Solar battery generator math

gparkis
gparkis Registered Users Posts: 3
I am trying to build a battery generator with solar recharge and I need to work backwards since I have specific needs.
family member received a new heart pump better known as an LVAD, and his life is completely dependent on electricity to charge the batteries for the device. when he goes home it will be with 8 batteries providing 3 days of power for the device.

This is what I know:
Battery charger that comes with pump can simultaneously charge 4 batteries and it takes 4 hours to fully charge but they usually go in half depleted so half that time. family member always has the other two batteries attached to device. to charge all 6 batteries would at max take 7 hours. then there is no more charging for over two days since we have 3 days power so that gives us two days to use solar to recharge battery backup.
when charger is plugged into kill o meter and four batteries are charging it reads:
111w
119.3 v
1.400 a

back of charger reads
100 - 240v ~ 3a
T5A 250V

now I am trying to determine the smallest version of battery, panel, inverter etc to make this backup charger in case of emergency.

the benefit is this does not need to run all day when electricity runs out, just enough to charge all his batteries for device and then we have two days to recharge battery with solar.

please advise!!!!

Comments

  • BB.
    BB. Super Moderators, Administrators Posts: 33,431 admin
    edited September 2016 #2
    Re: Solar battery generator math

    One more reading... the kWH (kilo Watt Hours) per day. That 111 watts is the 'Rate' (like miles per hour). The Watts*Time would be the amount Watt*Hours of use per day (like miles per hour * hours driving = miles driven).

    Also, need to know (roughly) where the system will be installed for the amount of sunlight per day.

    Say, looking at 120 watts * 10 hours per day in a pretty sunny region where you get at least 3 hours of sun per day (being conservative here). Battery with 2 days of storage and 50% maximum discharge (for long life):
    • 1,200 Watt*Hours * 1/12 volt batter bank * 1/0.85 inverter eff * 2 days of storage * 1/0.50 max discharge = 470 AH @ 12 volt battery bank

    The amount of solar array to recharge the 1,200 WH usage would be:
    • 1,200 watt*hours * 1/0.52 system eff * 1/3 hours minimum sun per day = 769 Watt array minimum

    And, to properly recharge the battery bank with a 5% to 13% rate of charge:
    • 470 AH * 14.5 volts charging * 1/0.77 panel+controller derating * 0.05 rate of charge = 442 watt array minimum
    • 470 AH * 14.5 volts charging * 1/0.77 panel+controller derating * 0.10 rate of charge = 885 watt array nominal
    • 470 AH * 14.5 volts charging * 1/0.77 panel+controller derating * 0.13 rate of charge = 1,151 watt array "max cost effective"

    And you should have a backup AC genset and AC battery charger... The rate of charge for the genset should be between 5% to 13% (or 25% not to exceed). For a 10% rate of charge (as an example):
    • 470 AH * 0.10 rate of charge = 47 amps nominal

    The generator needed to recharge this would be around:
    • 470 AH * 14.5 volts charging * 0.10 rate of charge * 1/0.80 charger eff * 1/0.67 PF = 1,271 VA rated genset minimum

    So, looking at hardware. A MorningStar TSW 300 watt inverter (12 volt), an automatic AC Transfer switch, A 45 amp battery charger with IQ4, and a Honda eu2000i genset (will possible natural gas/propane conversion, depending on what makes sense to you).

    The above is just a back of the envelope calculation with good equipment and would allow the person to live "off grid" indefinitely.

    If the region has reliable power... Then could ditch the solar array (or get it later). And use the battery bank+AC mains+genset for several weeks of emergency backup power. Or even dump the battery and just get one or two Honda eu1000i/eu2000i gensets. The old survivalist rule of "three is two, two is one, and one is none) philosophy of backup. Generators (usually) store better than batteries for emergency backup. You can store 10-20 gallons of gasoline with fuel stabilizer for ~1 year--and recycle to car/repeat (what I do). You would use probably ~1/2 to 1 gallon of gasoline per day with a eu2000i genset for charging (plus can run some lights, TV, computer, etc.).

    The full system is probably way over designed. But gives you three levels of backup power (battery, solar, genset). And, obviously, no company (or web forum) would rate/guaranty this system for "life support" use. You are on a do it yourself type forum.

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • Photowhit
    Photowhit Solar Expert Posts: 6,002 ✭✭✭✭✭
    Re: Solar battery generator math

    Well it's not a huge load, and it should be a resistance load so no big peak at the begining. It does actually plug in?

