701/electrical-circuit-protection/product.us0?sku=gp-batteries-gp380afhhb&_requestid=175747
Looking at the prices, one can see Brendan has got a good price on them.
Evan
I'd say he has bought from one of the following - http://www.gpbatteries.com/html/techinfo/racing_car.asp
Here is some catalogue ones I found - http://au.farnell.com/9772847/electrical-circuit-protection/product.us0?sku=gp-batteries-gp370schr&_requestid=175609
and http://au.farnell.com/174701/electrical-circuit-protection/product.us0?sku=gp-batteries-gp380afhhb&_requestid=175747
Looking at the prices, one can see Brendan has got a good price on them.
Evan
Note: It may or may not be a fake. Many NiMh's need a good long trickle charge the first time or so to attain full capacity. On a new battery, the sophisticated chargers, which are based on changes in the charge rate, are set for normal, used batteries. As I understand it, a brand spankin' new battery will spoof the intelligence into thinking more has been put in than has and cause an early shutoff. If you have not trickle charged the first time, sometimes running a few discharge/recharge sycles (many smart chargers can do this) will ease it up to full capacity.
I have seen 2000mAh batteies, new from a reputable distributor I use (batteryspace.com), take as little as 100mA, or even less in charge and quickly die. Then, after cycling a few times, take a full charge.
We use these a lot in RC Model Warship combat for smaller ships due to the increased power density. For larger ships (like the 6 foot battleships), we just use SLA's for ballast and get electricity as a happy byproduct.
Cheers,
Wreno