UPS Battery Question

[made2hack]

Ok,

So here's an overly simplified question regarding an Uninterrupted Power Supply unit. What are some of the built in limitations to just adding more batteries to the unit and have a higher capacity unit? Is there a reason why I just cant add say 6 extra batteries in parallel? What would hinder this? (in terms of the charging circuit, built in protection)?

Can this be achieved? If so, what do I need to look for? Are batteries charged equally at the same time in a UPS? Do I just need a charging circuit that can deliver more amperage at the same time? Or is it more complex? Is the circuit built in such a way that it will load balance only a predetermined number of batteries?

Any ideas on where to get started and what kind of info I need to gather will be greatly appreciated.

Thanks in advance,

[picandmix]

In simple terms yes you can add extra batteries but will need to modify the charging circuit for them.

It would be more helpful if you mentioned what type of UPS you are looking to use, eg a PC type ?, load ,  power and backup time ?

Example if its a simple PC UPS and you are trying to extend its backup time, be aware that some units will cut off automatically after a certain time, regardless of the battery state.

Lots of examples of such PC UPS upgrades on Ytube.

[rexxar]

You can add extra batteries if you like, but the charging circuit can only deliver so much current, so the batteries will charge slower. As for balancing, batteries in parallel get charged equally, so that's not really a major concern.

The problem with doing this with cheap consumer UPSes is that they are usually not designed to run for more than 10-15 minutes at a time. The one I have under my desk, for instance doesn't have heat sinks on the mosfets. They're simply bolted to a block of aluminum. The big thermal mass is enough to keep them from overheating for 15 minutes or so, but since there's no airflow through the case, extended runtime will be a problem.

Huge server UPSes often have connectors on the back where you can plug in extra batteries, and those extra battery boxes also have the connector, so you can daisy-chain extra batteries. Those, however are designed to run for hours at a time, and have active cooling.

I guess the question here is, what kind of load are you running, and for how long do you want to keep it up?

[made2hack]

The load is a computer server which needs to run more than the 10-15 min general. Because I don't want to have to deal with a diesel generator, I want to be able to extend the potential for the UPS to provide maybe up to 2 hrs of up-time. Now, the server doesn't run high wattage, maybe in the low 300s, but still.

So, essentially it's a matter of thermal management to keep them running longer? What about the "cheap" versions, do they turn off based on some kind of heat detection? Ie a thermistor? Or is the micro-controller logic just tell it to turn off after 10 - 15 minutes regardless? I'm referring to the "dumb" unmanaged types.

[picandmix]

For a critical application like that I would avoid any form of modifying a cheap UPS., apart from loosing your system there is an increased risk of fire etc if your mods do not work  well !

Suggest you visit the sites of some of the actual UPS manufactures and you will see there different types of UPS and I'm sure there will be such a device to meet your needs.

http://www.cyberpowersystems.com/products/ups-systems.html

[johansen]

The apc1000 and similar rack mount pure sine wave machines have heatsinks and fans and if you could easily add a heatsink and fan on the side of the transformer.
you can find them on ebay and sometimes craigslist for pretty cheap. here's one. http://www.ebay.com/itm/APC-SU1400RMXLB3U-SMART-UPS-1400-1400VA-120V-UNINTERRUPTIBLE-POWER-SUPPLY-TESTED-/231395330666

also, many pure sine wave back up supplies such as the one above do not have an external inductor but rather use the leakage inductance of the transformer for two purposes:
1) as a boost inductor for charging the battery.
2) as a filter for producing the sine wave output.

Installing a suitable inductor in series with the low voltage side of the transformer is worth the effort. you'll get a reduction in parasitic draw from the outlet of about 5-10 watts and the efficiency will increase during inverting operation significantly.

in that ebay link, the batteries (not included in the auction) connect through a 50 or 75 amp anderson power pole connector and they might be 10 awg wires.
if you have scrap copper lying around i would make them 8 awg minimum.

also the 1 foot leads between the transformer and the H bridge are 10 awg when they should be 8 awg wire. you can replace them too.. that's 1 more milli ohm removed from the system.

adding a second capacitor to the H bridge board is also possible because the ESR of the capacitor itself will suck up about a watt.

that particular unit probably has 16 mosfets.. you may get lucky and be able to add 4 more (some have room for 20)
if there are only 8 or 12 fets, you will almost certainly be able to add 4 or 8 more, this is certainly worth the effort.
i recommend buying the same fets, not mixing different part numbers.

Even if all you do is heatsink the transformer, you should still be able to get continuous duty operation from this series of inverters.

if you buy brand new AGM cells you shouldn't have any problem up to about 10 times the design amp hours of the system.
I would buy flooded cells and check the water level every 3 months... they will last a lot longer than the three+ years you'll get from agm cells.
the only danger of an infinite battery supply is the self discharge rate exceeding the charging current.
i don't recall if these inverters push the battery volts all the way up to 14.5 volts and then float it at 13.8.
but if the battery is flooded and very large, or if its agm and extreemely large, the inverter won't be able to provide enough amps to push the terminal volts all the way up to 14.6 volts or whatever it is.
thus it will eternally dump 1 or 2 amps into the battery and you'll be boiling the water out of the batteries and they won't last long if they are agm cells.. or you'll be adding water rather often to flooded cells.

an external charger will solve this problem but it creates a few more..