    So I'd go with;

    2 golf cart batteries (Sams or Costco) $80 each- $160
    @ 200 watts of panels in 12 volt nominal - (290watts $350 shipped)
    A small PWM charge controller for 25-30 amps (Northern Arizona Wind and Sun - @$90 shipped)
    A small reliable 300 watt morning star inverter @$240 shipped (Norther Arizona Wind and Sun) Likely you could use a cheap modified sine wave inverter, but I've quoted a pure sine to be sure it won't burn up an expensive charger.
    I would do this with just a 25 amp fuse(or what ever Morning star recomends) between the battery and inverter.

    I did my estimates as a backup system, Bill did his as a primary system. I don't think either of us is wrong, just how we read your post.
    Home system 4000 watt (Evergreen) array standing, with 2 Midnite Classic Lites,  Midnite E-panel, Magnum MS4024, Prosine 1800(now backup) and Exeltech 1100(former backup...lol), 660 ah 24v Forklift battery(now 10 years old). Off grid for 20 years (if I include 8 months on a bicycle).
    - Assorted other systems, pieces and to many panels in the closet to not do more projects.
  • Cariboocoot
    Cariboocoot Banned Posts: 17,615 ✭✭✭
    Re: Solar battery generator math

    Welcome to the forum.

    My first advice is not to fool around; this is critical medical equipment. The best thing you can do is supply the recharger with the power it was meant to run on. Do not try to circumvent the unit and charge the batteries directly from solar. It could go wrong.

    What you're seeking here is a back-up for the recharger. The question is: how unreliable is the utility power? How often will it go out? Would the supplied batteries cover the outage period?

    Otherwise, you were wise and took K-A-W meter readings! You need to supply a bit more than 110 Watts, pure sine (don't even think about MSW inverter here), for at least four hours (to be on the safe side).

    We can simplify the math by using 120 Watts @ 12 Volts which gives 10 Amps over 4 hours = 40 Amp hours for the charger. It will take a bit more to run the inverter, so the minimum battery bank size would be 100 Amp hours.

    That alone may be enough without any solar as it could easily be recharged when the grid comes back or from a generator or for that matter you could run a small inverter off the car if necessary. Adding solar panels to the mix would probably be an unnecessary expense ($300 worth of expense).

    Was that any help?
  • gparkis
    gparkis Registered Users Posts: 3
    Re: Solar battery generator math

    this is definately not to circumvent the grid. this is meant to be an emergency backup. You are correct in that this is a backup for the recharger not the device itself! normally the medical charger is plugged into the wall. when all batteries are charged, the device can run for 3 days without any new power. i am building a battery generator, to recharge the medical batteries which when plugged into wall takes 7 hours to charge all 6 batteries and then we are good for another 3 days without any new charging.

    the longest we did not have power was 6 days after sandy. otherwise, the utility is legit. just want a secure backup that is battery and not a gas generator.

    the first post was definitely overkill for our needs, the last two are going to be the way to go.

    again, just to plug the medical battery charger into the battery generator to charge the medical batteries and then chil for another 3 days. I was gonna use solar to recharge the back up system while we are hanging for the three days. even though this time period of no power is kinda unheard of around us but ya never know after something like sandy
  • gparkis
    gparkis Registered Users Posts: 3
    Re: Solar battery generator math

    thanks a million!
    what does providing a larger say 600watt inverter do to this kinda system? my guess is drain the battery faster since it will not charge the medical batteries any faster.
    Photowhit wrote: »
    Well it's not a huge load, and it should be a resistance load so no big peak at the begining. It does actually plug in?

    So I'd go with;

    2 golf cart batteries (Sams or Costco) $80 each- $160
    @ 200 watts of panels in 12 volt nominal - (290watts $350 shipped)
    A small PWM charge controller for 25-30 amps (Northern Arizona Wind and Sun - @$90 shipped)
    A small reliable 300 watt morning star inverter @$240 shipped (Norther Arizona Wind and Sun) Likely you could use a cheap modified sine wave inverter, but I've quoted a pure sine to be sure it won't burn up an expensive charger.
    I would do this with just a 25 amp fuse(or what ever Morning star recomends) between the battery and inverter.

    I did my estimates as a backup system, Bill did his as a primary system. I don't think either of us is wrong, just how we read your post.
  • BB.
    BB. Super Moderators, Administrators Posts: 33,431 admin
    Re: Solar battery generator math
    gparkis wrote: »
    thanks a million!
    what does providing a larger say 600watt inverter do to this kinda system? my guess is drain the battery faster since it will not charge the medical batteries any faster.

    Yep... More wasted power (larger inverter, generally, uses more power just to turn on).

    The reason several of us suggest the MorningStar 300 watt inverter (it does do 600 watts for ten minutes) is, besides being a true sine wave inverter (they only one I would suggest for this usage--MSW can be much "harder" on some devices--many times, the smaller electronic power supplies and battery chargers) is that this MorningStar unit includes several options that you would have to pay many times more for...

