Author Options:

UPS external battery setup? Answered

 I have a APC BACKUPS 500 with a dead battery, it can supply up to 300 watts continuously. However, at full tilt my computer can use up to 1000watts and the monitor uses about 150. With standard use my computer uses around 400watts or so. Would it be possible to use 2 UPS both wired in parralel to a single deep cycle lead acid battery. Would this work? The thing I'm worried about is wouldn't both UPSes try to charge the battery at the same t ime, and would that be ok? Red is positive and blue is negative in the picture. I would hopefully get around 1 hour of backup. Also, is there any way to make my computer go into energy conservation mode when the power goes out? I have a 2008 8 core mac pro.


.  I have been told by what I consider to be a reliable source (but I haven't verified) that the batteries in most home UPSs are the Sealed Lead-Acid (SLA) type and these are almost exactly like car batteries and just about any home UPS should be able to handle a car battery.
.  As Re-design points out, the added capacity of a car battery could lead to overheating problems, but a small fan should take care of that.
.  That said, you will not be able to connect two chargers to one battery as you show. If you can disable the charging circuit in one of the units, it might work.
.  UPSs are pretty cheap nowadays. If your data is that important, it's worth buying the proper UPS. I wouldn't suggest jury-rigging the brakes on your car, either.

Agreed. From memory VRLA, AGM. Valve Regulated Lead Acid, Absorbed Gel Mat.
Sealed, Spill proof (even if ruptued), lightly pressurised (to help with hydrogen reabsorbtion I think), and can be used upside down, sideways, whatever.
And a bit more expensive than the average car battery.
Also deep cycle, better for this kind of use. Long shelf life, long standby time, etc. 

Assuming you aren't intending to parallel the OUTPUTS which is a very very very bad idea, you can "stop" the battery using two chargers by putting a BIG diode between the unwanted charging UPS and the battery. Put the cathode towards the UPS.

The thought occurs to me that you're only using the UPS to convert to AC, which your PSU converts back to 12 & 5V DC. It could be more efficient to run DC direct with regulation etc. But PSUs can be complex in their operation... get the right UPS for the job.


Your picture shows wiring for the battery charging side of things, but it shows no wiring for the AC loads. 

The reason I am concerned about this is that if you're going to have two inverters pushing the same load (two UPSes feeding your single power hungry computer)  there has to be some sort of provision for keeping their AC outputs in phase.  This is analogous to a boat with two rowers.  Each rower has to put his oar in the water and pull at the same time.

Unless these two UPSes are designed to be "stackable", with some provision for keeping multiple UPSes in phase with one another, while the power is out, then you shouldn't be using more than one of them to drive a single load.  This is because bad things will happen if they drift out of phase.

Using a different UPS for each load is OK.  For example you could use one UPS for the monitor, and a separate one for the computer. 

Back to the subject of wiring two UPSes to the same battery:  Depending on how your UPS is wired the battery circuit might not be completely isolated from AC wiring.  The reason I suspect this is because I've got an old UPS, to which I have attached a new battery or two.  During this exercise, I felt an unexpected electric shock while touching one of the battery terminals.  It turns out that for this particular UPS ( a really cheap, no-name, brand, BTW), there is an AC voltage present between the battery terminals and the ground terminal of the AC plug.  A picture of me measuring this voltage is attached.

So if you thought you didn't have to worry about AC signals and phase on the battery charging side of things, because batteries are DC... well, now you have to think about that again, because there might be AC signals on the batteries too.


I can see all kinds of potential problems here.

1.  The upses are designed to charge a certain type and capacity battery.  You'll likely burn up either the chargers or the battery.

2.  During charging the battery out gasses hydrogen not a good thing to be venting in a house or near electrical equipment.

3.  The battery contains liquid acid and any spill will ruin the floor.

4.  Most upses were designed to run on a certain size of battery and the extended capacity/time the new battery would allow might cause the units to overheat.  They weren't meant for continuous operation for a long period of time just enough time to allow you to save your work and shut down.

5.  Connecting them in parallel to charge the new battery might be ok but their smart charging systems won't work right and may not work at all.

My suggestion would be to just replace the bad battery.

 The current battery in it is 7AH, not very much. Also, this APC is only rated for 300 Watts continuously. So this could power my monitor which uses up to 150 watts, but my computer can use up to 1kw, even though it is usualy around 300-400. I wanted to use one battery so that both my monitor and computer would run out around the same time and I wouldn't have wasted capacity, not much point of a computer without a monitor, or vice versa. Also, someone is selling a 1.5kw backup in my area for 50 dollars, which i'm going to buy.