Charging batteries with generator/alternator

Hi all

New to the forum.

There is 2 things I would like to ask.

1. If I was to attach my generator to my 12v deep cycle batteries would this cores any damage to my charge controller or solar panels?

2. I want to use a alternator to charge my batteries. Any advice, do's and dont's would be greatly appreciated.

Thanks in advance..


  • TrukinbearTrukinbear Registered Users Posts: 76 ✭✭
    What do you mean to attach your generator to a 12V deep cycle battery? Your battery is DC (and 12V) and your generator is (probably) AC and 120/240V - you'll need a charger.

    Automotive alternator won't likely bring your battery above 80%, they are better at making DC to be consumed than actually charging a battery. I guess it depends on what you want to do - just put some emergency power in a low battery bank, or bring it to a full charge - almost any automotive alternator will do the former, but you'll need a special unit to do the latter or use the alternator as a PMA and run the output through a charge controller. 
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  • BB.BB. Super Moderators, Administrators Posts: 31,452 admin
    As Trukinbear says--Details matter in solar power.

    The short answers are that you can have multiple charging sources for a single battery bank. As long as the solar charge controller and AC battery charger(s) are set to the correct charging voltage--They will work fine together.

    For flooded cell batteries... More or less:
    • 5% rate of charge (100 AH battery * 0.05 = 5 amps)--Can work for weekend/emergency use
    • 10% rate of charge--Good for full time off grid
    • 13% rate of charge--For solar, usually the "cost effective" maximum rate of charge for a typical system. (recommend remote battery temperature sensor--Hot batteries need charging voltage reduced)
    • 20-25% rate of charge--Maximum suggested (recommend remote battery temperature sensor)
    • 10%-20% rate of charge--Good range for genset charging
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • BB.BB. Super Moderators, Administrators Posts: 31,452 admin
    FLA batteries tend to take "whatever" current the charging source "offers" below ~80% state of charge. Over ~80-90% state of charge, the battery bank naturally limits charging current (assuming you have the correct charging voltage).

    Normally, with a genset, you would charge in the mornings (heavy discharge the day before, no sun in forecast for "today", etc.)... Start charging at 50% state of charge and discontinue genset charging around 80% SoC--That is the most efficient. Let the sun recharge your bank from ~80% to >90% state of charge (save fuel/wear & tear/noise from genset usage).

    Generally, you only need to go >90% SoC once a week if you are daily cycling... And typically Equalize Charge once a month.

    The above numbers are very approximate--Just some rules of thumbs to start your process. You generally want to let solar do all it can--And only run the genset if battery goes below ~50% SoC or you have other needs/issues. Running your AC loads when charging the battery bank (vacuuming, running a deep well pump, washing machine, etc.), can also be a good way to help a small battery system (run genset for heavy loads/low sun).

    If you have two charging sources (or more), you really want to avoid >~20-25%+ charging current--That can heat your batteries and cause excessive gassing (especially if charging voltage is set too high). FLA batteries are more efficient when charging below ~80% SoC (less heat, less gassing)--So that is a good time for genset usage (generators are typically most efficient between 50% and 80% electrical loading).

    Using an alternator--Can be OK... They tend to have high current when cold--But as they heat up (in the next 10 minutes or so), their output current capabilities are reduced.

    Standard car alternators have a set point around 14.2 volts or so... For charging deep cycle flooded cell batteries, they typically are charged around 14.75 VDC. So standard alternators (alternator+gas motor, alternator on an RV) may not charge as fast or as high of voltage as your solar charger would.

    There are marine alternators and voltage controllers that have adjustable charging voltages/better specifications--But not cheap.

    There are lots of different ways to charge your battery system... And they should all be based on your daily energy needs and battery bank AH and Voltage... That includes the genset and AC/DC battery chargers.

    Most batteries in the end fail because of undercharging... Using a hydrometer and separate voltmeter (or AC/DC Current Clamp DMM) will help you monitor and understand what is happening to your system.

    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
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