Correct Battery Charger Voltage for rechargable AAA/AA batteries

cepheus

To my surprise I tested the voltage on my AA/AAA charger yesterday and it was 5 volts (five) across every one of the individual sets of terminals! Surely this is far too high even for a rapid charger. I think rechargable batteries such as Li Ion are typically around 1.4v so the charge should be around 1.5-1.6v isn't it?

Jnelhams

It's more to do with Amp's (Pressure) of the electricity not the Voltage. For instance an AA battery is 1.5v and typically 2000mah (That's a tiny Ampage) Now for your battery to recharge you need a higher Ampage to pass through the battery and react with the chemicals inside the battery so that it is recharged. The voltage is of little relevance in this case, and it may come as a surprise to people that the same voltage in AA battery (1.5V) is enough to kill you, but as the Ampage (Pressure) supplied is only 2000mah (2amps in an hour) you are safe, but if the Ampage was .33ma it would kill you stone dead.

cepheus

So are you saying the charger might not be faulty?

I'd be interested if someone else could perform a voltage test on theirs.

Perhaps it is designed for an AAA/AA and allows for the internal resistance of the battery so when the battery is inserted it drops to around 1.5v?

Ectophile

Just tried it on my AA/AAA charger. About 6.2V with no batteries inserted, and it's worked fine for years.

The charger will have current regulation so as not to overcharge the batteries. If you measure the voltage with a battery inserted, it will be a lot less.

PS. Don't confuse battery capacity in mAH (milliamp-hours) or AH (amp-hours) with the current they can deliver (in Amps). They are completely different things.

cepheus

Thanks, I had a problem charging up some AAAs for a cordless phone so that is what made me suspicous. The Phone charger works fine with the same batteries.

gmgmgm

Jnelhams wrote: »

It's more to do with Amp's (Pressure) of the electricity not the Voltage. For instance an AA battery is 1.5v and typically 2000mah (That's a tiny Ampage) Now for your battery to recharge you need a higher Ampage to pass through the battery and react with the chemicals inside the battery so that it is recharged. The voltage is of little relevance in this case, and it may come as a surprise to people that the same voltage in AA battery (1.5V) is enough to kill you, but as the Ampage (Pressure) supplied is only 2000mah (2amps in an hour) you are safe, but if the Ampage was .33ma it would kill you stone dead.

Huh?

An AA battery should be able to give out 500mA sustained without too much trouble.

TimBuckTeeth

cepheus wrote: »

To my surprise I tested the voltage on my AA/AAA charger yesterday and it was 5 volts (five) across every one of the individual sets of terminals! Surely this is far too high even for a rapid charger. I think rechargable batteries such as Li Ion are typically around 1.4v so the charge should be around 1.5-1.6v isn't it?

Most battery chargers use cheap basic circuitry. Often just a diode (to prevent reverse current flow) and a resistor (to limit current) connected to the transformer. An off load voltage of around 5V is normal.
More advanced chargers with timers and full indicator LEDs often still use the basic diode and resistor as the charging part of the circuit.
The voltage of a fully charged NiCad or NiMh cell will not rise above about 1.3V due to the internal chemistry.
A resistor of 10 ohms will give a charge current of around 300mA.

(5 - 0.7 - 1.3 ) / 10 = 0.3A

(Supply voltage - diode Vf - cell voltage) / resistance = current

TimBuckTeeth

Jnelhams wrote: »

It's more to do with Amp's (Pressure) of the electricity not the Voltage. For instance an AA battery is 1.5v and typically 2000mah (That's a tiny Ampage)

As mentioned by Ectophile the 2000mAh rating of the battery is the capacity. The battery could supply 2000mA for 1 hour or 1000mAh for 2 hours etc. depending on the load resistance but does not give any idea of the maximum current available.

Now for your battery to recharge you need a higher Ampage to pass through the battery and react with the chemicals inside the battery so that it is recharged.

The standard (slow) charge current is typically 1/10th of the capacity. eg. 200mA. Higher currents are used for fast charging, around half the capacity eg. 1000mA, but can make the batteries quite warm.

The voltage is of little relevance in this case, and it may come as a surprise to people that the same voltage in AA battery (1.5V) is enough to kill you, but as the Ampage (Pressure) supplied is only 2000mah (2amps in an hour) you are safe, but if the Ampage was .33ma it would kill you stone dead.

A rechargeable AA battery can deliver over 5A if short circuited, limited by its internal resistance.

A 1.5V battery connected across dry skin would typically give less than 1mA current through the body due to the resistance of the skin (ohms law I=V/R) so would be imperceptible.

If the 1.5V was applied directly across the heart (in a surgical open chest scenario), due to the lower resistance, the current could be enough to disrupt the normal heart rhythm.

With a rechargeable single cell 1.2V battery the circumstances for electric shock are very specific, unlikely and nothing to worry about, however burns or fire due to short circuit are potential problems.

forgotmyname

NiCad and NiMH batteries need different charging conditions.

NiCad's drop power gradually with use, NiMH holds onto the power and it drops
rapidly at the end.

Different charger settings are required.

cepheus

Actually I forgot my old electrical engineering lessons, and mistook these chargers for transformers which output a fixed voltage. For these chargers measuring the voltage without current flow into the battery is meaningless. Once the battery is inserted the charging voltage drops to around the 1.5v level.