charging batteries without pv

alcatel99alcatel99 Registered Users Posts: 31 ✭✭
after 5 1/2 years with my first 12 volt system i have just had 4 days without enough insolation to fully charge my batteries which are 4 crown 6 volt 220 amp hrs  i am down to 12.4 volts  i am unsure if i need to disconnect my charge controller so i can charge using a generator or an auto battery charger i can shut the panels down with disconnect s and also switch the power off to the inverter but what if any thing do i need to do with the charge controller tristar 60 amp mppt apologies if this sounds like a stupid question butmy system that i purchased from NAWS has run perfectlu and i have not had to charge the batteries since installation.
thanks for any help


  • BB.BB. Super Moderators, Administrators Posts: 32,027 admin
    edited November 2020 #2
    You have a 440 AH @ 12 volt battery bank... The typical battery charging current would be:

    440 AH * 5% rate of charge = 22 Amps minimum suggested (weekend cabin, emergency use)
    440 AH * 10% rate of charge = 44 Amps nominal (recommended minimum by many battery mfgs for FLA type batteries)
    440 AH * 13% rate of charge = 57.2 Amps typical suggested max current (can do more, but watch battery temperatures)

    Given that your system has been operating from a well designed solar power system... You really don't need a minimum rate of charge for a few days (hopefully) of bad weather.

    More or less, Lead Acid batteries do not like sitting below 75% state of charge for many hours/day/weeks+... The batteries will sulfate (faster). So long as your bank is cycling and above 50% state of charge--You are OK for the short term (ideally, get the bank >90% SoC within a week, at least once--And all should be fine.

    You do not need to disconnect your solar charger--Multiple chargers (provided they are the correct voltage setpoint for charging--Such as 14.75 volts or a bit higher), just fire up the genset and charger. Ideally, you want to have each charger wired to the battery bus (star or home run wiring). You do not really want daisy chain wiring... Battery bus to charger A to charger B (not terrible, but less than optimum in some cases--Noise/cross talk between chargers, higher voltage drop in wiring slows down charging, etc.).

    The issue with some automotive chargers... They can have very high voltage setpoints >>15.0 volts (or even unregulated). Because they are cheaply made and/or they are setup to quickly recharge the car battery and get it out of the shop ASAP--Battery life--Nobody cares too much...

    So, if you have an automotive charger, monitor the battery bank voltage and turn off the AC charger if you see the battery voltage approaching 15.0 volts (and/or the batteries start to get hot).

    It will be very nice to keep your batteries cycling >50% state of charge (longer battery life). And never to go below <20% state of charge as a weak cell can actually approach 0% SoC or even start to reverse charge (pretty much death to most rechargeable chemistries).

    If your batteries are just cycling >50% state of charge (charging and discharging) and it has only be 4 days--You really do not need to throw on an AC charger... But if you wish to do that--Just do enough charging to get >~80% state of charge--That will be most fuel efficient for genset usage--And just let solar complete (use genset in morning, let solar complete late morning/afternoon).

    Don't stress over not reaching >90% SoC every day--Once or twice a week >90% is fine. Trying to reach 100% SoC everyday--Actually kind of hard on the batteries. And do an EQ once a month (5% to 2.5% rate of charge, typical recommendation)--Or EQ if SG range from high to low cell is >0.030 sg units.

    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • alcatel99alcatel99 Registered Users Posts: 31 ✭✭
    Thank you so much for the reply i guess i was panicking a bit early as the next day there was plenty of insolation and batteries went up to 14.5 and today i have them equalising so i think all is well  as batteries are getting on i have been equalising once a month if needed and usually i shut down at night at around 70% i avoided using the automotive charger and am saving up for some rolls surrette for when the Crowns need replacing .
    Thanks again
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