UPS conversion?

[South Easter]

Old UPS's are easy to come by, and usually its the battery thats gone.  So what I was thinking of is a way to run my computers off my battery bank (2 x 100Ah 12Vs = 24V)...

I could just remove the AC mains connection, remove the dud dinky batteries, and wire the battery connectors in the UPS to my battery bank (if I'm real lucky the UPS will be 24V too).  Now that seems simple, but catch is when my LVD cuts in, the computer will die.  So does anyone know if there is a (easy) way to reverse the operation of the UPS?  Ie. when battery voltage is there, use it, and when not, then use the mains supply?

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[jonas302]

check with damonhd thats how his is working http://www.fieldlines.com/story/2007/8/5/1980/01250

[nothing to lose]

"but catch is when my LVD cuts in, the computer will die."

Not sure what you mean. What LVD? One in the UPS or elsewhere? If the UPS is wired direct to the batteries a LVD on anything else should not effect it I think.

A good UPS should have a port you can connect to the PC to safely shut it down when the battery gets to low.

"if there is a (easy) way to reverse the operation of the UPS?  Ie. when battery voltage is there, use it, and when not, then use the mains supply?"

I don't think there is, but if there is I want to know too. I use UPSs for inverters  also at times. Certainly if there is away it will probably vary by UPS and not all be the same. I mostly use a meter and watch the battery voltage, if it's getting low I plug in the UPS to the grid if available or if not really needed right then I shut the system down till later.

 Also I have run a UPS on a UPS before. Wastes some power that way, but if the battery bank gets to low and the first UPS shuts down then the second UPS kicks to it's own battery power and starts beeping the warning. This has worked well for portable use when I would not have any grid at all and did not want the system to just shut down on me automatically.

When looking for a UPS for 24V look for the bigger ones of course, most the smaller ones I take apart use 12V. I have had good luck with APC Smart UPSs of 900watts and above and all so far I have messed with are 24V untill you get to the really big stuff then 48V.

 I think all the brands I took apart 400watt or less were 12V, I have not had many between 400watt and 900watt though so not sure about any of those.

[DamonHD]

Have a look at:

http://www.earth.org.uk/low-voltage-drop-out-circuit-design.html

Both the LVD/switchover part (revision 3), and the laptop-as-a-dump-load bit working as of today!

Rgds

Damon

[veewee77]

Many UPSs require mains voltage to be present before they will checkout and startup.  Some will start without finding mains voltage first.

The only way I know of to make a regular UPS work backwards would be to put a relay ahead of it in the mains supply and monitor the battery. When the battery gets low, energize the relay, switching in the mains supply to the inverter which will make it do it's regular switching thing and the voltage output will not drop out.

Also, remember that most UPSs are designed to power the device that is plugged into them for about as long as the dinky battery installed will last. If you run them much longer than that at close to their rated load, they will get hot and will require additional cooling and you may even need to add heatsinks to help dissipate the added heat. If you run them at a lower load, like about half of the rated capacity, they will probably survive.

JMHO - YMMV

Doug

[sdscott]

Good idea, Doug, and easy to implement with a simple circuit.  Come to think of it, the circuit that CompDoc designed as a load/charge controller will work and is about as simple as it gets, as it simply energizes a relay of choice between two fine-adjustment voltage set-points of battery bank.  The whole circuit is simply a 555 chip and a few discrete components!

[TomW]

Haven't really looked at it close, maybe Ghurds circuit could be adapted here too?

Poor thing will be retasked for many unforeseen uses.

Cheers.

TomW

[ghurd]

Yes, it can.

Change the power N-fet to a P-fet.  It becomes an anti-dump load, or an LVC (Low Voltage Connect?)

The power P-fet would turn on the relay coil, in the AC line of grid charger.

Battery gets low, relay comes on, grid charger gets power, battery charges.

The connections on the power P-fet need changed to a P-ch configuration.

G-

[TomW]

G-;

I gotta say, thats the first time I ever saw "anti-dump load" used in a sentence.

Great to know your little circuit will be so useful, despite some thinking you need to go to Xantrex for controllers.

Carry on.

Cheers.

TomW

[nothing to lose]

Hey that sounds great Ghurd.

Remember that big Rackmount UPS and all those little 1000watt ones also that we picked up in Eastlake when I was up there

All 24V units.

 What I want to switch is not the charging circuit but the AC feed line. ON and off with the relay, let the UPS switch itself as normal based on rather Grid is present or not. Is this what you meant?

 So if batteries are full UPS runs on batteries (grid off) if batteries get low relay turns on grid power and UPS runs off the grid as normal. Let the solar and wind charge the batteries while running on grid.

