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Replacing UPS Batteries with Marine/RV Batteries

  1. Mar 6, 2010 #1
    Have an APC Back-UPS XS 1500, Model BX1500LCD (same as BR1500LCD), computer battery backup unit. It came equipped with two 12V, 9AH batteries wired in series. After 2 years and a power outage resulting in the batteries being almost completely discharged (control software not running), they are at the end of their life as evidenced by drastically reduced run time.

    Rather than having to replace the batteries for $50-$80 every 2-3 years for 20-30 minutes of run time with my system, I would like to use 2 flooded, size 24M, 12V, 80AH Deep Cycle Marine/RV batteries. I haven't done the math, but I'm guesstimating that these batteries would provide 2.5-4 hours of run time for my system. If I were to do this, I would do it right, utilizing battery boxes, good connectors and wiring.

    I have 3 questions:

    1. Will the UPS be able to keep the marine batteries charged?
    2. After a deep discharge, will the UPS be able to recharge the batteries in a reasonable amount of time (36 hours max)?
    3. During a long power outage, would my UPS be able to supply power to my system for several hours without overheating? If not, can this be mitigated by modification (ventilation, heat-sink)?

    The UPS is a high-end consumer unit, so I'm hopeful that it is fairly well made and heavy duty enough to be able to handle the assignment.

    Here are the specifications for the UPS:

    Model: BR1500LCD
    Output Power Capacity: 865 Watts / 1500 VA
    Max Configurable Power: 865 Watts / 1500 VA
    Nominal Output Voltage: 120V
    Output Frequency (sync to mains): 60 Hz
    Crest Factor: 3 : 1
    Waveform Type: Stepped approximation to a sinewave
    Output Connections: (2) NEMA 5-15R (Surge Protection), (6) NEMA 5-15R (Battery Backup)
    Nominal Input Voltage: 120V
    Input Frequency: 60 Hz +/- 3 Hz
    Input Connections: NEMA 5-15P
    Cord Length: 6 feet (1.83 meters)
    Input voltage range for main operations: 88 - 139V
    Maximum Input Current: 12A
    Input Breaker Capacity: 15A
    Typical Recharge Time: 16 Hours
    Surge energy rating: 340 Joules
    Filtering: Full time multi-pole noise filtering : 5% IEEE surge let-through : zero clamping response time : meets UL 1449
    Data Line Protection: RJ-45 Modem/Fax/DSL/10-100 Base-T protection,Co-axial Video / Cable protection
    Regulatory Approvals: FCC Part 15 Class B,FCC Part 68,NOM,TUV

    Batteries I'm thinking of using (2 wired in series):

    DieHard Marine Deep Cycle/RV Battery, Group Size 24M
    Sears Item# 02827494000 | Model# 27494
    Voltage: 12.0
    Cold Cranking Amps (CCA at 0 deg.F): 500
    Reserve Capacity (RC): 135 min.
    Amp Hours at 20 Hour Rate: 80

    I don't think H2 gas would be a problem as batteries and computer would be in fairly large room with enough air changes to dissipate and I don't smoke and would keep ignition sources away.

    I know that doing this cannot be recommended and accept all risk and liability. I just want to know whether it would work from a theoretical perspective.

  2. jcsd
  3. Mar 7, 2010 #2


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    Welcome to the PF. The specs look generally okay. I think your questions #1 and #3 should be fine. On #2, it's different, and you can do the math to see if you're okay with it. Your new batteries have a larger capacity, and the charging current is limited according to the UPS specs. 9AH batteris get charged in 16H typ, versus 80AH batteries which would get charged in how many hours....? (it ratios)
  4. Mar 7, 2010 #3
    Thanks berkeman. The length of time it takes to recharge the batteries is actually only a minor concern, as this could only become an issue if there were multiple lengthy power outages within a few days of each other, which is exceedingly unlikely where I live.

