# combining multiple power sources for energy harvesting

#### DJ_AA

Joined Aug 6, 2021
305
Hi All,

I am working on a small project aimed at harvesting power from various sources, including solar panels and a small wind turbine, to charge a 12V battery. The charge circuit itself is not an issue. However, I am interested in knowing whether it is possible to combine the inputs from these two sources to maximize the input power.

Typically, the method to manage inputs involves placing two diodes before the battery charger, allowing the input with the highest voltage to be selected. This approach has a downside: if one source produces a lower voltage output but a higher current, it would not be utilized, leading to potential energy waste.

An issue arises when the wind turbine produces low energy simultaneously with the solar panels. Individually, they might offer a low charge current, but combining their inputs could potentially increase the charge current.

Is there a way to combine two power sources effectively to maximise input power?

#### bassbindevil

Joined Jan 23, 2014
828
Have a look at "maximum power point" (MPPT) for solar panels and wind turbines. Both of those sources have optimum operating points where the maximum power can be extracted, and those points vary according to how sunny or windy it is. The MPPT for solar can be approximated by loading a panel to about 17V, but the best MPPT charge controllers calculate the power while varying the load.

#### DJ_AA

Joined Aug 6, 2021
305
Yes, I aim to use an MPPT charger, but here is what I am trying to accomplish:

For example, suppose a solar panel is producing 10W and a wind turbine is producing 15W. By using a diode or some form of switching, the charge from the turbine would be utilized, so my charge current is 15W. However, I would like to see if there is a way to combine both power sources to achieve a 25W charge current.

Is this possible?

#### Sensacell

Joined Jun 19, 2012
3,451
Yes, I aim to use an MPPT charger, but here is what I am trying to accomplish:

For example, suppose a solar panel is producing 10W and a wind turbine is producing 15W. By using a diode or some form of switching, the charge from the turbine would be utilized, so my charge current is 15W. However, I would like to see if there is a way to combine both power sources to achieve a 25W charge current.

Is this possible?

"25 W charge current" is a misinomer, power is the product of current X voltage, batteries are charged with a current.

Each energy source will have it's own characteristic output voltage and impedance, to produce maximum power, it must be loaded with the correct impedance.
This load matching is the job of the MPPT controller.

Theoretically, if the two energy sources had similar impedances, and were both electrically floating, you could just hook them in series and be done.

The problem is that they won't have similar impedances, (totally different characteristics) and the MPPT controller in the charger expects a specific impedance from the power source.
It's all theoretically possible, but not very practical, otherwise every MPPT controller would proudly have an "Auxiliary Wind Turbine Power Input" listed in the marketing hype.

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#### Ya’akov

Joined Jan 27, 2019
9,170
While it would be less efficient, the source currently producing lower output could be used to charge a super cap, or an auxiliary battery as an accumulator. When the source that was producing more stops doing that, the accumulator could be switched in as a third source until it stopped being the one offering the most power.

Alternatively, you could have each source charge their own battery and use them in parallel for the load. Both of these schemes lack the elegance of being able to just add whatever power each has in real time, but using integration over time with an accumulator seems reasonable.

#### BobTPH

Joined Jun 5, 2013
8,996
Brainstorming here. I do not assert that this is workable, just would like to hear what others say.

I don’t see why it could not be done in a way similar to the way grid tied inverters work. Each source could adjust it’s voltage output until it is the current drawn from the source is at its MPPT point. But something would need to happen if the combined charging current is too much for the battery. I guess you could give priority to one of the sources, and the other would reduce its output at that point.

In any case, it is not simple.

#### Alec_t

Joined Sep 17, 2013
14,329
More brainstormimg. How about have each source drive its own boost converter to give the same output voltage something above the battery's fully-charged voltage, then OR the converter outputs to drive a battery charger?

#### camerart

Joined Feb 25, 2013
3,730
Hi D,
I think along the lines as #5
In my van, there are 2x separate solar panels, chargers and batteries, I use one till it gets low, then switch to the other, and so on.

I think each charger use a Capacitor, which charges up, then once at the correct voltage charges the battery, then switches back to the Cap. When there is low light, the Cap takes longer to charge to the correct voltage.
C.

#### DJ_AA

Joined Aug 6, 2021
305
What happens if I have two separate identical charge circuits going to the same 12V battery? There will be a load always connected to the battery.

#### camerart

Joined Feb 25, 2013
3,730
What happens if I have two separate identical charge circuits going to the same 12V battery? There will be a load always connected to the battery.
Hi D,
I think that as the different charging systems may have differing outputs, then each would charge a capacitor, and as each cap achieved the correct battery charging voltage it would switch to pulse charge the battery. Then if there was no wind, then that charger wouldn't achieve its charging voltage, so wouldn't switch to charge and simply wait, while the high one would pulse charge.
C.

#### MrAl

Joined Jun 17, 2014
11,489
In theory you can connect two current sources in parallel even if they put out different current levels.
That would mean you would have to convert each source into a current source. As someone said, this is similar to how line tied converters work, and there could be a lot of them connected to one single power line, each with a different DC buss voltage, and each putting out a different current level.
Solar power converters work the same way even though the panel max power point is being maintained. When line tied, they could all put out a different current level into the line while working the panel at the max power point.

