Voltage drop when using inverter

tanyamr
tanyamr Registered Users Posts: 1 ✭
in Off Grid Solar & Battery Systems #1
I have a 100ah agm battery for off grid camping. I purchased a 1000w inverter. I went bigger so I can use it when I upgrade to lithium in my van in the future. 

I plugged in a 500w milk frother to test it out and my battery went from 12.8 down to 11.1-11.5 when I turned it on and ran it for a few seconds. 
I turned it off worried I would kill my battery. After a few minutes it went back up to 12.7. 
My TV which is much lower, dropped it to 12.4 while using it but again bounced back up to 12.8 when I switched it off. 
Do these voltage drops hurt my battery or is it just the normal process of drawing the power from it?  Or is a 500w appliance too much for 100ah  agm battery?
i don’t want to run anything for long periods, just a few minutes here and there.

I should mention, the battery is in a battery box and the inverter is connected via a 175 anderson plug with 6 (I think) gauge wires…..they are quite thick. 
Thank you. 
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  • Photowhit
    Photowhit Solar Expert Posts: 6,006 ✭✭✭✭✭
    #2
    A few things...

    Lead acid batteries are rated for a discharge rate of 1/20th of their capacity per hour, so a 100 ah 12 volt battery has a capacity of roughly 100 x 12 = 1200 watts. So applying a load of 500 watts is trying to draw 42% of the capacity (per hour) at once! This has to do with something called the Peukert effect/formula. It's easy to see if your manufacturer has an effective capacity in it's manual.

    Here's Trojan's for their 100 ah AGM battery.


    When you try to draw 500 watts from your 100 ah battery, you are pulling 500 watts divided by 12 = @42 amps, 6 gauge is about right for a 42 amp load, but would guess your inverter would like 4 gauge for a 1000 watt load. There is still some voltage drop in the wire, if it's 6 gauge and 10 feet away, you will lose about 3.4%. Your invert should always be close to your battery bank;


    Link to a voltage drop calculator;

    https://www.calculator.net/voltage-drop-calculator.html

    There is less resistance with Lithium batteries, but 500 watts for a similar size battery is quite a load, a bit larger than my 6000btu window air conditions. Things that heat and cool tend to be your largest loads. Might look at your expected loads when designing your system.

    Voltage is always difficult way of determining battery capacity, A bit easier with lithium's very flat curve, here's info about charging and system voltage for Lead acid batteries;

    The voltage you are seeing is the system voltage and not the battery voltage. If you are connected to charging or a load it will effect the system voltage.

    During charging, there are basically 3 stages of charging, Bulk, Absorb, and Float.

    BULK;
    First thing when charging starts you will be in bulk, the voltage rises from what ever the system voltage was to a set point, around 14.5 volts. At that point the Charge controller stops the voltage from rising. Higher voltage can damage sealed batteries.

    ABSORB;
    Once the battery hits the preset point the charge controller keeps it at that point. Your batteries are roughly 80% full. Flooded batteries will start accepting less current at 80-85% full AGM/Sealed may go a little longer before accepting less current.

    On many controllers you can set this point, Some will have different presets for Flooded, and sealed batteries, or flooded, AGM, and sealed batteries. 

    The charge controller has a couple ways to know when to switch to float, Most inexpensive Charge controller are just timed for 1.5-2 hours. Some will also see less current flowing through the charge controller and shut it down when minimal current is flowing through the controller. On more expensive charge controller. You can set battery capacity to give the Controller a better idea of when to stop. you can also set a longer Absorb time. Or set 'end amps' a amount of amps flowing through the charge controller to stop Absorb and switch to the final stage.

    FLOAT;
    Once the Controller has determined the battery is fully charged it reduces the voltage to a point where very little current is flowing to the battery. This will prevent the battery from over charging and heating up.

    While in 'Float' the charge controller watch for voltage drop, which would indicate a load. If the voltage begins to drop the charge controller will allow as much current to flow from the panels/array to compensate and maintain the voltage. If the voltage can be maintained, the load will in essence be running directly off the array/solar. If the voltage drops below the preset float voltage, the controller may start a whole new cycle if it stays there for a period of time.

    The system voltage drop you see at night when the sun goes down is the charge controller moving into a resting mode with no energy to contribute to the system.

    The morning voltage may reflect a load present that is effecting the voltage level. With sealed batteries, you would want to disconnect the battery from the system and allow it to 'rest' for a while to get an accurate idea of it's SOC (State Of Charge) from the voltage
    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.
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