Using automotive batteries for a short "boost"

bbbuddy
bbbuddy Solar Expert Posts: 135 ✭✭✭
in Off Grid Solar & Battery Systems #1
Hi, we plan to try starting and running our 230volt 1hp well pump once a day this spring off of our solar system, for 2 to 3 hours of irrigation, but had planned on doubling our battery capacity from 390 amps at 24 volts to 780 amps at 24 volts to take into account the need for about 41 amps of 230 volts for the first 300 milliseconds of pump start...

however, we really have no need for the extra 390 amps otherwise...so I've been thinking...oops, danger ahead!... that maybe we could save some $$ and just add 2 12 volt car batteries on an on/off Blue Sea switch, and just add their amps to the battery circuit for the 2 minutes or so it would take to walk over to the well pump switch, start the well pump, then walk back to the battery shed and turn off the switch.

The car batteries are MEANT to have a high but short amp demand, unlike the deep cell batteries, so wouldn't this be the perfect use for them? During the rest of the day they could be trickle charged from a regular car charger plugged into the 120 volt circuit...not from the Midnight 150 solar charger, thus not interfering in any way with the solar system deep cell battery charging profile...

What problems am I missing in this plan?

Thanks!
Magnum4024PAE, 2 Midnite Classic 150s, 3100watts solar, 432ah lifepo4 battery.  Off grid since 2004.
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Comments

  • Blackcherry04
    Blackcherry04 Solar Expert Posts: 2,490 ✭✭✭
    #2
    Re: Using automotive batteries for a short "boost"
    bbbuddy wrote: »
    Hi, we plan to try starting and running our 230volt 1hp well pump once a day this spring off of our solar system, for 2 to 3 hours of irrigation, but had planned on doubling our battery capacity from 390 amps at 24 volts to 780 amps at 24 volts to take into account the need for about 41 amps of 230 volts for the first 300 milliseconds of pump start...

    however, we really have no need for the extra 390 amps otherwise...so I've been thinking...oops, danger ahead!... that maybe we could save some $$ and just add 2 12 volt car batteries on an on/off Blue Sea switch, and just add their amps to the battery circuit for the 2 minutes or so it would take to walk over to the well pump switch, start the well pump, then walk back to the battery shed and turn off the switch.

    The car batteries are MEANT to have a high but short amp demand, unlike the deep cell batteries, so wouldn't this be the perfect use for them? During the rest of the day they could be trickle charged from a regular car charger plugged into the 120 volt circuit...not from the Midnight 150 solar charger, thus not interfering in any way with the solar system deep cell battery charging profile...

    What problems am I missing in this plan?

    Thanks!
    It will work, how about a couple GC-2 Golf cart batteries from Sams or Cosco. They are no more expensive than a automotive battery, but they are only 6 V so you'd need 4. You actually buy 3 , 8 volt Golf Cart batteries . Your combiner switch will work fine.
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  • bbbuddy
    bbbuddy Solar Expert Posts: 135 ✭✭✭
    #3
    Re: Using automotive batteries for a short "boost"

    Yeah, I thought about those, but the car/truck batteries are MADE for giving the high amps for a short time, and seems it would be easier to recharge separately.
    Magnum4024PAE, 2 Midnite Classic 150s, 3100watts solar, 432ah lifepo4 battery.  Off grid since 2004.
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  • mike95490
    mike95490 Solar Expert Posts: 9,583 ✭✭✭✭✭
    #4
    Re: Using automotive batteries for a short "boost"

    Deep cycle and car batteries have different recharge voltage profiles. Don't mix them.

    And don't worry about a decent battery bank making the surge current, it's your cables, and connections that will generally not meet the spec to supply enough surge without a voltage sag. And your inverter has to be able to supply the starting surge, if it can't - well, you need a bigger inverter.

