System Efficiency 12V vs 24V battery bank

Discussion in 'General Questions' started by timber, Mar 8, 2011.

  1. timber

    timber WindyNation Engineer

    I'm weighing out the pros and cons of going with a 24VDC battery bank vs a 12VDC bank.
    Any thoughts or input would be helpful ... thanks!

    Regarding efficiency, 24V is, no doubt, going to win hands down.
    - Proposed overall system is hybrid off-grid with grid power available on site. (No grid-tie)
    - 12VDC is status quo and existing inverter is 12VDC.
    - Current Load Diversion is 80A and can operate at 12VDC or 24VDC
    - Several other system components operate at 12VDC so would need to be changed out.
    - Current batteries are 6VDC and can be used in several capacities other than battery bank.

    24VDC Pros:
    Input ..
    - DC from solar panels could be 48VDC for very efficient wire runs and low loss.
    - MPPT controller can work more efficiently to convert 48VDC to 24VDC battery charge.
    Output ..
    - 24VDC to 120VAC Inverter will be more efficient and require smaller wire sizes.
    - 24VDC supply circuit to house will have more efficient runs.
    ----- (So for producing AC, 24VDC has little disadvantage.)

    24VDC Cons:
    -Battery bank configuration must accommodate 24V so requires 4 or 8 (6V) batteries vs 2, 4, 6 or 8 (6V) batteries.
    - 24VDC supply circuit to house is non-standard voltage and requires 24VDC to 12VDC conversion for many DC applications.
    - 24VDC requires purchase of new inverter and several other system components.

    Unknown issues:

    - Operating WindTura at 24VDC input may or may not work well in lower wind speeds typical at this location.

    After viewing the above, I'm very inclined to go with the 24VDC battery bank.
    The efficiencies, over time, will offset additional system costs and make this a more reliable system.
    I may even have a 24VDC battery bank to supply AC and a small 12VDC battery bank to supply 12VDC circuit to house.
    No equipment will go unused. 12VDC-120VAC inverter will work for RV use or other future projects.

    Any thoughts or suggestions?????????????????????????????????? (Please, need your input here.) :?:

    Thanks, ;)

  2. Minnesota

    Minnesota WindyNation Engineer

    One other concern is that to recharge a 24V bank from the grid or from a generator after days with no sun or wind, then it is not so straight-forward, as 24V chargers/gensets are rare (may have to make your own). Mostly likely one would have to split the bank and use dual 12V chargers, and in the meantime the solar/wind input is offline. :eek:

    Good list there ...
  3. timber

    timber WindyNation Engineer

    Hi Minnesota,

    Good point I hadn't considered here,
    Splitting the bank to charge at 12V is doable but a bit of a hassle.
    I have to do this with my home-built 24V elect bike.
    I ended up (after a bit of head scratching) using a DPDT switch to split 24V into two 12V sets for charge or run at 24V.
    I think it would take a hefty (thus very expensive) DPDT to do the same here.

    IOTA does make a decent 24V charger at $200 for 25A or $400 for a 40A but it is an expense I hadn't considered. ... EUQ8wIwAw#

    There are other components I already have that will also need to change if going 24V, Dummy Load, Low Voltage Reconnect , not to mention inverter.
    Good thing I'm not in a hurry, lots to consider.

    Thanks for the input!

  4. timber

    timber WindyNation Engineer

    I thought Paul Blyth made a compelling argument for a 12V battery bank at the following link, if interested. ... 82344.html

    12VDC Pros:
    Input ..
    - using the right MPPT controller would still allow a high solar panel voltage.
    - we know that the Windtura 500 will charge a 12VDC battery bank in low wind.

    Output ..
    - 12VDC to 110VAC Inverter although somewhat less efficient could be turned off unless/ until needed.
    - 12VDC supply circuit to house could be used directly without conversion (therefor less losses) for many household items, especially lighting.
    - 12VDC is common and grid charging relatively easy, in my case I already have a nice IOTA 12 charger.
    - Other 12V system components can be used, since I have quite a few of these.

    Storage ..
    -12VDC allows a greater number of combinations in battery configuration (i.e. using 6V batteries in groups of 2,4,6 or 8 etc.)
    ----- (So for 12VDC supply to house we can see some advantages.)

    12VDC Cons:
    Input ..
    - Wire sizes to batteries need to be sized to handle the extra current.
    - 12VDC to 120VAC Inverter will be less efficient and require larger wire sizes throughout DC system panel.

    Output ..
    - 12VDC will have less efficient runs through house (larger wire sizes, breakers and fuses required).

    So there we have it (I'm sure I missed a lot but we have some indications).
    Each voltage has definite advantages and disadvantages.
    With the purchase of the right controller, either voltage could be used in the future so this doesn't seem to be an issue.
    Battery configuration can change but may require additional batteries.
    Wire size and length of runs are going to be a challenge at 12V but should be workable.
    Initial and overall costs do carry a bit of weight for my system. I get the feeling that with a 12V system I can keep costs down some.
    To be perfectly honest, I'm not sold either way yet. Starting to lean to a 12V system, however.

  5. Tuicemen

    Tuicemen Solar Guru

    Did you ever move to a 24 volt system?
    I'm considering doing the same see:
    I over looked this post prior.
    My main reasons for moving to 24 volt is better efficiency and I can basically double my arrays without having to add extra charge controllers.
    In fact it will free up one of my controllers.:)

    I even contemplated moving to 48 volt however that would require even more changes and expense.:rolleyes:
  6. Tuicemen

    Tuicemen Solar Guru

    Since moving from 12 to 24 volt I've not been disappointed.
    12 volt lighting shouldn't even be a consideration here with the price of LEDs dropping.
    I too had my place wired for 12 volt lighting but converted it to AC.
    I replaced some 12 volt lights with 24 volt RV lights just as an experiment.
    Although these were nice and bright I soon fried them as peak input from charger was 30 volts far more then they would handle.
    The house lighting is now all LEDs with the odd CFL.

    Many of the top inverters have a charger built in, I went with Magnum and added the AGS module (already had a backup generator)
    So far the generator has only come on once as a result of low batteries. ( it still does its weekly 15 min maintenance run).
    Since my battery bank was already 12 X 6volt I only needed to reconfigure. I I had moved to 48 volt I would have had to replace the bank as adding new to old would mean the new batteries only performed as well as the old.
    Since I had few 12 appliances and these were dual voltage there was no need to replace those.

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