i am making a battery application where i only want the current to go in direction. I dont want to use a diode (too big a voltage drop). Are there ICs/ckt that allow current in one direction only?

i have a charger that is charging the battery, i want the charger to source the current to the battery. I do not want the battery to power up the charger.

Any help would be appreciated.

What kind of battery chemistry are you charging? Most rechargeable batteries require specific profiles (constant current/constant voltage, pre-charge conditioning, end of charge detection, over voltage protection, temperature monitoring...) that goes well beyond making sure the current goes in one direction. Doing it wrong can be dangerous (fire, explosion, etc.)

There are many chips & circuits available specifically designed to safely charge batteries of many chemistries. Check out Texas Instruments (their bq line of charger ICs), Maxim, Micrel, Linear Technologies, and Microchip all have charger chips and eval boards for verifying a design. I'm sure others do as well.

That said, Linear's LTC4358 is an "ideal diode" capable of conducting 5 amps with 20 milliOhms series resistance. .http://www.linear.com/product/LTC4358. They have ones with other features as well.

A diode is the simplest solution. A Schottky diode will give you the lowest forward voltage drop.

A more complicated way to do it is to sense the current flow and disconnect if reverse current flow is detected. This is the way many of the battery charge ICs will do it, but as jimkirk says, they do many things besides depending on your battery chemistry and other needs. One way to do it yourself is to use a MOSFET in series with the charging circuit and bias the gate such that a higher voltage on the battery side than the charger side will turn off the MOSFET. How you do this in practice depends on many, many factors.

I'd re-iterate jimkirk's comments - making sure current goes in one direction is just one small consideration when charging batteries. Make sure you've covered all the other bases too before doing something unsafe.

"I have a charger..."

If you have a charger, then it's already designed to fulfill the functions inherent in that role. It would be a very rare exception where a battery charger requires an external diode.

In those rare cases, one could use a relay such that when external (AC?) power is applied to the charger, the relay connects the charger to the battery. If the charger is off, then the relay disconnects the battery. This assumes that you have a power source (e.g. commercial AC) that can afford to power a relay. This concept wouldn't be optimal if the power source was a solar panel.

perts - you can use back-to-back connected MOSFETs with a charge pump IC to drive them. This is the preferred method of interconnecting two DC voltage sources (ie - battery and charger) together, as you can tailor the turn-on time of the MOSFETs to reduce the inrush current from the battery charging the output capacitance of the charger.

Go to www.linear.com, and search "Ideal Diode". There are a number of IC offerings that do this function. Maxim also makes some.

Know what you mean about the battery and charger. A number of years ago, I went on a church orphanage construction project to Haiti. When there, they learned I was an engineer, and the town hospital wanted me to troubleshoot a battery problem they had with an emergency shortwave radio setup they had. Turned-out that someone had donated an expensive top-end battery charger to the hospital along with some new communications equipment. But, this was Haiti, and power comes from generators and frequently power is not available. The fancy charger was drawing current from the battery every time power was off, discharging it, and ruining the car battery after a few weeks of constant charge/discharge. Fun to troubleshoot when you have absolutely no equipment, not even a screwdriver, other than a VU meter scrounged from a old broken receiver. The hospital also had a broken X-ray machine, but I passed on trying to debug it!

Learned an important lesson as an engineer for my future designs - assume even the AC power to your device is available. Also, when they learned I was an engineer, they wanted me to re-design the roof trusses for the orphanage, but that's a whole other story.