Battery recommendations for a remote telescope observatory?

[SnowSailor]

Hi,

I am interested in building a remote telescope observatory somewhere outside of Phoenix, AZ. This observatory would need to be powered by solar since it's the only way to get electricity up in the mountains. The observatory structure would not be heated, so I am a little worried about the batteries in the winter. It's possible for the temperature to drop to 10F or lower in the winters, and I have seen that lead-acid batteries typically only have 60% of their rated capacity at that temperature. On top of that, batteries should not be discharged more than 60%, as that's where they will usually dip below their rated voltage and it can damage the batteries.

Given this, my question is how many amp-hours should I be looking for in a 12V battery bank? I estimate that my observatory structure will use 150W during the day on average and 250W during the nights on average. I think it's pretty safe to assume that means 200W on average throughout an entire day (4800 watt-hours, or 400Ah at 12V). Fortunately, all of my equipment can be powered with 12VDC so I don't need to run an inverter that would waste some power.

My planned circuit is: solar panels -> charge controller -> batteries -> comparator/mosfet cut-off switch -> 12VDC voltage regulator -> my equipment

I was told that I should decide on batteries before I decide on solar panels because the batteries are more important when it comes to actually powering my equipment. They're especially important because I won't have any sunlight when I'll be using the most power (at night). What do you all recommend I get for batteries given the extreme temperatures? I don't want to have to heat the batteries because that would probably use more power than its worth and at night I'll have the observatory roof open which will allow all the hot air to escape anyway. I'm looking to not spend too much money, but I understand that I'll probably be spending several thousand on the entire solar setup with batteries, panels, and a charge controller. Ideally, the batteries would require little to no maintenance because it may take several hours to drive to the observatory site and do anything.

Please ask any clarifying questions if I wasn't clear about something.

Thanks!

[Mike90250]

Does that location get snowed in, and have to survive for a week or 3 on it's own, or will it be visited daily ?

400ah daily needs 12,000ah in battery storage to get decent battery life without draining them deeply daily. That's going to need expensive 2V cells. Or maybe consider a 48V system with a DC - DC downconverter to 12V for your loads.

[SnowSailor]

Does that location get snowed in, and have to survive for a week or 3 on it's own, or will it be visited daily ?

400ah daily needs 12,000ah in battery storage to get decent battery life without draining them deeply daily. That's going to need expensive 2V cells. Or maybe consider a 48V system with a DC - DC downconverter to 12V for your loads.

I'm not sure if it will be snowed in, but the idea is to have the entire system managed remotely so no one has to go up there every day. During the day the charge controller will allow the batteries to charge, and they will be used at night. If ever the system dips below 12V output, all power will be cut off so no more is used (this is what the comparator/mosfet switch is for). This means all the equipment will turn off and no more power will be drawn. Ideally, this will never happen but it's possible if there are several cloudy/rainy days in a row. However, when the sun comes back out and the batteries are recharged to the point where they can provide enough power, the electronics will begin to draw energy from the system again.

The solar panels will be able to provide enough power to fully recharge the batteries during the daytime. 12,000Ah at 12V seems like quite a bit of overkill for just 400Ah of maximum daily usage. That's 30x what I'll be using and it would probably cost $20k or more. I was thinking somewhere around 1,200Ah should be enough. If the batteries only have 60% of their capacity in the winter (0 degrees F) and they shouldn't be used past 60% of their total charge, I think that means worst case I would only want to use 36% (60% of 60%) of the battery bank's rated capacity. I'm not sure where you got the 12,000Ah number from but 36% of 1,200Ah is ~400Ah and that's how I got my estimate.

If you could provide some explanation that would be great. Thanks for the help!

[BoloMKXXVIII]

I know they are expensive, but what about Lithium Titanate batteries? They have extremely long life and are supposed to be able to handle lower temperatures.

[SnowSailor]

I know they are expensive, but what about Lithium Titanate batteries? They have extremely long life and are supposed to be able to handle lower temperatures.

That's a tough call. I mean the the lithium batteries do tend to last longer and they allow you to pull much more charge from the battery without hurting it. I don't plan to build this until 2024 or 2025, so the prices of lithium batteries could go down. I saw one estimate that suggested it's possible for the cost per kWh to halve. At that point I would be more comfortable spending money on lithium since it would be nearly the same price per available Ah of output (considering cold weather performance and depth of discharge when compared to lead-acid). At this point I'm more looking for a suggestion about how much capacity I should be looking to get.

[Mike90250]

... 12,000Ah at 12V seems like quite a bit of overkill for just 400Ah of maximum daily usage......

Right, my decimal place jumped, 1,200ah, 3 days worth. The gold standard is 5 days. So what's the risk/loss if the system shuts down ?

[SnowSailor]

Right, my decimal place jumped, 1,200ah, 3 days worth. The gold standard is 5 days. So what's the risk/loss if the system shuts down ?

Ah, great. So everything that's running should be able to power back up automatically once power returns. Most modern computer motherboards have the ability to automatically restart when power returns, and all the other systems should be fine as long as the power switches stay on (which they would unless I went up there and manually turned them off for some reason). I'll have a telescope mount, satellite dish, computer, and a few cameras. All of those should be fine. I know a guy that runs his entire telescope system on 32W (not including a satellite dish), so I may actually be overestimating my power consumption by ~40% (a satellite dish should add ~30-50W and a few cameras might add some extra as well). So I may only need ~800-1000Ah of batteries to be safe.

