The self-discharge is 5–20% on the first day and stabilizes around 0.5–4% per day at room temperature. But at 45 °C it is approximately three times as high.
anorlunda said:https://en.wikipedia.org/wiki/Nickel–metal_hydride_battery#Self-dischargeIs the rate you see consistent with that?
anorlunda said:That translates to 32% per day if I did the calculation right. I suspect that your battery is damaged. NiMh batteries are easy to damage, especially if they are allowed to get too hot during charging.
Nickel Cadmium batteries have lower self discharge rates than NiMh. Most solar-powered garden lights use Nickel Cadmium batteries.
grasscut said:t shouldn't drop within few mins after its been removed from the holder right?
What source is the energy you're collecting?grasscut said:, I am working on a piezoelectric energy harvesting project.
Im using vibrational energy to convert it to electrical energy via piezoNascentOxygen said:What source is the energy you're collecting?
Thank you guys for the reply! Sorry for not being clear. Anyway, it is correct that 1.2V that i obtained is not fully charged. Its assume to be 90% charged. Because to detect whether the battery is fully charged, which can be done by sensing of temperature within the cell and voltage dip, additonal controller will need to be implemented which will dissipate power from the energy harvesting and its not ideal. Thus, i assume the battery to be fully charged when it reaches 1.2V of it cell voltage. As for current used to charged the battery, i wasnt able to get reading when i used a multimeter to measure the current. I am not sure why either. I am not quite sure what is wrong with the battery asit discharge almost within min after removed from the charging. I charged the battery from 0.93V to 1.2V in abt 5 hours time. If the battery is damaged, will it affect the charging time too?meBigGuy said:I suspect you are not fully charging the battery. You should read about NiMH charging. It is essentially a constant current process.
What current are you charging at? Note that when you are fully charged, the above chart shows 1.4V even when discharging.
Yeah.NascentOxygen said:If you are unable to get a reading of the current going into the cell it must be miniscule and you may need to charge it for a couple of weeks. You do have a diode in the circuit?
NascentOxygen said:Ordinary NiMH AA cells seem not to be affected adversely by limited overcharge, providing they don't get hot. Warm is fine. I would guess you are barely charging the cell, given the source. Try leaving it charging for 48 hrs, monitoring its temp just by feel. If it feels no warmer than body heat, it's okay.
If you are unable to get a reading of the current going into the cell it must be miniscule and you may need to charge it for a couple of weeks. You do have a diode in the circuit?
Can you give more details about your setup? What provides the vibrations?
meBigGuy said:You need to charge it to a higher voltage. 1.2V even at very low current is barely charged. The above graph says 10%. What makes you think you are 90% at 1.2V?
It is also possible that self discharge is significant compared to your charge current.
Leave it on your low current charger until the voltage stops rising (or the battery gets warm). It won't get warm until it is fully charged.
During the experiments the
time required for the battery to charge past the
cell voltage of 1.2 volts was measured in each
case. This is not a complete charge but is approximately 90% full
CWatters said:Take a look at this charge/discharge curve..
http://www.cobasys.com/pdf/tutorial/InsideNimhBattery/inside_nimh_battery_technology_files/image018.gif
Work out how full the cell is in % when it reaches 1.2V on the charge curve. Then look up that % on the discharge curve to see what voltage it might be when taken off charge.
meBigGuy said:I would question their unsubstantiated statement that "charging past 1.2V" represents a 90% charge. I would charge until the voltage stopped changing and assume that is fully charged. Then, based on that and typical curves, decide what voltage might indicate 90%, or, do some calibrated discharge experiments to determine the battery's true capacity at its rated discharge. Also, you need to determine its self-discharge rate.
Try leaving out the smoothing capacitor. This may deliver a voltage which some of the time is greater than what you're using, and may achieve better charging. Though I think you just need to give it a few weeks to charge; your source is a high impedance, of low power.grasscut said:i used a full bridge rectifier using diodes and a smoothing capacitor before connecting to the rechargeable battery.
