Wanting to replace UPS Capacitors

Discussion in 'Power Electronics' started by Korishan, Sep 14, 2016.

  1. Korishan

    Thread Starter New Member

    Jun 29, 2016
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    Hey everyone,

    I have an APC SMART Back UPS 2200 Power Supply SU2200 that I'm upgrading and replacing some parts in it.

    It currently has 2 1500uF 75V 3 prong capacitors in the one area. There is room for a 3rd. I've seen several youtube videos about UPS work and adding the extra cap is ok to do, even recommended (also is adding the extra mosfets to help with the loads; which I'm also going to add).

    The caps that are installed are these: http://www.ebay.com/itm/201445231125?_trksid=p2055119.m1438.l2649&ssPageName=STRK:MEBIDX:IT

    I'm wondering if it would be advised to just replace the 2 (and add the 3rd) with a brand new caps. I can't find the exact replacement uF and voltage in new form. I know I can put in ones that are of higher voltage and still be ok. But would it hurt to add 1 with higher voltage to the other 2, or should I replace them to all be the same voltage?

    Also, I've noticed that there are some cap that have, ie 50v 1500uF are of one size, and others larger. Why are they so different in sizes?

    The main reason I'm asking about replacing with new ones is because I don't want to possibly install just one identical one, then shortly afterwards when needing to use the UPS they blow out because they are close to EoL. I'd hate to have to take it all apart after putting it into service again.

    Side Note: This UPS is part of a larger project that will involve alternative energy charging methods to charge the batteries and run off the inverter for AC required devices. I got the UPS for about $60, free shipping :D


    Thanks for the help,
    Kori
     
  2. tcmtech

    Well-Known Member

    Nov 4, 2013
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    Unless it has some very cheaply made capacitors and it has been ran at it's maximum working capacity for years on end it's very doubtful that your present capacitors are reaching the end of their life.

    If they are not bulging or leaking and you do not have an actual capacitor tester to determine if they are going above their design ESR specs you're wasting your time and money replacing a perfectly good part.

    As for adding more capacitance and more switching devices to a UPS I don't see the point.

    By design, unless its a commercial unit specifically built to run for hours off of a large external battery bank system, a common unit is built to reach its overheating point just about the same time its little internal battery set goes dead so adding more switching device to gain more capacity wont do anything gainful for it.

    To be Honest with those small cheap self contained units if converted over to run off much larger battery or other DC power sources they are only good for about 1/4 - 1/3 their rated power on continuous duty use due to their very limited transformer capacity.

    You can add them if you want but you won't gain anything for having done so. The main power transformer is what sets the units working limits and by design they are only capable of running at a fraction of their peak ratings for continuous duty load work.
     
    Last edited: Sep 15, 2016
  3. MrSoftware

    Active Member

    Oct 29, 2013
    501
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    Is that one of the rack mount SmartUPS units? It might have fan cooling as well as a jack for external batteries, in which case it might be fine with extended run times.

    I don't think I would personally add additional part, but replacing known bad parts couldn't hurt.

    Maybe back in the day "hardware was heavy duty", but during our last batch of hurricanes I used an old APC BackUPS 600 (the ugly tan rectangular ones) connected to a car battery to run the TV for hours without issue, though the load was not hear maximum output for the UPS. The original battery in those was I believe a 12AH 12v battery. Those old APC units were crazy reliable, but we've had a large number of the newer ones die after just a year or two. I've got several of those old tan boxes, of various capacities, still in service after 20+ years, only requiring periodic battery replacement.
     
  4. tcmtech

    Well-Known Member

    Nov 4, 2013
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    I've got a bunch of those tan or black box units as well. Pretty much got all of them for free due to bad battery issues. :D

    Early on as with so many people, I too hooked them up to larger external battery and or power systems figuring they would make good cheap power inverter units. At light loads they were fine but I killed a few just running them at half power or less simply due to either transformer or switching devices overheating. :oops:

    That's when I started reading up on them and found that unless they were specifically designed to run off of large external battery systems they were designed to run out of battery just before they overheated to the point of damage. :(
     
  5. Korishan

    Thread Starter New Member

    Jun 29, 2016
    11
    0
    Thanks for the replies.