    What is your solar array ? It will also be helping power the inverter too.
    Powerfab top of pole PV mount | Listeroid 6/1 w/st5 gen head | XW6048 inverter/chgr | Iota 48V/15A charger | Morningstar 60A MPPT | 48V, 800A NiFe Battery (in series)| 15, Evergreen 205w "12V" PV array on pole | Midnight ePanel | Grundfos 10 SO5-9 with 3 wire Franklin Electric motor (1/2hp 240V 1ph ) on a timer for 3 hr noontime run - Runs off PV ||
    || Midnight Classic 200 | 10, Evergreen 200w in a 160VOC array ||
    || VEC1093 12V Charger | Maha C401 aa/aaa Charger | SureSine | Sunsaver MPPT 15A

    solar: http://tinyurl.com/LMR-Solar
    gen: http://tinyurl.com/LMR-Lister ,

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  • BB.
    BB. Super Moderators, Administrators Posts: 33,613 admin
    #5
    Re: Using automotive batteries for a short "boost"

    A good rough estimate for reliable surge current/power from a standard flooded cell battery would be a maximum discharge rate of C/2.5 (or 40% of AH rating of bank):
    • 390 AH @ 24 volts * 1/2.5 rate of discharge * 0.85 eff inverter = 3,182 peak watts

    That should be enough to start a 1 HP pump. You can look up VFDs (Variable Frequency Drive) for reducing surge current and adjusting the pump RPM for optimum pumping. Of course there is the issue of having the right pump motor for the VFD to drive (3 phase, or single phase with an external starting capacitor).
    BB. wrote: »
    Some discussions about VFD (Variable Frequency Drives)... Basically a variable frequency inverter with (typically) three phase output. Used to soft start motors (handy for 3 phase well pumps, or pumps with well head starting capacitor) and can also turn an AC motor into a variable speed motor (very handy for pumping applications).

    WELL PUMP and Inverter QUESTION
    Wind/solar for large scale pumping etc (out of my depth!)
    could use knowledge - using Gould jet pump - transfering from 230vAC to ? DC (new link/thread 10/27/2012)
    Help required to design off grid system (information on possibilities to connect "standard VFDs direct to solar panels) (new link 1/13/2013)

    -Bill

    Another big issue is simply pumping for 2-3 hours per day. The volume of water and pressure you will be pumping all adds to the amount of power/energy you will need to pump.

    Say your pumping system takes 1,200 watts and you want to pump three hours per day. The amount of energy needed would be:
    • 1,200 watts * 3 hours = 3,600 kWH per day

    Battery size (note, 1-3 days of backup/no sun storage with 50% maximum discharge--pick two days for now):
    • 1,200 watts * 1/0.85 inverter eff * 1/24 volt battery bank * 2 days of backup power * 50% maximum discharge = 235 AH @ 48 volts

    Assume that you get a minimum of 4 hours of sun per day in "pumping season":
    • 3,600 WH per day * 1/0.52 system efficiency * 1/4 hours of sun per day = 1,731 watt array minimum

    Now--If you can use a solar power direct pump (such as Grundfos, Lorentz, or others)--You can skip the whole Battery Bank, Charge Controller, AC Inverter costs and losses... You will pay more for the pumps (a near 1 HP Grundfos submersible pump is probably $2,000+ each). Or, read the VFD threads above and see if any of that can help you (VFDs are basically variable frequency inverters that run from 0 to 400 Hz--Soft start the pump, vary pump RPM based on pressure flow/speeds, etc.).

    Conservation and knowing your loads will really help you here. Note that measuring surge current actually requires special meters--And how specific AC inverters support surge current vs type/brand/etc. of pump loads is not always clear either.

    Asking questions about specific hardware you want to use, and getting good support from your Solar RE supplier (such as the host of this forum) can give you some good pointers too. Experience counts.

    Other than "Windsun" (our admin), everyone else here (including us moderators) are volunteers--mostly end users, but some solar/RE professionals too.