    Another concern I have involves the manner in which the UPS charges the batteries. Does it trickle charge or float charge? Would it still be advisable to give the batteries a topping/equalizing charge a couple of times a year to maximize their lifespan? If I were to do this, I would imagine I would have to disconnect the batteries from the UPS and I would only be able to charge one battery at a time with a 12 volt battery charger. If I make the investment in the batteries, I wouldn't want them sulfating on me. I could periodically check the open circuit voltage to see if they are being fully charged and act accordingly.
  5. Mar 7, 2010 #4
    Have another follow-up. In an article I read, the author suggested replacing the regular electrical outlet a UPS is plugged into with a GFCI outlet for testing purposes. He indicated that using the test button on the GFCI is better than unplugging the UPS, as the UPS remains grounded at all times. Makes sense to me, but thought I would see if anyone here has any thoughts on this.
  6. Mar 7, 2010 #5
    "9AH batteris get charged in 16H typ, versus 80AH batteries which would get charged in how many hours....? (it ratios)"

    80/9=8.88, 8.88x16=142 hours? Is that how it works?
  7. Mar 7, 2010 #6


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    I believe so. Remember that they are giving the 16 hours as a typical time.

    On the GFCI test method, that sounds good. I like the fact that the unit stays grounded.

    I don't know the answer to your trickle/float charge question. You may need to call the UPS manufacturer's tech support folks to find that out.
  8. Mar 21, 2010 #7
    Well, I did it. In the end I decided to play it super safe and go with sealed AGM deep cycle batteries, rather than flooded, even though I would have gotten twice the backup time for about the same price with flooded batteries.

    I purchased two Werker, 33 AH, deep cycle, AGM batteries from a local Batteries Plus store for about $80 each. Also purchased 5 feet each of red and black, stranded, 10 gauge wire; a 1 foot piece of stranded 6 gauge wire; 2 copper terminal lug connectors; crimp (butt splice) connectors; and spade terminal connectors.






    After removing the old batteries, I cut the existing battery connectors off the battery leads in the UPS batter compartment.

    I stripped a little less than a cm of insulation from the ends of the leads and connected the 5-foot wire extensions with crimp connectors. CAUTION: I DISCOVERED THAT THE LEADS HAVE VOLTAGE EVEN WHEN THE UNIT WAS UNPLUGGED AND WITH THE BATTERIES DISCONNECTED! SO KEEP THEM APART! Must be an internal battery.

    I filed two small slots in the battery compartment cover to allow the extension wires to pass through the case. I made sure I placed the slots a few inches apart.


    I wired the batteries in series to achieve the 24 volt input my UPS requires, using the lug connectors and the piece of 6-gauge wire.

    I positioned the UPS and batteries on apposite sides of a small chest located next to the computer, ran the wires and made the final connections.

    During testing, I ran my system for 1.5 hours on the new batteries and everything worked fine. The batteries, battery leads, connectors and series connection remained cool to the touch.

    The USP is equipped with cooling fans that came on intermittently during the test and kept the UPS from getting too hot. However, these fans did not come on during the subsequent recharging and the top of the UPS was hot to the touch during this time.

    4451720584_31a886a0d7_o_d.jpg 4450948591_fa688173b0_o_d.jpg
    I measured the battery voltage under load during the test and found that it slowly dropped over the course of the discharge as expected. After 1.5 hours, the voltage was down to 23.5 volts.

    The only issue I have now is that the UPS doesn't detect that it is now attached to much higher capacity batteries and as a result, shuts down the PC after only 10 minutes on the batteries if connect with the USP cable. If I'm the PC is unattended at the time of an outage, this behavior is fine as the PC shuts down gracefully and the batteries are protected from too deep of a discharge. However, if I'm working on the computer, this behavior is undesirable to say the least. My workaround is to disconnect the signaling cable, thus preventing the UPS from sending the kill signal.

  9. Mar 24, 2010 #8
    Update: I was able to run my system (150-175 watt load) for 1.5 hours on the batteries and still had about 15% charge left!

