Battery Open circuit voltage just after charging

[Super_sonic]

Does any one know why the open circuit voltage of a battery decreases steadily, just after charging it. My battery seems to be decreasing about 0.05V every 10 seconds.

 

[Nigel Goodwin]

Does any one know why the open circuit voltage of a battery decreases steadily, just after charging it. My battery seems to be decreasing about 0.05V every 10 seconds.

I couldn't give you a chemical reason for it, but it's perfectly normal - why are you concerned about it? - it will only drop to the voltage it's supposed to be.

 

[Super_sonic]

well its alwayse good to know that i didnt harm my battery. I was looking at some graphs of NiMH battery voltages when charging like the one attached. Im trying to get the same graph for my battery. I havent built a charger yet but im doing it manually through a power supply by adjusting voltage and keeping the charging curent constant. Im taking the cell voltage reading by turning off the power supply every couple of minutes and taking the open circuit cell voltage, but rate of decrease in this cell voltage at the begining is around 0.01A per second. I was wondering if the graph is doing the same as me i.e by taking the initial value of the cell voltage and not worrying about the decrease in voltage.

cheers

 

[Nigel Goodwin]

well its alwayse good to know that i didnt harm my battery. I was looking at some graphs of NiMH battery voltages when charging like the one attached. Im trying to get the same graph for my battery. I havent built a charger yet but im doing it manually through a power supply by adjusting voltage and keeping the charging curent constant. Im taking the cell voltage reading by turning off the power supply every couple of minutes and taking the open circuit cell voltage, but rate of decrease in this cell voltage at the begining is around 0.01A per second. I was wondering if the graph is doing the same as me i.e by taking the initial value of the cell voltage and not worrying about the decrease in voltage.

Your problem is you're disconnecting the charger, the charger should be feeding the battery all the time with a constant current, and you can monitor the battery voltage while it's charging - once it's fully charged the voltage will drop slightly (as on the graph), you detect this and turn the charging OFF.

 

[Super_sonic]

The only problem with that is that you cant monitor the battery voltage while its charging, cause the battery terminal voltage changes to the voltage being applied to it, i was thinking that when i want to take measurements i decrease the voltage being applied right to the point where current just starts flowing outof my power supply, so that would mean that my power supply just got above my battery voltage, You think that would do it?

 

[Nigel Goodwin]

The only problem with that is that you cant monitor the battery voltage while its charging, cause the battery terminal voltage changes to the voltage being applied to it, i was thinking that when i want to take measurements i decrease the voltage being applied right to the point where current just starts flowing outof my power supply, so that would mean that my power supply just got above my battery voltage, You think that would do it?

NO!!! - read the previous post!.

 

[JohnBrown]

Listen to what Nigel says. Yes, the voltage that you measure will be higher when the charging current is applied, but it will still follow the same sort of graph. If you are pulsing the charge, for a slower charge rate, you need to make sure that you always take the reading at the same time relative to the pulse, just before turning the pulse off, for example.

 

[bananasiong]

can i ask some questions?
after my nimH AA batteries are fully charged, i measured them one by one using multimeter. they gives 1.385V, or some even 1.4V. but as i know, AA rechargeable is only 1.2V right?

 

[Nigel Goodwin]

can i ask some questions?
after my nimH AA batteries are fully charged, i measured them one by one using multimeter. they gives 1.385V, or some even 1.4V. but as i know, AA rechargeable is only 1.2V right?

WRONG! - 1.2V is the nominal voltage, they will be higher when fully charged, and lower as they discharge,

 

[spuffock]

When all the active material is used up, the battery fully charged, the current will cause some water to be electrolysed. Some of the hydrogen produced will remain, and slowly dissipate. Since hydrogen has a higher electrode potential than the other cathode materials, it will produce a higher voltage until it is all gone. At least, that's what I remember from school. It was a long time ago.

 

[bananasiong]

can i charge my rechargeable batteries before they r fully discharged? For example the AA, when the voltage drops, around 0.8V is left, can i charge it?
fow how long should i charge it? i mean for those AA which is not fully discharged, since my energizer charger doesn't show it.

 

[audioguru]

Hi Banana,
Energizer have many chargers. Cheap ones that fully charge batteries overnight and you can leave batteries in them for a long time without damage nor shutoff. They also have expensive "fast chargers" that sense the battery's temperature and voltage to detect full charge then shut off.
Which charger do you have?

 

[bananasiong]

https://www.energizer.com/products/rechargeables/sizes.aspx
i couldn't find my charger from there, it looks like the value charger and the car charger, but "Compact Charger" is written on it, and my charger can charge 9V batteries, nimh and nicd also. For AA or AAA, 2 batteries must be charged together, the led won't turn on when only one battery is there.

Do u know which charger i'm using so far? I just bought it, i think it is new product of energizer, coz it came with 2 AA's 2500mAh.

 

[audioguru]

Ni-Cad and Ni-MH battery cells last longer if they are not overcharged.
When the voltage runs down to 0.8V per cell when loaded then they are discharged and need the full amount of charging time. If your charger doesn't indicate that they are fully charged then it probably doesn't have a detector or a timer so the cells should be removed from the charger after the recommended amount of charging time.

 

[raybo]

How complicated can it be to charge a battery. It needs I to charge not voltage. Voltage has to be greater then when the battery voltage is fully charged. Never bother to regulate the voltage just regulate the current. We all know if you leave a battery on a shelf it will slowly discharge that is the nature of the beast. Bottom line regulate the current and forget about the voltage it means nothing. Actualy a battery can have good voltage present but without current drain it means nothing it is just there.

 

[bananasiong]

When the voltage runs down to 0.8V per cell when loaded then they are discharged and need the full amount of charging time. If your charger doesn't indicate that they are fully charged then it probably doesn't have a detector or a timer so the cells should be removed from the charger after the recommended amount of charging time.

Okay, let's say (just for an example), my device can only be used 1V and above. If the AA drops from 1.2V to less than 1V, let's say it 0.8V. So i have to charge the battery by refering the charging time table given with the charger. As long as the charging time is not over the given chargiong time, no matter the battery is fully discharged or not, everything will be okay. Am i right?

i've heard that there is one charger which will discharge the battery fully before charging it. Has anyone heard this before?

 

[bananasiong]

another question

Instead of using a rechargeable 9V 150mAh battery (7.2V), i want to use 6 AA rechargeable battery, 1.2V (combination of a few 1600 and a few 1850 mAh). I will get the same supply voltage by 1.2V * 6 = 7.2V. Will i get longer lasting?

 

[Nigel Goodwin]

Instead of using a rechargeable 9V 150mAh battery (7.2V), i want to use 6 AA rechargeable battery, 1.2V (combination of a few 1600 and a few 1850 mAh). I will get the same supply voltage by 1.2V * 6 = 7.2V. Will i get longer lasting?

Yes, your batteries are rated at TEN TIMES the 9V one, so it's hardly a difficult question?.

However, NEVER MIX DIFFERENT BATTERIES, and I mean NEVER!!!, use either six 1600mAh or six 1850mAh, don't ever mix them!!.

 

[audioguru]

The 9V battery has six AAAA very skinny and small cells inside. That's why its capacity is only 150mA-hrs.

 

[bananasiong]

NEVER MIX DIFFERENT BATTERIES, and I mean NEVER!!!, use either six 1600mAh or six 1850mAh, don't ever mix them!!.

Why don't? My lecturer said that it is ok? Am i going to destroy it?

The 9V battery has six AAAA very skinny and small cells inside. That's why its capacity is only 150mA-hrs.

What is AAAA type battery?