Voltage Drop

czars
czars Registered Users Posts: 16 ✭
in General Solar Power Topics #1
Is there an industry accepted value for the maximum acceptable percentage voltage drop between a combiner and a charge controller. I am assuming, of course, that the voltage at the charge controller input is above the minimum charge controller value.
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  • BB.
    BB. Super Moderators, Administrators Posts: 33,613 admin
    #2
    Re: Voltage Drop

    Normally, we aim for ~1-3% wiring (voltage) drop. More than 3% loss, and you can start having issues (especially with PWM controllers getting enough voltage/current to properly charge a battery bank on a hot day).

    Less than 1% drop is just a lot of money to drop on copper wiring.

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
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  • BB.
    BB. Super Moderators, Administrators Posts: 33,613 admin
    #3
    Re: Voltage Drop

    I should add--the 1-3% drop is for circuit where we "don't care" about the exact voltage at the end of the run (solar panels to charge controller as an example).

    For "critical voltage" run, such as from charge controller to battery bank, you need very low drop for the controller to accuratly measure the battery voltage.

    Too much drop, and the battery will charge slowly, and possibly, not fully.

    12 volt batteries, roughly, run 0.1 volts for every 10% change in resting charging.

    When charging batteries, the rough voltage difference between "float" and "vigorous charging" with AGM batteries is a range from 13.8 to 14.2 volts (0.4 spread).

    Charging lead acid batteries (at room temperature) should be around 14.5 (Trojan is ~14.85 volts).

    So--you can see that these voltage drops are critical for happy batteries and happy end users.

    My suggestion, for 12 volt bank, no more than 0.1 volt drop from charge controller to battery bank, and I would suggest that you aim for 0.05 volt maximum drop.

    There are a few controllers that support Remote Battery Voltage Sense Leads... MorningStar TS series (both PWM and MPPT I believe) have remote voltage sense... A very nice thing to connect to your battery bank.

    Charge Controllers could be programmed to account for actual resistance on the Charger to Battery bank connection--but I have not seen anyone do that yet... Would be a nice option to have (but charge controllers can be very confusing to setup anyway--adding another configuration entry for battery wiring resistance may be too much to ask).

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
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  • niel
    niel Solar Expert Posts: 10,300 ✭✭✭✭
    #4
    Re: Voltage Drop

    voltage drop unfortunately doesn't just occur between the combiner and cc as it is additive from source to load. the nec specifies 5% being acceptable, but our wattage is more precious and costly so we opt for 2% or 3% with many leaning for 2% from the source to the load. source to load can be from the pvs to the batteries, but it probably should be taken farther to the dc items being powered even if it is an inverter. i guess it all depends on what you determine to be a load and most take it to the batteries only, but keeping losses down allows the solar power to do more work for you and safer too.

    read up on v drops with the link in my signature line.
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  • czars
    czars Registered Users Posts: 16 ✭
    #5
    Re: Voltage Drop

    Thanks for the info. You've been most helpful!
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