How does a battery charger work?

[kky]

I have little idea on how a battery charger works but i'll just state here what I think is the case and then state my queries.

When you receive an AC input from the mains there is first a step down transformer which lowers the voltage - then there is some regulation and it is converted to DC. This current is sent in the opposite direction of a battery which charges it.

Now my questions are...
1. Can we use a much smaller AC signal for this purpose - Something like the signal generated with a bicycle dynamo and then step it up\down as required. (what is typically the voltage produced by a bicycle dynamo)
2. What types of batteries can be charged in this manner?

Thanks a lot.

 

[TheDestroyer]

The types of batteries that support this are batteries with reversible chemical reactions, meaning you can go forward by applying negative voltage (connecting to a resistor or consumer), and backward by a positive one (charger)

For your first question, you need to understand how the current flows, to answer this question I'm going to compare it to 2 containers that contain gases, connected with a pipe, and every one has a piston. Now if you and your friend pressed on the pistons causing the same pressure on the gas, no gas will flow in the pipe, but if you press harder, the gas will start flowing from yours to his, until your container loses an amount of gas equivalent to the extra pressure you're applying. This should be very intuitive (hopefully for you, if not, say) to understand how gases flow.

In the same way, we consider electricity, what you do when you connect opposite directions is that you cause higher voltage (pressure) on the current (gas) so it flows from the source (adapter) to the battery, and keeps flowing until the voltage of the battery equals the voltage of the the source, and then the flow stops.

About the AC current, it's not possible because for the same reason, the voltage is going up and down, and actually this may harm the battery, as I think.

I hope this answers your question :), if not, just ask :)

Good luck :)

 

[rcgldr]

Any AC input is converted to DC. The charger senses the no load voltage then increases the voltage until some fixed of amount of current is going into the battery to be charged. For some batteries and chargers, a temperature sensor is used along with voltage and current readings. During charging, the smarter chargers will shut off the charging current and sense voltage, with a target voltage per cell depending on battery type (lead acid, nicd, nimh, li-poly-ion, ...). For lead acid batteries, the charging current is reduced as voltage increases, while the other battery types can utilize a near constant charging current.