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UPS with 24V External Batteries

Discussion in 'Power Electronics' started by FrankG, Jul 8, 2011.

  1. FrankG

    FrankG

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    Jul 8, 2011
    I'm sorry if this is a simplistic question. I don't have a lot of knowledge when it comes to these things. I'm just trying to set up a reliable, long-running UPS without spending a fortune.

    I have a UPS (Tripp Lite Omni VS1500XL) that has a port to plug in an optional 14Ah external battery pack for extended run time. I have that battery pack (Tripp Lite BP24V14), which is 24V. I also added two 12V 100Ah batteries, which are connected in series. I've been running it like this for a few months, and it seems to work great. I tested it running my computer and network equipment, and it was still over 90% capacity after 6 hours. I assume the UPS is charging the external batteries, but I also have a 12V charger that I switch between the 12V batteries every few days. I drew up a diagram of how everything is hooked up. Here are my questions:

    1. Is it okay to hook everything up like this, or should there be a diode somewhere to prevent power from flowing the wrong way?

    2. Is it okay to charge the 12V batteries while they are still connected in series and to the UPS?

    3. Is there any way to get the 12V charger to charge both 12V batteries at the same time while still keeping them connected in series and to the UPS?

    Thanks!

    [​IMG]
     
  2. duke37

    duke37

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    Jan 9, 2011
    I would disconnect the two additional batteries, connect them in parallel and charge them to 14V for a day or two. Then reconnect to your UPS and put your charger in a drawer. The UPS charges the internal 24V battery and should do the job for an external one.
    Charging one battery unbalances the system
    If your charger is not voltage controlled, then keep a constant eye on the voltage.
     
  3. FrankG

    FrankG

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    Jul 8, 2011
    Thanks for the advice. I do disconnect the batteries and charge them individually once in a while. My concern was that the two large batteries would draw more charging current than the UPS could supply, considering the internal battery is 14Ah and the external batteries total over 214Ah (they weigh around 70 pounds each). Is this not a concern? I figured keeping the 12V charger on one of the batteries would reduce the draw on the UPS charging circuit. It is a voltage-controlled charger designed specifically for sealed batteries that automatically switches to float mode when the battery is charged, and it even has temperature compensation.
     
    Last edited: Jul 8, 2011
  4. Resqueline

    Resqueline

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    Jul 31, 2009
    There seems to be confusion about Ah ratings. The internal battery pack is 7Ah (2 each of the typical 2.6kg 12V 7Ah batteries). Connected in series this is not 14Ah.
    There is an original 70Ah (allegedly, it weighs 63kg anyway) daisy-chainable battery pack available, and your external battery capacity is now 114Ah (or 107Ah).
    The instructions mentions no limit to the number of 70Ah (139lbs) packs that can be connected in parallell (daisy-chained), so I'd say don't worry about the extra 100Ah.
    The 70Ah user manual only says: "Note that multiple battery pack arrays will provide longer runtimes, but will also require longer recharge times.
    So I'd say just forget about the extra tender and leave the system to itself, as long as the system recharges sufficiently fast by itself for your needs.
    Just measure how long it takes for the batteries to reach full voltage (~29V) after a certain (typical) discharge period. If it's short enough then it's good enough.
    The spec's say nothing about the available charging current, it only says it can fully recharge fully depleted batteries in 4.5 hours with the included batteries (7Ah).
    While it is correct that more battery capacity requires more quiescent end-of-charge current, this is insignificant compared to the full available charging current anyway.
    What does your external 14Ah actually contain btw.? Its weight of 12.3kg says it might contain up to 4 each of the typical 2.6kg 12V 7Ah batteries.

    1. Yes, except you (& the batteries) would be better served using an extra battery tender (if needed at all). The single setup unbalances the batteries.
    2. Yes, provided it's done with an automatic tender.
    3. No, not unless you want a relay clicking back & forth every other second. This might confuse the tender however, so; no.
     
  5. FrankG

    FrankG

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    Jul 8, 2011
    Where did you read that there is an original 70Ah external battery for this model UPS? The only one I've seen is the 14Ah that I have. I assumed the UPS itself has the same capacity as the 14Ah expansion battery, but I'll take your word that it's 7Ah instead. One thing I know for sure is that when I connect the 100Ah batteries, the UPS get very hot and the fan runs fast when I hook up the 100Ah batteries. That had me concerned that they were drawing too much current from the charging circuit.
     
