12V input, 12V output battery charger (does it exist?)

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_OS_
_OS_ Solar Expert Posts: 207 ✭✭✭
I will soon have a spare bank of 12V batteries that I plan to use as a backup battery bank for my cabin system. The idea is to charge the spare bank when the main bank is full and use the spare bank to charge the main bank when the main bank is empty. I have seen several intricate systems that use two battery banks and now I want to build another intricate system :-) I could of course use a battery selector and manually switch between the battery banks but I would like to have a solution that is automatic. I will make the electronics myself (or just use a simple voltage controlled switch) but I need a 12V input (nom) 12V charger. Pref. one with temperature compensation. Does anybody know if such chargers exist?

Regards,
Ole

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  • _OS_
    _OS_ Solar Expert Posts: 207 ✭✭✭
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    Re: 12V input, 12V output battery charger (does it exist?)

    Found one but it only works down to 0 deg/C: http://www.powerstream.com/DCC.htm
  • BB.
    BB. Super Moderators, Administrators Posts: 33,479 admin
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    Re: 12V input, 12V output battery charger (does it exist?)

    If purely spare--Then the separate battery charger is probably a good solution (if not very cheap).

    Are the two battery banks of similar constructions (i.e., both flooded cell, but of different age/possibly different brands/AH ratings)?

    I know I tell everyone to avoid putting to different battery banks together in parallel--But in your case, assuming you have enough solar power to properly charge the bank--I would probably just connect them in parallel (with appropriate fusing/breakers if they are way different in AH capacity).

    Having an unused bank is a "waste".

    If the banks are of substantially different AH rating >>2x difference--Then even connecting the second bank is probably of little help. Combining a 100 AH string with a 20 AH (5x difference in this example) string is almost within 10-20% margin of error of an actual 100 AH battery bank capacity. And would not add much capacity or surge capability anyway.

    A good engineering rule of thumb is two things that are 2x or smaller in "difference"--they are "significant" or "similar". Things that are 10x or larger in difference--You can usually ignore the effect of the smaller one in any useful calculations.

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • _OS_
    _OS_ Solar Expert Posts: 207 ✭✭✭
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    Re: 12V input, 12V output battery charger (does it exist?)

    Hi Bill,

    Thanks for the reply. My current bank consists of two four years old Concorde Sun Xtender 305Ah (120h rating) in parallel and the spare bank consists of two almost new 290Ah (100h rating) branded AGM batteries. Since both banks are AGM and of nearly equal capacity I am fine connecting them in parallel?

    Ole
  • BB.
    BB. Super Moderators, Administrators Posts: 33,479 admin
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    Re: 12V input, 12V output battery charger (does it exist?)

    Personally, I would. But you may need/want to increase the amount of solar power.

    Also, I would use a DC Current Clamp meter (I know you are in Norway--but here is an inexpensive version from Sears in the US) and monitor the current sharing between the strings. If/when the current sharing becomes greater than an average of ~2:1 difference (i.e., 20 amps in one string and less than 10 amps in the second when under heavy charging/loading)--I would review connections (make sure clean and tight) and the actual battery condition (too sulfated, etc.). You might see the current sharing vary over time and temperature.

    If you have consistent difference in current--You could try adding a length or several of series cable in the "high current" string to force the low current string to process more current (if you have the interest) to balance out any internal battery resistance due to age/condition.

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • _OS_
    _OS_ Solar Expert Posts: 207 ✭✭✭
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    Re: 12V input, 12V output battery charger (does it exist?)

    Hi Bill,

    Yes I am in Norway but last week I was in CA on a technical convention and had a Rogue MPT-3024 delivered to my hotel! I installed it in my cabin system two days ago and will write a small review here later. Anyway I can get clamp-on ammeters over here and I will buy one as soon as I am in town again.

    BTW: I am writing this from my cabin way up in the mountains in Norway. We have no mobile coverage, no water, no grid power but we have internet connection! Almost two meters of snow now and the temp is -5C (23F).