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
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  • bbbuddy
    bbbuddy Solar Expert Posts: 135 ✭✭✭
    #6
    Re: Using automotive batteries for a short "boost"
    mike95490 wrote: »
    Deep cycle and car batteries have different recharge voltage profiles. Don't mix them.

    And don't worry about a decent battery bank making the surge current, it's your cables, and connections that will generally not meet the spec to supply enough surge without a voltage sag. And your inverter has to be able to supply the starting surge, if it can't - well, you need a bigger inverter.

    What is your solar array ? It will also be helping power the inverter too.

    Right, that's the reason to have the car batteries on a switch, to just add them to the bank ONLY to get the pump started....then just trickle charge them the rest of the time using a car charger. Cables are all 4/0. Array is 2200 watts. Once the pump starts, it will run pretty much off the array.
    Magnum4024PAE, 2 Midnite Classic 150s, 3100watts solar, 432ah lifepo4 battery.  Off grid since 2004.
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  • bbbuddy
    bbbuddy Solar Expert Posts: 135 ✭✭✭
    #7
    Re: Using automotive batteries for a short "boost"
    BB. wrote: »
    A good rough estimate for reliable surge current/power from a standard flooded cell battery would be a maximum discharge rate of C/2.5 (or 40% of AH rating of bank):
    • 390 AH @ 24 volts * 1/2.5 rate of discharge * 0.85 eff inverter = 3,182 peak watts


    That should be enough to start a 1 HP pump. You can look up VFDs (Variable Frequency Drive) for reducing surge current and adjusting the pump RPM for optimum pumping. Of course there is the issue of having the right pump motor for the VFD to drive (3 phase, or single phase with an external starting capacitor).



    Another big issue is simply pumping for 2-3 hours per day. The volume of water and pressure you will be pumping all adds to the amount of power/energy you will need to pump.

    Say your pumping system takes 1,200 watts and you want to pump three hours per day. The amount of energy needed would be:
    • 1,200 watts * 3 hours = 3,600 kWH per day

    Battery size (note, 1-3 days of backup/no sun storage with 50% maximum discharge--pick two days for now):
    • 1,200 watts * 1/0.85 inverter eff * 1/24 volt battery bank * 2 days of backup power * 50% maximum discharge = 235 AH @ 48 volts

    Assume that you get a minimum of 4 hours of sun per day in "pumping season":
    • 3,600 WH per day * 1/0.52 system efficiency * 1/4 hours of sun per day = 1,731 watt array minimum

    Now--If you can use a solar power direct pump (such as Grundfos, Lorentz, or others)--You can skip the whole Battery Bank, Charge Controller, AC Inverter costs and losses... You will pay more for the pumps (a near 1 HP Grundfos submersible pump is probably $2,000+ each). Or, read the VFD threads above and see if any of that can help you (VFDs are basically variable frequency inverters that run from 0 to 400 Hz--Soft start the pump, vary pump RPM based on pressure flow/speeds, etc.).

    Conservation and knowing your loads will really help you here. Note that measuring surge current actually requires special meters--And how specific AC inverters support surge current vs type/brand/etc. of pump loads is not always clear either.

    Asking questions about specific hardware you want to use, and getting good support from your Solar RE supplier (such as the host of this forum) can give you some good pointers too. Experience counts.

    Other than "Windsun" (our admin), everyone else here (including us moderators) are volunteers--mostly end users, but some solar/RE professionals too.

    -Bill

    Yes, I was taking into account the 2.5 maximum discharge for the inrush current....which because the pump has a locked rotor amp profile of 41 amps at 230 volts means I need 9430 watts the first 300 milliseconds....rather than provide this from more otherwise un-needed deep cycle batteries, I "rethought" it and figured the car batteries would do fine, but thought I'd double check here first.

    The pump uses 2,000 watts running. We have checked this with our multimeter. So once started it would run pretty much off the solar. The house batteries would be close to full when the pump is started, and would have at least 4 hours to finish absorb and get to float after the pump is turned off by about 1 pm.

