charger size

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System
System Posts: 2,511 admin
in Solar Beginners Corner #1
I'm in the beginning stages of cobbling together a Frankenstein's monster of an alternative energy system. I have 4 t105's that I want to use to power things like outdoor accent lights, recharge cordless tools, irrigation pump, etc. Charging the batteries via PV panels will be dicey since I have a lot of trees. Thus, my next addition is a charger that I can use from a generator that will run on biodiesel. I am planning on wiring the 4 batteries for a 12 volt system (two pairs of two). I think this gives me ~440 amp hours. Do I size my charger based on the entire bank or based on the individual batteries? If my math is right and my bank has a 440 Ah capacity, a 40 amp charger would work. Otherwise, I would drop to the 20 amp and charge them as two separate banks. I like what I've read about the Truecharge products.

At this point, I'm still in the project development phase and hope to start learning about wind and PV systems while I mess around with different components. What I'm describing here is definitely overkill. Thanks.

Todd
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  • BB.
    BB. Super Moderators, Administrators Posts: 33,447 admin
    #2
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    Re: charger size

    Ideally, you size the battery bank to your load requirements. And size the Charger(s) to the size of the battery bank and your daily loads.

    Roughly, a good rule of thumb for charging a flooded cell battery bank is around 5% to 13% (you can go larger than 13%--but the battery bank will not take all of the available current).

    So, if you have a 440 AH battery bank (12 volts)--By the way, these are rough recommendations, you can round up or down to standard ratings:
    • 440 AH * 0.05 = ~22 amps min
    • 440 AH * 0.13 = ~57 amps max
    Regarding dividing the battery bank in two--I would not do that unless you have a reason (Bank A for power tools, Bank B for emergency power, etc.).

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
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  • System2
    System2 Posts: 6,290 admin
    #3
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    Re: charger size

    I guess I should get a better handle on my load requirements. I'll pick up a Kill-a-Watt or something and see what my anticipated requirements are. If two batteries can do what I need, I could get by with a 20 amp charger. The 40 amp isn't much more expensive but, as I understand things, a 40 amp charger would force me to use a bank of four 6V batteries configured as a 12V system. A 40 amp charger hooked to a 12V 220 Ah battery (two 6V in series) would give a charge rate of ~28 amps which is too high for the two battery configuration. Right? The inverse is true for a 20 amp charger in that it would be below the minimum of 22 amps for a 4 battery bank wired for 12V.

    Todd
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  • Cariboocoot
    Cariboocoot Banned Posts: 17,615 ✭✭✭
    #4
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    Re: charger size

    A good charger will not put more current to the batteries than they can take. That "40 Amp" is a maximum rating. As the battery charges up, the current goes down.

    If they were to put 40 Amps no matter what the state of charge of the batteries, there would be a lot more fried batteries and a lot more angry customers about!:p

    But yes, get your loads worked out. A nice bank of 4 T105's is good for roughly 2.4 kW hours a day! But it would 'want' around 800 Watts of panel, so if you can get by with less do so (the battle-cry of the off-gridder! :p )
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  • System2
    System2 Posts: 6,290 admin
    #5
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    Re: charger size
    Cariboocoot wrote: »
    A good charger will not put more current to the batteries than they can take. That "40 Amp" is a maximum rating. As the battery charges up, the current goes down.

    Of course .. I should have known that. So only the second part of my interpretation holds up. That is that a 20 amp charger would have difficulty charging a bank of 4 6V batteries configured for 12V at 440Ah. However, a 40 amp charger would be okay to use on a 2 battery bank at 220Ah. Am I getting closer?

    Todd
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  • Cariboocoot
    Cariboocoot Banned Posts: 17,615 ✭✭✭
    #6
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    Re: charger size
    wapitiscat wrote: »
    Of course .. I should have known that. So only the second part of my interpretation holds up. That is that a 20 amp charger would have difficulty charging a bank of 4 6V batteries configured for 12V at 440Ah. However, a 40 amp charger would be okay to use on a 2 battery bank at 220Ah. Am I getting closer?

    Todd

    Yes. The 20 Amp unit would take longer, and depending on how deeply discharged the batteries are might run hotter, use more power, not last as long, et cetera. But it's marginal; the 20 A would charge a 440 Amp/hr bank. Just not as quickly or efficiently. It also may not be as effective in slowing the inevitable sulphation or in remixing the electrolyte.
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  • BB.
    BB. Super Moderators, Administrators Posts: 33,447 admin
    #7
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    Re: charger size

    Another thing to consider is generator size too... For the most part, you are better off choosing a charger/bank that will load your genset to around 50% minimum at full load.

