Is Parallel Charging Safe?
For those of you unfamiliar with the term, parallel charging refers to connecting multiple batteries in what is known as a parallel circuit. That means if you have two or more battery packs you connect the plus terminals of all your batteries together and connect that to the plus on the charger. You also connect all the minus leads of your battery packs together which goes to the minus lead of the charger. So basically you have plus to plus to plus etc. and minus to minus to minus etc... If you drew the batteries side by side on a piece of paper the connections would look like two parallel lines.
OK that is simple enough but why do this and is it safe? The big reason to do this is that IF you have enough power in your charger you can charge multiple battery packs in the same time it takes to charge one on a single charger. For example our fully decked out Xtrema charger can balance charge up to four 6s Li batteries simultaneously! If they are under 2,000 mah each they can all be ready in about one hour or less.
So this sounds exciting but is it safe? The quick answer is yes if you exercise a little bit of care. OK, so what does that mean? Well the first thing to understand is that when you parallel packs together they MUST be the same cell count. That means you cannot mix 2s and 3s packs together or any other combination. They MUST be all 2s or all 3s or all 4s etc. The reason is that the voltage (or cell pressure) must be very close or the larger cell pack will dump its charge into the lower cell pack and it will do this with a spark, heat and possibly fire will follow. Not a good time.
The good news is that they can be different mah ratings. So you can mix say a 1,000 mah pack with 2 1500 mah packs at the same time. So what do you set the charger to? Well for a safe one hour 1C charge you simply add up the mah and set your charger to the total. Example: 1,000 mah + 1,500 mah + 1,500 mah = 4,000 ma charger setting. If your batteries can handle a 2C charge rate then just double that.
OK, so you think you can pretty much feel comfortable that you will charge only same cell count packs since they all probably need to be the same to match one or more of your models. Now lets say you have a fully charged 6s pack and you connect it to another fully DIScharged 6s pack. The voltages are pretty different here and the end result is the fully charged pack will dump its charge into the fully discharged pack in a big hurry (again) causing a big spark, heat and possible fire may follow.
So how can this be safe? Well, the trick to remember is that you simply need to keep track of your discharged batteries by any method you chose. (moving them into a special box or LIPO sack that is separate from the fully charged packs is a typical way to keep track) Now you may be thinking what if my flights were a little longer one flight and less another and the voltages are different, what do I do now? In our experience we have found that if you time your flights or discharge the batteries until the speed controller cuts off then they are close enough to connect in parallel.
You see when batteries are near the end of the discharge curve they do not have much remaining power. In general we find that if the voltage is within 10% of each other it will be perfectly safe. One nice thing about the Xtrema charger is that the built in wattmeter can be quickly used as a VOLT meter to do a quick test before you connect them in parallel.
For more information and to actually see how to parallel charge your packs, check out our video below: