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Back up power for small load, extended run time

Discussion in 'General Electronics Discussion' started by Madsalts, Oct 19, 2016.

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  1. Madsalts

    Madsalts

    73
    2
    Sep 25, 2014
    I need to run several devices (a heater and a few pumps) for an aquarium, more-or-less constantly, in the event of a power failure. The total consumption of these devices, is about 50 watts per hour.

    I'd like to implement a backup system that, ideally, will run on external AGM batteries, so that I can keep the system going as long as possible. In a worst-case scenario, the power might be down for a week. In a more likely scenario, the power will be out for several hours once or twice during the winter, with mild sags in power every few weeks. I'd need the batteries to support the system for 20 hours, ideally, before being recharged by a generator. I say 20 hours because if the power fails right after I fall asleep at night, and I don't have time to address the issue before getting home from work, about 20 hours will have passed.

    So, I'm wondering how cheaply and reliably I can do this. I'd rather not get a double conversion UPS, simply because of the price. If anyone could put me in the ballpark of what I should be looking at, I'd appreciate it.


    I addressed this issue elsewhere, and was told that no "consumer grade" solutions exist for this problem. I was told that consumer-grade UPS systems are designed to provide power for short periods of time, and will fail if used for longer. I am told that I need to look at a "professional grade" solution. I have absolutely no idea what such a "professional grade" solution might be. If anyone can offer any insight, I'd really appreciate it. Also, I am wondering if it's wise to buy used equipment for such an application.

    Thanks.
     
  2. LightShow

    LightShow

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    17
    Jun 9, 2016
    Not saying that this is the best solution for you, but i use a few Deep cycle batteries and a simple power inverter made for Cars to run my Second workshop. the batteries charge via solar panels and the inverter has up to 1000w of output probably slightly overkill for your needs but my system on full load can run for 5hrs using about 800w.
     
  3. Madsalts

    Madsalts

    73
    2
    Sep 25, 2014
    I can get a 500 watt inverter for about $40. I already have a 35 Ah deep-cycle battery, in pretty good shape. To do a quick and dirty conversion, it seems like 50 watts AC being drawn in an hour, divided by 120 V, equals 0.417 Ah. Assuming that I can drain the battery down half way, I can get 42 hours of run time. Assuming that conversion efficiency will be 85%, I get 36 hours. Assuming that at some point, I will not be able to draw the required current as the battery approaches 50% capacity, it seems that I can get 24 hours of run time from the battery. If I forget about recharging with the generator, and simply get a second battery, I think that I'll be able to make it through all "reasonably" foreseeable outages. This is because the vast majority of outages would last less than one day. Others, of greater duration, could be dealt with by recharging the first battery, or buying an additional battery.

    Now, there are two issues. First, I need a device that will prompt the inverter to become active in the event of an AC outage. Additionally, the inverter must turn off and allow normal AC power to reach my appliances when AC power returns. It seems like such a device is probably common and reliable, I just don't know where to look for one, or what it might be called. Can anyone suggest something?

    Second, I need such a device to be safe. I'd rather have the aquarium fail than for the house to burn down. I have my doubts about a $40 inverter running for two days (or more) straight and not failing. Would such a device be safe? If not, is there some variation on the $40 inverter, likely costing more, that would be safe?

    The only issue remaining is that a solution involving an inverter would not filter power. What should I do, if anything, about "brown outs" lasting a few seconds? I could get a power conditioner, or simply not worry about it.

    Thanks.

    Edit: I came across the following:

    The way that the inverter/charger will work is that the inverter will work when the power is off and the battery charger will charge and maintain the batteries while the power is on. Since the battery charger that is included is a three stage battery charger with smart charging capabilities it is safe to leave the batteries hooked up to this at all times. The internal transfer switch will allow for automatic switching. This means that you will not have to do anything when the power goes out. The inverter/charger will start running all of the appliances you have selected all by itself. Even though the inverter/chargers are more expensive they are much more convenient and you won't even know that the power went out.

    Well, this is one solution. The only thing that I don't like is having the battery hooked up to be charged all of the time. I'd rather charge it myself, and then top it off every few weeks when I'm actually in the house. I don't want to leave it to chance that an unattended battery will charge perfectly well when I'm not around. Are there any modifications to this way of doing things that might work?

    Thanks.
     
    Last edited: Oct 20, 2016
  4. LightShow

    LightShow

    111
    17
    Jun 9, 2016
    Some sort of automatic transfer switch (not 100% sure so i wont give advice on this)

    Depends if you go for the cheap crappy brands or buy once and forget.. my system has been running for 2 years without issues allthough my "lab" is not physically connected to the house the batteries and inverter are completely shielded so if anything were to "go wrong" all i would have to do is build a new enclosure

    im pretty sure good quality transfer switches have features for most of this built in..
     
  5. Madsalts

    Madsalts

    73
    2
    Sep 25, 2014
    The simplest solution seems to get a computer UPS. With computers, such UPS systems have run times rated at about two hours, give or take. With my low power requirements, it seems that such a device could run for 24 hrs. Others have frowned upon this idea, saying that such UPS devices are not meant to run this long, and will suffer or fail if I push them in this way. Is there any truth to this? After all, I'm only talking about 50 watts.
     
  6. LightShow

    LightShow

    111
    17
    Jun 9, 2016
    Now here is something i know about :D

    Yes UPS systems are not meant to be run 24hrs straight especially if it may happen regularly for most it can decrease the life of the battery and the components inside are not rated for long usage times

    Although there are some ups's out there that are made to run for longer periods of time usually installed in offices or larger pc setup's in areas that have frequent brown outs they also end up costing alot more

    your standard everyday ups will work with your setup and most likely quite efficiently but i wouldnt trust it for longer periods of time over 10+ hours and if it happens regularly the components arent going to last and from experience neither is the battery.
     
