Battery Chargers

[Macka]

Hi,

i want to make a battery charger, but first i want some info, please...

1) Are battery chargers conected up in reverse polarity as i think they might be...

2) Most battery chargers take 8 hours to charge the battery, but i saw one in the shop that garunteed a full charge in 15 mins, how do they do it (do they increase the voltage or the amps)

3) What voltage and amperage do i need to charge batteries.

4) Is there anything else i should know?

Thankyou

 

[k7elp60]

Hi,

i want to make a battery charger, but first i want some info, please...

1) Are battery chargers conected up in reverse polarity as i think they might be...

2) Most battery chargers take 8 hours to charge the battery, but i saw one in the shop that garunteed a full charge in 15 mins, how do they do it (do they increase the voltage or the amps)

3) What voltage and amperage do i need to charge batteries.

4) Is there anything else i should know?

Thankyou

To answer your first question. No the chargers are not connected up in the reverse polarity. The + of the charger goes to the + of the battery and - of the charger goes to the - of the battery.
To answer the rest of your questions we need to know what type of batteries. Are the NiCad, NiMh, Lead-acid or what. Also the number of cells or battery voltage, and capacity in Ma/H or Ampere/Hours.

 

[Oznog]

2) Most battery chargers take 8 hours to charge the battery, but i saw one in the shop that garunteed a full charge in 15 mins, how do they do it (do they increase the voltage or the amps)

The amount of time needed is inversely proportional to the amps. Twice the current, half the time, more or less. The voltage needs to go up *a little* to make that happen.

Fast charging usually requires special procedures specific to the battery technology you have. It's easy to keep blindly pumping amps into a battery that is already fully charged (called overcharging) and this can damage a battery.

 

[Macka]

No the chargers are not connected up in the reverse polarity. The + of the charger goes to the + of the battery and - of the charger goes to the - of the battery.

I should have said what i meant diffrently, but thats what i meant by reverse polarity.

To answer the rest of your questions we need to know what type of batteries. Are the NiCad, NiMh, Lead-acid or what. Also the number of cells or battery voltage, and capacity in Ma/H or Ampere/Hours.

The batteries I want to recharge are AAA NiMh, i want to be able to charge 1 or 2 at a time and the voltage is 1.2V and 850ma/h

 

[Macka]

2) Most battery chargers take 8 hours to charge the battery, but i saw one in the shop that garunteed a full charge in 15 mins, how do they do it (do they increase the voltage or the amps)

The amount of time needed is inversely proportional to the amps. Twice the current, half the time, more or less. The voltage needs to go up *a little* to make that happen.

is this right...

t=1/850
so to charge it in half the time
t=1/1700
how many volts will i need?

 

[audioguru]

High pressure and temperature build up quickly just before a Ni-MH cell reaches full charge. At higher charge currents detection of full charge is important to prevent damage and even an explosion.
Fast chargers for Ni-MH rechargable batteries detect the tiny drop in charge voltage that occurs near full charge then stop the charging. Backup temperature and pressure sensors ensure cutoff if the charging circuit misses detecting the tiny voltage drop.

Energizer recommends charging at 1/10th the battery's rated capacity if sensors aren't used, with an additional 20% extra time to make-up for losses. They say a battery's life is reduced if overcharging continues.

Ni-MH cells are charged with current, not voltage. The voltage must be high enough for the current to occur.

On graphs, rated capacity is at a current called "C". Here are a couple of graphs from Energizer:

 

[Macka]

How do you detect the voltage drop?

So the ideal charging voltage is 1.5v? and if so how long would it take to charge.

By the way, im sure you already know this, but just incase, overcharging damages the battery because you begin to break the acid up into its elements like when you run electricity through water you split it into Hydrogen and Oxygen. the split up elements are gaseous and increase the pressure and can explode...... KAABOOMB!!!!

 

[audioguru]

How do you detect the voltage drop?

With a battery charger IC! Maxim and other manufacturers have them.

So the ideal charging voltage is 1.5v? and if so how long would it take to charge.

