I started taking old radios apart as a very young kid, began my career in electronics at 16, was a licensed broadcast engineer at 19. That led to computers and all kinds of electronic gimcrack design. Having been making sparks for so many years it is easy to forget that some of the basic stuff might seem scary or mystical to folks just getting started. Today, let’s look at a couple of very basic and very important terms: series and parallel.
You will run into the use of series and parallel connections in solar power work, most importantly, perhaps, in the connections of the batteries and the PV modules. We’ll start with batteries.
Series: Have you ever seen a picture or video of a bunch of elephants walking in a line, trunks wrapped around the tail of the beasts in front of them? If you can conjure up that image, then you can get the idea of a series connection. Let’s say the trucking company just delivered 8 Trojan T105 batteries that Roberto sold you when you called Sun Electronics. What do you do with them?
They are 6 volt batteries, meaning there are 3 cells of 2 volts each connected in series. Now, 6 volts is not terribly useful in most solar power arrangements, which are usually 12, 24, or 48 volt systems on the DC side. So if you take 2 of the 6volt batteries you can make 12 volts. With 4 you can have 24 volts. 8 X 6 makes 48 volts. Easy, right? BUT, how do you connect them?
In series connections, you connect a cable from the first battery’s “+” positive terminal to the next battery’s “-” negative terminal. Just like the elephants, but not as heavy. Now, if you take a meter and put the black lead on the first battery’s negative terminal and the second battery’s positive terminal, you’ll find that the meter should read something over 12 volts. Ta Dahhh! You’ve just made a series connection. Add two more batteries in the same fashion and you have 24 volts. Add the rest of the batteries, trunk to tail, just like the first 4 and you end up with over 48 volts. If you put one in the string of batteries backwards, the 6 volts of that battery will be subtracted and the voltage will be below 48v. Pay attention to what you are doing!
Note: These cables need to be pretty fat because you may be dealing with hundreds of amps to feed your inverter. We’ll talk of that another time, but if you ordered a kit from Sunelec, you’ll have properly sized cables.
Parallel: Ok, you have 8 batteries and you have a 24 volt system. You placed 4 of your T105s in series to make 24 volts, but you still have 4 batteries left. What to do with them? Simple. Just make another 24 volt series of batteries right next to the first set. Make sure both lines have the “+” at the same end and connect those positive terminals together. Same thing with the 2 negative terminals. Now you have 8 batteries and 24 volts. This is called a series-parallel connection because you are connecting series strings of batteries in parallel to increase POWER (not voltage).
If you only have 24 volts and you can get that with 4 batteries, why do we use 8? We double the storage capacity this way. Let’s say the T105 is rated at a capacity of 220 amp-hours (AH). With one string we can multiply 220 AH X 24 Volts to discover a capacity of 5280 watt-hours. Let’s just round that off to 5000 or 5KWH (KiloWattHours). When we add the second set of 4 batteries, we double our storage to 10KWH. 10’s gotta be better than 5, right? You betcha.
Wait a minute, you are saying, I have a 12 volt system! You know you get 12 volts from two batteries, so do you connect 4 sets of two batteries? Exactly! With this connection, you now have 880 AH of storage and still have the 10KWH power capacity of the 24 volt system, just at a different voltage. If that’s confusing, don’t worry about it…it’s a good thing.
Here are some tips.
- Take a can of white paint and mark the POSITIVE corner of your new batteries. Trojans are dark red and the Sun batteries (and most others) are black, so the white paint will stand out better to avoid confusion.
- Have a multimeter. Even a cheap (or free with a coupon) one from your favorite Chinese tool store is all you need. Check each connection as you go to avoid sparks later.
- Wear safety glasses…some batteries can make sparks and hydrogen and under certain conditions that can be a bad thing.
- Cables that are too big are better than cables that are too small.
- Put shrink wrap or tape around the transition of where your cable goes into the terminal.
- Don’t make your cables any longer than they need to be. Big wire costs big money and long wire has more resistance.
- Put a coating of grease or battery spray on the connections to avoid corrosion.
- Cheap bolt cutter make dandy wire cutters when working with large battery cables.
If connecting that last cable to an inverter or other device containing a large capacitor, you WILL get a spark, so tap the cable to a flat spot on the terminal and not the screw stud (if applicable) to avoid making a weld burn that will make it impossible to tighten the battery nut. A better plan is to make the last connection at the inverter or fuse/breaker. Good Luck
By: Neal Collier