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Stepping down ~120V DC to approx 24-36v for solar charge controller?

micromaniac

May 16, 2016
3
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3
Hi and thanks in advance for any help with this problem.

I'll prefix this by saying my electrical knowledge is average, I'm aware of the difference and relationship between V & I but perhaps not always so good at connecting things together without releasing oodles of magic smoke :)

A brief bit of background on my problem first - I'm attempting to use a 125W solar panel to charge a 12v lead acid battery, for use with an inverter to occasionally run low power appliances. Not the most unique of dilemmas I appreciate but I'm struggling to find an answer using my usually fairly dependable google skills.

The panel outputs approx 120V in full sun at around 1-1.2 amps. I believe it's part of a larger grid tie system so isn't designed to charge batteries, but the tinkerer in me is insisting it WILL charge a battery before I'm finished with it. I have a cheap MPPT charge controller that will accept up to 72V input and handles 10A.

I'm wondering if anyone has any good, low cost suggestions on the best way to step the voltage down to suit the charge controller I have?

I've looked into dc-dc buck converters etc but they get a touch too pricey for me for anything that will accept a 120v input.

The best idea I've come up with so far would be to perhaps connect 2 lower rated (lower cost!) dc-dc converters in series with the panel supply and then connect the lower voltage outputs from these before they hit the charge controller, perhaps with some diodes before they join if required. Unfortunately, I don't know whether that would work or if I'd just end up cooking 2 converters.

I'm aware that doing this will cause fairly substantial losses but I paid almost nothing for the panel and if I can get anything out of it, even 40% I'll be happy!
 

Harald Kapp

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Nov 17, 2011
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Welcome to EP.

I have a cheap MPPT charge controller that will accept up to 72V input and handles 10A
The MPPT controller is designed to interact directly with the solar panel, whci is not possible here due to the mismatch of votage levels. When you put a step-down converter between the panel and the controller, you'll no longer be able to use the MPPT feature because the V-I characteristic of the step-down converter is different from the solar panel's characteristic.

The best idea I've come up with so far would be to perhaps connect 2 lower rated (lower cost!) dc-dc converters in series with the panel supply and then connect the lower voltage outputs from these before they hit the charge controller, perhaps with some diodes before they join if required. Unfortunately, I don't know whether that would work or if I'd just end up cooking 2 converters.
The latter.
There are at least two issues with this approach:
  1. You assume an equal distribution of the input voltage among the two converters. Any mismatch (and these converters are typically not made to be operated in this way) will lead to an uneven distribution of the input voltage among the converters, resulting very probably in destruction of at least one of the two.
  2. Inexpensive DC-DC step-down converters commonly do not have isolated outputs. Rather Ground (0V) at the input is the same as ground on the output. Assuming the input voltage is evenly distributed among the inputs (which it probably isn't, see #1), the 'ground' of the upper converter (the one connected to the panel's '+' output is at 1/2*Vpanel=60V.
    As the lower converter is connected to the panel's '-' output, connecting the two grounds will effectively short-circuit the lower converter's input side, thus applying the full 120V to the upper converter's input.
    Bang!
Imho you have two choices:
  1. Use a MPPT controller rated for operation with a 120V panel.
  2. Forget about the MPPT feature, step-down from 120V to a save level (e.g. 18V) and use a charge controller rated for this low voltage (at consequently higher current).
Always keep in mind that you're dealing with lethal voltage and current levels. Observe the applicable standards and regulations, have your work checked by an expert vefore putting it to work.

Regards,
Harald
 

Gryd3

Jun 25, 2014
4,098
Joined
Jun 25, 2014
Messages
4,098
Hi and thanks in advance for any help with this problem.

I'll prefix this by saying my electrical knowledge is average, I'm aware of the difference and relationship between V & I but perhaps not always so good at connecting things together without releasing oodles of magic smoke :)

A brief bit of background on my problem first - I'm attempting to use a 125W solar panel to charge a 12v lead acid battery, for use with an inverter to occasionally run low power appliances. Not the most unique of dilemmas I appreciate but I'm struggling to find an answer using my usually fairly dependable google skills.

The panel outputs approx 120V in full sun at around 1-1.2 amps. I believe it's part of a larger grid tie system so isn't designed to charge batteries, but the tinkerer in me is insisting it WILL charge a battery before I'm finished with it. I have a cheap MPPT charge controller that will accept up to 72V input and handles 10A.

I'm wondering if anyone has any good, low cost suggestions on the best way to step the voltage down to suit the charge controller I have?

I've looked into dc-dc buck converters etc but they get a touch too pricey for me for anything that will accept a 120v input.

The best idea I've come up with so far would be to perhaps connect 2 lower rated (lower cost!) dc-dc converters in series with the panel supply and then connect the lower voltage outputs from these before they hit the charge controller, perhaps with some diodes before they join if required. Unfortunately, I don't know whether that would work or if I'd just end up cooking 2 converters.

