Back-up power for DC loads only
August 18, 2015 6:46 PM Subscribe
My property's at the end of a PG&E power line and during the winter, fallen trees and mud slides cut the power for a few days at a time. Most utilities (heating, refrigeration, cooking, water heater) have propane or wood-fired back-up but Internet connectivity has not. I would like a power supply for +5 VDC and +12 VDC, capable of about 60 Ah, and rechargeable with solar cells.
The power supply is meant to keep Internet available while power is out for an extended time. It needs to:
1) Power a DSL modem and a router that use +12 VDC. Each has a 1A "wall wart" power supply.
2) Charge tablets via a USB power connection (+5 VDC). The tablets have 3200mAh batteries. Eight complete charging cycles is enough.
3) Keep the 12V loads up for not less than 24 hr spread across five cloudy days.
4) Accept charge from a 110 VAC connection *and* from solar cells. It is all right if this is switched by hand from line power to solar cells. It would be nice if two back-to-back sunny days could completely re-charge the battery/ies. Latitude 37N, huge space available for solar cells, good view of sun in winter (10 am - 4 pm).
110 VAC output is not needed.
The battery/ies will live in a well-ventilated, unheated place. The temperature here rarely drops to freezing. Car battery/ies are fine.
Either canned or I can DIY this but I lack the knowledge to search out the right equipment. Go-to recommendations welcome.
Once there is stock and a few reviews we plan to put in a Tesla superbattery so this solution needs to last not longer than three years.
The power supply is meant to keep Internet available while power is out for an extended time. It needs to:
1) Power a DSL modem and a router that use +12 VDC. Each has a 1A "wall wart" power supply.
2) Charge tablets via a USB power connection (+5 VDC). The tablets have 3200mAh batteries. Eight complete charging cycles is enough.
3) Keep the 12V loads up for not less than 24 hr spread across five cloudy days.
4) Accept charge from a 110 VAC connection *and* from solar cells. It is all right if this is switched by hand from line power to solar cells. It would be nice if two back-to-back sunny days could completely re-charge the battery/ies. Latitude 37N, huge space available for solar cells, good view of sun in winter (10 am - 4 pm).
110 VAC output is not needed.
The battery/ies will live in a well-ventilated, unheated place. The temperature here rarely drops to freezing. Car battery/ies are fine.
Either canned or I can DIY this but I lack the knowledge to search out the right equipment. Go-to recommendations welcome.
Once there is stock and a few reviews we plan to put in a Tesla superbattery so this solution needs to last not longer than three years.
If you want to do this cheaply, you can wire up a 12 VDC plug for the modem directly off the 12 V car battery, and wire up a car socket onto the battery. Use car chargers to charge the tablets.
Each has a 1A "wall wart" power supply
You won't be using these if you are running off the car battery.
You'll be able to charge it from a 110 VAC connection with a standard car battery charger, which you might already own.
I don't know that much about solar cells but I have friends who use car batteries for their solar setups so I assume they work out fine.
posted by yohko at 7:32 PM on August 18, 2015
Each has a 1A "wall wart" power supply
You won't be using these if you are running off the car battery.
You'll be able to charge it from a 110 VAC connection with a standard car battery charger, which you might already own.
I don't know that much about solar cells but I have friends who use car batteries for their solar setups so I assume they work out fine.
posted by yohko at 7:32 PM on August 18, 2015
Best answer: You don't want a car battery, you're looking for a deep cycle / marine battery, which is better designed to provide long-term slow drain loads, as opposed to a car battery which is more suited to providing instant power for starting.
Here is a handy online tool that you can use to determine the minimum size battery you need based on your power consumption. Any of these can be hooked up using alligator clips to whatever charging solution you have, and same for power usage. You'll just need to be sure you have appropriate adapters. An adapter that has a 12v 'car lighter' adapter on one end will suit you fine for charging your devices; just get a USB car charger. Internet stuff might be a little trickier; you just would need to be sure you have an adapter that provides the right polarity and power output to your devices.
This is the solution I use to power my CPAP on our long (3-day) camping trip. Battery is connected to via alligator clips, with a 12v cig liter on the other end. Then I have a 12v adapter made specifically for my CPAP machine that just plugs into the lighter socket.
Or on preview, what the Real Dan said.
posted by SquidLips at 7:32 PM on August 18, 2015 [1 favorite]
Here is a handy online tool that you can use to determine the minimum size battery you need based on your power consumption. Any of these can be hooked up using alligator clips to whatever charging solution you have, and same for power usage. You'll just need to be sure you have appropriate adapters. An adapter that has a 12v 'car lighter' adapter on one end will suit you fine for charging your devices; just get a USB car charger. Internet stuff might be a little trickier; you just would need to be sure you have an adapter that provides the right polarity and power output to your devices.
