Series or Parallel - Which current flow option do you make when installing solar panels or battery banks?
There seems to be an increasing need to explain the differences between series and parallel electrical circuits recently, especially regarding battery and solar panel installations. So this week’s blog will be an effort to better explain the advantages and disadvantages of each configuration, and where better to start than with the water analogy … again.
Let’s say we have three water-cooled air conditioning units and they will be sharing one sea water pump. We can connect hoses between the pump, the units, and the discharge thru-hull in one of two ways - series or parallel.
A series connection will direct the water feed from the pump to the inlet of the first unit, through that unit, and then from the outlet of that unit to the inlet of the next one in line. The water will then pass through the middle unit, on through the last one in line, and then on to the discharge thru-hull. This makes the plumbing simple, but is not such a smart idea because the water coming out of a water-cooled air conditioning unit is at a higher temperature than the water going in. So the middle unit will be using water that has been pre-heated by the previous unit, and the poor last unit is left struggling to survive on water that has been superheated by the two previous units. Not good.
So we must make our air conditioning water connections in a parallel configuration. This requires splitting the water discharge from the pump into three individual feeds to the three units at a manifold, and then each unit will be receiving an equal flow of cool sea water. The discharges from the units can either come back together in a manifold and then to a single discharge thru-hull, or to individual through-hulls if more convenient.
Looking at the difference in water flow with each configuration, in the series example the full flow from the pump is forced through each unit, slowed down only by resistance from restrictions, etc. In a parallel plumbed set-up, the flow from the pump is split into three branches, so each unit will be receiving only 1/3 of the flow from the pump.
Now, moving to Solar Panels:
The silicon cells in solar panels are wired in series, i.e. the positive side of one cell connected to the negative side of the next and so on. We have to wire them that way because each cell is only capable of producing between ½ volt and one volt, so by stringing at least 32 of them in series it will increase the voltage incrementally through each cell, like the water getting increasingly warmer through the air conditioning units plumbed in series. So we will have at least 16v measured between the wire going into the positive side of the first cell and the wire coming out of the negative side of the last of the 32 cells. And just like our air conditioning units, where any restriction anywhere like a kinked hose will slow down water flow through the units, a shaded cell(s) anywhere will slow the electrical flow (amps) through the series string of solar cells.
Silicon cells must be wired in series in the panel to achieve the necessary voltage, but if installing two or more solar panels we have the choice to connect them either in series or in parallel.
Wiring two panels in series will effectively connect all the cells in both panels into one long series chain, so any shading on any cell(s) anywhere will slow down the current flow. Wiring panels in series effectively multiplies the voltage output while keeping the current flow low and constant, so wiring can be accomplished using relatively small gauge cable.
Wiring two panels in parallel will multiply the current output but keep the voltage constant. This requires larger gauge cable between the parallel point and the controller than when connecting panels in series due to the higher current output, but the potential effect from shading is less than with a series configuration. This is because any shading on any cell(s) in one panel will affect just that panel alone, and this is accomplished with the installation of a Blocking Diode on the positive lead from each panel to prevent back-feeding. This unfortunately drops the voltage output of each panel by a little over half a volt, but the Blocking Diode is a safety feature that should not be omitted due to the potential fire risk.
Moving the discussion to Battery Banks:
Battery banks can also be wired in series or parallel. I’m not a big fan of wiring low voltage cells in series due to the points raised here in an earlier bog: http://www.coastalclimatecontrol.com/index.php/blog/233-dead-cool-battery-banks.html. Utilizing a battery bank of individual 12v, 24v or 48v batteries wired in parallel to increase amp/hour capacity means that each battery is asked to accept or deliver a much reduced current flow than low voltage, high capacity batteries wired in series to increase voltage. Just like the water flow in parallel-plumbed air conditioning units, the charge and discharge current in a parallel-wired battery system is evenly split between each battery, allowing currents to be kept well within the battery manufacturer’s suggested maximum, even with high output charge sources and high current draw loads.
Ah, you say, but batteries are only generally available in 12v or lower voltages. Well that may be true of the lead acid variety, but exciting things are happening in the lithium ion world, where higher voltage batteries are available in standard BCI group sizes.
Coastal Climate Control is right on the cusp of diving into the lithium ion world, and will soon be offering a complete line of marine Lithium Iron Phosphate batteries that are now safer than lead acid. Watch this space!