    Generators running at less that 50% capacity typically still use 50% fuel flow. So for a 20 amp vs 60 amp charger... Assuming 80% charger efficiency and 14.5 volts charging:
    • 20 amps * 14.5 volts * 1/0.80 eff * 1/0.50 min gen loading = 725 watt
    • 60 amps * 14.5 volts * 1/0.80 eff * 1/0.50 min gen loading = 2,175 watt
    So, depending on what charger size you pick, you are looking at between a small Honda eu2000 class and a 2.5+ kW genset. You may also choose a 40 amp; genset and run a Honda eu2000i (1,600 watt).

    One other thing to watch out for with most battery chargers is that they have poor power factor... Basically, they pull "spikey" AC current and require a larger genset to run lower power ratings. Power Factor can be as poor as 0.6 for a battery charger (and usually they don't list their PF or VA rating): So, for example a 20 amp charger with 0.6 PF would need a generator rated at:
    • 20 amps * 14.5 volts * 1/0.80 eff * 1/0.6 PF = 604 VA
    • 40 amps * 14.5 volts * 1/0.80 eff * 1/0.6 PF = 1,208 VA
    • 40 amps * 14.5 volts * 1/0.80 eff * 1/1.0 PF = 725 Watts
    So, for example a 40 amp 12 volt charge controller, if power factor corrected should be able to just about run on a 1,000 watt genset (900 continuous). However, because of poor power factor, it would draw "too much current" and overload/pop the breaker on a smaller genset.

    It does get a bit overwhelming trying to balance everything out with a solar RE system. But having too large a genset can really cost you a lot in fuel (especially if you end up running the genset a lot).

    Many time,s the solution is a small genset for bank recharging, and a larger unit to run optional loads (such as shop tools). Also, gives you an emergency backup genset too.

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
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  • Cariboocoot
    Cariboocoot Banned Posts: 17,615 ✭✭✭
    #8
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    Re: charger size

    What Bill says is true: too many people buy too big of a generator, usually because they get some clunker cheap. Then they throw away money on fuel producing several kilowatts more than they need (so to speak - en potentia). If you want to size a generator right, take a look at your maximum cumulative load - i.e. the size of your inverter. Chances are the generator doesn't need to be any bigger, unless you have specific "high power" loads you intend to run only with the generator. Often you don't need as big a unit, because you can cut back power use while running it (turn off the TV!)

    If it can supply the maximum charge current and the average loads it'll be sized about right. No sense buying too big or spending all your money on fuel for cloudy days. :p
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  • System2
    System2 Posts: 6,290 admin
    #9
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    Re: charger size

    That's it. My head just exploded! I'll pick up the pieces and work through this info some more. What I need to keep in mind is ...

    1. Electric rates are currently between $0.095 and $0.105 per kWh
    2. I'm starting out wanting to use PV to charge batteries to run landscape lights at night and maybe a small fountain pump during the day.
    3. I have a very limited amount of sunlight exposure so would need some sort of generator to maintain/charge the batteries.

    Given these facts, I could easily have a payback time of 100 years! :D

    As I mentioned in the original post, this is still in hobby mode but any hobby worth doing is worth spending a bunch of money on. Thanks for all the information so far.


    Todd
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  • BB.
    BB. Super Moderators, Administrators Posts: 33,447 admin
    #10
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    Re: charger size

    That is always where my off-grid aspirations ended up... Spending dollars to save pennies.

    It is kind of interesting to figure out how much power costs you... For example, a gasoline genset running fairly efficiently will generate around 5kWH per gallon of fuel. A typical AC charger is around 80% efficient, lead acid around 80% efficient, and inverter around 85% efficient:
    • $/kWH = $3 gallon fuel * 1/5kW per gallon * 1/0.8 * 1/0.8 * 1/0.85 = $1.10 per kWHr
    And you can do similar things by adding generator costs to the $ line (and maintenance). And for solar power, just the capital costs + battery replacement costs over 20 years / 20 years of power used... etc.
    • $1,000 genset / 2,000 hour run time = $0.50 per hour
    • $3 qt oil / 25 hours = $0.12 per hour
    • $/kWH = 1.10 per kWH + $0.50 + $0.12 = $1.72 per kWHr total costs
    -Bill :cry:

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