No. Any voltage could be used for charging, as long as its voltage is high enough for your current-limiting circuit. One million volts could be used through a 12M resistor. When the battery is fully charged, each cell will be about 1.45V. Frequently, a constant-current circuit is used to limit the cuurent so a low voltage or a high voltage battery can be charged.

As explained before, the charging time is dependant on the amount of charge current. Divide the value of its rated capacity (850mA/hrs?) by the charging current (85mA?) and add 20%. This example will be fully charged in 12 hours. If the charging current is 170mA then it will be fully charged in 6 hours but will be damaged if its charging isn't stopped.

By the way, im sure you already know this, but just incase, overcharging damages the battery because you begin to break the acid up into its elements like when you run electricity through water you split it into Hydrogen and Oxygen. the split up elements are gaseous and increase the pressure and can explode...... KAABOOMB!!!!

Ni-MH batteries don't have acid, but gases and corrosive electrolyte squirts out of a vent if they get too hot with too much internal pressure during overcharge. If the current is high, KAABOOMB!
You can trickle-charge them with a low current forever.

 

[dinofx]

Can't you make a peak detector with an Op-Amp, diode, and capacitor? How long does this type of detection store the peak voltage? Can you use a second op-amp to hold and store the peak, almost like analog memory?

 

[BrianG]

Since I have been into hobby-class R/C for a little while I've learned that peak-detection chargers are by far the best. These types of chargers charge at a constant current (voltage will vary) and takes advantage of the fact that the battery charging voltage will drop a little at the end of a full charge cycle. They see this drop (usually .2v or less) and trigger the end of charging. The MRC Superbrain 959 (**broken link removed**) is an excellent, relatively inexpensive charger that can charge from 1 to 8 cells of NiMH or NiCD batteries, with an adjustable current, peak detection threshold, and/or time limit - or use the "Set and forget it" option and it will pick the best settings automatically. It can be plugged in to house 120v AC or use a 12v car battery if desired. I got mine specifically for R/C charging needs, but I've since made some adaptors so I can charge all the batteries in the house and have thrown away all those crappy wall-wart chargers. Incidentally, I've taken apart a couple of those cheaper chargers and all they had for "circuitry" was a small AC transformer with two diodes for full-wave rectification directly hooked to the battery terminals. Ugg.

Generally, a higher charge rate of less time will give the battery a higher voltage peak for more "punch", while a long, low current charge will provide a longer run-time, but with a slightly less battery voltage. For example, when I charge my GP3300 sub-C cells (3300mAh), I charge them at 4.5 amps for as long as it takes (about 20-25 minutes). This will give my 7.2v (6 cell) packs more punch for electric motors. But, when I charge my receiver pack (6v 1100mAh), I want a longer runtime so I charge those at .5A or 1A. No matter what, a peak charger will charge them only as much as they need without overcharging or undercharging. When charging, the battery should be a little warm at the end of the charge. NiMH cells tend to be more sensitive to high temperatures, so it's best to keep them from getting too hot. NiCDs are more forgiving, but they generally don't have the high capacities found in NiMH cells.

Hope this helps...

[Edit]: I almost forgot: It's not good to run NiMH cells down until they are totally dead. I've read this causes "cell-reversal" and may render the cell unusable. For best results, NiMH cells should be discharged to no lower than .9v per cell.

 

[audioguru]

Can't you make a peak detector with an Op-Amp, diode, and capacitor?

Sure, but what is the peak voltage? It depends on current, temperature and even the phase of the moon. You need a circuit to detect the peak then a small drop in the voltage.

How long does this type of detection store the peak voltage? Can you use a second op-amp to hold and store the peak, almost like analog memory?

You could calculate the RC time constant, but you are re-inventing the wheel since battery charger ICs do everything properly for you.

When I was young, the 1st voltage regulator ICs were released. I couldn't buy one so I made one with about 30 transistors. It didn't work. :cry:

 

[leogc]

The art of charging

Charging batteries correctly is a fine art. There’s a lot of info out there and a lot of dedicated ICs for the job. Check out www.national.com/appinfo/power/files/f7.pdf
to start.