Need assistance with Free loader style project.

The project is to make a phone charger similar to the free loader.

I have a low voltage supply (0.4) which I am converting to 5V using a charge pump and DC-DC Converter. The problem is that the circuit is unable to supply enough current directly from the source. So the plan is to have a battery which gets charged from the source. When a phone is connected the battery will be used in parallel with the source to charge the phone. The battery will potentially be charged continuously from the source, even when a phone is not connected.

My questions are:

1) What type of battery is best for this type of application? Should I use a battery charge management chip?
2) When a phone is connected and being charged is there anyway to set the source as the main supply and only top up the current from the battery if required.
3) I should probably go for a 4.8V battery as this will still be within the USB Spec. Otherwise it would be a 3.6V Battery and then use another DC-DC converter to boost it to 5 V. (Not very efficient)

Any tips / advice would be appreciated!

Thanks in advance!

First, how much current can the 0.4V supply?

Hi thanks for the reply. It can supply approximately 300mA @ 5V

What is the 0.4 supply? It would need to be about 4A.

blueroomelectronics said:

What is the 0.4 supply? It would need to be about 4A.

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That is what I was trying to ask????

MikeMl said:

That is what I was trying to ask????

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Oh I know that. Lets see what the OP has to say.

Its 1.2W so I should have said around 200mA.

No, 0.4V * 0.2A = 0.08W, or 1.2W/0.4V = 3A. Which is it?

Its 1.2W so I should have said around 200mA.

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My math says 1.2 watts at .4 volts is 3 amps.

Yes, 1.2W at 0.4V is 3A. At 5V its 240mA but its not 100% efficient so its around 200mA

What is the source of the 0.4V supply?