    It includes a remote on/off switch (just a little 12 volt signal switch) to turn on/off the inverter. And second it includes a "search" mode option. You set search mode on, and the inverter only turns on for a fraction of a second every 1-2 seconds looking for a >6 watt AC load. If there is no load, the inverter stays in "sleep" and using very little standby power. Plug in the battery charger (or use an AC Auto Transfer Switch), and the inverter fires right up.

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • Photowhit
    Photowhit Solar Expert Posts: 6,002 ✭✭✭✭✭
    Re: Solar battery generator math

    Yep, just more wasted power, you could invest a bit more money and get a charge controller with a meter. The system links I have suggested should provide about 600watt hours of available power daily with a couple days of clouds built in, so you could safely use a bit more power a small fan maybe a couple hours of Cf light bulb at night, but if your not using it all the time, it would be hard to understand the information.

    I asked about the charger, the batteries do plug into it? the reason is there are these proximity charging systems now, which use a magnettic field, but I would guess in a medical field this would be frowned upon.

    Only disadvantage to the morning Star is it doesn't have an outlet, ExelTech and Samlex make some true sine wave inverters with outlets, I'd be sure to turn these off after you use them, though I think they both have some sort of search mode. Also I would stay above the rated wattage of the charger you have (200 watts?) any of the inverters could provide a bit of extra when not charging the batteries, just understand the primary use.

    If you remember back to this during the next outage, if the sun is shining you will draw little of nothing from the batteries if you charge while the sun is hitting the panels, providing for more 'extra' energy.

    Also note the batteries I originally suggest will require some maintenance, this might be a situation that is best handled with AGM batteries (sealed batteries) either way I would check the batteries capacity regularly, like running the battery charger off of them!
    Home system 4000 watt (Evergreen) array standing, with 2 Midnite Classic Lites,  Midnite E-panel, Magnum MS4024, Prosine 1800(now backup) and Exeltech 1100(former backup...lol), 660 ah 24v Forklift battery(now 10 years old). Off grid for 20 years (if I include 8 months on a bicycle).
    - Assorted other systems, pieces and to many panels in the closet to not do more projects.
  • westbranch
    westbranch Solar Expert Posts: 5,183 ✭✭✭✭
    Re: Solar battery generator math

    re the batteries: If it was me, being that it is a Life/Death situation, I would do a lot of reading on the various types of long 'idle period' low use batteries. Some types discussed here are AGM, Li(thium) types, not as used in the Boeing Dreamliner, NiCd, and some other less seen types. A type that is used for telecom standby would probably be the best for sitting on the shelf and on 'a maintenance charge'.

    HTH
     
    KID #51B  4s 140W to 24V 900Ah C&D AGM
    CL#29032 FW 2126/ 2073/ 2133 175A E-Panel WBjr, 3 x 4s 140W to 24V 900Ah C&D AGM 
    Cotek ST1500W 24V Inverter,OmniCharge 3024,
    2 x Cisco WRT54GL i/c DD-WRT Rtr & Bridge,
    Eu3/2/1000i Gens, 1680W & E-Panel/WBjr to come, CL #647 asleep
    West Chilcotin, BC, Canada
  • deltarose48
    deltarose48 Registered Users Posts: 1
    About the LVad Photowit wrote. I need to know what type of golf cart batteries, what voltage and what AMp hrs? And I don't know what the rest of it means, perhaps you can go into more detail for me as I also need this for myself. The only thing I do understand is the inverter, the rest is another language to me unfortunately. ....Deltarose48
  • vtmaps
    vtmaps Solar Expert Posts: 3,741 ✭✭✭✭
    About the LVad Photowit wrote. I need to know what type of golf cart batteries, what voltage and what AMp hrs?
    Welcome to the forum,

    Golf cart batteries are 6 volt batteries, typically with a capacity of 200-225 amphours.   Amphours is a measure of capacity... how many amps for how many hours.  if a battery can produce 10 amps for 20 hours, then it is a 200 amphour battery. 

    Amphours can be a deceptive measurement... it is not a measure of energy.  If you want to know how much energy a battery holds you must multiply the amphours times the voltage.  Thus, a 200 amphour, 6 volt battery holds 1200 watthours (or 1.2 kilowatthours) of energy.

    If you put 2 GC batteries in series, you have 12 volts with 200 amphours ( = 2.4 kwh).

    --vtMaps
    4 X 235watt Samsung, Midnite ePanel, Outback VFX3524 FM60 & mate, 4 Interstate L16, trimetric, Honda eu2000i