Author Topic: Offgrid 48V solar to 24V battery setup  (Read 607 times)

Offline Mads Barnkob

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Offgrid 48V solar to 24V battery setup
« on: August 15, 2018, 03:26:52 PM »
Hi All

I hope someone can guide me in the jungle of PWM/MPPT Western/Chinese solar battery chargers.

First a list of what I got

3x 250 Watt 48V solar panels
2x 750 Watt 24VDC/230VAC inverters
?x 24V Gel UPS batteries

So what I need is a 1000 Watt 48V solar system to 24 V battery charger. Having looked at ebay/amazon etc. its a jungle of missing specs, wild claims and no real good comparisons. I hope someone else can share their experience on this.

This is something in the price range that I find suitable and the user feedback there shows it can be used with a wide range of PV input voltage and battery bank voltage can be selected: https://www.amazon.com/ZHCSolar-Controller-Regulators-Temperature-Compensation/dp/B077TSVRDP/

Attached picture is from the solar panel.
« Last Edit: August 15, 2018, 03:36:37 PM by Mads Barnkob »
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Offline the_anomaly

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Re: Offgrid 48V solar to 24V battery setup
« Reply #1 on: August 15, 2018, 05:57:38 PM »
Are you going to include sun tracking?  I think its worth it and I've seen people build simple tracking units using CdS cells to sense radiation angle.  Here is a neat source of info: https://www.eia.gov/todayinenergy/detail.php?id=18871

I don't have any product experience unfortunately.  Have you looked up any brand names like Siemens / GE / Bosch?  I would think they build a quality product.  String inverters seem to be popular in the industry.  Since each panels output can vary under the same conditions, an inverter tied to each panel can maximize that panels mppt.  If you build an array of panels and feed that into a single inverter, the weakest panel limits the output of the array.  That's all I remember from my research a year ago. 

Offline Mads Barnkob

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Re: Offgrid 48V solar to 24V battery setup
« Reply #2 on: August 16, 2018, 12:18:54 PM »
I do not have space for making a tracking mount for the panels, unfortunately, the wife-acceptance-factor is simply too low on a project like that :)

Brand names are costly, if I was going to build a long term investment system that was also grid-tied, I would go for quality products.

Here I already got panels and inverters for free, so i also wanted to find a matching solution for the charger.

Thanks for the hint on string inverters vs. micro inverters, though the micro inverters is a larger investment compared to a string inverter. From what I read up on just now microinverters is a preable choice if you have serious shading issues, installations that are more spread out so all panels are not next ot eachother. Considering I only got 3, that is not my main concern.
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Offline rikkitikkitavi

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Re: Offgrid 48V solar to 24V battery setup
« Reply #3 on: August 17, 2018, 03:38:01 PM »
Why a 48V 1000W to 24V charger?

Do you plan to parallell the three panels?

250W is their peak capacity, it is not necessary to design an inverter / charger for mor than 80% of the peak power , you loose maybe 5% of total solar energy (in kWh) but you will have a smaller inverter running closer to its maximum , ie higher efficiency and hence losses. Its a bit trade of between current related losses and simple switching losses which are more related to the size of the inverter (ie close to idle losses)

A man can not have to many variacs

Offline Mads Barnkob

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Re: Offgrid 48V solar to 24V battery setup
« Reply #4 on: August 21, 2018, 08:28:37 AM »
The PV panels that I have comes from a 6 kW system that was dismantled, so I only have a small part of the original system. They were all tied up in 2 parallel strings running at some 400-500 Volt maybe?

So to be able to use them, I will have to parallel them with blocking diodes as there is really no 150V input chargers, without getting unnecessarily expensive.

The inverters I have is also 24VDC to 230VAC, so the choice of a 48V to 24V charger is based on my parts.

From what I understand from this Victron calculator ( https://www.victronenergy.com/upload/software/VE-MPPT-Calc-2_7.xlsx ) there is some temperature dependent parameters regarding current.
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Offline the_anomaly

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Re: Offgrid 48V solar to 24V battery setup
« Reply #5 on: August 21, 2018, 02:11:42 PM »
Quote
So to be able to use them, I will have to parallel them with blocking diodes as there is really no 150V input chargers, without getting unnecessarily expensive.

