Portable Solar Power Generators, Revisited

Today, I came across a portable solar power generator that offered a battery expansion kit.  And at first glance, it seemed like a really good feature.  It allows you to buy the generator today that can grow in size with your needs.  After all, how hard is it to add batteries to your battery bank for more capacity? 

Unfortunately, it is not as simple as plugging in another battery bank.   Here is a list of items you need to be aware of before you invest in an expandable solar power generator:

1)      Performance – adding new batteries to an existing battery bank will automatically reduce the new battery’s performance.  For example, your existing batteries may only be able to achieve a 90% state of charge due to age.  Adding a new battery will cause the battery to reduce to a 90% state of charge.

2)      Uneven charging/ discharging – adding new batteries will generally be done by paralleling the batteries.  The number one rule when paralleling batteries is to keep the resistance in each parallel feed as equal as possible.  Failure to do so causes uneven charging and discharging, which will destroy a battery bank.

3)      Battery Maintenance – batteries need to be brought back up to a full charge.  Adding batteries to an existing battery bank means that both your solar array and charge controller will most likely need to be upgraded. 

There are more items for consideration when adding to an existing battery bank.  This list is only meant to highlight some of the bigger issues that can cause more harm than good to your portable solar power generator.  As always, take care of your system and it will perform for many years to come.

Micro-Inverters and the Apocalypse

For those fans of “The Walking Dead”, you may have noticed a micro inverter in last night’s episode.  Therefore, we felt that it is a good time to talk about what a micro-inverter is.

In order to use the power from your solar panels for appliances and household electronics, you have to utilize an inverter.  An inverter inverts the DC power from the solar panels into AC power.  Currently, in the residential market, there are two types of inverters.  There is the string inverter and there is the micro-inverter.  While they both serve the same function in the end, they both do it in different ways.  With a string inverter, the groups of solar panels are wired together in series to form a string.  With a micro-inverter, each solar panel is connected to one micro-inverter.

Why the two options?  Both inverters have their merits and there downfalls.  A string inverter is the most cost effective way to go solar and if the string inverter fails, it is easy to replace.  The downside to a string inverter is that any shading on one solar panel in the string can seriously hurt the power production of that string.  A micro-inverter doesn’t have the shading issues of a string inverter.  If one solar panel is shaded, only that panel and inverter are reduced in power production.  The remaining solar panels will still perform.  With this same idea, if one micro-inverter fails, it doesn’t affect the rest of the solar panels and their production.  The downside to micro-inverters is both their cost and replacement.  Since micro-inverters are installed under the solar panels, it can be difficult to replace them.

In the next article, we will discuss how shading impacts a solar array in more detail.  At least now, you know what a micro-inverter was doing in the apocalypse.

Ground Fault Protection

What is ground fault protection and do you really need it?  A ground fault is an abnormal condition in which current is traveling along the ground wire, which is normally a non-current carrying conductor.  This means that something in the Solar Array is shorting out, causing the current to flow into the ground wire.  If the condition is left to remain, the point where the short is happening will eventually fail causing a fire.  This can be seen in two recent devastating fires caused by Solar Array Ground faults. (Bakersfield, CA and Delanco, New Jersey)

Now is where the question arises, doesn't a circuit breaker of fuse remove the ground fault?  In a ground fault, a circuit breaker of fuse may not open clearing the ground fault.  The reason behind this is that in order for a circuit breaker or fuse to trip, the current flowing through that device must be higher than the devices rating.  For example, a 20 ampere circuit breaker needs over 20 amperes to trip.  In a ground fault there may not be enough current flow to trip the breaker.

The problem with ground faults is that not all of the current is flowing into the ground wire.  There is always a certain amount of resistance that happens in a ground fault that reduces the amount of possible current.  This means that even though there is a fault, it is not high enough to trip the circuit breaker of fuse that is normally in the system.  Therefore, ground fault protection is added to the circuit.  In general, a ground fault protective device is set to trip as low as 1 ampere.

A ground fault protection device will disconnect any loads on the Solar Array.  By disconnecting the loads, there is no longer a flow of current stopping the short circuit from persisting.  Now the Solar Array is brought to a safer condition until the short can be removed from the system.

Now for the million dollar question, do you need one?  If you are planning on putting solar panels on your dwelling, the answer is yes.  A ground fault protective device will help prevent your house from catching fire due to a ground fault.  Remember, ground faults generally do not trip your normal circuit breakers or fuses.  On the other hand, if your solar panels are going to be ground mounted, this isn’t a requirement, but still a good idea.

Having a ground fault protection device installed in your Solar Array is like having insurance.