# Charging 3.7V battery from 300W+ generator

I am doing a project involving a bike generator of 500W. All of my previous projects were small LiPo battery powered devices so I want to double check certain aspects of this relatively high power project with people who might know more.

All the basics are in place, the generators output is rectified and so on. The main issue I'm having is the dc/dc buck converter LMR16020 which is connected to a battery charger MCP73812 to charge 3.7V LiPo. The dc/dc out is at 5V and according to the datasheet of LMR16020 the max current output is 2A. So the max output power in this case is 10W.

Now assuming there is a constant 5W load connected to the output of the buck converter, to prevent issues when no battery is charged, and the MCP73812 battery charger is charging with a total of 400mA (battery charging current and the required supply current for the unit, the value is hypothetical). This should get us to a total load of 5W+2W=7W on the buck converter's output. For the sake of simplicity let's assume that there are no losses in rectifier and a dc/dc buck converter.

1. If this is all that is connected to the generator is it safe to assume that the output that generator sees is 7W and the buck converter is safe from high current damage? Since it's a bike generator the 7W load should only affect the difficulty to turn the rotor. And since it's a small load it should just be easy to rotate. The current of the generator should be determined by the load of the generator if I'm not mistaken.

2. If the output voltage of dc/dc is increased via FB resistor adjustment the max power that can be connected to the buck's output would increase because P=I*V? This is assuming the input power and voltage are high enough. In this exact scenario a different charger might be needed but that's besides the point.

I'm aware that some components and the generator are not ideal for the application, but they are the things that were "laying around" and they seem to meet minimum requirements for the project. There will be additional loads on the generator so this isn't a case where a 500W generator is used to charge a LiPo battery, it's just the aspect of the project I'm least certain about. I'm also aware that this isn't some unique or difficult concept, but I could't find the exact answer to this and as mentioned before I haven't done anything with this kind of high power so I would prefer to be called bad at searching or stupid than ruining components or generator in any way.

My main worry with the high power is that for whatever reason the dc/dc circuit will take the some of the extra power and fry itself and it's load. In previous projects LDO converters were enough so the dc/dc converter experience is lacking.

Edit1: Don't have the model or datasheet of the generator, but its fairly similar to this

• Is this a petrol engine driven generator or push bike? You should clarify in your question. I don't think any regular cyclist could output 500 W for very long. – Transistor Apr 29 '18 at 15:04
• I'm aware that 500W is not realistic, but it's the generator i had "lying around" and it should be ok with loads of up to 500W like ~150W or less which from what I read is reachable for an average person. It's rated at 30V so when it reaches it's max rpm there should be 30V but the load should decide the current and in the same way power output. – afff Apr 29 '18 at 15:13