    I had to unplug the data cable, as the UPS wanted to shut everything down after only 10 minutes. However, I was able to get the UPS to recalibrate by adding an additional 150 watt load (light) and running it on the batteries with the data cable unplugged for 45 minutes. Total load was about 330 watts and remaining charge was 14%. Now after recharging, the unit indicates I have 30 minutes backup time, which would be correct if I was using the specified 9AH batteries.

    Overall, I have to say I'm quite pleased with the results of the upgrade. Shouldn't have to worry about batteries for at least 4 years now.
  10. Apr 9, 2012 #9
    This is the very best write up on this topic I have been able to find on the Internet.

    Many thanks for taking the time to post this info and the great photos.

    I am currently (2 years after the original post by member dontosaw), following in his footsteps with the exact same APC battery backup unit so this thread is a gold mine.

    I recently went out of town for a week and within hours of leaving, we had the very first extended power outage that lasted 3.5 hours and all my equipment died after about 45 minutes. Server and all. Ironic that I had been nearby for 5 years with no power outage lasting longer than 5 minutes then only several hours after leaving, we had a 3.5 hour power outage. It just goes to show that you should always prepare for the unexpected :smile:

    Anyway, that event led me on a search for extending my battery backup time. Ideally, I would like 5 hours at 252watts load.

    I may need 2 battery backups and 4 batteries but it would still be FAR less expensive to do it the way dontosaw did this than to purchase a commercial backup unit.
  11. Apr 15, 2012 #10
    Today I got the Two Werker brand VRLA AGM batteries and modified the connections and all seems to be working as planned.

    Like the OP (dontosaw) I used an APC XS 1300 battery backup because I happened to have one.
    Also, since it is a 24 volt system, I figured it would have a better built in system to handle charging bigger batteries.

    The wires are cool to the touch and everything seems to be working.

    I used two 80ah batteries WKA12-80C/FR
    For every pair of these batteries I add, I get an additional 3 hours of runtime.
    These two batteries were $172 each less a 10% discount, plus tax so I paid right at $330
    for the batteries. I could have used cheaper batteries and saved a bit but these
    are inside the office for security and there is little venting of gases. Regular flooded lead / acid batteries
    can spill and there is generally more gassing I was told.
    Another concern with the batteries was how they provided power. I was told VRLA AGM batteries
    are better constructed for providing battery backup type power for computer applications.
    VRLA AGM batteries are better constructed with discharge states than regular flooded batteries.
    If the batteries face relatively frequent discharging, then they will outlast regular flooded lead/ acid batteries due to greatly reduced sulfation.
    My biggest concern was gassing inside my office.

    All I have connected is the critical things for maximum run time of the critical server.

    Server - 200watts (estimated)
    Watchguard Firebox - 50watts (published)
    Modem - 20watts (estimated)
    Monitor - optional (off as much as possible)

    So I'm providing backup essentially for 270watts give or take
    I plan to use a kill-A-watt power consumption device (which I already have) to measure the real world
    power usage of these components in the near future. That should give me an very accurate
    ideal of how much run time I'll have on batteries only.

    So my 80ah batteries should in theory provide almost 3 hours of runtime.
    Not bad. I may add two more batteries if I find I have to be out of town again. But 3 hours should be enough time for me to get the generator hooked up
    and running even if I'm away from the office when the power goes out. Or make other power arrangements.

    So at this point, I'm really glad to have that much extra run time for my critical server.
    I won't be quite as nervous when the power goes out.

    I'll check back down the road and report on the system.
  12. Apr 16, 2012 #11
    A quick update on one thing...

    After getting everything going nicely, I looked at the load (watts) and found to my surprise that the load for the Server, Firewall and modem only used 95 watts (monitor off). Or 130 with the monitor on.


    I was expecting a load of around 250 watts. I forgot that the server uses a much less powerful video card which in turn uses far less power than the average PC might.

    Instead of about 3 hours, run time is now estimated to be around an incredible 6 to 7 hours !