This is the solution I use to power my CPAP on our long (3-day) camping trip. Battery is connected to via alligator clips, with a 12v cig liter on the other end. Then I have a 12v adapter made specifically for my CPAP machine that just plugs into the lighter socket.
Or on preview, what the Real Dan said.
posted by SquidLips at 7:32 PM on August 18, 2015 [1 favorite]
Best answer: Check out mini PC power supplies meant for car computers. They take 6-24V (because car voltages do weird shit when cranking the engine), and output the usual PC voltages, including 12, 5, and 3.3.
This isn't a complete solution, but may be useful as a component.
Poke around that site for similar things. There is a lot of variety.
posted by ryanrs at 7:32 PM on August 18, 2015 [2 favorites]
This isn't a complete solution, but may be useful as a component.
Poke around that site for similar things. There is a lot of variety.
posted by ryanrs at 7:32 PM on August 18, 2015 [2 favorites]
There is an existing product that's almost what you want - generally search for "solar generators"; one example is the Yeti line of generators from Goal Zero. Looks like you're going to pay about $15 per amp hour capacity, and that's just for the "generator" pack itself, i.e., you need a solar panel as well. Which usually go for $2-5 per watt peak.
Besides a direct 12V out, this does have an AC source that is technically unnecessary for your stated needs, but I kind of doubt you'll save much by putting together your own system without that option, and it may be the easiest way to get a 5V source: just plug the existing wall warts into the generated AC power.
posted by Joey Buttafoucault at 7:37 PM on August 18, 2015
Besides a direct 12V out, this does have an AC source that is technically unnecessary for your stated needs, but I kind of doubt you'll save much by putting together your own system without that option, and it may be the easiest way to get a 5V source: just plug the existing wall warts into the generated AC power.
posted by Joey Buttafoucault at 7:37 PM on August 18, 2015
Best answer: What you need is called a UPS (uninterruptible power supply), specifically a DC 12V UPS. Here is one option, though I would make sure your 12V components can handle a range of voltage around 12V for this (most can). Then you need a battery (I would get a small AGM, i.e. no maintenance, battery intended for solar systems - 50Ah should cost about $100 and should last you for 24 hours). For the 5V, you need a DC/DC converter. Probably the easiest thing is to attach a cigarette lighter socket to your UPS and then plug in the adapter that you probably already have for use in your car. You'll also need to find the specific plugs for your router and modem and attach them to the UPS.
I would recommend using a watt meter to get a better idea of the actual power demand you have. I'm assuming about 20W, but I suspect it may be closer to 15W.
Here is another option on eBay. You could probably also build something yourself based on this board. There don't seem to be a lot of options for this in North America. Are you in Europe or North America?
For solar charging, you can just get a basic solar charge controller and panel and attach that directly to your battery. Something around 150W should be enough to keep you going indefinitely. If you just need a bit of battery top up, about half of that should work.
Realistically, it will probably be cheaper and simpler for you to spend $100 to get a battery with twice the capacity than it will be to add solar to this system because it only needs to last a few days.
posted by ssg at 8:35 PM on August 18, 2015
I would recommend using a watt meter to get a better idea of the actual power demand you have. I'm assuming about 20W, but I suspect it may be closer to 15W.
Here is another option on eBay. You could probably also build something yourself based on this board. There don't seem to be a lot of options for this in North America. Are you in Europe or North America?
For solar charging, you can just get a basic solar charge controller and panel and attach that directly to your battery. Something around 150W should be enough to keep you going indefinitely. If you just need a bit of battery top up, about half of that should work.
Realistically, it will probably be cheaper and simpler for you to spend $100 to get a battery with twice the capacity than it will be to add solar to this system because it only needs to last a few days.
posted by ssg at 8:35 PM on August 18, 2015
I think what you might want is the electronics out of a Mobile Home (or RV). I think they are built around a 12V system with intermittent charging, and to get 5V there are a plethora of adapters, as well as battery, solar, and "house power" 120VAC options.
posted by nickggully at 9:05 PM on August 18, 2015
posted by nickggully at 9:05 PM on August 18, 2015
Go here:
http://www.homepower.com/
... pay for a subscription, and then download the past year's 12 issues in PDF form. READ THEM, paging through them like you would a magazine. Within those last 12 months, everything you need to know will be covered. and better than any commenters here -- sorry, seven commenters above :)
They will cover topics you DON'T care about, like hydro and wind power and grid-connected solar, but in there you will find what you are looking for.
