One battery at a time.

[concrete_hed]

I have an appliance that uses 3 AAA batteries, and I have a charger that can only charge 2 or 4 AAA batteries at a time. I need a way of charging all 3 batteries at once using the charger, without overcharging or blowing something up. I could just buy another rechargeable battery and then charge all 4 of them, but i dont want to. I was thinking of maybe shoving something between the contacts of where the missing battery should go so it is a complete circuit, but that will probly blow something up, or overcharge the other batteries. Any ideas anyone?

[JesuBungle]

I think you could use a capacitor instead of a battery, and somehow include that in the circuitry of the charger. You might have to do some research to find out how big of a capacitor to use though. Then after you charge your 3 batteries, just discharge the capacitor and start a new charge sequence.

[gcol]

How would you overcome the problem that a capacitor is neither constant voltage nor current? Also, you will find that even 100farad (big!) is nowhere near enough for an AAA

[Externet]

Hi.

Depends on what the batteries chemistry requiere for charging; constant current or constant voltage, and which type of charger it is.

 

No way a common capacitor can be inserted in place of the missing cell.

 

Measure the current on a bay with 2 cells under charge and place a resistor to obtain the same current in place of the missing cell bay. A light bulb could be used too, and if the charge current is under 20mA; a red led could take its place too, as it would provide ~1.5V drop and handle the current.

 

Miguel

[wpenrose]

I have an appliance that uses 3 AAA batteries, and I have a charger that can only charge 2 or 4 AAA batteries at a time. I need a way of charging all 3 batteries at once using the charger, without overcharging or blowing something up...

 

Without knowing more about your battery types and your charger, I think there is only one safe solution here. Get two sets of batteries. When both sets are discharged, you can recharge both sets (4 and then 2).

 

Most cheap chargers for nickel-cadmium or nickel metal hydride batteries just use current limiting, ie, a simple resistor in series with a voltage source, to keep the current below the safe limit for the battery. For AAA's that would be about 50 milliamps per cell. Usually the cells are in series, so simply removing a battery and shorting the connectors would increase the current above safe levels.

 

For instance, to charge three AAA nicad cells in series from a 12 V source, the total voltage drop between source and batteries is 12 V - (3 x 1.4 V) = 7.8 V. To limit the current to safe levels, you need a resistor of 7.8/.05 = 156 ohms and 1/2 watt.

 

Combining charged and discharged batteries in the same charger can result in a 'reverse-polarity' fault, where one of the batteries will be destroyed by the others. It may even burst.

 

Rechargeable alkalines and lithium ion batties need more complex chargers.

 

Dangerous Bill

[concrete_hed]

The batteries are rated 1.2V, 700mah and they are Nimh type. The resistor idea might work, but then I had another idea. What if I could use one of the charging bays to charge 2 batteries at a time by placing them parellel to each other? Or what if I charge 2 of the batteries and then place a fully charged battery and an empty battery into the charger, so one of the batteries would be charged twice. Would this cause an overcharge? All i know about the charger is that It takes 16 hours to charge the batteries.

 

I know this is a problem that is easily resolved, but I like to do things the hard way.

[Externet]

Hi.

Mixing two cells in series -one charged, the other not- is not a good idea as the charged one may overheat, or dry out. You would be exceeding its charging recommendation time.

 

Two cells in parallel is not a good idea either, as by any unbalance reason current may pass mostly into one and poorly into the other.

Cells in parallel is fine for lead-acid and Lithium-ion, but not for Nickels.

 

So what is the charging current measured?

I believe for AAA cells should be under 20mA and the red LED can work just fine.

If the charger is really limiting the current, should work equally well with only one cell on the bay. Measure the current! It is the case of a shorted cell put to charge; it should not damage the charger nor affect its companion cell, then a resistor or a short could work equally well.

Miguel

[JesuBungle]

My apologies. I was drunk when I posted that:rolleyes: . But yeah, find a resistor at the same rating as the battery and just throw it in there.

[concrete_hed]

Ok, thanks for your contribution to my question. I'll try the resistor thing, and if it blows up then... well, that'll be one lesson learnt the hard way.