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Thread: Battery + inverter advice required

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    Gold Member daveob's Avatar
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    Battery + inverter advice required

    I hope that Dave_A or one of the other esteemed gurus on the forum can give me some advice here. I seem to get conflicting advice from salesmen, depending on what they are trying to sell / have stock of.

    Having just received our newly revised load shedding time-tables, I have decided that some backup power is going to be a must to ensure continual productivity as we are now due to be 'hit' at least 4 times a week during working hours.

    As I can't use a gennie ( townhouse complex ) I have decided that the best solution would be a charger / battery / inverter option.

    I'm looking at using 2 deep cycle batteries ( 94 Ah or 105 Ah, depending on supplier ) connected to a 20A charger and 1000W inverter, with household power required for essential items at a max of about 550 W. This should give me about 4 hours backup time.

    My question relates to the necessity of having a UPS for my PC equipment. My understanding after initial research, is that I could have a UPS so that the UPS supplies uninterrupted power to the PCs while I go flick the manual change-over switch. I assume that the inverter is always in the 'ON' state, but does nothing until the change-over

    switch is used and power starts to flow from the inverter to my equipment (?).

    However, do I need the UPS if I do the following :
    connect the charger to the battery to the inverter ( therefore continual top-up / maintenance charging of the battery ).
    then connect the inverter to the change-over switch, to my power board ( for supplying battery power for lights, TV, etc when required )
    but connect the seperate circuit that my PC equipment is on ( PCs only using about 300W - no printers ) directly to the inverter so that the PCs are always using the inverter power, thereby using the inverter instead of a UPS, and the batteries are always charged fully, and the inverter would therefore, in reality, be using the power supplied by the battery charger, and would also be isolated from fluctuations in the municipal power supply.
    When the power cuts, the PCs continue using the inverter's power ( uninterrupted ) while I go flich the switch to route the inverters power to the rest of the essential appliances in the house.

    Also :

    1. would you recommend an automatic change over switch ? if so, can anyone tell me a make / cost / source ?

    2. any recommendations ( make / cost / source ) for the best deep cycle batteries to use for this system.
    Watching the ships passing by.

  2. #2
    Site Caretaker Dave A's Avatar
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    The way you are planning it will keep your computers up and running. I can't see a problem there.

    Whether you go automatic or manual crossover really depends on how important it is for you to cross over to backup power when no-one is around to do it manually.
    The trouble with opportunity is it normally comes dressed up as work.

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    Gold Member daveob's Avatar
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    Hi Dave_A

    One of the other forum's comments I have received is :

    [quote]
    A 20 amp battery charger won't be able to sustain a 1kW inverter (unless it's supposed to be 20 amps @ 220V, doubtful thought). Remember if the Inverter is 90% efficient (example), it uses 1100kW of power, that is about 92amps @ 12volt. The charger wouldn't be sufficient, not even close.
    [end quote]

    I assume that if the charger pushes 12 v 20 A, that's only 240 W. so to get 1000 W, the charger would need to push 85 A. Based on this, thre is no way that the charger/battery/inverter could possibly work like a permanent inline ups.

    Or have I missed something ?
    Watching the ships passing by.

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    unfortunately i wrote a long detailed thread about invertors and batteries but the site was upgrading when i tried to upload it...so i will just give it in brief now.

    a modified sine wave invertor cannot run a UPS.

    buy an invertor with a built in charger.

    depending on the application and budget...use a modified sine wave if you cannot afford a pure sign wave invertor for your tv and a light.

    dont use a modified sinewave invertor on critical applications or sensitive electronic equipment

    most invertors work the same as UPS with a built in auto change over and charger...if you are going to use a 1000watt unit make sure it is a 24 volt system it will give you a longer back up time.

    think of a battery like a drum of water if you only open the tap slightly the drum will take a long time to empty...if you open the tap fully... the drum will empty within a couple minutes...same as a battery invertor backup...you load the unit with 1000watts it will only last a short period of time...if you only use 1 x 50 watt lamp it will last a long time...this is why you need to be careful of the tedelex unit they are saying the unit provides 6-8 hours backup time but what they dont tell you is the calculation of how they work this out...

