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UPS, Marine Battery, as Sump Pump backup

  1. May 26, 2017 #1
    Hey there! I'm an IT guy and found this forum through searching on replacing UPS batteries with deep cycle 12v marine batteries. The reason I was looking is I happen to have 4 spare Compaq rack mount UPSs with dead batteries taking up space in my basement. I was thinking of putting in a backup sump pump in case of a power outage, and then thought "wait a minute, a UPS will back up the existing electric sump pump".

    While I know regular UPS batteries are really just meant for systems to have a few minutes for a graceful shutdown, I want to set this up so it will be able to run a sump pump on and off for several hours after a power outage.

    The average time to drain the sump is about 30 seconds. It's a 6.5amp 115v pump. The batteries that the UPS is supposed to use are these. http://www.apc.com/shop/us/en/products/APC-Replacement-Battery-Cartridge-23/P-RBC23

    How many 12v Deep Cycle marine batteries would I have to use to guarantee 48 hours of service assuming the pump runs once every 30 minutes for 30 seconds during that time?

    From what I just read, if anyone can figure this out you folks can. I don't just want the answer, so if you can provide the math you use to get it I'd LOVE that for future projects. Thanks in advance!
  2. jcsd
  3. May 26, 2017 #2


    Staff: Mentor


    6.5 amps * 115 V = 747 Watts
    Lets say 800 watts from the DC side. 800/12V = 66 amps. That's comparable to the starter current to start an engine.

    In 30 seconds that draws 0.55 amp-hours. Round it to 0.6 amp-hours per cycle of the pump. 2 cycles per hour, 96 cycles in 48 hours, so 96*0.6 = 58 amp-hours is your total requirement.

    A group 31 marine battery, is nominally rated at 220 amp-hours, but it should not be discharged more than 25% for maximum lifetime.
    So say, 55 amp hours useful capacity.

    These numbers are rough, so 58 versus 55 sounds like a nearly exact match.

    I pay $110 for a group 31 marine battery flooded battery. But those need ventilation. A gel cell or AGM battery is sealed and needs no ventilation but they cost more (just saw an AGM one for $139 online. That's a good price).

    The marine battery may be bigger and heavier than your old battery.

    Group 31 size:
    Length: 12 15/16 in.
    Width: 6 3/4 in.
    Height: 9 3/8 in.

    Weight: 69 pounds

    Your sump could probably run for 96 hours on this battery, but doing so would drastically reduce the lifetime of the battery.

    One thing to check, is the efficiency of your DC to AC inverter. Those calculations assume 93% efficient. If the inverter is significantly less efficient than that, you need still more battery

    Good luck
    Last edited: May 26, 2017
  4. May 26, 2017 #3


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    Watch out for starting current (locked rotor current) of the pump motor, it could be five to ten times the running current. UPS's usually have protection built-in that causes them to shut down and require a manual restart after an overload. A sanity test would be to connect the UPS to your car battery using good quality, heavy duty, jumper cables and see if the pump will start under load.

    If the UPS can't handle the pump, there are inverters available (at least in the US) for automotive and Recreational Vehicle (RV) usage up to a few thousand watts. Cost is in the $100 - $200USD range for a 1KW unit. These often have a delayed overload shutdown specifically to allow motor starting.

    Just an associated story about prevention versus recovery. The building I'm in recently had a flooding problem. In February we had a record-breaking rain storm. The building fortunately had a battery backup system to keep the lights, heating, phones, Internet access, etc running. When the first floor flooded, the power was shut off there and the 10 apartments evacuated. The rest of the building went to battery backup. NOT connected to the backup were the elevator, door locks, gate to the parking lot...or the sump pumps. Some apartments had four feet of water. After 3 weeks of repairing the damage, some people returned to their apartments. Then a second storm hit. Same results. The apartment manager mentioned that the second flood "got the attention" of the owner. Sump pumps were added, drainage plumbing was modified, etc. Glad I'm on the second floor!
  5. May 26, 2017 #4


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    Here's something else to consider:
    from http://batteryuniversity.com/learn/article/absorbent_glass_mat_agm
    Battery University is a good place for learning battery technology.
  6. May 27, 2017 #5


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    2017 Award

    There speaks a yachting man! Ten times that for many car engines.
    Buying anything with 'Marine" written on the side tends to hurt your bank account - as does something involving data systems integrity. There are deep cycle batteries in SUVs and Winnebagos and they are probably cheapest of the lot. It's just a matter of reading the battery spec and manufacturers usually have accurate data sheets. Just buying something a bit bigger than you need could possibly be the cheapest solution for a static application.
    One power failure in a blue moon could be coped with and it may not matter if it knackers a cheaper battery (a disposable lifeboat, if you like) but one failure a week would be a different matter.
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