Stepping down high voltage panel to charge 12/24v battery

[geeksplus]

Hi and thanks in advance for any help anyone can offer on this Please bear in mind that my electrical knowledge is fairly limited...

I have a Pramac 125w solar panel with the following attributes:

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 would like to use this to charge a couple of standard 100Ah leisure batteries either in a 12v or 24v config via the cheapest method possible.

Charge controllers that will handle this voltage seem to be quite expensive so I was wondering whether I could step it down to a more sensible voltage before passing it through to the charge controller. I'm aware there will be losses involved but the panel cost very little so I'm happy to extract anything from it I can. I currently have a cheap MPPT charge controller that will accept up to 72v input to work with.

I've been looking into buck converters that will handle this voltage but again they're not particularly cheap. Is it possible to use say 2 buck converters that are rated at approx half of the panel voltage, wired in series to achieve a couple of ~12v-24v outputs that could then be tied together to feed into a charge controller? In my head at least, that should work but I suspect there may be some obvious law of physics that I'm missing that may prevent me doing so!

Alternatively, if that's not possible does anyone have any clever ideas how I can make use of this panel for a lower voltage charging system?

Any help greatly appreciated!

Thanks, Mark

[SunEagle]

Hello geeksplus and welcome to Solar Panel Talk

Unfortunately a single 125watt panel with a Peak Power Current of 1.25 amps will only charge a 12Ah battery. It does not generate enough amps for even one 100Ah let alone 2 in any configuration.

A battery needs to get 1/8 to 1/12 amps of it's Ah rating to properly charge it. A 100Ah battery needs at least 8.4amps to charge it.

[geeksplus]

Assuming it is possible to connect multiple dc-dc converters in series, would i be able to use 2-3 of the following converters to achieve it?

http://allelcdn.upshotcommerce.com//mas_assets/theme/allelectronics/spec/DC-2818.pdf

I'm looking specifically at the PAF500F48-28 model as, in my limited understanding at least, a couple of those should handle the maximum 133v supply the panel could manage split across them. Input voltage range of 36-76v, output nominal 28v & adjustable to 16.8v-30.8v as far as I can tell.

Please feel free to tell me that my amateur ramblings are way off course!

Thanks

Mark

[geeksplus]

Thanks SunEagle. Posted that last question before I saw your reply.

Would the current not increase with the voltage being stepped down? I'm happy to change the battery config to suit a bit better if required. The 2 batteries I have here are getting a bit long in the tooth so will need replacing at some point anyway.

[SunEagle]

While the voltage is important to be above the battery rating for proper charging the major item is the amount of amperage you generate to do the charging. Installing equipment to adjust the voltage will end up reducing the efficiency and increase the losses.

To make sure the battery is properly charged you also need a solar Charge Controller between the battery and the solar pv. Hooking up the panel directly to the battery even through voltage converters will not properly charge your battery.

[SunEagle]

Thanks SunEagle. Posted that last question before I saw your reply.

Would the current not increase with the voltage being stepped down? I'm happy to change the battery config to suit a bit better if required. The 2 batteries I have here are getting a bit long in the tooth so will need replacing at some point anyway.

Whatever you use to adjust the voltage may change the amperage but your initial amperage generation is the key to your system. IMO those 125w panels are not designed to charge batteries but were supposed to be used as a grid tie system with a lot of them connected to the inverter.

[geeksplus]

Thanks again for more info.

I believe you are wholly correct that these are designed for grid tie systems. I have happened upon one at a silly cost due to a small crack in the glass and the tinkerer inside me really wants to make some level of use out of it rather than it just being more landfill. I'm not overly bothered about losses but am wary of spending too great an amount on it when I could likely spend a small amount more and get a system designed for the purpose. The most demanding use I would have for it would be to run a sub-40w pond pump for a few hours a day.

Do you happen to know whether what I have suggested above with multiple dc converters is technically possible or am I way off the mark?

The little bit of electrical knowledge I do have makes me suspect there could be some issue with combining the outputs from them if it's even possible to use them in series. Maybe need some form of diode in-line before the outputs are combined?

Again, likely just inaccurate ramblings on my part but I like to try!

Thanks

[SunEagle]

Thanks again for more info.

I believe you are wholly correct that these are designed for grid tie systems. I have happened upon one at a silly cost due to a small crack in the glass and the tinkerer inside me really wants to make some level of use out of it rather than it just being more landfill. I'm not overly bothered about losses but am wary of spending too great an amount on it when I could likely spend a small amount more and get a system designed for the purpose. The most demanding use I would have for it would be to run a sub-40w pond pump for a few hours a day.

Do you happen to know whether what I have suggested above with multiple dc converters is technically possible or am I way off the mark?

The little bit of electrical knowledge I do have makes me suspect there could be some issue with combining the outputs from them if it's even possible to use them in series. Maybe need some form of diode in-line before the outputs are combined?

