Off-grid battery system for radio site

First mistake, you cannot use average, you have to use worse case month of either December or January. Otherwise all the months below 3 you go dark, or run on generator everyday.

Use 48 volt battery system and then use 48/12 buck converter for 12 volts. Accept the fact the site will be on generator if you expect 99% availability. To minimize generator run time and get 99% availability you over size batteries and panel wattage.

Throw out every deign practice you you have heard here as they do not apply mission critical systems. Battery Antimony = 10 days, panel wattage 50% larger than required in minimum winter month. Size generator to fully recharge batteries from 50% DOD in 4 to 8 hours.

I meant that the worst-case month seemed to average around 3 or just above, looking at PVWATTS and a few maps I found. Average for the year is around 5, max is around 7. Let me plug in 10 days and check out buck converters, I don't think we've ever used one.

Are these the Existing gear or your proposed new gear ?

A 15Kw array and a 5Kw array ? That's a massive theft target ! I'd add another Kw to run the electric fence !

Existing MW and proposed VHF. There are currently 140 110W panels, of which 28 are for 12V and the rest for 48. And there are no fences, it's on a mountain top in a national forest, remote yet frequented by 4WD and snowmobile drivers, and somehow in the whole 12 years they're been up, none have been taken. I can't explain it.

I like 10 days of autonomy. I need to figure out if the generator and rectifiers up there are up to it. Especially the rectifiers.

I'll come back with some new numbers.

If your battery hut temps don't get too cold, you may consider LFP batteries, more efficient than AGM, but would need tweaking to set the voltage setpoints for new chemistry.

Any new radios with higher efficencys than the existing gear ?

And I concur with Sunking, a straight 48V system with DC-DC downconverters for the 12V gear. Should be in the >90% eff ballpark.

While I agree with Mike and Sunking that it would be more efficient to use a single 48v battery system and use a 48 to 12 VDC converter for those 10 stations, you mentioned mission critical and IMO I would not put all my eggs in one basket.

I am not sure how each system interacts or is used by the Highway Patrol, but depending on how critical either system needs to be maybe having a solar battery system to run each type of equipment provides you with some additional margin of safety should a single system go down (for any reason) then you at least have a second system working.

The temps probably get down to 40F winter and up to about 80F summer. I'll look at LFP batteries, I'm not familiar with them, although I see a lot of threads here about them. Thanks for the tip.

I'm not too familiar with a buck boost charger either, would this be used to charge 12V batteries, or run 12V loads directly? The loads will vary from about 5A to 100A or so, if half of the stations transmit at the same time, which isn't likely, but possible. I'd feel better if they could all transmit at the same time, at about 200A. I think the existing power distribution (breakers, wire sizes) allows that. My short reading of Buck-boost converters seems to say they are best when the current is relatively constant, so would they be OK in an application like this?

I don't know if I want to go with one 48V system only either, but I'd like to look at what could be done. About 1/3 of the VHF stations will be controlled by the microwave, the rest being stand-alone repeaters. The Microwave has drops to the VHF radios, but also passes through to another important site. So there is about 1/3 functionality overlap between VHF and MW, with the rest being independent of each other.

I need to think about things like whether I want an unusual failure of one thing to take down both systems. For example, if a VHF station goes key-down 24/7 for some reason and we can't get up there, it would be bad if it pulled the MW down with it. Or if somebody shoots a hole in a waveguide and causes the (48VDC) compressor to run 24/7 and taking down the 48V battery, it would still be nice to have the VHF. There is a lot to think about.

I really appreciate the ideas, thank you!

Steve

The VHF radios are made in Canada by Daniels, and are optimized for solar sites. They have a lot of options like turning off the front panel LEDs, etc. Unfortunately in transmit, there isn't much to do to make them more efficient. The Microwave is long haul equipment that has a lot of power hungry features, like hot standby (two of almost everything, always running and ready to switch over). So I think we are stuck with that type. We don't use any or the cheaper, lower power tower-mounted stuff like cell companies use. That stuff might be OK, we just don't use it. A typical MW terminal is 500-700 watts of use.

Let's get you cleared up. To step down DC voltage there are two routes to take.

1. i am certain you are familiar with known as a shunt or series regulator. A LM 7805 is a good example. It acts like a resistor to take 48 volts input and output 12 volts. They are series devices which are antiquated because they are so inefficient. 48 to 12 volts is less than 25% efficient. Reason is Current Input = Current Output. So if you have a 12 volt 50 amp load (600 watts) means you input 50 amps @ 48 volts = 2400 watts. Makes for a great place to set a coffe pot and make coffee in a hurry, and doubles as a space heater.

2. The other modern type is a DC to DC converter and from the prospective of power at the input/output appears to operates like a transformer where Power input = Power Output minus some operational losses on the order of 2 to 5% for good converters and as much as 10% for cheap converters. So with a 12 volt 50 amp output, the input is 48 volts @ 12.5 amps. Instead of wasting 1800 watts as heat, down to 20 to 50 watts loss. A buck converter steps voltage down, and boost steps up voltage. The technology I am certain you have heard of is Switch Mode. It takes DC, converts to high frequency AC, to a step up or step down transformer, then converts back to DC.

If you have both 48 and 12 volt loads you would likely just step down to power the 12 volt equipment directly. Just make sure the unit is fully isolated because 48 vdc systems are positive ground, and 12 volt is negative ground. Are you aware of that? If not make a mental note, or sparks and smoke will fly.

Secondly if you loose 48 volts, microwave link is down and who cares if th e12 volt BTS has power or not. Not going to work if the link is down. Repeater will still work though.

If designed correct yes, otherwise you can make two systems, but that cost big bucks.

Another thing that will bite you in the but is voltage sag at 200 amps on a 12 volt system. If the 12 volt battery capacity is less than 1600 AH you will have significant sag. Should not be a problem with a 8 to 10 day Antimony. If using LFP would eliminate that worry and allow you to use smaller batteries.

Right, the -48V system is positive ground, I've been working with it for years but I have to admit I temporarily forgot about that when thinking of the converter operation. I'll keep that in mind when looking at converters.
We have a few -48V sites with -24V microwave radios and have switching -48V to -24V converters.

I don't think there is any reason to plan for 200A, because now that I think about it, our distribution breaker panels would trip if everything keyed at the same time.

I have a lot more to learn, let me go do some reading...

48 and 24 volt communication systems both use positive ground. 12 volt is the queer one.

The reason for the 12 volt systems using negative ground comes from vehicles. They use the same radios in vehicles and base stations. It has always been a problem in LMR and public safety radio systems. Once you cross the line into commercial radios everything is 24 and 48 volt for low power up to 200 to 400 watts, and switches to high voltage AC for high power like commercial TV/AM/FM. I suspect it will not be long before TV/AM/FM transmitters will be gone.

When you suggest a buck converter, do you mean something like this?



I wouldn't mind using something like this if I could parallel more than one, to provide the power and redundancy. We do use a lot of Newmar supplies/chargers.

Yes there is quite a few manufactures for them

Nothing wrong with that converter. But note, its not a simple "buck" converter which is cheap and highly efficient.
Downside is a buck can't change output polarity (I-O not isolated). Generally it can't raise voltage either.

The one on the site solves those problems, at a lower efficiency. Bruce Roe