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Old 07-09-2016, 04:58   #1
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Mppt backwards

Just wondering if it's possible using a Mppt to charge a 48 volt battery bank from a (1000 watt) 36 volt solar system or can it only go from a larger voltage to a smaller one. Also if it is possible what kind of power loss would I get?
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Old 07-09-2016, 05:10   #2
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Re: Mppt backwards

Nope. Doesn't work that way.
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Old 07-09-2016, 05:15   #3
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Re: Mppt backwards

Quote:
Originally Posted by jp.beauchemin71 View Post
Just wondering if it's possible using a Mppt to charge a 48 volt battery bank from a (1000 watt) 36 volt solar system or can it only go from a larger voltage to a smaller one. Also if it is possible what kind of power loss would I get?


Unfortunately not. In very simple terms, solar controllers are basically power supply units that accept variable input voltages and control the output according to an efficient charging scheme, and they'll typically use FETs as part of their control cycle, making them 'one way' (unlike simple induction/coil transformers). The input (PV) voltage is always higher than the output (battery) voltage.



However, MPPT controllers come in all shapes and sizes - some automatically adapting to the system (battery) voltage. Here's a 12/24/48V example:

https://www.victronenergy.com/solar-...rs/mppt-150-70



However, you'd have to ensure that your PV array was configured such to ensure that your input voltage is clearly above the required output voltage, as specified by the manufacturer of the MPPT controller.

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Old 07-09-2016, 06:01   #4
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Re: Mppt backwards

How come you have a "36 volt solar system" in the first place? Probably what you have is more than one panel, but connected in parallel. You'd just need to connect them in a series instead ... or a bit of both, e.g. 2 series of 5 panels each. This gives a higher voltage to the input side of the controller.

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Old 07-09-2016, 06:34   #5
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Re: Mppt backwards

Actually what you are asking for is available, but not very well known. There are two kinds of power supplies, boost and buck, the mppt section of the controller is the same as it is regulating the operating voltage of the panel for maximum power generation. The output of the mppt section is fed into the power supply section where the power is converted to a combination of Voltage and amperage required by the battery bank according to the SOC and the voltage of the bank. By far most MPPT solar controllers have buck power supplies which step down the voltage and step up the amperage. A few have boost power supplies that step up the voltage and step down the amperage. This is how car adapters for 19V laptops work. They step up the voltage from your 12V car power socket to the 19v required by the laptop. All of this comes at some cost. Most modern power supplies will operate in the 90-93% efficiency range with some doing a little better. When shopping for a boost solar controller beware of the misuse of the term boost in advertising and product naming. Blue sky controllers are said to boost your amperage, but actually operate as buck power supplies for instance. Gensun has an actual boost controller that will allow you to charge 48v banks from 12V panels, but if I remember correctly it is set up for LiFePo batteries, not LA. You might want to check with them and see if they have one for LA.

Your power loss will not be much different than any other mppt controller, assuming you are using the term power correctly, that is watts put into your batteries. Your amperage put into the batteries will be proportional to the voltage of your panels. If they are nominal 12V panels which are usually really 17V MPP you will get about 1/3 of their output at 48V, if they are 24 Volts nominal you will get about 1/2 of their amperage output.
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Old 07-09-2016, 08:26   #6
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Re: Mppt backwards

What Captain Bill said!

Genasun has one that boosts a "12V" panel to charge a 24/36/48V lead-acid battery: Genasun GV-Boost 105-350W Solar Boost Charge Controller with MPPTGenasun

I've not used it, but Genasun seems to build good stuff.
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Old 07-09-2016, 08:41   #7
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Re: Mppt backwards

Wow lots of info, Thanks a lot guys! The reason I don't wire it in series (I have 4X250w at 36v) is if I do it at 72 volts to 48 volts. I cant find a Mppt that has a input of more than 60 volts without going to super expensive models.
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Old 07-09-2016, 08:45   #8
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Re: Mppt backwards

The Genasun would work fine except its only 8A. On a bright sunny day I should be able to produce more than that. Shouldn't I?
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Old 07-09-2016, 09:11   #9
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Re: Mppt backwards

You could use one Genasun MPPT unit for each panel and combine the high-voltage MPPT outputs. While not as simple, you might get more usable power because of the series-panel shading problems often seen on a boat.

I assume your 36V panels put out about 7A max, and there should be reasonable margin since the Genasun folks are saying their 8A spec is probably good to at least 9A.
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Old 07-09-2016, 23:53   #10
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Re: Mppt backwards

The higher the battery voltage, the higher the panel voltage can be, and so can still be under 8A. We keep the Genasun GVB's in stock for 12V (for <12V panels), 24V, 36V, & 48V, for both Pb & Li.
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Old 08-09-2016, 02:12   #11
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Re: Mppt backwards

http://www.gsl.com.au/bmppt800.html


A range of sizes from 60 watts to 1500 watts.


google "boost mppt".
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Old 08-09-2016, 02:54   #12
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Re: Mppt backwards

Another controller that is interesting is the Western WMarine10, which is both an MPPT buck, and a boost controller. It is slightly less efficient either way than a Genasun, however in certain situations you can get more net Ah/day from some panels in particular shading conditions. We've tested them on the larger Solbian panels that are made of of two series strings, and that have the laminated-in bypass diodes. If these panels (for instance an SP125) are shaded on one half (the long way, not across the short way), the bypass diodes isolate the shaded side and the panel becomes a "smaller" panel at 1/2 the voltage. When used with a normal Buck MPPT (Genasun, etc.) the output to a 12V or 24V battery becomes 0A. However with the WMarine10 it will switch to a boost and keep working (albeit at 1/2 power). This can be helpful if you configure a Solbian panel fore/aft in an area that often shaded by the boom. But only if shaded lengthwise, the "long" way. If the shade falls across both of the panel's series strings, it is shut down no matter what controller is used.
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