[BoloMKXXVIII]

That's a tough call. I mean the the lithium batteries do tend to last longer and they allow you to pull much more charge from the battery without hurting it. I don't plan to build this until 2024 or 2025, so the prices of lithium batteries could go down. I saw one estimate that suggested it's possible for the cost per kWh to halve. At that point I would be more comfortable spending money on lithium since it would be nearly the same price per available Ah of output (considering cold weather performance and depth of discharge when compared to lead-acid). At this point I'm more looking for a suggestion about how much capacity I should be looking to get.

Not all types of lithium batteries are the same. Different formulations have very different properties. While the price may be higher up front, if you calculate the cost over the long term lithium batteries can be cheaper.

[Mike90250]

I know they are expensive, but what about Lithium Titanate batteries? They have extremely long life and are supposed to be able to handle lower temperatures.

https://batteryuniversity.com/learn/...of_lithium_ion Lithium Titanate ( LTO )

Last battery listed (about 1/4 of the way down the page, the rest is comments)
LTO cells support low temperature discharge

However, all the glowing reports of "low temp performance" fail to mention they cannot be recharged until they are warmed up. You will note that cold climate, low temp Li batteries require much longer recharge times, because the integrated BMS is using resistance heating to warm the battery for the 1st hour, then recharge can slowly start.

I don't know of any Li based batteries that can be recharged below freezing.

So a well maintained flooded bank may still be the best bet, and oversizing the PV a bit could be the best answer.

[SnowSailor]

https://batteryuniversity.com/learn/...of_lithium_ion Lithium Titanate ( LTO )

Last battery listed (about 1/4 of the way down the page, the rest is comments)
LTO cells support low temperature discharge

However, all the glowing reports of "low temp performance" fail to mention they cannot be recharged until they are warmed up. You will note that cold climate, low temp Li batteries require much longer recharge times, because the integrated BMS is using resistance heating to warm the battery for the 1st hour, then recharge can slowly start.

I don't know of any Li based batteries that can be recharged below freezing.

So a well maintained flooded bank may still be the best bet, and oversizing the PV a bit could be the best answer.

That's a good point. I didn't really consider that. But with enough wattage from the panels would the batteries heat up faster? In the location I'm looking at, I get about 5.5 peak solar hours at the minimum (winters) and 7.5 during the summers. If the batteries are warming up for 1 hour, that still leaves 4.5 peak hours to charge. If I had like 1500W of solar panels would that be enough to heat up and charge 2x200Ah 12V LTO batteries even in the winter? I'd probably want to go with the cheaper lithium batteries like LiFePo.

[Mike90250]

"Which batteries" is going to be your call. and in a couple years things may have changed quite a bit. Nailing down the energy consumption is going to be key. If you ever get snow that blocks the panels for several days, you run the risk of the charge controller's brains running the batteries down to LBC, and then the charge controller is shut off till you manually restart it.

[SnowSailor]

"Which batteries" is going to be your call. and in a couple years things may have changed quite a bit. Nailing down the energy consumption is going to be key. If you ever get snow that blocks the panels for several days, you run the risk of the charge controller's brains running the batteries down to LBC, and then the charge controller is shut off till you manually restart it.

Oh so the charge controller won't just come back on when the sun comes out again and power starts to flow into it?

[BoloMKXXVIII]

"Which batteries" is going to be your call. and in a couple years things may have changed quite a bit. Nailing down the energy consumption is going to be key. If you ever get snow that blocks the panels for several days, you run the risk of the charge controller's brains running the batteries down to LBC, and then the charge controller is shut off till you manually restart it.

You never know. In a few years we might be using lithium garnet batteries.

http://news.mit.edu/2019/enriching-solid-state-batteries-jennifer-rupp-mit-0711

[Mike90250]

Oh so the charge controller won't just come back on when the sun comes out again and power starts to flow into it?

A slow ramp up of power, does not always trigger a reboot. It may just hang. or reboot 1x out of 10, or 9x out of 10 But having PV powering a shut-off charge controller on a dead battery, is not a normal operating condition. And if the battery is drained and controller shut off, how will panels charge the battery to restart things ? Controllers are nearly always powered from the battery.

So to avoid that shut-down condition, you may need to have a larger battery bank to "flywheel" through cloudy days

[PNjunction]

NO battery prices ever go down. I wouldn't base a project on that kind of waiting.

As for lithium, the initial prospect was that it would totally wipe lead out. Didn't happen. Early users balked at the "upfront" price, and marketers fool the public by touting the cycle life being roughly 4 times that of lead, justifying the 4x in price. But this was only meant to be temporary and as lead was removed from the planet, the prices would come down.

That upfront price is also extremely expensive if you make a mistake in your original capacity / power budget calculations. Now you get to start over, or maybe sell the used trash to someone else.

Suckers. The total rip-off is that the materials and production of li-ion is about 4x *cheaper* than making a lead acid battery. And they all end up in the dump because there isn't much to salvage really in recycling - at least not enough to be profitable.

I *love* LFP tech, and built a few great projects, but after a decade of prices not falling - despite promises to early adopters - and manufacturers just raking us over the coals in manufacturing cost and cheap litihium (as compared to lead), made a fool out of me for trusting the industry.

Li-ion prices should be MUCH lower than lead by now - but since that will never be realized, lead-acid lovers can rest assured that they will NEVER go away. And if you choose lead, you just replace them a few times - with a much lower financial risk - than LFP and other lithium essentially coming solely from one country.