(even the 13.8 volts is too high a float voltage but there is little that can be done with APC equipment to reduce this voltage ... they enjoy selling replacement batteries too much)

[Cliff Matthews]

You might use a separate charger for the auxiliary battery bank and link it to the existing battery via one chunky stud diode and heavy DPDT relay. When AC mains is up, the relay disconnects the battery bank and permits it to charge, when AC mains is down, the relay disconnects the charge circuit and connects the battery bank via stud diode. The battery bank would begin assisting when the principle battery drops 0.6 volts - enough to forward bias the diode into auxiliary assist mode.

* You'll want to adapt the inverters cooling significantly, as most consumer UPS's are not designed to operate for extended periods.

[BradC]

Ok,

So here's an overly simplified question regarding an Uninterrupted Power Supply unit. What are some of the built in limitations to just adding more batteries to the unit and have a higher capacity unit? Is there a reason why I just cant add say 6 extra batteries in parallel? What would hinder this? (in terms of the charging circuit, built in protection)?

Can this be achieved? If so, what do I need to look for? Are batteries charged equally at the same time in a UPS? Do I just need a charging circuit that can deliver more amperage at the same time? Or is it more complex? Is the circuit built in such a way that it will load balance only a predetermined number of batteries?

Buy an old APC SmartUPS. Anything from 1000VA upwards. They have honking great iron transformers, big heatsinks and suitable fans. They almost _all_ have the option in the firmware to tell it you have extra batteries installed. APC extra battery units are measured in 17AH blocks, so for each extra 17AH you put on you just tell it you have another external battery. Of course you re-charge time drops proportionally but your run time increases exponentially (the lower the load on a lead acid, the more capacity you'll get out of it).

Most other smaller UPS's are built for short runtimes, therefore they don't have sufficient cooling and rely on thermal inertia to keep things under control. Once you tweak up the run time you start cooking things.

The other nice thing about the APC SmartUPS series is schematics are out there if you look hard enough, and you can fix them with basic hand tools. They are also pretty much bullet proof as they were designed before the "save 10c on that component" era.

My .2c worth.

[edpalmer42]

As has been stated, modifying a UPS is not a good idea.  But here are a couple of alternatives to consider.

Buy a second-hand UPS that's a lot larger (more wattage) than you need.  They're often thrown away when the batteries fail rather than just replacing the batteries.  I don't know why, but it happens.  Get replacement batteries and you'll find that since they're lightly loaded, the batteries will last a lot longer and the UPS will run (relatively) cool.

Or, buy a UPS that's smart enough to tell the server when the power fails.  Add the software to the server so that it will do a graceful shutdown before the batteries run out of power.  That's the best way to do it if you're worried about data integrity.

Ed

[Cliff Matthews]

I'll add to my previous post, since further thought (and guilt) now cause me to add a disclaimer:

1) Fire hazards should be avoided. Use short over-sized power coupling cables and connectors capable of the high DC current (sometimes exceeding 30 amperes).
2) A forward biased stud diode carrying 40amps needs to dissipate around 25 watts and still keep its junction well below its operating parameters (so a fan, mica insulator, standoff's, and thermal heat paste is in order).
3) Think about placing the original PCB and additions in another larger fire-proof enclosure, if you decide to augment some aluminum and add some fan-forced cooling and AC thermal shutdown.

Don't want to be down on the idea.. but it's not without risk.

[Hey_Allen]

Look in any local electronics salvage/surplus shops as well.  In the Seattle area, Re-PC is a good option, particularly the downtown one which has a much larger as-is section.

I have bought a couple of APC branded UPS' over the years, for $5-15, which only needed batteries.

Batteries for the larger one set me back around $60, but if you were already looking at buying batteries, putting them in a larger UPS makes more sense than trying to link them to a small UPS, and may even give you more functionality and reliability.

[madires]

I'd recommend to get an (used) APC SmartUPS supporting additional battery packs. They are built to run for several hours in case of a power outage.

[iamdarkyoshi]

This may sound silly, but we have UPS inverters connected to our solar panel and battery bank, and the UPS inverters are connected to whole circuits in the house. Keeps the heaters and refridgerators running when the power dies. Certainly not safe though, an electrician would probably expell brown liquid out of his pants if he saw what our basement looks like 

[johansen]

This may sound silly, but we have UPS inverters connected to our solar panel and battery bank, and the UPS inverters are connected to whole circuits in the house. Keeps the heaters and refridgerators running when the power dies. Certainly not safe though, an electrician would probably expell brown liquid out of his pants if he saw what our basement looks like 

what's not safe about it? usually when you connect the grid to the ups output smoke is expelled rapidly.

[iamdarkyoshi]

This may sound silly, but we have UPS inverters connected to our solar panel and battery bank, and the UPS inverters are connected to whole circuits in the house. Keeps the heaters and refridgerators running when the power dies. Certainly not safe though, an electrician would probably expell brown liquid out of his pants if he saw what our basement looks like 

what's not safe about it? usually when you connect the grid to the ups output smoke is expelled rapidly.

Well, the circuits in the breaker box are connected to some 3 way light switches that switch between mains and the UPS backups. There are also some switches that disconnect main from the ups backups input, simulating a power outage. But... the breaker box is full, so breakers are dangling out of the box, and the only thing keeping the ups backups from seeing potentially 100a of load is the breaker on the back of them... that and it is damp down there and the boards are corroding...