# How come my rechargeable battery self discharges so fast?

1. Feb 23, 2016

### grasscut

Hi, im working on a piezoelectric energy harvesting project. i was able to use my piezoelectric to charge up my AA NI MH battery 1300mAH from 0.93V to 1.201V. However, once i removed the battery from the battery holder, the cell voltage starts to drop without being connected to any load.
Anyone know what might be the reason to this?

2. Feb 23, 2016

### anorlunda

3. Feb 23, 2016

### grasscut

Last edited: Feb 23, 2016
4. Feb 23, 2016

### anorlunda

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.

5. Feb 23, 2016

### grasscut

I didnt use the battery much other than discharging it with a torchlight. i was surprised too when i removed the NiMH battery out of the battery holder and realised the battery are already starting to discharge. So let say i have a NiMh that is working fine, when i charged it up to 1.2V, it shouldnt drop within few mins after its been removed from the holder right?

6. Feb 23, 2016

### anorlunda

Right. At least not by that much.

7. Feb 23, 2016

### CWatters

Cell voltage isn't really a good indication of how full a NiMH cell is. Just because the voltage was 1.2V doesn't necessarily mean it was fully charged.

Best way is to start with it discharged (to say 1V) then measure the charge current. Multiply by time to get the number of mAH that you have put into it. Calculate how full it should be as a percentage of it's nominal capacity. The charging process is typically only 80% efficient.

Likewise if you want to know how much is in a battery the best way is to set it up on a load and measure how much you can take out until the voltage falls to 1V.

8. Feb 23, 2016

### CWatters

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.

9. Feb 23, 2016

### Staff: Mentor

What source is the energy you're collecting?

10. Feb 23, 2016

### meBigGuy

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.

11. Feb 23, 2016

### grasscut

Im using vibrational energy to convert it to electrical energy via piezo
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. Im not sure why either. Im 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?

12. Feb 23, 2016

### Staff: Mentor

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?

13. Feb 23, 2016

### Staff: Mentor

Yeah.

Try charging it with a real charger to verify the cell is working properly.

14. Feb 23, 2016

### meBigGuy

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.

15. Feb 24, 2016

### grasscut

Yes, im thinking whether the current is too insignificant, but somehow it shows that the battery voltage is increasing. For my circuit, i used a full bridge rectifier using diodes and a smoothing capacitor before connecting to the rechargeable battery. The vibration is provided via a speaker. The battery temperature doesnt really have any charges when im charging it. So are you saying that the battery voltage are just increasing because of the constant input of the source, but its not actually charging ?

90% at 1.2V is determined from a literature review.
https://institutes.lanl.gov/ei/pdf_files/JIMSS2005.pdf

16. Feb 24, 2016

### meBigGuy

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.

17. Feb 24, 2016

### grasscut

Am i right to say that based on the graph, at 1.2V of the battery when its charge, its only about 10-20%?
Thank you for sharing your knowledge with me. Since the battery that i charged almost discharged instantly when removed from the circuit, is it still considered that the battery was been charged?
Like you said, i might need to charge until the voltage stop or reaches it full capacity, which was 1.27V when i bought it.

18. Feb 25, 2016

### Staff: Mentor

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.

19. Feb 25, 2016

### grasscut

Isit because there is some power that is dissipated at the smoothing capacitor end? i cant leave it to charge for weeks though, i dont not have access to my lab for such a long hours. So i have quite limited choice. Yes, the piezoelectric has very high voltage output but the output power isnt too much. i can get approximately 5.8mW from one of my piezoelectric.

20. Feb 25, 2016

### Staff: Mentor

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.