Wanted: UPS that stops charging if batteries are hot

Discussion in 'General Electronics Chat' started by DMahalko, Jul 13, 2012.

  1. DMahalko

    Thread Starter Active Member

    Oct 5, 2008
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    Can anyone recommend a UPS that has a temperature sensor in the battery bay on the battery packs, and stops charging if the batteries get too hot?



    Yesterday my little 1300VA home APC UPS started beeping wildly, and apparently the batteries are totally dead now.

    So I open the battery bay, disconnect the leads and YEOUCH these batteries are HOT. Something like 160+F, almost sizzling hot.

    Y'know I may not be a battery charger expert here, but if the gel-cel AGM batteries are 90F over ambient, maybe the device should STOP CHARGING them. Maybe?

    Also, um, maybe cooking them at high temperature might be part of why the batteries are failing, after only 3-5 years of basically sitting there doing nothing but float-charging, and a 10-sec load test every two weeks?



    We've used APC model UPS at work for the last decade and this is very common. APC just doesn't do thermal monitoring of the batteries, and frequently cooks batteries to death.

    I've come into the server room to find the whole room smelling like rotten eggs, due to "sealed" dying lead acids boiling out anyway in an APC Smart-UPS.

    I'm really getting tired of this charging behavior.
     
  2. R!f@@

    AAC Fanatic!

    Apr 2, 2009
    8,750
    759
    Welcome to the group.

    Same story here
     
  3. DMahalko

    Thread Starter Active Member

    Oct 5, 2008
    174
    14
    Actually.... this being AAC... I wonder if I could put a temperature/charge limiter into an APC UPS, without it freaking out.

    [​IMG]

    Model: APC BX1300LCD

    APC specs say, 780 watts actual max output
    Two 12v AGM lead acid in series.. 24v
    So.. 780w / 24v = 32.5 A estimated max output to inverter

    APC specs say, typical recharge time: 16 hrs
    Batteries are 9.0 AH, so 24v @ 9AH = 216 watt/hours capacity
    216watt/hr / 16hr recharge time = 13.5 watts/hr charging
    13.5 watts/hr / 24v = 0.5625 amps max charge current

    So I just need to pick some parts with far higher ratings and call it good enough.



    Thermal charging limiter:

    THERMAL SWITCH, SPST-NC, 40°C, 250v, 10A, $3.78
    http://www.newark.com/multicomp/03en15t044-40-25/thermal-switch-spst-nc-40-c/dp/07WX4444


    Allow emergency power while thermally limited:

    STANDARD DIODE, 50A, 400V, DO-203AB, $4.24
    http://www.newark.com/vishay-formerly-i-r/vs-50pfr40/standard-diode-50a-400v-do-203ab/dp/53H8794


    Okay, now please go ahead and pile on me why this is all wrong. :p
     
  4. R!f@@

    AAC Fanatic!

    Apr 2, 2009
    8,750
    759
    Not bad, u might be getting some where.
    Since I am the only one here AFAIK dealing with APC, we might do something together.
    But I am working so I need some more time to go thru ur findings.

    Will post back ASAP.
     
  5. DMahalko

    Thread Starter Active Member

    Oct 5, 2008
    174
    14
    I am aware my calculations are fudged, likely thoroughly chocolate coated. But picking much higher rated parts should deal with it anyway.

    The UPS specs list the output available to the load, not the power used internally. It's probably only about an 80% efficient inverter.

    And 12v batteries are actually about 13.6 fully charged, so 24v is really 27.2v.

    So:
    780 watts + 20% inefficiency (156 watts) = 936 watts actual battery load
    936w / 27.2v = 34.4A

    ..which is 2A higher than the original estimation but still lower than the 50A diode rating.



    Meanwhile the thermal switch is rated 250v AC, 10 amps while the load is 24v DC. But the specs page doesn't SAY the thermal switch is rated for DC.

    Will a 250v rated AC switch (339v peak) successfully turn off a non-zero-crossing DC voltage (27v actual) twelve times lower than its peak voltage rating? ...probably, yes.

    (And even this more accurate assessment is still fudge, just with a thinner coating.)
     
    Last edited: Jul 14, 2012
  6. Dodgydave

    Distinguished Member

    Jun 22, 2012
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    Last edited: Jul 14, 2012
  7. DMahalko

    Thread Starter Active Member

    Oct 5, 2008
    174
    14
    After some more thought, if the thermal switch is closed and the inverter is activated, the power is going to prefer the switch path rather than going through the diode since the diode will drop the battery voltage by about 1 volt, vs the switch which is zero ohms when closed.

    So either the thermal switch path needs a high current rating too, or it needs a blocking diode so that discharge/inverter current doesn't take that path. Pop-disc thermal switches don't rate above 10-15A so either a relay is needed, or the diode.

    [​IMG]

    This second discharge blocking diode would also keep the battery voltage "seen" by the UPS constant.

    For the original circuit, when discharging if the switch suddenly opens the voltage will drop by about 1 volt due to the diode, and the UPS may unexpectedly drop the load if the batteries are nearly discharged. A second diode on the switch path forces battery discharge voltage seen by the UPS to stay constant regardless of whether the switch is open or closed.
     
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