I'm aware that doing this will cause fairly substantial losses but I paid almost nothing for the panel and if I can get anything out of it, even 40% I'll be happy!
You mention it may be part of a grid-tied system?
There is a bit of a problem here, and we need you to please clarify.
The 120V... is that AC or DC?
Grid-tied systems usually have an inverter that is connected to mains *and* connected to a DC source from the solar panels.
I think you have two options... and two options only...
Consult with an electrician for this one!
Remove the panels from the inverter, and install a battery charge controller and batteries onto the panels. You can then connect the grid-tied inverter to your battery bank... *but* this will result in the batteries running low during low light conditions. So using the batteries as a 'Back-up' is kind of useless here unless you can manage to connect the output of your solar panels to the inverter and battery charger in parallel... then you have a battery bank with no draw that you can use for emergencies... This is a guess... I've never seen it done before but it would in theory work...

Second super easy recommended idea.
Go buy a UPS... you can get them cheap or expensive. Plug it into 110VAC or 220VAC (depending on where you live, it's just a regular AC outlet... when power goes out, the UPS will continue providing power to it's outputs. Run-time depends on the load and size of the UPS, but this requires no trickery ;)
 

micromaniac

May 16, 2016
3
Joined
May 16, 2016
Messages
3
Welcome to EP.


The MPPT controller is designed to interact directly with the solar panel, whci is not possible here due to the mismatch of votage levels. When you put a step-down converter between the panel and the controller, you'll no longer be able to use the MPPT feature because the V-I characteristic of the step-down converter is different from the solar panel's characteristic.


The latter.
There are at least two issues with this approach:
  1. You assume an equal distribution of the input voltage among the two converters. Any mismatch (and these converters are typically not made to be operated in this way) will lead to an uneven distribution of the input voltage among the converters, resulting very probably in destruction of at least one of the two.
  2. Inexpensive DC-DC step-down converters commonly do not have isolated outputs. Rather Ground (0V) at the input is the same as ground on the output. Assuming the input voltage is evenly distributed among the inputs (which it probably isn't, see #1), the 'ground' of the upper converter (the one connected to the panel's '+' output is at 1/2*Vpanel=60V.
    As the lower converter is connected to the panel's '-' output, connecting the two grounds will effectively short-circuit the lower converter's input side, thus applying the full 120V to the upper converter's input.
    Bang!
Imho you have two choices:
  1. Use a MPPT controller rated for operation with a 120V panel.
  2. Forget about the MPPT feature, step-down from 120V to a save level (e.g. 18V) and use a charge controller rated for this low voltage (at consequently higher current).
Always keep in mind that you're dealing with lethal voltage and current levels. Observe the applicable standards and regulations, have your work checked by an expert vefore putting it to work.

Regards,
Harald

Hi Harald, thanks very much for responding.

I figured there would probably be massive holes in my thought process, thank you for pointing out where they lay.

Fear not, I'm very aware of the danger of this level DC voltage and will be taking all sensible precautions while working with it.
 
Last edited:

micromaniac

May 16, 2016
3
Joined
May 16, 2016
Messages
3
You mention it may be part of a grid-tied system?
There is a bit of a problem here, and we need you to please clarify.
The 120V... is that AC or DC?
Grid-tied systems usually have an inverter that is connected to mains *and* connected to a DC source from the solar panels.
I think you have two options... and two options only...
Consult with an electrician for this one!
Remove the panels from the inverter, and install a battery charge controller and batteries onto the panels. You can then connect the grid-tied inverter to your battery bank... *but* this will result in the batteries running low during low light conditions. So using the batteries as a 'Back-up' is kind of useless here unless you can manage to connect the output of your solar panels to the inverter and battery charger in parallel... then you have a battery bank with no draw that you can use for emergencies... This is a guess... I've never seen it done before but it would in theory work...

Second super easy recommended idea.
Go buy a UPS... you can get them cheap or expensive. Plug it into 110VAC or 220VAC (depending on where you live, it's just a regular AC outlet... when power goes out, the UPS will continue providing power to it's outputs. Run-time depends on the load and size of the UPS, but this requires no trickery ;)

Sorry, that should have read - "It WAS part of a larger grid tied system" :) It is no longer, it has a small crack in it so I got it for silly money. I just have a single 1.1m x 1.3m 125w solar panel that I'm trying to make use of rather than landfill.

The panel produces 120V DC, I'm simply trying to step it down to a usable and safer level for my charge controller via the most cost effective method possible. I've overcomplicated things I suspect with my amateur connecting dc converters together ramblings. I've simply found some $10 ones that look a lot more cost effective than the $100+ ones that are rated for 120V. These are the cheap ones I was envisioning connecting together if possible http://www.allelectronics.com/make-a-store/item/dc-2818/28v-18a-dc-dc-converter/1.html - It looks like this may be a dumb idea so I guess my question is better condensed down to the following:

The panel I have -

Pramac 125w
Module - LUCE MCPH P7
Peak Power - 125
Peak Power Voltage - 100
Peak Power Current - 1.25
Open Circuit Voltage - 133
Short Circuit Current - 1.47
Initial Vmpp - 107
Initial Impp - 1.38

I also have a charge controller capable of accepting 24/36/48/60/72V inputs and handling 10A

What's the cheapest/simplest and most efficient way to step the 120V down to a level that charge controller will handle?

Happy to break out a soldering iron if anyone can give me an idea of a circuit diagram to achieve it.

Thanks for your help.

Mark
 
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