Step down 14.2v to 6.8v to charge a single Trojan 6v battery

Outrunner
Outrunner Registered Users Posts: 4 ✭
edited February 2016 in Solar Beginners Corner #1
Hi all, totally newbie here, need some advice, I'm not sure I'm doing my schematics right...

I was thinking to get a L105 6v Trojan battery, and step down the Voltage from the regulator to 6.8v to feed it.

Input PV is a 240W Mono, 31V, 9.7A that goes into a 30A MPPT > only 12v or 24v out.
But at 14.2V output, amperage is already at 16A. Stepping down to 6.8 would crank it up to 35A.
and there are not many dc/dc converter that goes that far up. So instead of getting a single 6v battery,
I probably would be better off with a line of 2x6v Trojan L105>125 batteries.
Also don't want to oversize my bank, as I know my tot Wp usage is well within what single L105 battery can provide me.

Input PV is a 240W Mono, 31V, 9.7A that goes into a 30A MPPT > 13 to 16A peak out at 14.2v

So I thought.... I may split the current in two, or three lines, with an Amp breaker, than feed three equal cheap Dc/dc converter with max specs of 10a  and then recombine the output to feed my single 6v Trojan battery. I'd say 30 bucks for the splitter/converter/merger/ could do.

The downside in this config, is that I the MPPT would not read the battery Voltage state now, undervoltage now would be set into the dc/dc converter instead... I'm not totally sure I can bypass this, but it should... so there is the FLOAT function I would lose. And that may be a problem, how would I tell the controller to stop feed the battery once that is fully charged??

I would then use another dc/dc to get back 12v for my appliances.

But... could it works? Anyone has experience in using dc/dc to charge 6v batteries with this Amperage?
Do I have some alternative or it is just better to get two 6v and put them in line?




ty
Outrunner
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Comments

  • BB.
    BB. Super Moderators, Administrators Posts: 33,613 admin
    #2
    Unless you have some sort of special needs--Your best bet would be two 6 volt batteries in series.

    Can you tell us what you want to do with the power (how many watts, how many hours per day, etc.). Also, do you have other limits to your system (can only mount 1x 240 Watt panel, 2x batteries, etc.).

    Once you know what your power needs are (after you do a lot of conservation), and other limits... Then you can start designing a system to support those needs.

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
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  • zoneblue
    zoneblue Solar Expert Posts: 1,220 ✭✭✭✭
    #3
    A charge controller requires a battery (or something that behaves very much like one). This you cant do this:

    PV --> CC --> DC converter --> Battery

    With some advanced jiggery pokey you might be able to make it sort of go, but whats the benefit? And what are you going to do with 6VDC?
    1.8kWp CSUN, 10kWh AGM, Midnite Classic 150, Outback VFX3024E,
    http://zoneblue.org/cms/page.php?view=off-grid-solar


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  • Outrunner
    Outrunner Registered Users Posts: 4 ✭
    edited February 2016 #4
    Total power 370wp

    Splitted into separate banks
    130Wp to a 100 Ah 12v main Motorhome sustain bank, always connected.
    240Wp to I don't yet know but was thinking 250ah to 400ah x20h 6v singleTrojan battery.

    Appliances: 100Wh

    That is 500W per 5 hour or 1000W per 10h.   The first bank can stand the 100W for 5h before going under 50%, a dc\dc power inverter will go into protection at that point, preventing my battery to drain too much in case I forget my pico projector and pc on through the night, cutting the power off.

    The 240Wp bank will be standalone, meant for backup for fast recharging of extra devices, portable speakers, heaters, girls appliances absolutely not predictable... and so on...  A 12v 40W freezer would take 965W per day at peak. And for that 250ah is the minimum. Just the freezer alone.
    Adding another 240W and total up to 500ah standalone bank is possible, but as for now, a 250ah 6v battery is enough.
    For this I would like not to split in two smaller 125ah 6v battery, but stay with one 250ah battery that I can top with another 250ah battery in the future. Instead of 4x 125ah. That would also mean less wiring. From this the 6.8v dilemma.

    I have plenty of space for additional PV and batteries. That is not a problem. Lose the controller FLOAT function, is a problem.
    So it looks I'm better off with 2x 6v 125ah at the moment, unless a second 240W mono is under way...

    And one more question... do I need to put any circuit breaker in this array??
    I haven't thinked of any... ty.



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  • Outrunner
    Outrunner Registered Users Posts: 4 ✭
    #5
    zoneblue said:
    A charge controller requires a battery (or something that behaves very much like one). This you cant do this:

    PV --> CC --> DC converter --> Battery

    With some advanced jiggery pokey you might be able to make it sort of go, but whats the benefit? And what are you going to do with 6VDC?
    The main benefit is to have a single battery with lower voltage and higher capacity, rather than many, so it would save also a bit of space, less sensors, wiring. Less mantainance, but there's no CC rated for 6v out and 30+A. Not one. Reconverting 6v to 12v is pretty easy, but the jiggery on the CC ... the read/charge are the same that would go into the dc/dc... i can't see how to cheat on that..
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