    One caveat is recharge time. I estimate that if the batteries were fully drained, it would take nearly a week to fully recharge the battery with just the APC 1300.
    So anytime the batteries are drained for an hour or more, I'll supplement the charge with an external charger but one has to remember that VRLA batteries do not like to be fast charged so
    the external charger can charger faster, but not too fast. One also must remember that we're dealing with a 24 volt system whenever the batteries are conected and unless you have a 24v charger,
    you have to charge the batteries in parallel (12v) or one at a time.

    Also, I used 8 gauge wire for all connections. But instead of cutting the connectors at the APC, I added Male connectors to my wires that needed to connect to the APC. That way (for whatever reason) I could always go back to the OEM setup with two OEM batteriies if I wanted to.
    Last edited: Apr 16, 2012
  13. May 10, 2012 #12
    Came across this looking for something else - nice upgrade. But one really important consideration for anyone considering this approach is missing.

    *** Safety Note ***
    These batteries are capable of VERY high short circuit current so a fault in the UPS or in the wire between the batteries and the UPS could easily result in a fire or rupture of the batteries.
    A fuse, breaker or fusible link MUST be added right at the external batteries (in the battery box within inches of the battery terminal).

    A couple of $ for an inline automotive fuse holder and appropriate fuse could save your or your family's lives.

  14. May 12, 2012 #13
    Thanks Peter, good advice.
    But let's take that one step further.

    To determine the SIZE fuse you should use, you need to know the Watts and Volts

    In this case, we're dealing with 12 Volts and so we need to determine the maximum Watts the circuit would be asked to provide. In the case of the Battery Backup units both the OP and I used, these show the LOAD on the display so it's very easy to determine the load. You can also estimate the load based on individual values. For example a monitor may display the watts used and so forth.

    Once you have the maximum watts, add a safety margin ( I will use 25%) then divide then multiply the Watts times your safety factor. So, my maximum load is 150Watts TIMES 1.25 = 187.5 Watts.

    Now divide the peak Watts by the volts = 187.5 / 12 = 15 Amps
    So I will use a 15 Amp fuse inline.

    I could use a larger amperage fuse but that will cause the fuse not to blow until higher Power levels are reached which kin of defeats the purpose. It may take some experimentation to find the Goldilocks fuse size (ideal). If the 15A fuse blows too often, try a 20A fuse.

    Don't forget that whatever way you add a fuse holder, there should never be wire inline that is less than the rest of the circuit wire going to the batteries and the backup. In y case, 10 gauge.

    HINT: I may use an actual House Circuit Breaker. that way I can just insert the ends of my 10Ga wire and should it pop, I can just reset it.
  15. May 12, 2012 #14

    jim hardy

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    be very careful using house type breakers on DC . Since your batteries are small you might get away with it. But here's an interesting piece of trivia:

    Circuit breakers strike an arc between their contacts when the contacts first start to separate. Just like an electric welding arc.
    Two things break up that arc to extinguish it:
    A: some internal vanes, like miniature venetian blinds, cut the arc into segments as the contacts separate and the arc widens.
    B: The fact that the AC current sinewave crosses zero 120 times per second. Arc goes out then.

    Now on DC there's no zero crossing so the arc current must be wrestled to zero by brute force. You get no assist from a zero crossing.
    That's why the DC rating of a breaker or switch is ALWAYS a lot less than its AC rating.

    SO look carefully at that breaker to make sure it has DC rating at least equal to your battery's short circuit capacity. I'd back it up with a fuse, too.


    old jim
  16. May 13, 2012 #15
    Yes, also very good points - be sure to size both fuse and wire for the expected load. Since the fuse is really only to protect against a fault condition I would size it slightly higher than the maximum steady state current expected as you have done and use a slow blow type to allow for in-rush at start and possible load changes - if you have a shunt or current clamp and 'scope you could measure the peaks and size for that. You don't want any nuisance failures but you do want to blow below the threshold for any kind of electrical mayhem.