    Our array here provides over 10kw a day right now in the winter "if" I ask for it...meaning I have everything electric on all day without taking out any that is needed to fully charge the battery bank...so I know in the summer I will have at least an extra 6kw for my well pump, as our daily use including fully recharging the batteries is about 4500 watt hours.

    On our previous 800 watts I got as much as 5.1KW in the summer. This would be the equivalent of 14 KW with the current array. Way more than I need for the pump, recharging the batteries, and our daily use.

    The well pump is non-negotiable as it provides us with 40GPM which should be enough to irrigate close to 2 acres...and that's my goal, to be able to irrigate as much as possible just from the solar.
    Magnum4024PAE, 2 Midnite Classic 150s, 3100watts solar, 432ah lifepo4 battery.  Off grid since 2004.
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  • BB.
    BB. Super Moderators, Administrators Posts: 33,613 admin
    #8
    Re: Using automotive batteries for a short "boost"

    Actually, you pump probably does not use this many "Watts"... More than likely unless you have an expensive power meter, you are measuring "Amperes".

    We tend to think of power as:

    Power = Volts * Amps

    When, in reality, especially for AC power, (one of the equations) is:

    Power = Volts * Amps * Power Factor = Volts * Amps * Cosine (Phase Angle between Voltage and Current)

    For your pump, it is very easy for it to operate at around 0.67 Power Factor, which would mean:

    Power = Watts = VA * PF = 2,000 VA * 0.67 = 1,340 Watts

    And full locked rotor power, PF could very easily be less than 0.50 PF (just guessing, never have measured)....

    So--The AC wiring and Inverter Output tend to have peak ratings where Watts Max equals VA max.

    However, from the DC Side of inverter input and battery output, that ends more towards Watts (or VA * PF).

    You cannot measure Watts with just a current meter, or even a current meter + a volt meter. You need one device that can measure Amps+Voltage at exactly the same time and measure the phase angle between the two (and the RMS area under the curves).

    Got to go now--But it is probably not as bad as you think. Also, if you can use a VFD (or specialized well pumps/motors), you can reduce surge to near full run load, and PF to near 0.95 or better. In some ways, these devices have "near magical" performance improvements (as well as near magical prices too :cry:). VFD's themselves are not very expensive--If you can find one that fits your needs.

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
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  • mike95490
    mike95490 Solar Expert Posts: 9,583 ✭✭✭✭✭
    #9
    Re: Using automotive batteries for a short "boost"

    regarding your power used, my XW inverter, claims my 1/2 hp pump, consumes 1,000 watts. I'm not sure how the metering is configured, if power factor is applied, or what. But it stands that your 1hp will consume 2,000 watts.

    Can your inverter supply the locked rotor surge ? Also, larger inverters have a big capacitor in them to help unload some of the 120 Hz ripple from the batteries, that cap can be your friend for a slight starting boost.


    Grundfos 10 SO5-9 with 3 wire Franklin motor (1/2hp 240V 1ph # 214505 ) on a timer for 3 hr noontime run - Runs off PV
    Powerfab top of pole PV mount | Listeroid 6/1 w/st5 gen head | XW6048 inverter/chgr | Iota 48V/15A charger | Morningstar 60A MPPT | 48V, 800A NiFe Battery (in series)| 15, Evergreen 205w "12V" PV array on pole | Midnight ePanel | Grundfos 10 SO5-9 with 3 wire Franklin Electric motor (1/2hp 240V 1ph ) on a timer for 3 hr noontime run - Runs off PV ||
    || Midnight Classic 200 | 10, Evergreen 200w in a 160VOC array ||
    || VEC1093 12V Charger | Maha C401 aa/aaa Charger | SureSine | Sunsaver MPPT 15A

    solar: http://tinyurl.com/LMR-Solar
    gen: http://tinyurl.com/LMR-Lister ,

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