  6. davenn

    davenn Moderator

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    Sep 5, 2009
    with the external batteries in series to produce 24V you will only be getting the 100A/H

    if they were in parallel you would have 12V at 200 A/H

    Dave
     
  7. davenn

    davenn Moderator

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    Sep 5, 2009
    Thats because the UPS charger is struggling to charge the 100 A/H external battery supply and whatever is internal to the unit. You are running the risk of killing it.

    Dave
     
  8. FrankG

    FrankG

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    Jul 8, 2011
    Okay, 100Ah because they're in series and not parallel. Got it. Thanks.

    Right. This is why I added the 12V charger. I guess I should get another one so that there's one on each battery at all times.
     
  9. Resqueline

    Resqueline

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    Jul 31, 2009
    The info is right there at the Tripp-Lite site. I just Googled your model number and hit straight at it. The Ah ratings seems only to be found as a part of the part numbers.
    For sure the UPS has to work hard for much longer with 100Ah extra but it seems to me to be made to deal with even more. Does it ever "wind down"? Measured voltage?
     
  10. FrankG

    FrankG

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    Jul 8, 2011
    Wow, you're right, there is a 70Ah daisy-chainable OEM battery available for this UPS. I wasn't aware of that. So I guess it can handle 100Ah of external battery capacity. I will remove the 12V charger, and let the UPS maintain the batteries.

    With the 100Ah batteries and 14Ah external battery attached, when fully charged without the external charger attached, the battery voltage shows 27.1 volts on the Tripp Lite monitoring software, confirmed with a VOM reading of 27.07V. Is that an acceptable voltage? Earlier you mentioned full voltage should be ~29V.

    The UPS runs hotter (fan blows warm air) with the large batteries attached, at all times. Maybe that's okay, though. After all, that's what the fan is there for - to remove heat from the unit.
     
  11. Resqueline

    Resqueline

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    Jul 31, 2009
    27V is the correct long-term charging voltage. After a discharge some automatic chargers may be made to go up to 29V for a while before relaxing down to 27V.
    This is done to ensure a full charge in a short time. SLA's have a charging current limit spec if charged to 29V (to avoid thermal runaway), but none if charged to 27V.
    So since it's made to employ anything from 7Ah to 217Ah (or more) I guess your UPS is using only 27V to protect its internal 7Ah batteries from thermal runaway.

    If you want to see exactly what's going on you can measure the air inlet & outlet temperatures, as well as putting an Ammeter in series with the batteries.
    I'd expect a fully charged 100A SLA to draw somewhere between 0.5 and 2A quiescent charging current. A 7Ah usually draws between 0.05 and 0.15A.
    I've seen that old & worn/ damaged (deeply disharged once too many) batteries can exhibit excessive charging currents. This would also raise their temperature.
     
  12. FrankG

    FrankG

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    Jul 8, 2011
    The current is fluctuating wildly between 0 and 0.5A, so fast the digital meter can barely display a reading. I wish I had an analog meter. I left the batteries disconnected for a few minutes, and after reconnecting them they were drawing a steady 1.2A for a few seconds, then went back to swinging between 0 and 0.5A. I checked this with two different digital meters, thinking the first one wasn't working properly. Is it normal for the current to be fluctuating like that?
     
  13. Resqueline

    Resqueline

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    Jul 31, 2009
    Yes, such "bursting" sounds normal for a particular type of charging system. Anyway the batteries seems to be good and not placing a high demand on the UPS charger.
     
  14. genius122

    genius122

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    Jul 18, 2011
     
  15. Resqueline

    Resqueline

    2,848
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    Jul 31, 2009
    Nope, you're wrong. I think you may be thinking about/ confusing it with capacitors/ capacitance.
    Each 12V 100Ah battery contains 12V * 100Ah = 1200Wh, so two batteries contains 2400Wh in total.
    Parallel: 2400Wh / 12V = 200Ah
    Series: 2400Wh / 24V = 100Ah
     
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