posted by intermod at 9:33 PM on August 18, 2015 [2 favorites]
http://www.homepower.com/
... pay for a subscription, and then download the past year's 12 issues in PDF form. READ THEM, paging through them like you would a magazine. Within those last 12 months, everything you need to know will be covered. and better than any commenters here -- sorry, seven commenters above :)
They will cover topics you DON'T care about, like hydro and wind power and grid-connected solar, but in there you will find what you are looking for.
posted by intermod at 9:33 PM on August 18, 2015 [2 favorites]
If your power goes out you may find that your internet is likely to be out as well. If the power lines are downed by a tree the same tree has likely brought your DSL line down as well.
posted by jmsta at 2:59 AM on August 19, 2015
posted by jmsta at 2:59 AM on August 19, 2015
Okay.
You'll need to gain an understanding of basic electrical power. A few tips.
1) Again, the batteries you want are not car batteries. Car batteries are designed for very short, high current loads, in particular, turning over car engines. You need deep cycle batteries, which are very long draw, lower current loads.
2) Having said that, you can still be dealing with rather high current, so battery interconnections, esp. if you're using multiple batteries, want to be made with *big* conductors. Too small a conductor + too much current = heating element = at best, an inadvertent fuse, at worst, fire and explosion.
3) Explosion? Yes, explosion. Lead Acid batteries generate hydrogen, lots of it if the battery is overcharged. Fire+hydrogen+oxygen=BOOM. Bad.
So. Large enough conductors between the batteries and the charge controller, good ventilation, and that risk will be mitigated.
Make sure to inspect the system every so often. Batteries will fail over time, and can kill other batteries if left in circuit by allow them to overcharge. Good charge controllers will compensate for that, cheap ones will happily eat all the batteries.
The freezing point you need to worry about isn't water, it's the freezing point of the acid. Fortunately, that's 30% Sulfuric Acid, which is about -25°F.
The big question is "how big?"
You need to calculate your assumed base load in amps, because battery capacity is rated in amp-hours, since they battery voltage is fixed. For your 12V devices, this is easy -- just read the amps. Your DSL modem is 1A, your router is 1A. So, every hour, they'll each draw 1 amp hour.
Your charging devices are more complex. First, they're drawing off USB, which is 5V. That will downconvert from 12V, so we can't just read the amperage. The hand wave way is to look at the charging adapter, get the total wattage (either by reading directly, or multiply volts x amps), then dividing wattage by 12 to get the 12V amperage draw, then adding 10% as a fudge factor.
Then you figure out how long it takes to charge them. That gives you the amp-hour draw of charging, but unlike your DSL modem, this only lasts a couple of hours.
So, the DSL modem was 1 amp hour, you specced 2 days, that's 48 hours. You need 48 amp-hours of battery to run that for the specified time.
The router was also 1 amp hour. That's another 48 amp-hours. That's 96 total.
Assume that you need 3 amp-hours to charge a phone, 6 to charge a tablet (I'm *completely* making these up, BTW!) and you wanted eight cycles. 8x3=24, assume two phones, that's another 48. 6x8=48, two tablets, another 96. 96+48+96=240 amp-hours.
And so forth. Basically, anything you want to power on this system, you figure out what it will draw, how long it will draw, and that will tell you how many amp-hours the system will need to provide.
Once you have the total amp-hour draw, you can make some decisions. Are my specs feasible? (240 amp-hours is not that large a battery, BTW -- and even on a cloudy day, you'd be drawing some power from solar.) How much do I add for growth? Figure some for that as well.
Now, recharge factor. If you size the batteries completely to the load, that means you can run your load off that battery for the specified time with *nothing else*. But if you have solar panels, they'll provide power when it is sunny. That means you, at the very least, won't be drawing as much from the batteries when it is sunny.
How do you figure that? Well, how many amps do they put out?
Ideally, you want the output (again, in amps) to exceed the baseline draw, in amps, so that the batteries are charging. If the panels provide enough that the batteries fully recharge during the day, you basically have infinite runtime (until the batteries fail, of course.) If they don't, you'll have extended runtime, but eventually the batteries will go flat.
But if your baseline draw is, say, 6 amp hours, and you have peak draw of 10 amp hours, and your panels provide 12 amp hours, you'll put back at least 2 amp hours, at at most 6 amp hours, into the batteries. That's marginal. At best, you're matching your baseline, so at night, you're going to be losing. If you're panels are pushing 18, that's much better -- they're always charging more than baseline, even at peak draw, and the batteries are probably finishing every sunny day near fully charged. With batteries, you tend to see two cycles -- adequate recharge systems, which end the day near full charge, or not, which start the day with flat batteries and never get charged. The latter are really hard on batteries.
posted by eriko at 8:11 AM on August 19, 2015
You'll need to gain an understanding of basic electrical power. A few tips.
