Will a 12v system work for this sump pump?

thentro
thentro Registered Users Posts: 9
Hello, I am building a system to run some small electronics and one big septic system sump pump. It is rated as a 1/2 HP, 9.5 amp pump at 115v. It will run for about 1/2 hr pr week. The large size in needed to be able to pump the waste uphill and at the correct volume for the drainage field to work properly, so a smaller DC pump was not an option. We are currently running it off a generator.

I have a 12v system designed, that will run off a 200 amp/hr battery connected to a 1500 watt inverter, using a 150amp breaker. I am concerned that I won't have enough power to run the pump, and/or I will be putting too much stress on the batteries by using this kind of load on a medium/small system. I have considered converting the whole thing to a 24v setup to drop the amperage draw off the batteries, but I am not sure if that would really help much. Any thoughts? I would hate to flip the switch and have nothing happen!

Comments

  • inetdog
    inetdog Solar Expert Posts: 3,123 ✭✭✭✭
    Re: Will a 12v system work for this sump pump?
    thentro wrote: »
    It is rated as a 1/2 HP, 9.5 amp pump at 115v. ... We are currently running it off a generator.

    I have a 12v system designed, that will run off a 200 amp/hr battery connected to a 1500 watt inverter, using a 150amp breaker. I am concerned that I won't have enough power to run the pump, and/or I will be putting too much stress on the batteries by using this kind of load on a medium/small system. I have considered converting the whole thing to a 24v setup to drop the amperage draw off the batteries, but I am not sure if that would really help much. Any thoughts? I would hate to flip the switch and have nothing happen!

    1. If the pump will draw Full Load Amps of 9.5 for the whole half-hour, you will be dropping the battery to 50% Depth of Discharge (DOD) in one run. That is not good for the battery, especially if it happens when the sun is down and the batteries will not start to recharge for 12 hours or more. I hope that the 1/2 hour is cumulative and will consist of many several minute runs.
    2. The high current from the battery will not be efficient, and if you want to double your battery bank size then going to series and 24 volts with a 24 volt inverter will be easier to maintain.
    3. The starting current for your particular pump motor cannot be calculated from the running current alone. Look for the Locked Rotor Amps (LRA) on the name tag or find out more from the manufacturer of the pump motor. The starting current will determine whether or not your inverter can handle it. 20 Amps LRA, for example, will require very large battery wires at 12 volts even if the inverter itself can handle it. A soft start circuit for the pump would be another way to approach the problem.
    SMA SB 3000, old BP panels.
  • thentro
    thentro Registered Users Posts: 9
    Re: Will a 12v system work for this sump pump?

    Thanks for your reply. Indeed the pump only runs for a few minutes a day and at our whim so we can determine the best time to empty it. I have emailed the manufacture about the Locked Rotor Amps, we will see if I hear back. Currently it is starting off a honda 2000w max/1600w run generator without a hiccup.

    Question, I understand that high amperage/low voltage will cause some inefficiencies, and I would need some very large gauge wires to run the inverter, but why would a 24 system be easier to maintain? 24v equipment seems to be more expensive and needs more wires and connections at every step.
  • BB.
    BB. Super Moderators, Administrators Posts: 33,431 admin
    Re: Will a 12v system work for this sump pump?

    More or less, the rule of thumb I use... 100 amps is relatively easy and inexpensive to wire/fuse/breaker. Over 200 amps gets really expensive (lots of copper, larger protective devices, etc.) and physically difficult to make connections/bend wire, etc.

    So, take an example of a 1,200 watt inverter, 85% efficiency with a 10.5 volt cutoff voltage (or 21/42 volts for 24/48 volt banks):
    • 1,200 watts * 1/0.85 eff * 1/10.5 volts cutoff * 1.25 NEC derating = 168 amp branch circuit minimum
    • 1,200 watts * 1/0.85 eff * 1/21.0 volts cutoff * 1.25 NEC derating = 84 amp branch circuit minimum
    • 1,200 watts * 1/0.85 eff * 1/42.0 volts cutoff * 1.25 NEC derating = 42 amp branch circuit minimum

    So, roughly, I suggest around 1,200 watts maximum for a 12 volt battery bank, and 2,400 watts maximum for a 24 volt battery bank.

    And, many "good quality" inverters can supply upwards of 2x rated power for surge (less than a few seconds), so the surge currents would be even higher than above.

    So, your average loads may be light, but if you have heavy starting current, you may much heavier (and keep it short) DC cabling to start the pump and now "brown out" the DC input of the inverter. Adding capacitors at the input of the inverter will not really do any good (capacitors would need to be very large, and use a higher voltage swing that 1-2 volts to store significant amount of energy for the inverter to pull for a few seconds--just is not practical).

    Also, remember voltage drop... A 12 volt battery can be drawn down to, roughly, 11.5 volts under load (below that, the battery is getting heavily discharged and/or under very heavy current for its AH rating). And most devices cutout at around 10.5 volts, so that gives you ~1.0 volts for wiring/fuse/breaker drop (and hot batteries naturally output a slightly lower voltage).

