Rate this desert comm site system

Here's a desert comms site. It's a 48v-only site, no AC so there is less overall conversion waste.

The site has 10 Canadian Solar 400W CS6R-400MS-HL, two strings in 5 series each.

Each string plugs into its own MPPT charger on one Victron MPPT RS 450/100-Tr.

The site is using LiTime 51.2V 100Ah batteries in parallel to the Victron. The positive leads are fused with AMG fuses using an LMG fuse holder including the Victron itself. https://www.eaton.com/us/en-us/catalog/emobility/series-lmg.html The LMG "common input" will be used to connect the LMG's positive bus bar to a 48v telecom fuse panel.

It gets into the teens in the winter, and occasionally even negative temps. I'm still thinking about some heating pads for the batteries since there is no heated 48v option. I'm thinking one 400w panel wired direct to several pads in series/parallel with temp control relays but I haven't settled on anything yet.

The goal here is to power around 250w-350w load continuously, forever. On some really cloudy weeks we may have to plug in a generator. Hopefully this is rare, and the multitude of panels provides enough charging during short periods of sun during the fall or winter that the site can survive with only rare use of a generator.

You didn’t mention how you handle max amps (charging limits) of the batteries.

chriscappuccio said:

I'm still thinking about some heating pads for the batteries since there is no heated 48v option

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It seems 19.2kWh of battery should do it for two days of inadequate sunlight with a day+ of headroom; the battery heating however might crowd you.

Max charge? I am depending on the charger to stop based on typical CC/CV profile since the state of charge information isn't available from the LiTime BMC. The configurable items include the time for absorption charge and the voltages for absorption, float, etc. I hope the charger still provides enough current during float to power equipment but i'm not quite sure what it will do.

The battery heater would run off a separate panel that isn't installed yet. Just gonna run a single 400w panel through a temperature controlled relay which will connect to five 24v/150w pads wired in series. That's a variation of the heating system posted here. I am gonna slide the heater pads between the batteries, glued to metal plates. The pads are from batteryhookup.

I have an army of these solar panels and of these Victron MPPT RS 450/100-Tr chargers for future use. I intend to use them for off grid living. This is a mini version of what I want to keep doing. If anyone has recommendations on how to better use them, I'm all ears.

Looks good. 20kWh of storage should carry 350w of loads for 3 days during cloudy weather. I would add auto starting generator and chargeverter. The only thing I don't like are those batteries not reporting SOC for telemetry.

Is a Victron BMV-712 is worth adding? I'm not sure if this with its shunt can provide better information for charging.

During the some ~75% cloudy days last week (as rated by wunderground.com) I was still getting around 7-8kWh from the panels each day, that was pretty nice. That's almost enough to match a 350w load for 24 hours, but not quite.

Since I don't have any battery heaters I turned the charger off for now. I checked the battery box last night and it was still around 42F while outside was 30F. This was after turning the charger off for 3 days. The only load on the batteries right now is a Cerbo GX and some occasional lights.

Now I'm imagining the summer/winter maintenance will be uncovering and covering the battery box. It seems like charging will get too hot to leave it covered during the summer. I'm a little torn on whether the LiTime is better to use, or a heated server rack battery (if such a thing exists.)

chriscappuccio said:

I have an army of these solar panels

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Have you considered mounting some panels vertically on your south facing wall? That would be great for low winter sun so you could find a battery heat solution.

For a cheap and easy heat solution, which i'd consider if i had a need, i'd heat water in a 55 gallon barrel directly from a few panels with a $10 hot water heating element. Maybe it'd keep the ambient temp in your building warm enough as it dissipates heat passively. A blanket over the battery box and barrel so they'd more directly share heat would help too.

So just stick a barrel and a heating element in the building? That's slick. Panels in series right to the heating element? Do you have any suggestions on barrels or heating elements that are particularly suited to the task? Perhaps just the standard blue water barrel? What would you set the temperature limits on the relay to? This is probably a much better idea than heating pads...

Here is a thread that goes thru the numbers on setting up an array and sizing it for different heating elements.


@AntronX understands this so i will conjure him.

What are the specs of your panels? How many could/would you be able to install? Is there a snow depth expected where you'd need to keep the panels above?

For a barrel, i'd strongly consider metal for ability to handle a hot heating element and to transfer heat to air or any "heat transfer" system. Plastic a good insulator so not what you want (i think).

I think even slicker idea is to use waste heat from 350W of communications equipment to keep batteries warm. Figure out how to capture and circulate warm air from equipment rack into battery enclosure.

Most of the equipment will be outdoor, on a pole. The indoor equipment is going to be only a fraction of the usage.

Another option is to install Victron charge controller inside insulated battery box. At 4000W it will generate 80 - 120W of heat from dc-dc conversion loss. It may just be enough to keep batteries at 50F and above.

Is there foam under the batteries? You can improve battery box design to retain more heat by wrapping inside of the box with shiny reflective film to reflect infrared radiation back towards the batteries and lift the batteries on a shelf to minimize thermal bridging to bottom foam. LFP batteries have 97% charge/discharge efficiency and will self-heat at the rate of about 60 watts while being charged at 4kW.

I might improve the batt box and do a solar electric water heater since that way you could work inside the room in the freezing cold and not be freezing cold.

MisterSandals said:

Here is a thread that goes thru the numbers on setting up an array and sizing it for different heating elements.

Dumb heating element and water pump

I'm just hoping I don't damage the panel in some way. A resistive load pulling as much power as it can is exactly what an MPPT charge controller is designed to do. Get a heating element that will handle the number of watts for the panels you intend to use. Like a 2000W element for 2000W of solar.

diysolarforum.com

@AntronX understands this so i will conjure him.

What are the specs of your panels? How many could/would you be able to install? Is there a snow depth expected where you'd need to keep the panels above?

For a barrel, i'd strongly consider metal for ability to handle a hot heating element and to transfer heat to air or any "heat transfer" system. Plastic a good insulator so not what you want (i think).

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The panels are 400 watt. They are operating voltage 30.8 and open circuit voltage 36.8. I could put two of them on the side of the building vertically for winter. I wouldn't expect more than 4ft of snow to pile up around this shed, maybe 5ft, and that would be very rare.