Do you need 24v panels to run a 24 volt battery bank?

nobody928
nobody928 Solar Expert Posts: 62 ✭✭✭✭
I would like to convert to a 24 volt battery bank by running 4 105 deka agm batteries in both series and parrellel. What do I need other than a 24v power inverter? What would be advantages and disadvantages?

Is this a good idea for a small 560 watt array?





4 x deka 105 amp hour a.g.m.'s, 4x 140 watt ul 12 volt panels, xantrex c40 charge controller.

Comments

  • Cariboocoot
    Cariboocoot Banned Posts: 17,615 ✭✭✭
    Re: Do you need 24v panels to run a 24 volt battery bank?

    First a couple of basic information threads:

    http://forum.solar-electric.com/showthread.php?15989-Battery-System-Voltages-and-equivalent-power

    http://forum.solar-electric.com/showthread.php?16241-Different-Panel-Configurations-on-an-MPPT-Controller

    So as you rearrange your 12 Volt 105 Amp hour batteries to two parallel strings of two in series to get 210 Amp hours @ 24 Volts you must also arrange your array to deliver Vmp around 35 in order to have enough Voltage available to charge the batteries. With "true 12 Volt" panels you wouldn't even need an MPPT controller to do this: simply change the four panels in parallel to two parallel strings of two panels in series - the same reconfigure as the batteries. If the array worked @ 12 Volts it will work @ 24.

    If those panels are Kyocera 140's they produce a Vmp of 17.7 and an Imp of 7.9. So on the 12 Volt system using a PWM type controller you would have got a peak current of 4 * 7.9 or 31.6 Amps. With 420 Amp hours @ 12 Volts that would be a charge rate of 7.5%. A tad low, but workable.

    Rearranged for 24 Volts the array would now be Vmp of 35.4 and the current 15.8 Amps on a PWM controller. The battery bank becomes 210 Amp hours @ 24 Volts and the charge rate is still 7.5%.

    Efficiency goes up slightly, with less V-drop through the (same size) wires do to the change in ratio of V to A for the power delivered. With an MPPT type controller you might squeeze a couple-three more amps out of the array, which would increase the charge rate another 1%. How much better will depend on things like distance between array and controller and operating conditions (chiefly temperature).
  • niel
    niel Solar Expert Posts: 10,300 ✭✭✭✭
    Re: Do you need 24v panels to run a 24 volt battery bank?

    i'll try to clarify this a bit for him as it gets confusing when some 24v pvs can't be used to charge 24v batteries. the confusion comes as a result of the voltage open circuit (voc) and the voltage max power (vmp) are referenced and can mean the difference between working or not. this also does not encompass the higher buffer voltage one would want to operate the controller and the headroom to account for high temps.

    generally the 12v pvs will have about 17.0v vmp or a bit more while 24v pvs will have 34.0v vmp or a bit more if referenced to being suitable for the same in battery voltage. these voltages are typically about 18v to 18.5v typically for 12v pvs and 35v to 36v for 24v pvs these days that are used in pwm applications for 12v and 24v battery systems respectively. another way is to go by the number of cells in the pv. most 12v pvs suitable for 12v batteries have 36 cells while those suitable for the 24v batteries will have double that for 72 cells. the number of cells is the best overall indicator if a pv is 12v, 24v, or something else. there are a few exceptions of course such as those using thin film technology for example.

    gt pvs have cells in numbers outside of those i've stated such as 60 cells being typical. these are often referred to as 24v pvs, but not in reference to the appropriate battery voltage. these types are not well suited for pwm controllers and therefor not for the same battery voltage because there is either a higher power loss due to excessive volts available or insufficient voltage for consistent operation.