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4 AA battery charger

by Trevo
Tags: battery, charger
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Jul16-14, 01:09 PM
P: 22
Hi I need a circuit to charge 4 aa lithium ion batteries. I need it to charge at 5 volts and when the batteries are done charging, the circuit cuts off. It would help a lot if I could get this in a small space around 2 quarter's. Thank You.
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Jul16-14, 02:27 PM
berkeman's Avatar
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Quote Quote by Trevo View Post
Hi I need a circuit to charge 4 aa lithium ion batteries. I need it to charge at 5 volts and when the batteries are done charging, the circuit cuts off. It would help a lot if I could get this in a small space around 2 quarter's. Thank You.
Welcome to the PF.

What have you found in your web searching so far? Can you use an off-the-shelf charger?
Jul16-14, 02:45 PM
P: 22
Well I'm trying to make a portable cellphone charger that I can recharge so I don't want to get an external charger and I am just beginning in circuitry so the Web is a little confusing

Jul17-14, 04:33 PM
P: 245
4 AA battery charger

The proper way to charge a battery is to do it in stages depending on the charge state of the battery. When the battery is dead it must be fed by a current source. The reason is that if you put a voltage source on a dead battery it will draw too much current and cause damage. The current has to be limited in the first stage. At some point the battery will have enough charge so that it won't draw a high current from the voltage source. At that point it needs be switched to the voltage supply so that it can be topped off.

AA batteries have a nominal voltage of 1.5V and you said you have 5V to work with. You're going to need a boost circuit to get up to 6V if you want to use a single charge unit for all 4 batteries. Alternatively, you could use a buck circuit to create a 1.5V source if you wanted to charge each battery separately. That would require 4 charge units. You could even buck to 3V and charge 2 batteries at a time with 2 charge units.

The best way to get it done would be to buy something off-the-shelf. If you really want to make something then you should consider buying an all-in-one-chip for battery charging. Texas Instruments makes a line of Li chemistry chargers. They have great documentation with a complete circuit that will probably do what you want. Check out this datasheet and look at figure 7. You can buy that chip at Mouser or Digikey. You'll still need to boost your voltage to 6V for that circuit.
jim hardy
Jul17-14, 05:33 PM
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P: 3,735
whoah - thanks for that link ! A charger ic so smart it can figure out which type battery it's charging ?
and in 8 pin dip !

That one's a keeper ! Thanks..
Jul17-14, 06:34 PM
P: 245
By the way. They make what you want Trevo and it's not terribly expensive.

Battery backups on Amazon.
Jul18-14, 12:22 AM
P: 1,084
lithium-ion is nominal 3.6V per cell, BTW

If you want to charge 4 batteries, you need a charger that can deal with 4 batteries in different states of charge. I don't personally know how the consumer chargers do that. You are better off with a larger single cell battery pack and a boost converter. Also, if you over-discharge a li-ion cell it is destroyed. Some li-ion battery packs have built in protection circuits.

If you overcharge a li-ion battery it explodes or catches fire. Very dangerous technology.

Properly charging LI-ion batteries is non trivial. They start with constant current charge until they reach 4.2V +- 10mV, and then you reduce current to maintain 4.2V until the current becomes very small. You can imagine trying to do that for 4 batteries in differing statesw of charge.

To create a "USB Compatible" charger is also not trivial.

Also, getting a cellphone to charge is not trivial. Many will only charge if connected to a charger that obeys the correct charger protocol. Iphone, nokia, etc are all different. Look at figure 3 (page 16) in this datasheet. It show what is needed to fool different phones into charging. (usually shorting DP to DM is adequate)

After you solve the battery charger issue, you need to create 5V +- 5% power supply or the phone won't chage. And, depending on the phone the current demand can be from 500ma to 1.5A.

You should buy a commercial pack to do what you want for the phone you own. Robust, tested with the phone, etc.
I make battery chargers, and I would not build my own due to packaging, reliability testing, etc. You won't be able to match a commercial solution for ruggedness and abuse.

BTW, The TI bq2000 chip has a design example for a single li-ion cell in figure 7. Not sure how to expand that to 4 cells. It also describes the 2 stage (actually there are three) charge algorithm for li-ion.
Jul20-14, 06:50 PM
P: 22
Okefenokee with the bq2000 would i need to use all pins?

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