Question about solar vs grid charge current

adrenochrome
adrenochrome Registered Users Posts: 1
Hello,

Why is it OK to charge with current up to 25% of battery capacity when using an AC battery charger (such as an Iota), however when using a solar charge controller current should be about 10%?

This comment by BB in a different thread is what got me thinking about this:

"Regarding charging your battery bank, we usually recommend 5% to 13% or so for solar charging current... If you have an AC charger (utility or genset), you can go 10-20-25% or so rate of charge."

Please help me understand this. Thanks.
Dave

Comments

  • Cariboocoot
    Cariboocoot Banned Posts: 17,615 ✭✭✭
    Re: Question about solar vs grid charge current

    Welcome to the forum Dave.

    It's more a matter of economics.
    We're talking about peak charge current here, not a constant charge rate. 10% of the battery capacity works well with solar because it gives you enough power to recharge during a good day as well as compensating for most average loads. You could in fact run at 25% peak current, but that would mean buying 2.5 times as many panels and possibly a larger or second charge controller to handle that current potential. Most of the time that extra equipment would be doing nothing, making it a poor investment.

    Now consider charging from a generator. They are most efficient when loaded above 50% of their rating. As such they are most economical when Bulk charging at high currents. When you get into the lower current demand at the end of the charging cycle you're burning more gas per Watt hour than at the beginning. So a shorter time starting at a higher current works out better in financial terms.

    If the batteries can take 25% peak current you can charge them that way regardless of the charge source. It's just a matter of how much you want to spend up front.
  • BB.
    BB. Super Moderators, Administrators Posts: 33,433 admin
    Re: Question about solar vs grid charge current

    Also, there is one limit of ~13% (C/8) rate of charge for deep cycle batteries (i.e., 100 AH battery bank, 12.5 amps of charging current)... If you exceed this rate for charging a deeply cycled battery (i.e., many hours at >C/8 rate), the battery bank can start to overheat.

    That is a real issue for industrial chargers which may charge based on a Current Rate + Time with an "unlimited" amount of Utility Power to keep at that high rate.

    For solar, most chargers are based on Absorb voltage set points (like 14.75 volts for 2-6 hours)--Depending on the voltage and time, the batteries usually don't get as hot. The down side is a hot battery has reduced charging voltage (-5 mVolts per degree C per Cell). So a "hot battery bank" may look under charged to a "dumb" charge controller.

    Add a remote battery temperature sensor, the charge controller will dial back on the output voltage to help prevent the battery from overheating and boiling/catching fire.

    For those folks running generators (and feeding them expensive fuel), generally, they only run the generators long enough to get the battery bank to ~80-90% maximum state of charge... Below 80%, the battery is almost 100% efficient (at reasonable current levels) in charging. As you approach 100% state of charge the battery becomes near 0% efficient (perphaps 50% goes to heat and 50% goes to making hydrogen and oxygen).

    That is why even a 2.5-5% rate of charge used to equalize a battery bank needs to be carefully monitored for temperature. A full battery bank can easily be overheated at even those low of charging current.

    In the end, for most "typical" off grid folks that use most of their power in the evening, their battery banks are usually >90% SOC in the afternoons (basically >90% is "full"). And unless you have needs for more power, the 13% seems to be a reasonably "cost effective" upper bound.

    For generators, as Marc says, keeping them well loaded (>50%) and charging to ~80% or so SOC (at "full available current") for a couple of hours is fine. AC chargers are not "that expensive" and if you already have good sized AC generator, then 20-25% for OFF GRID charging is usually OK...

    Just monitor your charging to make sure that all is working well.

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