I'm concerned about this.  If the blocking diode is reverse biased protecting the solar panel from incoming current, won't that completely eliminate it from the array?  The weakest panels will never contribute.  How about using a standard SMPS to drop 150V (from series config) to 24, then use the battery charger?  Sure you lose a bit of power but should be much less than losing an entire panel.

Offline Mads Barnkob

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Re: Offgrid 48V solar to 24V battery setup
« Reply #6 on: August 21, 2018, 03:31:35 PM »
Quote
So to be able to use them, I will have to parallel them with blocking diodes as there is really no 150V input chargers, without getting unnecessarily expensive.

I'm concerned about this.  If the blocking diode is reverse biased protecting the solar panel from incoming current, won't that completely eliminate it from the array?  The weakest panels will never contribute.  How about using a standard SMPS to drop 150V (from series config) to 24, then use the battery charger?  Sure you lose a bit of power but should be much less than losing an entire panel.

I can not really find anything with a quick search that actually goes against paralleling panels. The panels already has 8 diodes built-in, maybe that is already enough to take care of these issues.

I like your idea about the DC-DC converter, will see what I have in stock :)
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Offline Hydron

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Re: Offgrid 48V solar to 24V battery setup
« Reply #7 on: August 22, 2018, 01:15:39 PM »
The maximum voltage of a panel will always be higher than the ideal operating voltage (which should be set at the MPP, "maximum power point" where the V*I product is greatest), so assuming identical panels the diodes should always be forward biased, with each panel running at the same voltage but slightly different currents.

Mads: have you looked into maximum power point trackers? just running a series or parallel string of panels into the input of a standard DC-DC converter will not give good results - you want to be manipulating the duty cycle to get the maximum power from the panels.
You can probably approximate this with a simple controller by looking the V-I curve of the panel and regulating the _input_ voltage of the DC-DC converter to this value (output is essentially constant, as it's running into a battery bank, though you'd want to add over-charging protection), which will run the panel near the ideal point.
A better controller would perturb it's operating point while measuring output (or input) power to actually hit the ideal panel voltage for the exact conditions it's running in (will vary with temperature, sunlight etc).

I did a bit of this stuff at uni, designed/built a system of three 200W+ MPPTs (maximum power point trackers) and a 600W battery charger. The charger was a bit iffy (kept blowing up output diodes) but the MPPTs worked well. Mine just used a hall effect current sensor and a voltage divider, both sampled by a microcontroller, which perturbed the PWM duty cycle in one direction until the I*V product started dropping, then went the other way. This simple algorithm meant it would oscillate a little (trivial amount) around the maximum power point and would track it if conditions changed.

Offline the_anomaly

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Re: Offgrid 48V solar to 24V battery setup
« Reply #8 on: August 25, 2018, 06:15:45 PM »
I can't find anything bad about parallel either which surprises me.  I tried to set up a test but I forgot that a single cell only produces ~0.5 volts with no load and I just don't have time to solder a bunch together and try it.

If one of the panels becomes shaded or has another defect and the system is asked to supply (for example) 5amps, that one panel may produce 5amps but at a lower voltage than the other panels because its maximum power point has been shifted. That is what made me think a protection diode could become reverse biased.  I could still be wrong about this but wiring the panels in series would surely allow them all to contribute power even when they are not all operating at their maximum power point.


Offline station240

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Re: Offgrid 48V solar to 24V battery setup
« Reply #9 on: October 16, 2018, 02:57:07 PM »
Victron have not unreasonable pricing
https://www.victronenergy.com/solar-charge-controllers

I rewrote the instructions at the top of the page to be clearer.

In our MPPT model names,  the first number is the maximum PV open circuit voltage. The second number is the maximum charge current.
For example MPPT 75/50, maximum PV open circuit voltage 75V.  maximum charge current 50A.

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Re: Offgrid 48V solar to 24V battery setup
« Reply #9 on: October 16, 2018, 02:57:07 PM »

 


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