    The reason for adding the extra parts (which there are spaces on the PCB for such parts) is to help with thermal load and reliance of the switching. This one specifically is not the "rack mount" unit, however, the parts in it are the same parts they used in the rack mount unit, just arranged slightly differently in the case to accomodate the batteries internally.

    There are 4 large heatsinks that the mosfets screw to and I plan on adding a fan directly atop the heatsinks. By design, there is a plastic diverter card that catches some of the case fan air and directs over the heatsinks. The unit is designed to run at 1.6Kwatt. Also, one of the KEY reasons I'm using this unit, instead of some of those small brick sized ones, is that it is a Pure Sine Wave inverter. A definate for running motors (A/C, ceiling fans, fridge, etc.). And, it's rated at 97+% effiecient. A deffinate winner for something so cheap :)

    I actually plan on getting a second unit and run them in tandem to get a full 240V setup. Which is possible with this unit.

    So, if there is no need to replace the caps if they are in good shape, then could I install a cap close to the others? I really only need 1, not 4 (as listed in the ebay listing above). Such as, could I install a 1500uF 100v and still be ok? Or would that compromise the other 2?

    Another question, why are some caps that 1500uF 75v electrolitic different in size, drastically different, than another of the same spec?

    Thanks for your replies

    Kori
     
  6. MrSoftware

    Active Member

    Oct 29, 2013
    501
    123
    There's more to caps than just voltage and capacity. Temperature rating, ESR, leakage, expected service life, etc.. all these things also affect the design.

    Also if the caps are used for any sort of noise filtering, then the size may be important. I'm not an expert on the topic by any stretch, but the specifications of the capacitor can affect how well it filters certain frequencies. It may be possible that the spots on the PCB are blank because they use the same PCB for larger units and just populate those spots to get more output, but I don't know how you could tell that for sure without a schematic or having the larger unit to compare to.
     
  7. tcmtech

    Well-Known Member

    Nov 4, 2013
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    Unfortunately adding more switching devices in parallel wont do much for heat.

    It could very well make things worse if the circuitry that drives them is not sized properly to handle the added devices gate loading as well and could actually make things worse due to decreases rise and fall times when the device are switching on and off.

    My guess is that APC uses the sma bse circuit board for multiple units of which some are higher capacity than yours and those units would use the added capacitors and switching devices but have a different larger heatsink array and possibly different switching device driver circuitry as well.

    My point is adding the additional parts could improve things, make no measurable difference or make things worse.

    As for the 1.6 KW rating is that at 100% duty cycle or for the 5 - 6 minutes it would typically take to drain the stock battery at that level? There's a huge difference between the two.
     
  8. Korishan

    Thread Starter New Member

    Jun 29, 2016
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    Here's the load graph from APC for this unit:

    http://www.apc.com/products/runtimegraph/runtime_graph.cfm?base_sku=SU2200NET&chartSize=large

    Under a full load (1.6KW), the default batteries will last about 10mins. But I plan on adding quite a bit more capacity than what was originally in the unit. Then use solar/wind to keep the batteries charged up.

    I guess I can just set the unit up and start doing load testing and see where it gets hot and what the current efficiencies are at the various loads.

    Kori
     
  9. tcmtech

    Well-Known Member

    Nov 4, 2013
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    I see nothing on their graph. I just get run time and model number headers and that's it. The rest of the page is blank.
     
  10. Korishan

    Thread Starter New Member

    Jun 29, 2016
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  11. tcmtech

    Well-Known Member

    Nov 4, 2013
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    That worked. All that tells me is the run time estimates on the given battery it comes with. Nothing about load capacity Vs run times with an external power source.

    APC tend to build their units fairly well so more than likely yours will just overheat and shut off if ran to hard for too long. Most of the cheap one I have collected had little to no thermal protection.
     
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