Adding A Generator to a PV System

I have an off-grid cottage equipped with 3 185W panels, 4 6V 420 ah batteries, a charge controller and an inverter. I often run a 3Kw genertor to pump water and run larger tools. Can I use it to help charge the batteries at the same time? Thanks !

Comments

  • CariboocootCariboocoot Banned Posts: 17,615 ✭✭
    Re: Adding A Generator to a PV System

    Welcome to the forum.

    You certainly can and should use that gen to charge the batteries. It's practically standard operating procedure off grid. :D

    If the inverter doesn't have a built-in charger you will need a stand alone battery charger. Without knowing the system Voltage it's hard to say what one, but I can tell you that three 185 Watt panels will have a tough time with four 420 Amp hour 6 Volt batteries. One example:

    24 Volt configuration (all in series) 420 Amp hours @ 24 Volts. 42 Amp peak charging rate * 24 Volts = 1008 Watts, which would be a 1300 Watt array once typical losses are factored in (with MPPT controller). The array size would be similar for a 12 Volt system, but you would have difficulty handling the peak current for 840 Amp hours @ 12 Volts (MidNite Classic to handle the output; other controllers will not manage so much current).

    If this is a 12 Volt system, consider re-evaluating the loads and disconnecting half the battery bank; you may get better over-all performance.

    As it is you could expect about 35 Amps @ 12 Volts with an MPPT controller, which would be pretty low for 840 Amp hours of battery (4% - below the recommended 5% minimum). With a PWM controller it would be even less (can't imagine they are 12 Volt panels @ 185 Watts though).

    More details about the system would allow us to make a better evaluation.
  • stephendvstephendv Solar Expert Posts: 1,571 ✭✭
    Re: Adding A Generator to a PV System
    Tombsl wrote: »
    I have an off-grid cottage equipped with 3 185W panels, 4 6V 420 ah batteries, a charge controller and an inverter. I often run a 3Kw genertor to pump water and run larger tools. Can I use it to help charge the batteries at the same time? Thanks !

    Yes, but you'll need an AC charger to do so. Some inverters have an integrated charger built in, if yours doesn't then you'll need an external charger.
  • BB.BB. Super Moderators, Administrators Posts: 32,612 admin
    Re: Adding A Generator to a PV System

    Welcome to the forum Tombsl!

    And yes, you can have multiple charging sources with a battery bank. However, there are a few things to watch out for.

    First, you should run each set of charging cables back directly to the battery bank/bus bar connections. If you run the AC charger to the output of the solar charge controller, then to the battery bank (daisy chain wiring), the electrical "noise" from the AC charger could confuse the solar charger.

    Next, while it is handy to use the battery charger clips to charge the battery bank--You really should hard wire them. The clips generally do not make good long term high current electrical connections. Also, you don't want to be connecting/disconnecting the clips on a high current battery bank (short circuits can be "exciting" to say the least with a large lead acid battery bank).

    And, if you permanently attach the charger, you should have a fuse/breaker at the battery bank to protect the charger's wiring (ideally, you should have a fuse/breaker on each positive wire that leaves the battery bank). Fuses/breakers are there to protect the wiring, not protect the loads (in general).

    Lastly, picking the size/brand/model of battery charger to connect with the generator.

    First, Amp rating. A typical lead acid battery needs around 5% to 13% rate of charge. You can charge upwards of 25% rate of charge--But you need to monitor the battery temperature to prevent over heating/thermal runaway (hot batteries use lower charging voltages--which can confuse a battery charger that does not use a remote battery temperature sensor to reduce charging voltage on a hot battery bank). The "over 13% caution" can be an issue if you charge with the AC charger and Solar Panels at the same time (and no loads on the battery bank). In general, the thermal issue is probably more of an issue if the batteries are "deeply" discharged (less than 75% state of charge to 20% state of charge) when the batteries are subject to hours of >>13% rate of charge. Above 80-90% SOC, the batteries will naturally limit current (if chargers are programmed to the correct voltage).

    Lastly, there is the loading on the generator. Ideally, you want the generator loaded to round 50% to 80% of rated load. If too high, your genset may have problems with overheating the alternator. If loads are too low, then you waste fuel (gasoline generators tends to use around 50% fuel flow for 0-50% of rated loads, inverter-generators tend to be a bit more fuel efficient down to the 25% rated load range). Diesels tend to have issues with "wet stack" and cylinder glazing if operated below ~40-60% of rated load (commercial installations will have resistor load banks to properly load diesel gensets).

    And, while we usually work with "Watts" to plan loads... With AC loads, you also have to work with Volt*Amps (VA). Watts are:

    Watts (power) = Volts * Amps * Power Factor

    "Good" AC loads have Power Factor >0.95 or so... "Bad" Power Factor loads can be in the 0.50 to 0.67 range pretty easily (CFL lighting, computer power supplies, AC induction motors, AC battery chargers). While we see generators and inverters generally rated in "Watts"--In reality, most are rated as Watts=VA maximum. So many loads you will connect can "pop the breaker" on your genset even though the watt rating is not exceeded.

    So, in the end, yes, you can put a nominal to relatively large AC battery charger on your genset to help recharge the battery bank during poor weather/heavy loads (or if you nave a relatively small solar array).

    Question about battery charger selection with EU2000 generator.

    Regarding the "optimum" generator to AC battery charger selection--This thread has lots of information (if you are interested in the details and the search)... If not, we don't have to go down that level of detail to answer your questions.

    The reasons we ask so many questions about your overall system and usage is that a "balanced" system will usually give you the best results for the minimum amount of money (and money spent on fuel usage).

