Lithium battery chemistry is volatile. If one battery can catch fire and cause a lot of damage because of improper charging, imagine what several batteries hooked together and charging in close proximity to one another can do if anything goes wrong.
Parallel charging might sound like a convenient way to charge multiple batteries at the same time, but it can be dangerous. Parallel LiPo charging is an advanced method of charging and requires a certain understanding of wiring and electricity. You should not attempt parallel charging if you are uncomfortable with calculating charges, sizing wire, or building safe parallel charging and balancing boards and harnesses. If you’re not an expert at hooking up the batteries in parallel, please don’t attempt it. When you connect batteries in parallel, you are actually doubling the capacity of the batteries while maintaining the voltage of “one” of the individual batteries. The charger can only “see” one battery instead of all the individual battery charges and is unable to prevent overcharging.
The only way parallel charging can be done safely is when each battery has exactly the same production parameters. As you might know, each battery is different regarding its date of production, chemical composition, age, and charge history. The more these parameters differ, the higher the chance of overcharging and thermal runaway.
Series charging is safer than parallel charging methods (keeping in mind it’s no less dangerous than normal charging). It is crucial that the charger is set up correctly. To charge six single-cell LiPo batteries with the same capacity, for example, the batteries must be wired in series. The charger should be set up as if the batteries were a six-cell LiPo. Only then is it safe to balance-charge your LiPos. Balance charging means that the charger charges each cell individually and safely. While series charging is a relatively safe method to charge multiple batteries at the same time, proper wiring can be complex and should not be attempted by beginners.
Multiport chargers can include their own balancing ports, eliminating the need for a separate balancer when charging lithium batteries. These chargers are the safest when it comes to charging multiple lithium batteries.