If the trigger voltage is set to switch before the batteries are to low for the UPS to shut down then there would also be some power for backup as normal if the grid went down while on it. I need to check the shut down voltage later, but say if the UPS shuts down when batteries drop to 20V then set the trigger for 22V, so the UPS would still be a UPS as normal just with less battery power when they are low enough to be running on grid. So if grid goes down, UPS just switches back to batteries as normal even though they are low.

Could you put together a simple controller with the relay to switch on the 120Vac line when the batteries drop to say 21V or so. Since load and charging should be equal on all batteries I guess monitoring just one 12V battery in a bank should work as well, perhaps better since it would also work for a 48V Matrix controll also to switch 240Vac with the correct relay perhaps.

This might be another product for you Ghurd

 Make an outlet type box that plugs into the wall, connects to batteries, plug in your AC device (UPS) to box. If batteries are charged outlet is off, outlet on when batteries are low. This could be used for other stuff also.

 If you want an area lighted use a 12V light and have a AC light plugged in for standby. AC light only turns on when when battery is low and 12V shuts off. Course you could do that with a light sensor also, just kicking out ideas for other uses.

Heating? 12V heater normally, low batteries AC heater turns on. Maybe good for chicken coops and pump houses

Build it like a Watt O Meter with leads for battery connection.

[rossw]

>> Battery gets low, relay comes on, grid charger gets power, battery charges.

There's a problem of concept here, gents...

Sounds great in theory, but when the batt gets low and the relay operates to turn on the grid, the UPS will start charging the batteries - which will by its very nature, raise the volts on the batteries and probably reverse the process (unless there is a LOT of hysteresis - probably enough to cause other problems)

[ghurd]

Good point.  

I thought of that, but he said "dinky batteries" and I figured `dinky AC charger' too.  Might take a while to get it to raise a volt, not much hysteresis though maybe enough.  It may cycle a lot.  The hysteresis of the circuit could be increased with a few parts value swaps.

Dinky charger? I kind of wonder if a dinky AC charger will keep up with the power the batteries are supplying to the computer.  If the dinky charger supplies 75W to slowly recharge the dinky factory batteries, while the computer is drawing 150W... Not good.

I may have something out of whack.  Not sure I've seen 2 UPS the same inside.  But I haven't seen many.

G-

[nothing to lose]

Hey Ghurd,

This is why I would like the AC to turn on off not the charging circuit itself. When batteries are low, kick on the grid to power everything as normal. Shut the grid off when batteries are charged.

This would work well for the SU2200 rack mount unit I use and most anything else. Let the UPS switch to grid or batteries as normal. Just turn grid on and off by state of charge.

[pepa]

   hi NTL, i bought a circuit board from tom in NH that turns on and off a dedicated grid circuit by the state of charge of the battery bank. i use it to keep a spare battery bank charged and it has been working well for me. if you used one to tie your ups in, it may work for what you need. when the batteries are full the grid would drop out and the ups would take over as normal until the batteries are used up and the cycle would start again after the batteries are charged using your re. tom called it a reverse grid tie system, i hope this helps, pepa.

[sdscott]

South Easter, Ghurd;  The circuit I mentioned (CompDoc's) has two independent trigger-points (fine-adjustable via two pots) for energizing the relay at a low voltage and maintaining until the high voltage threshold is reached. I.e. energize at 12.2v and disengage at 14.2volts.  His design also has delay to eliminate false triggering (eg. Power-spike or brown-out from momentary large loading).

Also, most UPSs have a separate charging circuit that can be disabled by removing the protective fuse for that circuit (internal).  It would defeat the purpose of RE to allow the UPS to supply charging to the battery...

[vincecastro]

Here is the circuit your looking for.  Took me a month to find it.  Hope it helps.  I decided to get one assembled.

http://www.yourgreendream.com/diy_gp_bat_charger.php

Vince

[ghurd]

Well... I can't find a CompDoc circuit.  Best guess at which site says it was sold.

Sounds like it would be about the same.  Where it uses 2 pots, I used resistor X to determine the difference between the voltages, and a pot to move those voltages up and down the scale.  The cap in my circuit will take care of very short duration spikes or micro-mini brown-outs, a bigger cap could be used if there was a problem. A bigger cap will deal with longer variations.

With my circuit, set it to 1V hysteresis (RX=1.8K), change it to anti-dump load control or LVC, connected to a relay coil.  The relay contacts go in series with the fuse sdscott mentions (to use the internal charger, if it is set up that way).  Monitor one 12V battery.  If that battery dropped 1V, so did the other, meaning a 2V hysteresis on the 24V bank.

