Ghetto 12v UPS

[dmmcintyre3]

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Oh, do tell about your UPS.  Self made, off grid or some sort of busted gear hack? 12V DC is like telco lite (typically 48V)

It's just an AGM lead acid battery and a charger. I pulled the charger out of an old RV. I estimate I'll get over 10 hours runtime from it, since I used a 115 amp hour battery.

A few solar panels wouldn't be hard to add, but I'm not currently running off grid.

 

[acd]

It's just an AGM lead acid battery and a charger. I pulled the charger out of an old RV. I estimate I'll get over 10 hours runtime from it, since I used a 115 amp hour battery.


A few solar panels wouldn't be hard to add, but I'm not currently running off grid.

Now that we're derailing the topic, what does your 12V output look like? Is it just unregulated output from the charger or do you use a pi/buck step down to regulate at 12V?

(if a mod could fork us off this thread, I'd appreciate it.)

 

[dmmcintyre3]

Now that we're derailing the topic, what does your 12V output look like? Is it just unregulated output from the charger or do you use a pi/buck step down to regulate at 12V?

(if a mod could fork us off this thread, I'd appreciate it.)

The battery charger outputs ~13.5v (it's a float type charger) and I use the computer PSU for regulation to other devices like the monitor and speakers.

 

[acd]

So it's just single battery charger input set to float charge to a battery, then a 12V input PSU on the back side w/ 12V, 5W, 3.3V out to whatever else? How much current does that setup draw? Does the PSU step up voltage when the 12V cells drop under 12V or does it just shut down? What's the ripple like on PSU output?

 

[drmike]

I do a similar thing with an old UPS and a deep cycle battery much larger than intended to be in UPS.  Problems with that since when grid power goes down, it doesn't deal with things and goes offline real quick.  Battery is fine, so the unit is being smart and expected less battery.

Problem I always had with this trickle + battery is my draw is too high and the charger doesn't keep up.

I'd like to find something cheap and buyable that would look at the power being pulled from battery and  adjust the trickle rate charge to correspond.  Draw 5A, the trickle charger bumps to 5A

Have multiple needs for such a module and solution.

I have solar, but not using it for this, today.

 

[notFound]

What I use as my UPS is a very simple setup to make use of my unused BT phone line which is around 60V and not many amps, but it's enough to charge a 12V battery over a period of a day or so, possibly more and run an inverter for emergencies. My uncle has also made a really complex setup which automatically switches to a bank of batteries and provides power indefinately with juice for at least a day, overkill but it was a great project.

 

[titanicsaled]

Batteries are just so darn expensive!

 

[MartinD]

What I use as my UPS is a very simple setup to make use of my unused BT phone line which is around 60V and not many amps, but it's enough to charge a 12V battery over a period of a day or so, possibly more and run an inverter for emergencies. My uncle has also made a really complex setup which automatically switches to a bank of batteries and provides power indefinately with juice for at least a day, overkill but it was a great project.

Any schematics for that?

 

[notFound]

Any schematics for that?

I got it from my uncle [idea and method].

But it'd be nice compiling my own schematic for the first time, mine is made out of spare parts and crap I found laying around stuffed together.

 

[MartinD]

So, share the method?

 

[acd]

That power is there to make phone calls in emergencies. You're also not paying for it. So moral issues aside, this is what you would do:

Measure open circuit voltage across pins 2 & 5. Then switch your DMM over to current mode and measure short circuit current. If the line is smarter than you and disables more than a few mA of current in short circuit, you're SOL. But it probably won't. Use the short circuit current and the open circuit voltage to estimate the thevenin equivalent voltage source. For optimal power transfer, you want to match the load resistance to the voltage source internal resistance (impedance matching, theory, etcetc.).

First thing's put a diode in your line, probably shottky with at least 100V reverse breakdown to hopefully prevent the AC from burning your converter if you get a phone call. Put a buck converter after that, but regulate the voltage output based on the voltage at the input (you're making a constant current consumer) to keep the input voltage around half of your open circuit voltage. If the output voltage is under that value, you'll probably get around a watt of transferred current. You may need to detect when the output voltage is too high to prevent damage to your device being charged. Most devices will charge with more than 1W of charging power so it should be OK to trickle charge in this configuration.

Don't test with devices you care about.

 

[drmike]

Batteries are indeed very expensive and have terribly short life.

What I do these days is spec batteries based on the end need.

Some examples:

Offgrid out building - 15 watt solar panel on the roof - (4) SLA 12V 6A(?) batteries --- use = motion sensor (24/7 draw) and LED lights

Bicycle lighting - no power source - (1) 12V 6.xAH lithium technology pack - use for running lights at night and accessories

Other bicycle light / portable lighting - no power source (charges from main power) - (1) 12V pack made of 8 AA batteries in a project battery pack.  Rechargeable alkaline.

The SLA batteries are blah.  Not so good. Perhaps age and environment or perhaps not enough high A charge from time to time.  Panel maxes the battery and goes into over volt discharge for protection on a good sunny day.  More panels?  Not such a good idea.

Lithium pack - pretty good value.  It is a Chinese blue wrapped pack.  Few years old by now.  It's light infrequent use.  So far, very good.  Cost was roughly $7 per AH.

AA Alkaline packs - active project.  Use these in all sorts of things and have them laying around for as-needed power.  Have 20 empty packs waiting to be built  Downside is AH of the packs are 1-1.5AH.   Packs are made of (8) AA cells - standard off the shelf AA's.  Roughly $5+ tax for 20 cells. 25 cents US each or $2 total in cells.   Downside is the discharge needs to be regulated (cut off voltage regulator with 20% approximately storage left in cells).  Charger for these is not great and standard main power.  Mind you, recharging standard alkaline cells which everyone has been trained are throwaway single discharge.

There are rechargeable alkalines, but one line I've bought that are manufactured in Canada are horrendous.  Obviously, NiMH needs revisited in this design but the pack requires more cells and is iffy if not regulated as NiMH has top volatage that can be higher by several volts when charged, but retreats quickly, so have to run a pack of 10 NiMH cells (I need a AA big gang charger and 10AA holder packs).

 

[pcan]

What I use as my UPS is a very simple setup to make use of my unused BT phone line which is around 60V and not many amps, but it's enough to charge a 12V battery over a period of a day or so, possibly more and run an inverter for emergencies. My uncle has also made a really complex setup which automatically switches to a bank of batteries and provides power indefinately with juice for at least a day, overkill but it was a great project.

About 20 years ago I saw a lot of projects to harvest energy from the telephone line on electronics magazines. Of course, this is not autorized by the telephone operator. The line voltage should never drop under 40-45 volts to avoid interference with the phone service. This usually means that the maximum safe draw is in the range of 3-4 milliamperes; easily done with a LM317 current regulator with a 330 ohm feedback resistor (please check the exact value on the datasheet - I am recalling a old memory). The phone line is feed to the regulator trough a full wave rectifier rated at 200 volts, because the ring voltage could spike to 110 volt. A overvoltage protection is also useful/needed. The LM317 output could be connected to the required step-down switching regulator. This is easy to buy from RS or any electronics distributor; I guess that some modern PSU modules can be current limited on the input to avoid the LM317 entirely.