# When you charge a rechargeable AA battery... does it applies voltage reversed?

#### rambomhtri

Joined Nov 9, 2015
481
Hi, I have a quick question. Your average rechargeable AA battery 1.2V:

When it's dead, I believe it gets to 0.8V or so, and you put it in a charger, does it applies voltage in the opposite direction to charge it?

What is the maximum V a charger would apply to a battery 1.2V AA to charge it (assuming I made up those constant 1.2V and it's probably a variable V with different values)?

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#### Tonyr1084

Joined Sep 24, 2015
6,894
No. Reverse charging would destroy the battery. As far as the proper voltage for charging batteries depends on battery chemistry. A Nickel Metal Hydride requires specific charging voltage and current. Off hand I don't know that information but you can look it up on line.

#### BobTPH

Joined Jun 5, 2013
4,937
The voltage changes as the battery charges. The charger places enough voltage on it to push a certain amount of current. The final voltage for a NiMH cell is about 1.4V

Bob

#### rambomhtri

Joined Nov 9, 2015
481
So a battery charges if you put external voltage in the same direction?
Without knowing it, it really sounds like it's just pushing the battery to discharge more, isn't it?

Then... if you put a battery in parallel with another battery, both get charged?
No, of course not, but why?

#### MrChips

Joined Oct 2, 2009
26,145
Think about it in this way.

When the battery is being discharged you are taking charge out of the battery.
In order to restore the battery to its charged state you have to put charge back in to the battery. Hence the +ve terminal of the charger has to be connected to the +ve terminal of the battery.

So a battery charges if you put external voltage in the same direction?
Without knowing it, it really sounds like it's just pushing the battery to discharge more, isn't it?

Then... if you put a battery in parallel with another battery, both get charged?
No, of course not, but why?
You cannot get something from nothing, i.e. there is no free lunch.

The battery with the higher voltage will lose charge and its voltage will decrease.
The battery with the lower voltage will gain charge. Its voltage will increase until both batteries are at the same voltage.

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#### BobTPH

Joined Jun 5, 2013
4,937
Current flows from plus to minus. If You connect the minus of a voltage source to the plus of s battery, current will flow out of the battery. To make current flow into the battery, you need to connect a higher plus to the battery plus.

Bob

#### nsaspook

Joined Aug 27, 2009
9,745
Think about it in this way.

When the battery is being discharged you are taking charge out of the battery.
In order to restore the battery to its charged state you have to put charge back in to the battery. Hence the +ve terminal of the charger has to be connected to the +ve terminal of the battery.

You cannot get something from nothing, i.e. there is no free lunch.

The battery with the higher voltage will lose charge and its voltage will decrease.
The battery with the lower voltage will gain charge. Its voltage will increase until both batteries are at the same voltage.

View attachment 249500
My pedantic angel says "you have to put charge back in to the battery" is amiss. You put (charge with) energy back into the battery by separating existing charge in the battery.

#### crutschow

Joined Mar 14, 2008
29,825
Technically, to charge the battery, you put current with energy through the battery in the reverse direction of the discharge, to reverse the chemical change that occurred when the battery was discharging and delivering energy.

To generate this (reverse) charge current the charging voltage has to be slightly higher than the intrinsic battery voltage (which changes with its particular state of charge).
Usually this current is limited in some manner to not exceed the battery's maximum rating.

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#### MrChips

Joined Oct 2, 2009
26,145
To add to that, if the charging voltage is the same as the battery voltage then no current would flow. In other words, the battery is not being charged.

#### nsaspook

Joined Aug 27, 2009
9,745
Technically, to charge the battery, you put current with energy through the battery in the reverse direction of the discharge, to reverse the chemical change that occurred when the battery was discharging and delivering energy.

To generate this (reverse) charge current the charging voltage has to be slightly higher than the intrinsic battery voltage (which changes with its particular state of charge).
Usually this current is limited in some manner to not exceed the battery's maximum rating.
Sure, the current circulates in the charging circuit with the same amount in and the same amount out of the battery but the electrical field (not the tiny KE of charged particles) energy transferred remains in the battery.

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#### rambomhtri

Joined Nov 9, 2015
481
OK I am lost. Can you do an electric circuit with the power supply with its polarity, the battery's polarity and its connection?

Parting from a dead 0.8V battery, how is it connected to recharge it?

#### MrChips

Joined Oct 2, 2009
26,145
If you took one battery and connected a wire between the +ve and -ve ends, you create a short. The battery would not like it and would be sure to let you know. A fully charged battery would become very hot very quickly.

If you stack two batteries, one on top of the other, the combined voltage would be double that of one battery.

If you connected a wire from the top +ve terminal to the bottom -ve terminal, both batteries would be very unhappy with you.

If we were to unstack the two batteries while maintaining the same electrical connections we would still have the same circuit as before which is still not good for the batteries.

