connecting two generators in parallel or series

[plusmartin]

Hi!


I want to charge a battery with two small generators that will spin on wind power.

At low speeds the voltage across the generators will not be high enough to surpass the 12V battery, therefore I want to connect the generators in series.

When the generators spin faster the voltage will be higher and each of them will be able to charge the battery by itself. I will connect them in parallel when this happens.

I designed a simple circuit, please give me your comments

 

[mister_rf]

You can use also a DPDT relay... :-D

 

[enjunear]

1. Use FETs (or a relay); you'll get a lower on resistance, which will improve your efficiency... each active BJT will drop ~(0.2V * charging current) watts, each.

2. Be careful with two generators in series. You'll limit yourself to the minimum current produced between the pair of them. You might consider keeping them in parallel and running the output through a buck/boost converter. Then you can set your output voltage accurately and minimize the power lost in the series/parallel switching network.

You'd need to run some simulations on the efficiency of both designs to see which would win (I have a feeling a well-tuned SMPS could beat the inherent losses of the series/parallel generator network).

 

[plusmartin]

Thanks!

relays were my first choice, but I was worried that they would switch too many times and eventually malfunction... plus they are noisy.
Thanks for the schematic, I will get some relays and try it.

Using a converter also sounds great, a little more elegant, but also more complex. Do you have some examples with chosen parts (inductors, caps, diodes, etc). Or can I go and buy an integrated SMPS ready for action?

 

[Ogu Reginald]

Like you said,connecting them in series is a good design as it is effective at low speed but at high speed it is not safe. I support connecting them in series but add a voltage regulator so that at high speed it will also be safe.

 

[enjunear]

Thanks!

relays were my first choice, but I was worried that they would switch too many times and eventually malfunction... plus they are noisy.
Thanks for the schematic, I will get some relays and try it.

Using a converter also sounds great, a little more elegant, but also more complex. Do you have some examples with chosen parts (inductors, caps, diodes, etc). Or can I go and buy an integrated SMPS ready for action?

Check out TI/National's WebBench tool. It'll give you a range of options and spit out ready-to-build designs.