    With a 24 volt system, you now have ~23 volt batter vs 21 volt cutout--so ~2 volt drop (at 1/2 the current).

    And a 48 volt system, ~46 volts vs 42 volts-- roughly ~4 volt drop at 1/4 the current...

    So, it is just much easier to do the DC wiring, plus you can send more power, farther, without having to spend a whole bunch on heavy copper cables.

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • vtmaps
    vtmaps Solar Expert Posts: 3,741 ✭✭✭✭
    Re: Will a 12v system work for this sump pump?
    thentro wrote: »
    but why would a 24 system be easier to maintain? 24v equipment seems to be more expensive and needs more wires and connections at every step.

    Whether or not 24 volts system is easier to maintain depends on the batteries. The key is to have one series string of batteries.

    If you find yourself looking at two parallel strings at 12 volts, you could take those same batteries and put them in series for 24 volts. That would mean less connections, thinner wires, and less chance for something to go wrong. When batteries are in parallel it is difficult to charge them equally and they usually don't last as long as they would in a single series string. With parallel battery strings you need to frequently use a clamp ammeter to check that current is equal in each string. If its not equal, then what? You my be able to rotate the batteries, but that usually doesn't work.

    Also, when batteries are in parallel you should have a fuse or breaker in each parallel string. This is because a common way for old batteries to fail is a shorted cell. when that happens one string will start discharging into the other string. Even if you see the wires glowing there's not much you can do about it.

    Getting back to your question, a 12 volt system is not necessarily more maintenance than a 24 volt system. If your need for battery capacity leads to parallel strings, than you would be better off with 24 volts.

    --vtMaps
    4 X 235watt Samsung, Midnite ePanel, Outback VFX3524 FM60 & mate, 4 Interstate L16, trimetric, Honda eu2000i
  • thentro
    thentro Registered Users Posts: 9
    Re: Will a 12v system work for this sump pump?

    I was planning on getting one 200amp/hr 12v battery (an AGM since it will freeze in the winter), so for me making a 24v system would make more complexity in that regard. I would also like to stick with one panel for now, if possible.

    From all of your helpful responses, it sounds like I am right on the cusp of 12 vs 24 volts. I am hung up on getting the Locked Rotor Amps for the pump. I know it can't be any more than 2200 watts, since that is the breaking point of the generator it has been running on for the past year. So I think if I got a 1500w inverter with a 3000w surge, and attached that to my 200amp battery with some big #2/0 AWG cables, that could handle a momentary surge without bottoming out in voltage.

    Once it gets through that half second, then it can hum along at 9.5 amps for 2 minutes before shutting off and get back to charging for another few hours.
  • Cariboocoot
    Cariboocoot Banned Posts: 17,615 ✭✭✭
    Re: Will a 12v system work for this sump pump?
    thentro wrote: »
    I was planning on getting one 200amp/hr 12v battery (an AGM since it will freeze in the winter), so for me making a 24v system would make more complexity in that regard. I would also like to stick with one panel for now, if possible.

    Technical point: charged batteries do not freeze in the winter. There is no reason to spend extra money on an AGM on account of this. You could buy less expensive flooded cell batteries and have your 24 Volt system.

    200 Amp hour 12 Volt: http://www.solar-electric.com/pvx-12210.html $570
    225 Amp hour 6 Volt (*4): http://www.solar-electric.com/cr225am6vode.html $556

    Twice the available power with the 24 Volts system, fewer current issues, and you may find "warehouse" GC2 batteries for even less money than the Crowns.
  • inetdog
    inetdog Solar Expert Posts: 3,123 ✭✭✭✭
    Re: Will a 12v system work for this sump pump?
    thentro wrote: »
    I am hung up on getting the Locked Rotor Amps for the pump. I know it can't be any more than 2200 watts, since that is the breaking point of the generator it has been running on for the past year. So I think if I got a 1500w inverter with a 3000w surge, and attached that to my 200amp battery with some big #2/0 AWG cables, that could handle a momentary surge without bottoming out in voltage.

    A generator has two main limits:
    1. The amount of current the windings of the generator can safely carry and
    2. the power of the engine to drive it under load.

    In the case of the current capability, the windings and the circuit breaker(s) should be able to handle the sort of surge a starting motor will require. The engine probably could not and would drop in speed under heavy load even though at maximum throttle.
    But the engine and generator have a flywheel effect which can supply some extra mechanical energy for a very short time as the rotation speed decreases.

    The inverter has three primary limits:
    1. The long term output which is usually set by heating of the electronic components, and
    2. The short term output which it can deliver as things are getting hotter, as long as the surge is short and/or the inverter components are cold to start with.
    3. The amount of current it can pull from the battery bank before the voltage goes so low that the inverter shuts itself down.

    There are so many differences among generators and inverters that it is hard to make a simple comparison in overload/surge capacity between two with the same continuous output rating.
    SMA SB 3000, old BP panels.
  • thentro
    thentro Registered Users Posts: 9
    Re: Will a 12v system work for this sump pump?

