12V inverter with 24V battery bank?

I am seeking knowledge in how to best maximize electrical power production efficiency. It seems less efficient to power a small ac load (say <50W) with a large inverter (2500W) than with a small inverter (like Morningstar's 300W unit). Hence my thought was to have two inverters (the large one for large loads - microwave...etc.) and a small inverter for small loads (laptop computer, tv, stereo, shaver, curling iron ... all less than 100W). I favor Outback for the large inverter and Morningstar for the small inverter. However, I want a 24V system (battery bank) and the Moringstar inverter is 12V. I have read where it is possible to use a 24-to-12 volt step-down converter as a battery equilizer which is said to allow large 12V loads to be pulled from a 24V battery bank. That is what I am considering doing, but worry that the battery bank's life my in fact be compromized with such use. Experts, what is your opinion?

PS: I don't like the small inverter options at 24V (Exeltech expensive and high phantom load, Samlex inexpensive, but worry about reliability)
Thanks,
Kevin

Comments

  • BB.BB. Posts: 28,083Super Moderators, Administrators admin
    Re: 12V inverter with 24V battery bank?

    Inverter efficiencies... Best thing to do is run the calculations from the data sheet and/or testing...

    However, you do want to look at the overall size of the system too... If the system is going to supply 2.5kW for serveral hours per day, loosing 6 watts in 24 hours is only 6*24/2*2,500=2.88% of "waste"... Almost small enough to ignore in day to day operation...

    However, if you have lots of little 6w*24hour loads (say 10 of them), then you are all of a sudden up to 30% of your total system load.

    Things like deciding if your main inverter is running 24 hours per day with variable loads (search mode) vs turning the inverter on only when you need it--may safe the ~6-8 watt load that some inverters burn on "search for load" mode.

    Regarding the old 12 volts from a 24 volt battery bank... Don't do it if they are AGM, and resist doing it if they are wet cell (where you can equalize and add water every so often)... You can do it--but you are basically converting water to H2 and O2 on the other series battery to recharge the lost current used for your 12 volt inverter... Not efficient or battery friendly at all.

    But, if you still want a small inverter, Wind-Sun Exeltech line has a nice range of 24 volt inverters in the 125-600 watt range--plus they are running a 5% avoid the inventory tax sale. (but I see you don't like their standby loads and that they are expensive--Understand the standby load, but they are true sine wave inverters--which tend to be much more expensive design and also run many motor and electronic loads more efficiently than a mod-square wave inverter would)...

    But, in any case, if you pay for 5-7 watt "standby" loads for a small inverter, and 6-8 watts for search mode in a large inverter--it seems to be hard to justify the small inverter.

    The Morning Star 300 watt pure sine inverter has about the same 8 watt zero load power and same ~90% efficiency at full load--but at 12 vdc... It would seem that the Exeltech would be similar ratings--but at your desired 24 VDC...

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • icarusicarus Posts: 5,108Solar Expert ✭✭✭✭
    Re: 12V inverter with 24V battery bank?

    Not to argue with Bill,

    I have a Prosine 300 from Morningstar and love it.

    It draws 450mA (5.4 [email protected]) on with no load, 25mA (.3 watts) with the inverter turned off, and 55mA (.66 watts) in stand by, not the ~8 watts suggested. My only complaint is it induces a significant RF noise on AM.

    I do agree about the 12-24vdc transformer however.

    Icarus
  • System2System2 Posts: 6,290 admin
    Re: 12V inverter with 24V battery bank?

    I also have a Prosine 300 and am very impressed with it. I use the remote start function and like the 25 ma amp draw when off. Another thing I like is that the instant it turns on, it's already producing full power, unlike many other inverters I have experience with, which require a couple of seconds to stabilize before they're really ready for a big load. I also love the fact that it doesn't have or need a fan, making it silent :) The only thing I would improve, would be to lift those indicator LEDs up out of the holes they're in, so you can see the status without having to line up your eyes with the holes so you can see down the pipes to the LEDs.
    Wayne
  • BB.BB. Posts: 28,083Super Moderators, Administrators admin
    Re: 12V inverter with 24V battery bank?

    I was not knocking the MorningStar inverter... But 5.4 watts on with no load is pretty much in the 5-7 watt no-load curve that I typed about.

    The bigger issue for me was that it appears that there is only a 12 VDC Prosine inverter, whereas there is a 24 VDC Exeltech inverter (with basically the same power on/no load/efficiency ratings). Anything that converts from 24VDC down to 12 VDC is going to completely swamp the 1-3% rated efficiency differences between the MS and Ex inverters (that I see from the specs... I have not used or even seen in the "flesh" either of these devices--let alone worked on them).

    It does appear that the Prosine Morning Star inverter does have a lower power search mode (55mAmp at 12 vdc=0.66 watts)--so if you have small, intermittent/switched 120 VAC loads--then it may make good sense for 24x7 powered inverter.

    However, if it is a, for example, 50 watt continuous load at 120 VAC--on a 24 VDC system I would take a very good look at 24 VDC input inverters instead of trying to figure out how to get 12 VDC for 120 VAC conversion from a central 24 VDC battery bank.

    Granted, this is just my 2 cents worth of opinion.

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