    here is an example
    if you load a 12 volt 800 watt invertor using a 105 amp/hr battery (rated at 50 % duty cycle) with 350 watts it will last for just under 2 hours

    if you use the same load 350 watt with a 24 volt 1400 watt invertor using 2 x 105 amp/hr battery (rated at 50 % duty cycle) it will last 3 hours.

    there are just toooo many experts out there at the moment who know just a little more than most and are selling generators and invertors because it is a big money making racket....it is just so unfortunate for the public that where there is a demand there will be a person selling regardless of wether or not they have an idea of what is going on.
    Last edited by murdock; 01-May-08 at 09:43 AM.

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    Site Caretaker Dave A's Avatar
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    Quote Originally Posted by murdock View Post
    unfortunately i wrote a long detailed thread about invertors and batteries but the site was upgrading when i tried to upload it...so i will just give it in brief now.
    Sorry Murdock. I know how frustrating that can be.

    I was trying to get the upgrade part where I had to disable the board done early before anyone started posting. But obviously you're an early bird...

    Thanks for taking the time to give it a go a second time.
    The trouble with opportunity is it normally comes dressed up as work.

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    I would not go for that option due to the cost and the short life of batteries.

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    Gold Member daveob's Avatar
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    Alternate solution ?

    OK. Firstly, a big Thank You to all for the replies, advice and input.

    In summary, the following seems apparent :

    1. I can't use the battery as a permanent UPS as the battery charger can not recharge the battery as fast as I would be draining it if I ran the PCs from the battery all the time.

    2. I can't hook-up a UPS between the battery and the PCs as a UPS should not be run from a modified sine wave converter on the battery pack.

    So can anyone give me any advice on what I see as the logical solution :

    I purchase a standard offline UPS, which includes an 840W modified sine wave inverter (sold by Mecer for PCs, so reasonably sure it will work on PCs ) and also includes 2 batteries ( 12 V 9 Ah x 2 batt. = 216 W @ 85% eff. = 183 W ). This will give my 2 PCs ( total 700 W ) about 15 minutes running time ( printers isolated from this system ).

    What's to stop me from opening the case and connecting a deep cycle 94 Ah battery ( with an additional charger ) into the circut to give me additional running time ( still at a max of 840 W ).

    This way, the big battery is not used until the power dies and the UPS kicks in and supplies the power from the total available in the UPS batteries and the external batteries.
    Watching the ships passing by.

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    Some more input:

    1. Try and run 24V or 48V if possible, your batteries will last longer, and your cables won't need to be as thick / switches & relays as expensive as on 12V. The less Volts you have, the more Amps you need to get the same Watts. Watts is the important thing that you're after. 12V x 2A = 24W BUT 24V x 2A = 48W

    2. Don't ever run a PC, or anything directly off a inverter constantly, inverters are built for that purpose, and you will shorten it's life span. Rather has a automatic fail over circuit. This can easily be done with a 2-stage relay. Basically, when eskom is on, the relay is "closed" and you use eskom power. When eskom is off, the relay is "open" (use the NOC side), and you use the inverter + battery. On the same relay, you can make the charger (use a more expensive trickle / intelligent charger) to charge the batteries when Eskom is on. The use of the relays will avoid "cross-talk" and negative feedback - which could damage your equipment.

    3. If you need to run a UPS of an inverter, then you'll need a PURE since wave inverter. UPS' won't work on square since wave / half sine wave / modified sine wave inverters.

    4. Use FUSES!!!!

    5. If possible, get 1 or 2 sizes bigger inverters than what you need right now. Soon you'll see how well it works, and connect more stuff to it.

    And if you need to add more batteries to the bank, then purchase them in pairs of 2 / 4 at a time.
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    Gold Member daveob's Avatar
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    Confused

    I SoftDux. Thanks for the extra info, but now I am getting even more confused.