    For this application with short runs the 10ga wire is actually overkill - yes, it is rated 15A for power transmission but over 10' the resistance is only 0.01ohm so wire losses and heating are tiny - it could carry 50A on this run easily. Similarly a couple of inches of leader on a fuse holder that is a gauge or two smaller isn't going to be a problem. 4" of 14ga would be around 0.001ohm. I usually go in for overkill too though and if I were doing this mod I'd probably use one of those car-audio type fuse block with recessed screw terminals and yes, I probably would have used 10ga too.

    Jim raises a really good point on the breaker DC rating. I wouldn't use a household AC breaker, most are designed to plug into a specific panel anyway on the line side - but an automotive or RV type for DC would be okay. It is also important to get the line (input) and load (output) terminals correct with breakers.

    Happy mod'ing,
  17. Jul 28, 2013 #16
    I know this is an older thread but I was wondering one question that was never answered here...how do these UPS systems charge the batteries? Do they trickle or float? How do you know that these systems will not overcharge and ruin these batteries?

    I'm thinking of incorporating one of these systems and no where can I find the information on the UPS battery charging capabilities.
  18. Jul 28, 2013 #17

    jim hardy

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    It's hard to speculate what the UPS designers did.

    It's a safe bet they use a "smart" charging technique suited to the batteries they provided with the ups

    and "smart" charger IC's do just that:
    They monitor battery voltage and temperature to determine its state of charge.
    They can apply several levels of current.
    When battery is sensed at low state of charge they charge quickly with fixed current , then switch to a lower rate sometimes with current dependent on voltage until battery is fully charged, then switch to just a trickle for 'hold' .

    Here's an application note from TI that'll give you insight to what the designer has available :

    you might open the device and see whether you find a "smart" charger IC. Nowadays it could be done all by microprocessor instead, but algorithms will be similar I would think.

    As stated earlier, a "quick" charging current to a 9AH battery will be a pretty slow charge for a 90AH one.

    Nice upgrades, fellows ! Have fun.

    ps - you might think about making sure the batteries and UPS are at the same temperature so the 'smart' charger applies the right temperature adjustment. In other words don't put the batteries outdoors if UPS is indoors, and avoid placing either right over an airconditioning duct..
    old jim
    Last edited: Jul 28, 2013
  19. Jul 28, 2015 #18
    Hi Guys,
    I'm back. Was doing a search and came upon this article I posted 5 years ago. Update: Werker batteries died. Just replaced with 2 35 Ah Duracell Ultra AGM batteries.

    Regarding the suggestion for a safety fuse, wouldn't the built in UPS circuit breaker and the GFCI outlet I have it plugged into provide adequate protection? If not, please elaborate. I'm all about safety and will add a fuse if it is really necessary for safety.

    Having issues with the setup now. It looks as though the batteries are being drained rather than charged, but I don't want to replace the unit without confirming that the batteries are indeed both good. The are wired in series for 24v and I measure 24.8 volts for both and 12.51 volts and 12.41 individually hooked to the setup while running. Seems that the UPS charging circuit has failed. I too the batteries back to Batteries+Bulbs to have them tested and charged once. The said the batteries are good. I do wonder why they don't have equal voltages.

  20. Jul 28, 2015 #19


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    A big part of why breakers and fuses are used is to protect the wires from overheating. If something shorted out in the wiring going to the UPS the you would have on the order of hundreds of amps flowing through the wires and batteries. It is best practice to put current safety devices closest to the source. Often in packs with multiple batteries you will find a fuse in the middle of the stack somewhere.

    A quick search finds:

  21. Jul 29, 2015 #20

    jim hardy

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    Technically, you need to protect the wires that carry power from your batteries up to the UPS.
    A good sized fuse in that middle wire between your batteries will do that.
    The UPS should protect itself so you can use a fuse sized for the wire. 30 amp will be okay for #10. Auto parts stores have convenient inline fuseholders, be sure to get one rated 30 amps.
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