Again, likely just inaccurate ramblings on my part but I like to try!

Thanks

There are a number of DC voltage pumps that can run directly from a solar panel. It comes down to what that pump needs for both a voltage rating and amp rating.

What can fool some people is just looking at the total panel wattage. That number is calculate by multiplying the Max Power Voltage x Max Power Current. If the voltage is real high like your panel then the current is relatively small which will not be enough to power anything because most small DC pumps might be rated around 12v to 15v DC but still require a few amps to start and run.

[geeksplus]

My apologies as I haven't fully explained my intended use for the system. The pump suggestion was more to give an idea of the maximum draw I would ever expect it to have to handle. The full plan is for it to be used on a remote area of my garden that is too far from my house for mains power. Ideally I would like to rig this into a battery and have the ability to run additional loads such as chargers, led lights and even a low powered audio system for short periods of time.

[SunEagle]

My apologies as I haven't fully explained my intended use for the system. The pump suggestion was more to give an idea of the maximum draw I would ever expect it to have to handle. The full plan is for it to be used on a remote area of my garden that is too far from my house for mains power. Ideally I would like to rig this into a battery and have the ability to run additional loads such as chargers, led lights and even a low powered audio system for short periods of time.

Based on the cost of a solar / battery system to run even small loads it may be less expensive to run the grid power out to where you want it. It all depends on the cost to run the power and the size of your load.

If you know what your total daily watt hour usage will be then you can size your battery & solar pv panel wattage. You would then compare the cost of that system (which the batteries may only last a few years) to the cost of running a circuit from your home to that load.

I have found that even if that load was 300 feet away from the home and needed only 250 watt hours a day it would be cheaper to run the circuit from the home.

[geeksplus]

Thank you again for your input SunEagle. However, I'm not really looking for alternative ways to achieve power to the remote area. I take a certain pleasure in just playing around with electrical components and have some new ones and new problems to solve because of them.

Setup is as follows I guess,

- High voltage & low amperage varying DC supply from approx 120v panel

- Existing charge controller that will handle roughly half of the above supply without producing magic smoke.

- Batteries that, while old still work, and are happy to be charged by the above charge controller, assuming sufficient current present to do so.

- Inverter to extract said juice from battery for use with various small loads.

The problem I'm trying to solve lies directly between item 1 and item 2. How to convert the DC voltage down to a sensible voltage that my charge controller will be happy with?

[SunEagle]

Well good luck. I am sure you will find a way to reduce the voltage going to your charge controller but unless you increase the number of those panels or use different ones your batteries will have to be small.

[Technologist]

Hi and thanks in advance for any help anyone can offer on this Please bear in mind that my electrical knowledge is fairly limited...

I have a Pramac 125w solar panel with the following attributes:

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 would like to use this to charge a couple of standard 100Ah leisure batteries either in a 12v or 24v config via the cheapest method possible.

Charge controllers that will handle this voltage seem to be quite expensive so I was wondering whether I could step it down to a more sensible voltage before passing it through to the charge controller. I'm aware there will be losses involved but the panel cost very little so I'm happy to extract anything from it I can. I currently have a cheap MPPT charge controller that will accept up to 72v input to work with.

I've been looking into buck converters that will handle this voltage but again they're not particularly cheap. Is it possible to use say 2 buck converters that are rated at approx half of the panel voltage, wired in series to achieve a couple of ~12v-24v outputs that could then be tied together to feed into a charge controller? In my head at least, that should work but I suspect there may be some obvious law of physics that I'm missing that may prevent me doing so!

Alternatively, if that's not possible does anyone have any clever ideas how I can make use of this panel for a lower voltage charging system?

Any help greatly appreciated!

Thanks, Mark

Reading your posts you sound like you have fun messing and tinkering with electronics, the panel is 100 Volts at peak power. You want a lower voltage panel with more amps and it might end up in the dump anyway as it's a cracked panel.

So here's my idea if you have the soldering skills.

Pictures of the LUCE MCPH P7 panel shows it has what looks like 5 separate sections of "series" run cell banks totaling 100 Volts at peak power with bus bars on the sides of the panels. Most panels are very hard to tap into but some are easy if the layout is perfect and the bus bars are accessible. "If" and that's a big if you could get to the bus bars on the sides and wire those 5 series sections into 5 parallel sections. You would have one 20 Volt 6.25 Amp Panel. I only say this because cheap MPPT controllers that can handle 133 Volts are hard to come by and trying to step 100 Volts down to 12 wastes efficiency while bringing down 20 Volts to 12 with the right MPPT controller can be in the 96-98% efficiency range.

Also you mentioned you have a "cheap" MPPT controller. Most of the cheap MPPT controllers are really PWM and lie about them being MPPT so you might want to post the make and model so people to see if it's real.