1) Again, the batteries you want are not car batteries. Car batteries are designed for very short, high current loads, in particular, turning over car engines. You need deep cycle batteries, which are very long draw, lower current loads.
2) Having said that, you can still be dealing with rather high current, so battery interconnections, esp. if you're using multiple batteries, want to be made with *big* conductors. Too small a conductor + too much current = heating element = at best, an inadvertent fuse, at worst, fire and explosion.
3) Explosion? Yes, explosion. Lead Acid batteries generate hydrogen, lots of it if the battery is overcharged. Fire+hydrogen+oxygen=BOOM. Bad.
So. Large enough conductors between the batteries and the charge controller, good ventilation, and that risk will be mitigated.
Make sure to inspect the system every so often. Batteries will fail over time, and can kill other batteries if left in circuit by allow them to overcharge. Good charge controllers will compensate for that, cheap ones will happily eat all the batteries.
The freezing point you need to worry about isn't water, it's the freezing point of the acid. Fortunately, that's 30% Sulfuric Acid, which is about -25°F.
The big question is "how big?"
You need to calculate your assumed base load in amps, because battery capacity is rated in amp-hours, since they battery voltage is fixed. For your 12V devices, this is easy -- just read the amps. Your DSL modem is 1A, your router is 1A. So, every hour, they'll each draw 1 amp hour.
Your charging devices are more complex. First, they're drawing off USB, which is 5V. That will downconvert from 12V, so we can't just read the amperage. The hand wave way is to look at the charging adapter, get the total wattage (either by reading directly, or multiply volts x amps), then dividing wattage by 12 to get the 12V amperage draw, then adding 10% as a fudge factor.
Then you figure out how long it takes to charge them. That gives you the amp-hour draw of charging, but unlike your DSL modem, this only lasts a couple of hours.
So, the DSL modem was 1 amp hour, you specced 2 days, that's 48 hours. You need 48 amp-hours of battery to run that for the specified time.
The router was also 1 amp hour. That's another 48 amp-hours. That's 96 total.
Assume that you need 3 amp-hours to charge a phone, 6 to charge a tablet (I'm *completely* making these up, BTW!) and you wanted eight cycles. 8x3=24, assume two phones, that's another 48. 6x8=48, two tablets, another 96. 96+48+96=240 amp-hours.
And so forth. Basically, anything you want to power on this system, you figure out what it will draw, how long it will draw, and that will tell you how many amp-hours the system will need to provide.
Once you have the total amp-hour draw, you can make some decisions. Are my specs feasible? (240 amp-hours is not that large a battery, BTW -- and even on a cloudy day, you'd be drawing some power from solar.) How much do I add for growth? Figure some for that as well.
Now, recharge factor. If you size the batteries completely to the load, that means you can run your load off that battery for the specified time with *nothing else*. But if you have solar panels, they'll provide power when it is sunny. That means you, at the very least, won't be drawing as much from the batteries when it is sunny.
How do you figure that? Well, how many amps do they put out?
Ideally, you want the output (again, in amps) to exceed the baseline draw, in amps, so that the batteries are charging. If the panels provide enough that the batteries fully recharge during the day, you basically have infinite runtime (until the batteries fail, of course.) If they don't, you'll have extended runtime, but eventually the batteries will go flat.
But if your baseline draw is, say, 6 amp hours, and you have peak draw of 10 amp hours, and your panels provide 12 amp hours, you'll put back at least 2 amp hours, at at most 6 amp hours, into the batteries. That's marginal. At best, you're matching your baseline, so at night, you're going to be losing. If you're panels are pushing 18, that's much better -- they're always charging more than baseline, even at peak draw, and the batteries are probably finishing every sunny day near fully charged. With batteries, you tend to see two cycles -- adequate recharge systems, which end the day near full charge, or not, which start the day with flat batteries and never get charged. The latter are really hard on batteries.
posted by eriko at 8:11 AM on August 19, 2015
Best answer: Go to your local computer recycling shop, and your local weird-tech-recycling shop/electrical junk supply place that sells surplus stuff. At least in and around my city there's maybe 4 places that would fit one or several of those descriptions.
You want a 12/24/48v UPS with 12/24v input. This is not a non existent item.
