battery charger

On an small island where we run a Organic Farm we use a rather small solar system 4 x 55W pannels connected to a 150 Amp maintenance free battery, to produce energy (220V via sinus inverter) to operate communication devices (mobile phones/ CB set, computer and some energy saving lights for a couple of hours at night.
It works fine but some times during the monsun when it can rain for a couple of days and there is little sunshine the battery gets low and we have to utilise as back up a small Honda generator (220V 750W).
I would like to be able to use this generator to charge the our battery which is connected to the solar pannels whenever there is no sunlight.
Could anyone please adivse me which type of a battery charger would be the right one to buy to charge this battery.
Thank you in advance


  • BB.BB. Super Moderators, Administrators Posts: 32,019 admin
    Re: battery charger


    Regarding the Maintenance Free battery... In the US--that typically is a flooded cell automotive battery with calcium (or other additive) to reduce hydrogen gas production when charging--and reduces the need to add distilled water during the normal life of the battery.

    For Solar RE batteries, we highly recommend those designed for Deep Cycle applications. The batteries may be flooded cell or various types of sealed batteries (AGM, Gell, VRLA--Valve Regulated Lead Acid, etc.).

    So, assuming this is a deep cycle battery of some sort and is rated at a 150 Amp*Hours at 12 volts. Things we can recommend (rough rules of thumb):
    • 150 AH * 0.05 = 7.5 amps minimum charger
    • 150 AH * 0.13 = 19.5 amps maximum charger (20 or 25 amp is fine too)
    Recharging your battery... From generator efficiency point of view, the battery is going to take the maximum current from 0% charge (dead) to ~80-90% state of charge. Normally, don't take a battery below 50% state of charge (for longer life), and never below 20% state of charge (may end up damaging battery). And never store a battery below 75% state of charge for days, weeks, months--will sulfate and lose capacity quickly).

    Regarding your genset, your optimum power point (for various reasons) would be at 50% of rated load. Which, assuming 80% charger efficiency and 14.5 volts maximum charging voltage would be:
    • 750 Watts * 0.50 loading * 0.80 chrgr eff * 1/14.5 volts charging = 21 amps
    A 20 amp charger will match nicely with your 750 watt charger.

    You don't need a fancy charger with a "float"/trickle stage--that is only useful for use when you have Grid Power and electricity 24x7. You need a simple charger that can output high charging voltage (14-14.5 volts) and 10-20 amps of current until the battery reaches approximately 80-90% charged.

    And you should be able to connect the AC charger and Solar Charger to the same bank and even operate at the same time... The chargers should prevent overcharging of the battery bank (basically, pick somewhere around 14.0-14.5 volts for "target voltage" -- see battery mfg. specifications for proper charging voltage and current).

    At that point, the current from the battery charger will start to be reduced (because the battery is at maximum charging voltage)... At some point, you will want to shut down the genset and let the solar array take the charging the rest of the way.

    If you get back down to 50% state of charge, crank the genset up again.

    Time wise:
    • 150 AH (85%-50% state of charge) * 1/20 amp charger rate = 2.6 hours of run-time
    You may need to run more (not all chargers will output full current for several hours)--but it gives you an idea of the minimum run-time you should aim for during bad weather and a 50% discharged battery. (do not let the battery set for more than 1/2 or 1 day below 75% state of charge without recharging >80% state of charge to prevent sulfate hardening).

    By the way, how do you know the state of charge of our battery (measuring resting voltage, specific gravity, Amp*Hour meter)?

    At least--that is my guess at what I would do. Above numbers are approximate and can be tweaked a bit.

    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • icarusicarus Solar Expert Posts: 5,433 ✭✭✭✭
    Re: battery charger

    An IOTA or Xantrex Truecharge 20 amp charger would be perfect.
    The Xantrex has been hard to source of late, but the older generation was a great charger and is still available a bunch of places.

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