  7. Madsalts

    Madsalts

    73
    2
    Sep 25, 2014
    Well, now we're back to the inverter, transfer switch, and battery. It looks inexpensive enough. I'll have to poke around at it some more to see if it's something that I can do myself, or if I'll need to hire an electrician.

    Would this be an appropriate inverter?
    https://www.amazon.com/BESTEK-Power...=UTF8&qid=1476930112&sr=8-1&keywords=inverter

    Would either of these be appropriate switches?
    https://www.amazon.com/gp/product/B00153EYTO/ref=ox_sc_act_title_3?ie=UTF8&psc=1&smid=ATVPDKIKX0DER

    https://www.amazon.com/gp/product/B0039T0518/ref=ox_sc_act_title_2?ie=UTF8&psc=1&smid=ATVPDKIKX0DER

    Thanks.

    By the way, since someone asked ( think) the 50 Watt load is due to a combination of a heater and some pumps.
     
    Last edited: Oct 20, 2016
  8. LightShow

    LightShow

    111
    17
    Jun 9, 2016
    if you are not confident with wiring it all up definitely consult an electrician..

    the inverter is slightly overkill but for the price its good :)

    the switches seem solid enough. either or would suffice :)
     
  9. Madsalts

    Madsalts

    73
    2
    Sep 25, 2014
    thanks, I appreciate it.
     
  10. Madsalts

    Madsalts

    73
    2
    Sep 25, 2014
    Even though the inverter is overill, I'm going with a larger one yet. For $20 more, I can get a pure sine wave inverter.

    Below is my math, and I'm hoping someone can tell me if I'm making any mistakes.

    At any one time, the max load that will be drawn is 50 watts. I would like the system to run for a maximum of 24 hours. With this power, at this length of time, 1200 Watt-hours will be used. Dividing by a voltage of 110 (the voltage output of the inverter, we get 10.9 Amp-hours.

    I have a battery that is rated at 35 AH, but is only 82 percent efficient at 50 degrees. The conversion loss with the inverter is 17%. Therefore, 35 (.82) (.83) = 24 AH.

    This means that I not only meet the 10.9 AH requirement, but also surpass it such that I can run the setup for over 48 hrs. All this is a rough estimate, of course.

    Is my reasoning sound? Have I overlooked anything major?
     
  11. BobK

    BobK

    7,682
    1,688
    Jan 5, 2010
    No your math is off by a factor of 10!

    You calculated how many amps are required at 120V. But the battery (I assume) is 12V. It will require 10 times the required amps from the battery to get the current needed at 120V. The correct way to do the calculation is by using Watt Hours.

    You battery is 12V x 35AH which is 420WH.

    You need 50W for 20 Hours which his 1000WH.

    So you battery, even with a 100% efficient inverter would last less than 1/2 hour.

    Now do you begin to see why such a system is going to be expensive?

    Bob
     
  12. Madsalts

    Madsalts

    73
    2
    Sep 25, 2014
    I see that 420 is roughly half of 1,000, but I wonder why it would last for 1/2 hr? If I'm drawing 50 watts per hour, and a 12V 35AH battery gives me 420 AH, wouldn't I then be able to run for 8.4 hrs?

    Given the losses I listed, I will assume that I need 1471 WH.
    With a 12V battery, this translates to (1471 WH) / (12 V) = 123 AH, right?

    Assuming all this is correct, the least expensive way might be to wire the 35 AH battery in parallel with a new 100 AH battery. I'd guess that this would result in some loss, but I'll contact the manufacturer.
     
    Last edited: Oct 21, 2016
  13. BobK

    BobK

    7,682
    1,688
    Jan 5, 2010
    Never mind, I had a mind fart. Yes, with a battery providing 420 WH you can draw 50W for about 8 hours.

    What I meant to say was less that 1/2 of the 20 hours, not 1/2 hour.

    Bob
     
  14. BeeGee

    BeeGee

    1
    0
    Oct 22, 2016
    Hi Madsalts,
    Here's my numbers for your battery requirements based on the inverter efficiency of 83%.

    1. Battery load power is 60.24 watts (50W load to heater & pump etc + inefficiency of the inverter)

    2. Based on the battery voltage of 12v, the battery current would be 5.02 Amps.

    3. The cuttoff voltage of the inverter is specified as 10V

    4. When looking at battery specifications & performance, the Ah rating is specified at a particular c rating and temperature i.e., 50Ah at c10 , c specifying in this case 10hour @ 25C. At any other time the Ah rating of the battery will be lower if the hours are longer.

    5. In addition to 4 above, discharge ratings for batteries are given at a particular vpc (Volts per cell) so consider a 12V batttery is made up of 6 cells therefore, the discharge vpc your looking for is in the region of 1.75vpc which will give you a 10.5V value the battery is allowed to go down to before the inverter drops its output.

    Based on the above, I'd be looking for a battery with something in the region of 110Ah to achieve the 20Hour backup time you require.

    Some other considerations for the inverter - go for one with a soft start so it cranks the output voltage up slowly - avoid high inrush currents that trip the overcurrent protection.

    Hope this helps a little

    Bill G
     
    Last edited: Oct 22, 2016
  15. Madsalts

    Madsalts

    73
    2
    Sep 25, 2014
    thanks
     
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