NascentOxygen said:Try leaving out the smoothing capacitor. This may deliver a voltage which some of the time is greater than what you're using, and may achieve better charging. Though I think you just need to give it a few weeks to charge; your source is a high impedance, of low power.
From a high impedance source, the capacitor would be averaging the potential; its average value is lower than its peak value. There may also be a bit of capacitor leakage, and you don't have any current to waste.grasscut said:Isit because there is some power that is dissipated at the smoothing capacitor end?
NascentOxygen said:From a high impedance source, the capacitor would be averaging the potential; its average value is lower than its peak value. There may also be a bit of capacitor leakage, and you don't have any current to waste.
5.8mW, if that were indeed the power delivered, means at 1.2v you have only 5mA. So a 2500mAh AA cell would need around 600 hours to fully charge, though in practice this low charging rate may not even be able to keep up with the cell's losses, meaning it can never charge.
You measured 21 volts across 80kΩ?grasscut said:The 5.8mW was obtained from a resistive load of 80Kohm when i test the performance of the piezoelectric.
Apologies, its 68K Ohm resistor.NascentOxygen said:You measured 21 volts across 80kΩ?
meBigGuy said:5.2mW /1.2V = 4.3ma. At 100% efficiency that would charge your battery in 1300/4.3 = 302 hours = 12 days
I've seen it said that NiMH self discharge is 2% per day, which would be 26mah, which over 24 hours would be about a 1ma load.
Assuming 1ma leakage, and 1.3V average voltage that would increase to 433 hours = 18 days. (1300/((5.2/1.3) - 1)/24)
CWatters said:The graph I posted suggests that at 1.2v the cell might only be 1% charged...but its not safe to assume even that much.
I think you should concentrate on trying to measure a meaningful charge current. Until you can do that best assume its not being charged at all.
grasscut said:my battery at full capacity only reads 1.27V,
meBigGuy said:You have not explained how you know it is at full capacity when it reads 1.27V. Do you also have a real NiMH charger? (BTW, I am doubting your conclusion that it is fully charged)
You are placing too much reliance on that voltage. It's more of a guide for a typical cell. The best way to estimate a cell's state of charge is knowledge of the charge you have put into it, and it seems you are delivering minimal charge from your novel source.grasscut said:I measured the cell voltage of 1.27V when i first bought it. without using it at all, is brand new.
NascentOxygen said:You are placing too much reliance on that voltage. It's more of a guide for a typical cell. The best way to estimate a cell's state of charge is knowledge of the charge you have put into it, and it seems you are delivering minimal charge from your novel source.
meBigGuy said:When brand new, the cell has been self-discharging on the shelf for as long as it has been since it was charged at the factory. And, they may not have charged it to 100%. 1.27 volts is just what it happened to be at when you measured it. It has no meaning.
You need to fully charge it with a NiMH charger if you want to characterize it.
Rechargeable batteries can lose their charge quickly due to a process called self-discharge. This occurs when the battery is not in use and slowly drains its energy over time.
The rate of self-discharge varies depending on the type of battery and its age. Generally, it can take anywhere from a few days to a few months for a rechargeable battery to self-discharge completely.
While self-discharge is a natural process, there are ways to prevent it from happening quickly. Storing the battery in a cool and dry place can slow down the rate of self-discharge. Additionally, using the battery regularly can also help maintain its charge.
Yes, the type of device can have an impact on the self-discharge rate of a rechargeable battery. Devices that have a standby or sleep mode, such as smartphones and laptops, can cause the battery to self-discharge faster compared to devices that are completely turned off when not in use.
To extend the lifespan of a rechargeable battery, it is important to properly maintain and use it. This includes fully charging and discharging the battery at least once every few months, avoiding extreme temperatures, and using the battery regularly. It is also recommended to replace the battery when it starts to show signs of decreased performance.