    Thanks for all the helpful advice, especially the frozen battery info.

    I got a call back from someone at the pump company who told me that the Locked Rotor Amps for our 1/2 HP pump is 30A so a 12v system would be out of the question. Now that throws me into another problem, since a 30A draw means about a 3500 watt draw for a fraction of a second before dropping down to 1100w.

    But what if I attached it to something that cuts down the start up surge? Such as this divice for amplifiers:
    http://www.ameritron.com/Product.php?productid=ICP-120

    Or a real variable frequency drive such as this?
    http://www.driveswarehouse.com/p-102-l100-004mfu.aspx

    I believe they would soft start the pump, which would eliminate that huge spike that is causing me so much trouble, and keep me in the 1400watt rage.
  • inetdog
    inetdog Solar Expert Posts: 3,123 ✭✭✭✭
    Re: Will a 12v system work for this sump pump?
    thentro wrote: »
    But what if I attached it to something that cuts down the start up surge? Such as this divice for amplifiers:
    http://www.ameritron.com/Product.php?productid=ICP-120

    Or a real variable frequency drive such as this?
    http://www.driveswarehouse.com/p-102-l100-004mfu.aspx

    I believe they would soft start the pump, which would eliminate that huge spike that is causing me so much trouble, and keep me in the 1400watt rage.

    That spike is what the motor would draw if it could. If the inverter drops the voltage down when it sees the sudden overload instead of cutting off completely, it might ride through it. It is a matter of the inverter design.

    A real VFD would be a sure bet, but expensive. Something that just limits the inrush current for a single cycle or a few cycles of the AC waveform like the Amp device could instead keep the motor from starting or else just delay the surge for a few cycles. Not designed for the same purpose.

    There are other soft-start circuits short of a full VFD which would be cheaper and might work. But you want something that does not require modifications to the motor, etc. One common method just uses a transformer and a relay to drop the voltage to the motor during starting.

    I can't recommend any specific solution for your motor and for any particular inverter. (Some inverters have better surge capability than others.)
    SMA SB 3000, old BP panels.
  • thentro
    thentro Registered Users Posts: 9
    Re: Will a 12v system work for this sump pump?

    VFD's seem interesting but I don't understand them. This looks like a good one too but I don't understand the output. Is it 0-240v as required by the motor, or does it up convert from 120 to 240?
    http://www.factorymation.com/s.nl/it.A/id.192/.f?sc=2&category=30

    Either way, if I am spending that much money I might as well up the inverter size and be done with it. I just feel silly sticking on a super expensive 2000w inverter worth almost half the system to start one pump! And on a battery bank that could never sustain that kind of draw if I wanted to use it for something else. I guess if we ever were to upgrade it would be useful.

    I am thinking about getting the Samlex 1500w 24v inverter and seeing what happens. The theoretical peak of the pump starting is 3500 watts, which this wouldn't cover, but someone mentioned that it might be able to take the spike if it is a good one:
    http://www.solar-electric.com/sa15wa12vosi.html

    Or should I just go for the 2000w version.
  • vtmaps
    vtmaps Solar Expert Posts: 3,741 ✭✭✭✭
    Re: Will a 12v system work for this sump pump?
    thentro wrote: »
    I am thinking about getting the Samlex 1500w 24v inverter and seeing what happens.
    <snip>
    Or should I just go for the 2000w version.

    That question gets asked alot on this forum with respect to running motors, air conditioners, etc. Unfortunately no one knows, unless they have your exact same pump. Please follow up and let us all know what does or does not work. --vtMaps
    4 X 235watt Samsung, Midnite ePanel, Outback VFX3524 FM60 & mate, 4 Interstate L16, trimetric, Honda eu2000i
  • inetdog
    inetdog Solar Expert Posts: 3,123 ✭✭✭✭
    Re: Will a 12v system work for this sump pump?
    thentro wrote: »
    VFD's seem interesting but I don't understand them. This looks like a good one too but I don't understand the output. Is it 0-240v as required by the motor, or does it up convert from 120 to 240?

    Unfortunately for your needs, it not only ups the voltage from 120 to 240, it outputs 3-phase. It will only work for a 3-phase 240 volt motor. and will only deliver 2.2 Amps per phase.
    (VFDs are most often used with 3-phase motors because the really big motors tend to be 3-phase)
    SMA SB 3000, old BP panels.
  • thentro
    thentro Registered Users Posts: 9
    Re: Will a 12v system work for this sump pump?
    vtmaps wrote: »
    That question gets asked alot on this forum with respect to running motors, air conditioners, etc. Unfortunately no one knows, unless they have your exact same pump. Please follow up and let us all know what does or does not work. --vtMaps

    Followup! I bought the 1500w Samlex 24v and it was able to start the 1/2 HP, 115v 9.5 amp effluent pump with a 30 locked rotor amps load with out complaint or hesitation. That is theoretically 450 watts over the max load of the inverter (3000) but either the pump does not draw that much, or the inverter is able to handle the short spike. Thanks to everyone for their help.