building small solar generator

Re: building small solar generator

Welcome to the forum TH,

Regarding the system you wish to build. In general, it is not very practical to add to an existing system. Adding new batteries to an existing system, or adding mixed AH rated batteries (like you are thinking about) can make it difficult for the batteries to properly share charging and discharging current.

Either, design a system to support your needed loads. Or, define a system based on solar array or battery size, and build the rest of the system around the basic components.

Regarding 24 volt solar panels and such... Basically, if you are using an PWM (inexpensive) solar charge controller, the Vmp of the array should be ~17.5 volts for a 12 volt battery bank or ~35 volts for a 24 volt battery bank. If you install a "24 volt" panel on a 12 volt battery bank with a PWM controller, then you will lose around 50% of the array's capabilities.

With MPPT (Maximum power point tracking) charge controllers, they can take "high voltage" / low current from the Solar Array and efficiently down convert it to low voltage / high current required to recharge the battery bank. The Vmp-array needs to be around 17.5 volts minimum for a 12 volt battery bank, but can be much higher (depending on specific MPPT controller).

There are several reasons why MPPT controllers can be a better choice. One is a higher Vmp-array voltage allows an array to be farther away and/or use smaller gauge wire back to the battery shed/charge controller.

Second is that most solar panels > ~100 watts are much cheaper per watt ($$$/watt) than smaller solar panels. However, these higher wattage solar panel are not usually of the 17.5/35 volt Vmp panels but "random" voltages that work very well with Grid Tied solar system which can have Vmp-array in the 200-400+ VDC range.

By itself, taking two 17.5 volt panels to recharge a 12 volt battery bank will charge just as fast in series or parallel when using a MPPT type charge controller (Power=Voltage*Current). So, if you double the voltage (series connection) it will be the same power as if you doubled the current instead (parallel). Of course, if you have a 24 or 48 volt battery bank, you will need a higher voltage Vmp-array to properly charge the higher voltage battery banks.

Here are a couple threads that you may find interesting... The first is a stand alone system with lots of discussions about choices and details of the parts needed. The second is a thread (with video) about installing a small system in an RV trailer:

Emergency Power
Kevin's RV

And here is another example by Mike90045 called the Solar Monolith:





Update pictures/information here.

If you can define your needs (battery capacity, solar array, or Watt*Hours/Amp*Hours per day), it will be a bit easier to show you the choices/options you have . A generic discussion could be anywhere from a 20 watt panel to a 4kW solar array. And the differences/choices are much different depending on which it is we are discussing.

-Bill