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • TombslTombsl Registered Users Posts: 13 ✭✭
    Re: Adding A Generator to a PV System

    Thanks folks! I didn't expect such fast and thoughtful responses. All your suggestions were very helpful.

    If it would help, here is more info about my set up: three 7.84 A (Ipm) , 23.6 V (Vpm), 185 W panels; a Xantrex C35 controller set at 13.4V float and 14.5V bulk, and low voltage dissconnect set at 10.5V; 4 420 Ah deep cycle batteries set up in series/parallel to deliver 12 volts; and, a 2000 W pure sine wave inverter without an add-on charger source. We are now consuming about 2.5 KwH per day. In the summer the system works great, but in the mid to late fall it starts to have low voltage problems (we're up north and get an average of 4.5 sun-hours per day in the fall). That's why I'm looking for back up.

    Once again -- thanks for your help !
  • CariboocootCariboocoot Banned Posts: 17,615 ✭✭
    Re: Adding A Generator to a PV System

    Yes, you have a problem.

    840 Amp hours @ 12 Volts is not going to recharge properly from 23 Amps. That's a 2.7% peak charge rate.
    555 Watt array for 4 hours of good sun will realize about 1.1 kW hours AC per day. You are using more than 2X that.
    This is a classic case of chronic deficit charging. You are still going because you've got 840 Amp hours to work with and only use about 200 per day. Now look at that charge current again: 23 * 4 hours = 92 Amp hours. Slowly but surely you are draining the batteries. In Summer you've got more sun so more charging.

    Frankly you would be better off disconnecting half the batteries, even though it means greater DOD (about 50%) it will also improve the charging. But this system is way short on panels.

    Those are probably flooded cells, right? Do yourself a favour; get a hydrometer and check every cell's specific gravity ASAP.
  • TombslTombsl Registered Users Posts: 13 ✭✭
    Re: Adding A Generator to a PV System

    Thanks again for the input. Actually the system works pretty well. In the fall we tend to be there for a couple of days at a time, so it has 7 days to charge for three days of use, which balances the 2 to 1 usage/charging deficit if we have 4 batteries in operation. With a generator back up it should work out ok. We're only there a few weekends in the winter, and have a full propane parallel system for lights etc. Hydrometer readings are great so far.
  • inetdoginetdog Solar Expert Posts: 3,123 ✭✭✭✭
    Re: Adding A Generator to a PV System
    Tombsl wrote: »
    Thanks again for the input. Actually the system works pretty well. In the fall we tend to be there for a couple of days at a time, so it has 7 days to charge for three days of use, which balances the 2 to 1 usage/charging deficit if we have 4 batteries in operation. With a generator back up it should work out ok. We're only there a few weekends in the winter, and have a full propane parallel system for lights etc. Hydrometer readings are great so far.

    Glad to hear that! Please keep in mind that if the 3 days of use take the battery bank below 50% DOD (and to some extent below even 20% DOD) there can still be some sulphation damage during the time it takes to bring them back up to 80% or to full charge.

    If you don't do too many of these unbalanced use cycles during each year, you may not notice any problems before it is time to replace the batteries anyway. :-)
    SMA SB 3000, old BP panels.
  • westbranchwestbranch Solar Expert Posts: 5,183 ✭✭✭✭
    Re: Adding A Generator to a PV System

    I see that you have 4 batteries to use but it sounds like you only use 2 at a time???

    " 4 420 Ah deep cycle batteries set up in series/parallel to deliver 12 volts"


    " if we have 4 batteries in operation"


    did I read that correctly?

    PS I see your LVD is set to 10.5v, consider moving it up to 12.0v. At 10.5v your battery may not be able to recover...
     
    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
  • TombslTombsl Registered Users Posts: 13 ✭✭
    Re: Adding A Generator to a PV System

    Thanks again all! We do use all 4 at the same time -- two in series to generate 12 V paralled with two others in series.
  • TombslTombsl Registered Users Posts: 13 ✭✭
    Re: Adding A Generator to a PV System

    A question for Fizzycist. The inverter shuts down at 10.5 V to protect the batteries. Do you know how this compares to the 50 % DOD level? Take care.
  • CariboocootCariboocoot Banned Posts: 17,615 ✭✭
    Re: Adding A Generator to a PV System
    Tombsl wrote: »
    In the fall we tend to be there for a couple of days at a time, so it has 7 days to charge for three days of use,

    This is not really a good practice, even though it's how many systems were set up to be used. In those seven days with 'whatever' sun might occur the batteries could spend too much time under 75% SOC, leading to premature sulphation. The lower the SOC and the longer time spent there the greater the risk.

    I always make sure my batteries are fully charged before I leave the cabin for winter, even if it means running the gen all the time I'm there closing things up. It's much easier to keep a battery charged than to recharge it.
  • CariboocootCariboocoot Banned Posts: 17,615 ✭✭
    Re: Adding A Generator to a PV System
    Tombsl wrote: »
    A question for Fizzycist. The inverter shuts down at 10.5 V to protect the batteries. Do you know how this compares to the 50 % DOD level? Take care.

    A 12 Volt system is at 50% DOD with a resting Voltage of around 12 Volts. At 10.5 it is essentially dead and may not recover or not completely. Have a look at our host's battery FAQ's here: http://www.solar-electric.com/decybafaq1.html
  • TombslTombsl Registered Users Posts: 13 ✭✭
    Re: Adding A Generator to a PV System

    What a great article/FAQ! It should be required reading for anyone getting into the PV game. Thanks again to everyone -- I know what I need to do now. Take care, Tom
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