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Thread: Off-Grid Inverter Battery Charging

  1. #1

    Default Off-Grid Inverter Battery Charging

    I'm trying to understand how batteries are charged. Could someone explain how batteries are charged when using an inverter with charger?

    Can the batteries still accept a charge while the inverter is supplying power? What I means is, if the inverter is supplying 120v output from batteries and you have a generator connected to the inverter, is the generator powering the charger system to charge the batteries?

    I'm sure every inverter may have different capabilities, but in general is this possible?

    Where I'm missing it is, there are only two wires connecting the batteries to the inverter. Can the inverter draw power from the batteries and at the same time provide power to charge them.

    I've read here that some use portable generators to do supplemental charging. Just wondering how it works.

    Thanks.

  2. #2
    Join Date
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    SF Bay Area (California)
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    Default Re: Off-Grid Inverter Battery Charging

    Basically, the standard inverter/charger operates either as an inverter (battery->AC power), or as a battery charger (AC->battery + AC-> loads)... More or less, like a standard UPS (uninterruptable power supply) for a computer.

    There is a type that can be a bit more "nuanced" in operation... For example the Xantrex/Schneider XW Hybrid Inverter can operate in "generator support mode".

    Basically, as above, plus if the Generator is supplying AC power--The load can draw more power than the generator can supply buy drawing power from the battery bank and inverting it to AC which adds to the generator output (you program the XW to draw a maximum of 15 amps of AC from the generator, and if the load draws 20 amps, the inverter will supply the extra 5 amps of 120/240 VAC).

    Also, some inverter/chargers can "share" the generator output... Say you program for 10 amps @ 120 VAC maximum from the generator. The battery bank is discharged. So, the inverter/charger is drawing 10 amps from the generator to recharge the battery bank. If you now turn on a 5 amp AC load, the inverter charger will cut back on its draw (back to 5 amps) and allow the load to draw the balance of the 5 amps from the generator (at the expensive of charging the battery bank slower).

    When reading the manuals, you also have to be clear about "which amps" they are typing about... Sometimes they will mix 20 amp 120 VAC current limit and 40 amp DC battery current limit (which limits the AC input current to the charger via the VAC/Vbatt voltage + conversion losses + PF issues--confused yet?)--Which adds to the confusion (

    Clear as mud?

    -Bill
    20x BP 4175B panels (replacement) + Xantrex GT 3.3 inverter for 3kW Grid Tied system + Honda eu2000i Inverter/Generator for emergency backup.

  3. #3

    Default Re: Off-Grid Inverter Battery Charging

    Quote Originally Posted by Toby View Post
    I'm trying to understand how batteries are charged. Could someone explain how batteries are charged when using an inverter with charger?

    Can the batteries still accept a charge while the inverter is supplying power? What I means is, if the inverter is supplying 120v output from batteries and you have a generator connected to the inverter, is the generator powering the charger system to charge the batteries?

    I'm sure every inverter may have different capabilities, but in general is this possible?

    Where I'm missing it is, there are only two wires connecting the batteries to the inverter. Can the inverter draw power from the batteries and at the same time provide power to charge them.

    I've read here that some use portable generators to do supplemental charging. Just wondering how it works.

    Thanks.
    When an inverter/charger detects power at its AC IN it switches the loads to the AC source and changes to charging mode. It does not supply power from the batteries in that mode. Never mind the more complex units with load sharing that Bill mentioned; the effect is the same.

    Most off-gridders have at least one generator for providing back-up charging when the sun refuses to shine.
    1220 Watts of PV, OB MX60, 232 Amp hrs, OB 3524, Honda eu2000.

    Ohm's Law: Amps = Volts / Ohms
    Power Formula: Watts = Volts * Amps

  4. #4

    Default Re: Off-Grid Inverter Battery Charging

    Quote Originally Posted by Toby View Post
    Can the batteries still accept a charge while the inverter is supplying power?
    Some can, some can't. In bidirectional inverters like the Outback GTFX or the old Trace SW series, it's continuous, and the inverter will either subtract or add power to the AC line to keep the generator under its maximum current limit. (I have an SW4024 that I use in this mode for blackouts.) Most inverter/chargers will limit input power to prevent overcurrent faults to the generator.

    What I means is, if the inverter is supplying 120v output from batteries and you have a generator connected to the inverter, is the generator powering the charger system to charge the batteries?
    In many cases yes.

    Where I'm missing it is, there are only two wires connecting the batteries to the inverter. Can the inverter draw power from the batteries and at the same time provide power to charge them.
    In many cases yes. (Not at the same time - but it can switch back and forth very quickly.)
    Last edited by bill von novak; August 3rd, 2012 at 12:12 PDT.

  5. #5

    Default Re: Off-Grid Inverter Battery Charging

    Still a bit confused.

    Let's say the inverter is a 4000 watt unit. Shouldn't the generator be at least the same watt output (or greater) as the inverter if the inverter switches to generator if it sees the input?

    I noticed that both of you have Honda eu2000's but if I read the inverter models correctly, output over 3000 watt.