Then you add say, a pigtail adapter and a dual port USB car charger, and like this with some adapters, and you're done. That's someeethinggg like an 6amp or so charger? That should top off even a large one of those in 48 hours.
That's how i'd do this at least, and i bet i'd be done for $150-200.
The good place i'd actually buy one of these from locally would let me do a rundown test on the batteries if i was serious/cared. Definitely bring some sort of 12/24v load. These sorts of things were designed to drive 48v telco equipment usually, and some of them are older and have tired batteries. However, that stuff is still readily in use and it might be a pretty new unit.
These will have, if the batteries are healthy, expensive as hell nice batteries designed for exactly this in them that will last years.
posted by emptythought at 12:19 PM on August 19, 2015
You want a 12/24/48v UPS with 12/24v input. This is not a non existent item.
Then you add say, a pigtail adapter and a dual port USB car charger, and like this with some adapters, and you're done. That's someeethinggg like an 6amp or so charger? That should top off even a large one of those in 48 hours.
That's how i'd do this at least, and i bet i'd be done for $150-200.
The good place i'd actually buy one of these from locally would let me do a rundown test on the batteries if i was serious/cared. Definitely bring some sort of 12/24v load. These sorts of things were designed to drive 48v telco equipment usually, and some of them are older and have tired batteries. However, that stuff is still readily in use and it might be a pretty new unit.
These will have, if the batteries are healthy, expensive as hell nice batteries designed for exactly this in them that will last years.
posted by emptythought at 12:19 PM on August 19, 2015
Response by poster: Lots of good information here including several products that are new to me and a couple of approaches that might work *spectacularly*.
My daily power budget, after considering the above, is: 320 W*hr for DSL modem + router, using them 12 hr per day, plus 75 W*hr to charge one tablet and one phone. With a 250 Ah, 12 V battery, the battery alone should last five days at 75% discharge.
A 200W solar installation, according to the PVWatts calculator at http://pvwatts.nrel.gov/, should deliver 500W*hr/day at my location in December. Given one of the criteria above, this will exceed, on the average, the 400W*hr/day power budget thereby gaining on the load by 100W*hr/day.
If emptythought's idea works out - Halted Specialties might have what they're talking about - this could even be reasonably inexpensive.
Thanks to all commenters.
posted by jet_silver at 8:15 PM on August 20, 2015
My daily power budget, after considering the above, is: 320 W*hr for DSL modem + router, using them 12 hr per day, plus 75 W*hr to charge one tablet and one phone. With a 250 Ah, 12 V battery, the battery alone should last five days at 75% discharge.
A 200W solar installation, according to the PVWatts calculator at http://pvwatts.nrel.gov/, should deliver 500W*hr/day at my location in December. Given one of the criteria above, this will exceed, on the average, the 400W*hr/day power budget thereby gaining on the load by 100W*hr/day.
If emptythought's idea works out - Halted Specialties might have what they're talking about - this could even be reasonably inexpensive.
Thanks to all commenters.
posted by jet_silver at 8:15 PM on August 20, 2015
Bear in mind that your router and modem use at the very most 1A and likely in normal operation far less, so 320Wh is more than you need for 12h operation. Half of that is a good guess to start, but testing is the best way to know for sure.
posted by ssg at 8:30 PM on August 20, 2015
posted by ssg at 8:30 PM on August 20, 2015
This thread is closed to new comments.
Chilled lead-acid batteries have significantly reduced low temperature capacity at medium to high discharge, but as long as you keep your loads small, like what you've described, you'll be fine.
Get a buck converter to go from DC 21-11 volts to a nice 5 volts, for the USB, like the "DROK® DC-DC Buck Voltage Converter 4.5-40V " at Amazon for your tablet/phone charging.
You'll need a charge controller for the lead-acid battery. The Xantrex/Schneider C35 or C40 charge controller is robust and settable for what the battery wants.
An RV-style converter (120 volt AC to 13.8-ish DC) is what you want for the line-powered DC part of this. An Iota DLS-30 is up to the task.
You're probably seriously oversizing your solar array if you want this to be charged up in 2 days in the wintertime. Irradiance, even as low as 37 degrees latitude is very seriously impacted in the wintertime. I'd ditch them entirely, and just charge from the converter.
The details are devilish, though. Converters always run a little more than the 13.25 V that makes a deep cycle battery happy, so you'll want a RV diode-style isolator between the converter and the charge controller. You'll have to check your battery water every now and again, but if you're living with propane and firewood, you are used to that sort of thing. And you need fuses, for safety: a lead-acid battery is capable of terrifying short-circuit currents.
posted by the Real Dan at 7:31 PM on August 18, 2015 [2 favorites]