Hence you want the +ve end of the battery to go to the +ve terminal of the battery charger. -ve goes to -ve.
Put a power resistor in series with the power supply and the battery in order to limit the charging current.
Measure the current with an ammeter in series with the battery and set the power supply voltage to give the desired charging current.
Do not charge with a current greater than C/10. For example, if the battery capacity C is 2500mAh, don't charge with a current greater than 250mA. As a rough estimate, an almost dead 2500mAh battery will be charged at 250mA for 10 hours.

#### BobTPH

Joined Jun 5, 2013
4,937
What the previous post shows is what we would call a trickle charger. They can be safely used to charge an NiMH battery in about 24 hours. A faster charger requires monitoring the current and adjusting the voltage to keep it charging at the same rate. It also requires detection of when the battery is fully changed and cutting it off at that point.

Bob

#### rambomhtri

Joined Nov 9, 2015
481
If you took one battery and connected a wire between the +ve and -ve ends, you create a short. The battery would not like it and would be sure to let you know. A fully charged battery would become very hot very quickly.
View attachment 249534

If you stack two batteries, one on top of the other, the combined voltage would be double that of one battery.

View attachment 249537

If you connected a wire from the top +ve terminal to the bottom -ve terminal, both batteries would be very unhappy with you.
View attachment 249538
If we were to unstack the two batteries while maintaining the same electrical connections we would still have the same circuit as before which is still not good for the batteries.
View attachment 249539
Hence you want the +ve end of the battery to go to the +ve terminal of the battery charger. -ve goes to -ve.
Put a power resistor in series with the power supply and the battery in order to limit the charging current.
Measure the current with an ammeter in series with the battery and set the power supply voltage to give the desired charging current.
Do not charge with a current greater than C/10. For example, if the battery capacity C is 2500mAh, don't charge with a current greater than 250mA. As a rough estimate, an almost dead 2500mAh battery will be charged at 250mA for 10 hours.
View attachment 249543
THANKS!

#### Audioguru again

Joined Oct 21, 2019
4,696
A Ni-MH cell like yours is charged with a constant current. Its voltage gradually rises until it is fully charged when the voltage reaches a peak then drops a little. At a full charge the internal pressure and temperature increase and the charger should be disconnected. Most chargers detect the voltage drop then turn off.

Here is a graph from Energizer battery company. Its lines are 1C for the capacity of the cell, 2500mA for your cell. The voltage rises to 1.55V while charging. With a charging current that is 0.3 times the cell's capacity the voltage peak is 1.42V. With an overnight charge at 0.1 times the capacity there is no voltage peak but the cell is fully charged at about 1.38V.

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#### rambomhtri

Joined Nov 9, 2015
481
A Ni-MH cell like yours is charged with a constant current. Its voltage gradually rises until it is fully charged when the voltage reaches a peak then drops a little. At a full charge the internal pressure and temperature increase and the charger should be disconnected. Most chargers detect the voltage drop then turn off.

Here is a graph from Energizer battery company. Its lines are 1C for the capacity of the cell, 2500mA for your cell. The voltage rises to 1.55V while charging. With a charging current that is 0.3 times the cell's capacity the voltage peak is 1.42V. With an overnight charge at 0.1 times the capacity there is no voltage peak but the cell is fully charged at about 1.38V.
Wow, so the voltage charging peak is roughly the same as the battery output?

BTW, there's one thing that I don't understand. If putting AA batteries in parallel makes them charge themselves until equilibrium, how is it that sometimes a 3 parallel battery devices stop working because two batteries are at 1.2V and another at 0.8V?

#### ericgibbs

Joined Jan 29, 2010
15,542
how is it that sometimes a 3 parallel battery devices stop working because two batteries are at 1.2V and another at 0.8V?
hi ram,
Consider the current requirement of the device being powered.
Can the 0.8V battery output enough current to help power the load device.?
E

#### Ya’akov

Joined Jan 27, 2019
5,676
OK I am lost. Can you do an electric circuit with the power supply with its polarity, the battery's polarity and its connection?

Parting from a dead 0.8V battery, how is it connected to recharge it?
Because a secondary (rechargeable) cell uses a reversible chemical reaction to store and release energy, you put the current into the cell in the same way it comes out. Positive is always positive and negative is always negative.

You can thing of each half of the charge-discharge cycle as “undoing“ the other. Push energy in, and like an electrochemical spring, push it back out just how it came in.

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#### BobTPH

Joined Jun 5, 2013
4,937
how is it that sometimes a 3 parallel battery devices stop working because two batteries are at 1.2V and another at 0.8V?
Batteries in parallel cannot have different voltages. If they have been charged and discharged while in parallel, they will always stay at the same voltage. The weak cell will just produce less current than the others.

If you put three cells in parallel when 2 are at 1.2V and one is at 0.8V, the voltage will fall to some voltage in between and current will flow from the charged ones to the unchanged one.

Bob

#### crutschow

Joined Mar 14, 2008
29,825
Wow, so the voltage charging peak is roughly the same as the battery output?
Slightly higher, yes.
Why is that surprising?
how is it that sometimes a 3 parallel battery devices stop working because two batteries are at 1.2V and another at 0.8V?
That's not possible.
The batteries must be in series, not parallel.