What specifications for the battery charger?

RickeyHRickeyH Solar Expert Posts: 39
Howdy. I'm in the process of putting together a small 24 volt off grid solar system for the shed. Will be initially looking at around a 220ah battery bank using Trojan T105-RE 6 Volt 225 AH Deep Cycle Batteries but at a later date will want to double it to 440ah.

What determines the specifications for the battery charger? I'm guessing that I'm after a 24 volt charger as the batteries although 6 volt will be wired in series to give me 24 volts at 220ah initially and then series and parallel to give me 24 volts at 440ah. But what amperage charger should I be looking at? Do you need more information on my system before being able to advise?

Is there a thread or FAQ that covers this? (I'm not after an inverter/charger as we will be running 12 volt lighting, TV etc.)

Thanks

Rickey from sunny Australia

Comments

  • BB.BB. Super Moderators, Administrators Posts: 29,523 admin
    Re: What specifications for the battery charger?

    If you want to look at the "optimum" battery charger for a "smaller system" with a smallish genset, this a good thread to read through:

    Question about battery charger selection with EU2000 generator.


    So--is this going to be charged from the utility/grid power, or from a genset?

    And to be clear, you are not asking (at least right now) about a solar PV power charging system?

    The problem is--It is actually kind of difficult to find a programmable stand alone AC battery charger with Power Factor Correction. Xantrex has a few, but they appear to be quite difficult to find.

    Another option is the Inverter/Charger. There are some good choices out there--in the higher wattage (and not cheap) range (Magnum, Outback, and others).

    Otherwise, there are the tried and true "simple" battery chargers. Although it appears that PowerMax may be offering a Power Factor Corrected version.

    Do you have a genset already, or are you looking to size/pick one of those too...

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • CariboocootCariboocoot Banned Posts: 17,615 ✭✭
    Re: What specifications for the battery charger?

    About the same as you'd go for with PV and charge controller: minimum of 5% of the total Amp hours and up to the max the batteries can handle (which varies by battery but usually around 13%). Trojan recommends a 10% peak current for their batteries, so you'd be trying for 44 Amps @ 24 Volts. You can +/- it a bit of course. One example would be this Iota unit: http://www.solar-electric.com/ioen40amp24v.html
    When you're charging off a generator you do want to make good use of the gen's power, and that means bulking up as quickly as possible. Then you can usually let panels finish the job, even on a partly cloudy day. It's a pain and a waste of fuel if you have to run the gen all through Absorb (low current, long time) - but sometimes it has to be done if the weather won't co-operate.

    BTW, with eight Trojan 225 Amp hour 6 Volt batteries you'd have two parallel strings of four giving a total of 450 Amp hours @ 24 Volts.

    How are you going to be running 12 Volt lighting, TV, et cetera off a 24 Volt battery bank? That's a lot of DC to DC converting. And long wire runs can result in a lot of Voltage drop and/or the need for really thick (expensive) wiring.
  • icarusicarus Solar Expert Posts: 5,256 ✭✭✭✭
  • RickeyHRickeyH Solar Expert Posts: 39
    Re: What specifications for the battery charger?

    Thanks everyone. Yes I already have a generator. A no name brand 3500W max. output that I bought for when we are building. Cariboocoot has given me what I need I think. Something around 40 to 45 amps for a 450ah battery bank. I was planning use a 24 volt to 12 volt converter. The living area part of the shed is only 5.3m x 3.1m. (17 feet x 10 feet) so long runs of wire wont be an issue.

    The advantage that I see with 12 volt is that I can do all the wiring myself without too many safety concerns. If I go down the 240v inverter root, not only do I have the expense of buying an inverter but wiring becomes problematic without a licensed electrician involved.

    What do you think? Am I on the right track?

    Rickey
  • BB.BB. Super Moderators, Administrators Posts: 29,523 admin
    Re: What specifications for the battery charger?

    With a 3.5kW genset, the largest "typical" charge controller would be:
    • 3,500 Watts * 0.80 charger eff * 0.67 typical PF * 1/29 volts charging = 65 amps maximum (estimated)

    So a 45 to 55 amp 24 volt battery charger would be appropriate for that size genset (using typical numbers). As always, check the specifications for the battery charger.

    Choosing your battery bank voltage... How much current/power do you need to pull from the battery bank? My rule of thumb is to keep the current in the DC wiring to around 100 amps maximum (nominal)--If you have much higher draw, you are looking at some very thick cable to carry the current.

    Also--Sending low voltage/high power any distances over a hand full of meters is a problem. With 12 volt systems, you have about 0.5 volt drop (assuming 11.5 volts minimum battery voltage and 10.5 volts inverter/device cutout, with allowances for 2x surge current).

    A 240 VAC inverter is going to make wiring much simpler and cheaper... That 1,200 watts at 12 volts and 100 amps becomes ~5 amps at 240 VAC. You can send that power several hundred feet with a standard heavy duty extension cord. You would be hard pressed to send 100 amps of 12 VDC more than 5-10 feet without very heavy copper cable.

    There are sometimes secondary issues--For example, many 12 volt devices are designed to work with car electrical systems of ~12 to 14.2 volts. And it is not unusual for devices/DC adapters to fail at 15+ volts (during equalization). An AC inverter will make things much easier.

    I don't know about code/inspectors in your area--But I think you can build out a better, cheaper, and possibly even safer system. You can fuse/breaker individual 240 VAC circuits built with standard wiring and practices for your region.

    Building out a similar system in 12 or 24 VDC is going to cost a lot more unless your loads are very light.

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • RickeyHRickeyH Solar Expert Posts: 39
    Re: What specifications for the battery charger?

    This is my earlier post where we discussed the charge controller requirements http://forum.solar-electric.com/showthread.php?16390-Which-MPPT-Charge-Controller-for-a-24-volt-system

    My power needs will be approx. 1.0kwH per day but will increase in the future to around 1.5kwH per day. The panels will be on a shed and will initially be used for lighting (12v LED mainly) and TV with some water pumping required at a later date. Will also need to recharge so cordless tool batteries as well.

    I have purchased this charge controller http://www.pvsolarchina.com/45a-tracer-4215rn-ep-tracer-mppt-solar-controller.html

    I think you would agree that my expected loads are light. What do you think about running 24 volt lights, fridge and water pump (later). We have an Engel fridge which will run on 12V 24V or 240V. That would leave just the TV which I could run off a 24V to 12V converter installed next to the TV.

    I'm viewing the whole setup of the solar system on the shed as a learning experience. I was given a rough quote of $3K for a 'couple of 85 watt panels and a few batteries'. I'm budgeting the same amount ($3K) to do it myself with 6, 190W panels and 220ah of batteries (will up to 440ah later on).

    At the moment we camp on the block nearly every weekend and our power needs (fridge and lighting) are supplied by a couple of 12 volt jump start battery packs which we cart up with us. I think we will be able to improve on that system. :D

    Thanks for all your help

    Rickey
  • BB.BB. Super Moderators, Administrators Posts: 29,523 admin
    Re: What specifications for the battery charger?

    It does sound like you can use DC directly. You would be better off with a 24 VDC system if you have a larger battery bank... The typical "large" solar charge controller is around 60-90 amps (depending on brand and model).

    So, a 24 volt battery bank will allow a 12/24 volt capable controller to manage a 2x larger array at 24 volts (vs 12 volt battery bank).

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