I'm not a fan of monitoring 1 battery in a string, but it won't matter much in this case because the RE and grid charger will be looking at the overall string.  One battery will be supplying current to the circuit.  Solid state relay would help?

Would be a decent idea to add a DPDT switch to both batteries to decide which is supplying current to the circuit, and switch it once a week.  Keep them more balanced.

Still on my 1st pot of coffee.

Just throwing out an idea.

G-

[sdscott]

Ghurd:  CompDoc is a member and his files are here;

http://www.fieldlines.com/user/compdoc

[South Easter]

I opened up an old UPS and thought I would post a pic or two of the internals.  (I removed a heat sink to makes things more visible.

I am wondering if the transformer can be left out, i.e. if it is only used for AC-DC step down...

A bit of a worry is that the battery is at 12.7V, so maybe its not the battery thats a problem with this unit... might mean the inverter is dead.

Also, I run at 24V, so not sure how to provide the 12V DC this unit requires. Maybe I should look for a bigger UPS.

A very amusing bit was a little board with the two phone jacks. It was connected to... absolutely nothing! Looked good on the brochure maybe!

[oztules]

South Easter

I think you will find problems if you leave the transformer out as it appears from your pics that it is the heart of the 12-240v step up..... not down  for the inverter. The two empty tabs (p01 and (i'm Guessing p02) go to the power tranny low voltage winding, they appear to be switched by a "H" bridge arrangement (the four Fets)... this is what gives you your high voltage 50 hz (60hz) output when in inverter mode (no grid).

I suspect that you would not require such a large step down transformer to power the logic on the control board. They may derive their power to the control board from the batteries instead.

Of course I may be completly wrong as I have not played with many of these things, but it does seem to lack any high freq transformers and other fets and bits to drive it as a inverter like they do in modified sine types. There they seem to like to push pull at high freq to a high voltage in a small ferrite transformer (to say 350vdc) and then h bridge it to modified sine at approx 240vAC.

............oztules

[TomW]

South Easter;

Well, those phone jacks probably are in and out for a line surge protector on the phone line to the modem. Maybe just connected thru case screw to ground? Maybe just a MOV between a couple pins. Hard to say but that yellow disk thing near the right hand jack looks like a MOV [Metal Oxide Varistor] they are used to clamp excess voltage from getting into the rest of a circuit.

Just an idea.

Cheers.

TomW

[South Easter]

Oztules: You are quite right - no transformer, no work.

TomW: The case is plastic, and there was no metal connection between the phone jacks and the rest of the circuit.  Also no coil/magnet/optocoupler etc that I could see.

I looked at the AC wave form on an old scope, and it looks like a horrible sawtooth shape! So I assume this is not a pure sine wave inverter.

It does start up without mains voltage present and a load present, although only ran for a few seconds.  So I hooked up a new 100Ah battery in place of the suspect 7Ah one, and then it could start up 'cold' (no mains), and run a small flourescent tube as a test load. When the mains is switched on, a relay clicks and its back to using mains power.

Irritatingly, it seems to have a warning beep when no mains is present.  Nothing a pair of pliers won't fix!  Maybe I'll replace it with an LED.

Now to think about a simple way (within my abilities!) to switch back to mains when the LVD kicks in...

[joestue]

Those Mosfets are 60 volt, 12 or 14 milliohms rds on.

so you could rebuild the entire circuit as a full H-bridge, and run on 24 volts.

That transformer has a center tap connected to the positive side of the battery, two fets are in parallel pulling each coil to ground. This does not require a charge pump to drive the fets.

The transformer probably has an extra coil in series with the primary to reduce losses and lower output voltage as it recharges the battery through the 4 1N540x diodes.

i have gotten my hands on a lot of UPSes that just died, everything worked except for the microprocessor. I broke a 2400VA unit when I forced it to start a 1/2 horse motor by disabling the current sense, but everything was ok then too, i removed all the power components and built other smps from them.

is the transformer welded?

[South Easter]

Had a rethink, and wondered if there were any drop in replacements for computer PSUs which took in 24V DC instead of AC.  

I found exactly that at mp3car.com - has any used these, or similar products?  It seems to be this will give silent power (no fans), and also be more efficient since it leaves out the invertor/rectifier stages.

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[oztules]

Joestue,

Your explanation makes good sense (push pull) and would explain the terminal next to the fuses (CT + take off point). I can now see three wires from the secondary (Bl Br Red), only noticed two before. (didn't notice the blue one)

Thanks for setting me straight. Haven't had to work on small ups yet (and hope not to)

..............oztules