    How does powering the inverter's charger affect wattage available for use? Generator input is 2000 watts or 16 amps. Wouldn't the inverter's battery charger use some of that power thereby limiting what's available to the house loads? As an example, generator output is 16 amps. Inverter charger uses 5 amps. That leaves only 10 amps (1200 watts) for house loads.

  6. #6

    Default Re: Off-Grid Inverter Battery Charging

    That would be a miracle of physics.

    Current either flows from the inverter to the batteries or from the batteries to the inverter. It can not do both at the same time over one set of wires. I've yet to see any inverter with separate wires for charging and discharging the batteries.

    Even with those inverters that can do load sharing/shedding they are either drawing from the batteries or sending to them. This may change form moment to moment depending on the load draws, but it is still a one-way street. While charging no power is being drawn from the batteries. If power is being drawn from the batteries no charging is occurring.

    Note that you can have a separate stand-alone charger for the batteries (as many do with inverters that have no built-in charger) but that is not what the OP was asking about.
    1220 Watts of PV, OB MX60, 232 Amp hrs, OB 3524, Honda eu2000.

    Ohm's Law: Amps = Volts / Ohms
    Power Formula: Watts = Volts * Amps

  7. #7

    Default Re: Off-Grid Inverter Battery Charging

    Quote Originally Posted by Toby View Post
    Still a bit confused.

    Let's say the inverter is a 4000 watt unit. Shouldn't the generator be at least the same watt output (or greater) as the inverter if the inverter switches to generator if it sees the input?

    I noticed that both of you have Honda eu2000's but if I read the inverter models correctly, output over 3000 watt.

    How does powering the inverter's charger affect wattage available for use? Generator input is 2000 watts or 16 amps. Wouldn't the inverter's battery charger use some of that power thereby limiting what's available to the house loads? As an example, generator output is 16 amps. Inverter charger uses 5 amps. That leaves only 10 amps (1200 watts) for house loads.

    Not necessarily. An inverter's rating is based on its maximum continuous output; in this case 4kW. That does not mean it needs 4kW from the generator to function. I use a 1600 Watt Honda EU2000i for my system and the inverter is 3.5kW. You do need enough generator capacity to allow the inverter to charge at a reasonable rate and handle any loads that may be running at the same time. If the total load exceeds the gen's capacity, the inverter will "drop" the generator (either switching off the AC IN or going to load sharing if so equipped) and return to providing power from the batteries. At that point charging halts until the situation is rectified by a drop in load demand.

    As a matter of fact, my Outback's charger is set to 5 Amps AC limit and the Honda can produce up to 12 (high elevation loss). The overhead is more than enough for the regular loads. If I start the water pump the inverter will drop the gen momentarily, then resume charging once the heavy start-up surge has passed and the input detection recycles.
    1220 Watts of PV, OB MX60, 232 Amp hrs, OB 3524, Honda eu2000.

    Ohm's Law: Amps = Volts / Ohms
    Power Formula: Watts = Volts * Amps

  8. #8

    Default Re: Off-Grid Inverter Battery Charging

    I'm starting to get it.

    I think it was mentioned, but to me, wouldn't it be better to have a stand-along charger that would be powered off the generator? As long as the generator meets the amp needs of the charger, the charger would work at full capacity vs limited by the house load demands.

  9. #9
    Join Date
    Mar 2006
    Location
    SF Bay Area (California)
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    Default Re: Off-Grid Inverter Battery Charging

    In theory, Yes. Separate components, one failure does not take out multiple functions.

    In practice, may cost more and inverter/chargers can be more efficient and sophisticated than available standalone chargers.

    -Bill
    20x BP 4175B panels (replacement) + Xantrex GT 3.3 inverter for 3kW Grid Tied system + Honda eu2000i Inverter/Generator for emergency backup.

  10. #10

    Default Re: Off-Grid Inverter Battery Charging

    Quote Originally Posted by Toby View Post
    I think it was mentioned, but to me, wouldn't it be better to have a stand-along charger that would be powered off the generator? As long as the generator meets the amp needs of the charger, the charger would work at full capacity vs limited by the house load demands.
    Well, then you are converting 120VAC -> 24VDC -> 120VAC for your loads which isn't that efficient and puts more stress on the inverter. Most inverters are designed to be pretty smart about charging with loads attached.

    Let's take two cases that I am familiar with:

    Prosine 2.0. 2kW inverter, 1kW generator. AC breaker value set to 8 amps.

    Load 0W -> 1kW to battery, 0kW to load
    Load 500W -> 500W to battery, 500W to load
    Load 1000W -> 0W to battery, 1kW to load
    Load 1500W -> 0W to battery, 1.5kW to load (potentially) - generator overloaded

    In all cases the generator sees close to max load.


    Trace SW4024. 4kW inverter, 2kW generator. AC breaker value set to 16 amps.

    Load 0W -> 2kW to battery, 0W to load
    Load 1000W -> 1kW to battery, 1kW to load
    Load 2000W -> 0W to battery, 2kW to load
    Load 4000W -> 2kW FROM battery, 4kW to load
    Load 6000W -> 4kW FROM battery, 6kW to load
    Load 8000W -> 6kW FROM battery, 8kW to load - inverter overloaded

    Again in all cases the generator sees close to max load.

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