    Regards,
    Ole
  • waynefromnscanada
    waynefromnscanada Solar Expert Posts: 3,009 ✭✭✭✭
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    Re: 12V input, 12V output battery charger (does it exist?)
    _OS_ wrote: »
    BTW: I am writing this from my cabin way up in the mountains in Norway. We have no mobile coverage, no water, no grid power but we have internet connection! Almost two meters of snow now and the temp is -5C (23F).

    Regards,
    Ole

    Wow! Love that report from your part of the world. I find it very interesting, and in my mind it's almost like being there. Thanks for sharing!
  • _OS_
    _OS_ Solar Expert Posts: 207 ✭✭✭
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    Re: 12V input, 12V output battery charger (does it exist?)
    Wow! Love that report from your part of the world. I find it very interesting, and in my mind it's almost like being there. Thanks for sharing!
    Hi Wayne,

    Thanks. I am coming back with a more thorough report on the MPT-3024 later.

    Internet in my cabin was not so fun this morning when I entered this forum and got this message:
    Sorry. The administrator has banned your IP address. To contact the administrator click here

    I had to log into my work server to write this reply. Anybody know why I am banned? BB?

    Ole
  • Windsun
    Windsun Solar Expert Posts: 1,164 ✭✭
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    Re: 12V input, 12V output battery charger (does it exist?)
    _OS_ wrote: »
    Hi Wayne,

    Internet in my cabin was not so fun this morning when I entered this forum and got this message:
    Sorry. The administrator has banned your IP address. To contact the administrator click here

    I had to log into my work server to write this reply. Anybody know why I am banned? BB?

    Ole

    Not sure why that range was banned, probably got a bunch of spam from there in the past, but I have removed all 91.149.xxx blocks
  • BB.
    BB. Super Moderators, Administrators Posts: 33,479 admin
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    Re: 12V input, 12V output battery charger (does it exist?)

    Sorry,

    As Windsun said, we were getting a lot of (probably) Russian spammers from that part of the world (I don't recall the exact ban).

    Ole, if you are still having problems, just PM me your IP address and I will clear it from the list.

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • _OS_
    _OS_ Solar Expert Posts: 207 ✭✭✭
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    Re: 12V input, 12V output battery charger (does it exist?)

    Another thing that annoys me is that most temperature compensated battery chargers has a maximum output voltage of 14.7V or so but my Concorde AGM batteries call for a charge voltage of ~16V when the temperature is -20C (-4F). I know most of you live in Arizona and think that no batteries work and should be charged at -20C or 16V but the truth is that it works! I have e-mailed Concorde and they sent me the Technical manual of the Sun Xtender batteries and it confirms this. We get about half the capacity at -20C but my Sun Xtender have not failed me in 5 very cold winters now. The problem is when I charge the batteries in these temperatures with a generator and my Victron charger. It takes forever since the max output voltage is 14.7V.

    I have looked at the Xantrex TrueCharge2 chargers and they have a max output voltage of 15.5V but the lowest operating temperature of 0C.

    Any other suggestions?

    Ole
  • _OS_
    _OS_ Solar Expert Posts: 207 ✭✭✭
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    Re: 12V input, 12V output battery charger (does it exist?)

    Hi Bill,

    I works now! Thanks Bill and windsun! Also see my reply to my initial post above.

    Ole
  • vtmaps
    vtmaps Solar Expert Posts: 3,741 ✭✭✭✭
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    Re: 12V input, 12V output battery charger (does it exist?)
    _OS_ wrote: »
    Another thing that annoys me is that most temperature compensated battery chargers has a maximum output voltage of 14.7V or so but my Concorde AGM batteries call for a charge voltage of ~16V when the temperature is -20C (-4F).
    <snip>
    Any other suggestions?
    Since you have AGMs you don't need to vent them. Therefor you can superinsulate them. My batteries are in an uninsulated plywood box and stay about 10° F above ambient (35°F) in the winter. I can't superinsulate them because they are flooded. You should be able to keep them as warm as you want with enough insulation.
    --vtMaps
    4 X 235watt Samsung, Midnite ePanel, Outback VFX3524 FM60 & mate, 4 Interstate L16, trimetric, Honda eu2000i
  • _OS_
    _OS_ Solar Expert Posts: 207 ✭✭✭
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    Re: 12V input, 12V output battery charger (does it exist?)