#### DJ_AA

Joined Aug 6, 2021
305
In theory you can connect two current sources in parallel even if they put out different current levels.
That would mean you would have to convert each source into a current source. As someone said, this is similar to how line tied converters work, and there could be a lot of them connected to one single power line, each with a different DC buss voltage, and each putting out a different current level.
Solar power converters work the same way even though the panel max power point is being maintained. When line tied, they could all put out a different current level into the line while working the panel at the max power point.
well I will have the same identical mppt circuit, so what must I do as your suggestion?

#### MrAl

Joined Jun 17, 2014
11,489
well I will have the same identical mppt circuit, so what must I do as your suggestion?
Hi,

You need to think in terms of a current regulator rather than a voltage regulator. That would mean a converter that converts whatever input you have into a current source rather than a voltage source.
Maybe you can use the MPPT circuit to output the voltage, then a converter that converts that into a current source. I have to say though that I have never done this before. The line tied converters I did all started from some DC buss that came from a solar array, and the converter output connected to the line with a large contactor and put out a small current that ramped up to a larger current. The MPPT circuit used a small solar cell to adjust the power point because that is what Sandia Labs recommended at the time, and these were for them. The solar cell sampled the solar insolation and that provided the information for the MPPT circuit. The reason they wanted to use a solar cell rather than a sampling algorithm was for stability, it was much more stable doing it that way. It's possible MPPT circuits today may use a simpler idea to do the tracking, allowing some inefficiency to exist. With these things stability is more important than efficiency.

So all you would be looking for is a converter that regulates an output current rather than a voltage (with a voltage limit of course) and a way for it to disconnect from the common output automatically if needed. Obviously, this won't be super simple, but I think you would be able to find designs online. If not, you may have to buy a book on this.

#### MrAl

Joined Jun 17, 2014
11,489
I am thinking about using one of the following:

https://www.analog.com/en/products/ltc4162-s.html
or
https://www.analog.com/en/products/ltc4015.html

LTC4015 looks bit more promising as it also includes a counter.
Hello,

A battery charger looks like an interesting way to do it. Just have to make sure the target voltage is never reached.
You'll also have to check for stability once you get two connected in parallel. If you have two power supplies or two batteries you can test with those first.
Just wondering, did you mention the amount of current you are dealing with and the maximum voltage output?

#### DJ_AA

Joined Aug 6, 2021
305
Hello,

A battery charger looks like an interesting way to do it. Just have to make sure the target voltage is never reached.
You'll also have to check for stability once you get two connected in parallel. If you have two power supplies or two batteries you can test with those first.
Just wondering, did you mention the amount of current you are dealing with and the maximum voltage output?
I need to check I think max charge voltage is 14V and , current is 3A.

I am thinking if one charger was to output 14V, it might the other think the battery is fully charged

#### MrAl

Joined Jun 17, 2014
11,489
I need to check I think max charge voltage is 14V and , current is 3A.

I am thinking if one charger was to output 14V, it might the other think the battery is fully charged
It seems to me that the process would go something like this...
First, the two 'chargers' would be pumping current into the battery, and the voltage would keep rising.
When it gets to the max of 14 volts, you have to terminate the charge. That means if the other unit 'thinks' it is fully charged, then it really is fully charged, and you would have to end the charge process.
If you do not want to waste any power after that, you would have to supply a second battery.

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#### nsaspook

Joined Aug 27, 2009
13,307
You need to use actual battery current (charge or discharge) to decide the state of the charging system. When you have loads, batteries and supplies it can get complicated tying to track energy flows and conditions.
Look at the DL BAT current as conditions change. The DL BAT current/voltage sensor measures current in and out of the battery. DL PV current/voltage sensor measures the charge controller input current from a power supply or solar panels.

GTI power is the inverter load.

#### DJ_AA

Joined Aug 6, 2021
305
You need to use actual battery current (charge or discharge) to decide the state of the charging system. When you have loads, batteries and supplies it can get complicated tying to track energy flows and conditions.
Look at the DL BAT current as conditions change. The DL BAT current/voltage sensor measures current in and out of the battery. DL PV current/voltage sensor measures the charge controller input current from a power supply or solar panels.

GTI power is the inverter load.
LTC4015 includes a counter, which I believe monitors current in and out of the battery.

The aim was to do something similar combining multiple solar panels, but in this case a solar and turbine.

#### MrAl

Joined Jun 17, 2014
11,489
I re-read my post #17 and had to make a typo correction, but also realized that adding a second or even third battery would be an interesting idea if the main (first) battery always got charged and stayed charged for extended periods of time. The solar panel and/or turbine would be just sitting there doing nothing all that time. With a second battery, the charging could be diverted to that battery which would make use of the power source again. If that one does the same thing, a third battery would take over.
When it came time to use the power, the second (or third) battery would be able to supply more energy than just a single battery if needed.