    Quote Originally Posted by SoftDux View Post
    Don't ever run a PC, or anything directly off a inverter constantly, inverters are built for that purpose, and you will shorten it's life span.
    does a UPS not also have an inverter built into it ? so if I am running my PCs from a UPS for 1 hour, is this not the same thing ? ( or is 'constantly' a lot longer than that ? )

    Quote Originally Posted by SoftDux View Post
    Rather has a automatic fail over circuit.
    Is this not what is already built into the offline UPS that kicks in as soon as the mains power fails ?

    Quote Originally Posted by SoftDux View Post
    If you need to run a UPS of an inverter, then you'll need a PURE since wave inverter. UPS' won't work on square since wave / half sine wave / modified sine wave inverters.
    This bit I have heard before which is, because of the Pure wave inverter costs, made me look at adding more batteries to the UPS. ( ie. to boost the capacity of the existing UPS batteries, adding to the Ah and thereby increasing the total wattage available to power the PCs via the UPS's built in inverter.

    So my questions now are specific to adding additional battery power to the UPS unit (in parrallel), instead of trying to run the 2 systems seperately, in series, so to say. Any suggestions on this ?
    Watching the ships passing by.

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    Platinum Member SilverNodashi's Avatar
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    Quote Originally Posted by daveob View Post
    I SoftDux. Thanks for the extra info, but now I am getting even more confused.



    does a UPS not also have an inverter built into it ? so if I am running my PCs from a UPS for 1 hour, is this not the same thing ? ( or is 'constantly' a lot longer than that ? )
    Yes, a UPS has a built in inverter, and that's the "problem" For a UPS to actually start-up you'll need a pure sine wave / true sine wave inverter to work.

    Quote Originally Posted by daveob View Post
    Is this not what is already built into the offline UPS that kicks in as soon as the mains power fails ?
    Yes, but the battery chargers on those "all-in one" type inverters are normally only 4 / 5 AMP charger. So if you want to save money by getting one of those, you'll be very very limited in terms of how long it will run on batteries, and how much time it needs in order to recharge.
    For example, 1A charger will have to charge for 102hours to full 1 102A battery, or 204hours for both. 5A will run 20hours for 1 battery, 40hours for both. Now, this is fine if you only sustain 2 hours blackouts everyday. Chances are the batteries will be fully charged by the time the next blackout occurs. But what if the blackouts are 4 hours long, and you use 90% of the available power on the inverter? It will probably not last 4 hours, so you'll want to add 2 more batteries (this is now cheap, since you already have the expensive inverter / charger), then the built in charger won't be able to recharge the batteries fully before the next blackout.


    Quote Originally Posted by daveob View Post
    This bit I have heard before which is, because of the Pure wave inverter costs, made me look at adding more batteries to the UPS. ( ie. to boost the capacity of the existing UPS batteries, adding to the Ah and thereby increasing the total wattage available to power the PCs via the UPS's built in inverter.
    This, believe it or not, is actually the better way todo it. Rather get a 1000 / 2000VA UPS, which runs on 24V, and then add 2 / 4 more larger batteries to the system. You'll still need a bigger charger to recharge them quick enough, since the built in charger is just too small. With this setup you can run your PC, and some lights off it. I have 1000VA UPS in the office, with a server, ADSL router, 8 port switch, 3 PC's & 2x LCD monitors connected to it and it works ok. I have to add though, that I don't have extra batteries for it yet, but wanted to point out what kind of load the UPS can handle. I get about 5 - 10 minutes with it's standard batteries. Adding 2x 102AH batteries will easily get me 2 - 3 hours.

    Quote Originally Posted by daveob View Post
    So my questions now are specific to adding additional battery power to the UPS unit (in parrallel), instead of trying to run the 2 systems seperately, in series, so to say. Any suggestions on this ?
    This depend on the UPS. IF it's a 600 / 800VA, then you need to connect the batteries in parallel, to keep 12V. If the UPS is 1000VA and up, you'll need to connect 2 batteries in series to forum a 24V battery bank, and then connect that batter bank to the UPS. You'll need decent cables though, DON'T use cheap / thin wires for this. Also, get a seperate charger to recharge the big battery / batteries
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  12. Thank given for this post:

    Dave A (08-May-08), daveob (08-May-08)

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