    Thanks for the suggestion vtmaps but the problem is that my system stays unused for several months in the winter and even the most super insulation can't keep the cold out. I am seriously considering burying them into the ground which would help.
  • BB.
    BB. Super Moderators, Administrators Posts: 33,479 admin
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    Re: 12V input, 12V output battery charger (does it exist?)

    If you are not cycling the batteries when they are cold--Perhaps just holding at "float" voltage is "good enough" for the job.

    Once you get in, heat the place up, and start cycling the batteries, You can then use the lower charging voltage.

    I think a couple people here use an "battery heater" (I am sure you have more knowledge than I about cold weather aids) and their portable genset. Heat the batteries for a few hours, then they stay warm throughout the rest of the stay (between usage, warmer cabin, and insulated box).

    One issue I have seen is that very few "12 volt" devices will operate "reliably" above ~15-15.5 volts anyway--So charging/operating at 16 volts without damaging the loads is problematic.

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • Cariboocoot
    Cariboocoot Banned Posts: 17,615 ✭✭✭
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    Re: 12V input, 12V output battery charger (does it exist?)

    Late to the game but I'll repeat the standard mantra of those of us who live in the Frozen North: charged batteries don't freeze. Cold batteries actually last longer due to the slow-down of the electro-chemical action.
    SOP around here is to leave the panels & controller hooked up but turn the loads off. Since they don't get depleted, Bulk & Absorb times will not be long and Float is available on any good, sunny day. (Okay, we have a shortage of good, sunny days in Winter up here.)

    It isn't hard to keep a battery bank that is not being used safe through the Winter. Try to use it in Winter is a problem. :roll:
  • _OS_
    _OS_ Solar Expert Posts: 207 ✭✭✭
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    Re: 12V input, 12V output battery charger (does it exist?)

    How far north are you? Our cabin is about 300km south of the Arctic circle.
  • _OS_
    _OS_ Solar Expert Posts: 207 ✭✭✭
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    Re: 12V input, 12V output battery charger (does it exist?)

    Thanks for the suggestions Bill. My batteries are in the tool shed where there is no heating but I am seriously considering digging a hole in the ground and make some sort of "earth cellar" under the tools shed.

    I usually disconnect the loads when using the generator so 16V will be no problem. The problem with the charger I have now is that the temp compensation only compensate down to 0C and it takes for ever to charge my battery bank when it is as cold as now (-10C).

    Ole
  • Cariboocoot
    Cariboocoot Banned Posts: 17,615 ✭✭✭
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    Re: 12V input, 12V output battery charger (does it exist?)
    _OS_ wrote: »
    How far north are you? Our cabin is about 300km south of the Arctic circle.

    You're at about 63 N Lat, I'm at about 52 N Lat. -10C isn't unusual at all here. Right now it's a balmy -5! :D
    We get temps down to -40C. This Winter has been consistently cold; lakes have not unfrozen yet.

    My batteries are inside the cabin, surrounded by 2" of polyurethane insulation with 3" on top. Yes, they are FLA's and the enclosure is vented. The MX60 is able to keep them above freezing all Winter without trouble. Since they are not in use it does not matter that they "go dormant" and aren't brought up to "full" Voltage every day. Yes, temp sensors "run out of compensation" when the mercury plummets. But it is still enough to keep them from freezing.

    However, I like your idea of underground storage as it affords an additional advantage of keeping them cooler in Summer. Batteries don't like heat either.

    Let's face it: no one has batteries that are kept under ideal conditions. We'd have to be living in a laboratory for that! Inevitably whatever compromises we've had to make in terms of environment and cycling regime will take some time off the maximum lifespan of them. It's the nature of the beast. You can easily spend more on trying to supply ideal conditions than it costs to replace them after a few years. But your efforts to provide a more stable temperature by digging a hole sounds reasonable to me.