Connecting batteries in series – BatteryGuy Knowledge Base
Connecting in Series Increases Voltage onlyConnecting Batteries of Different Voltages in SeriesConnecting Batteries of Different Amp Hour Ratings in SeriesConnecting Batteries of Different Voltages and Amp Hour Ratings in SeriesThe Age Factor of BatteriesBest Practice When Connecting Batteries in SeriesWhen You Can Mix Different Rated Batteries in SeriesAs discussed in this article, the closer the voltages and amp hour capacities of the various batteries wired together match, the less damage they will do to each other. Age also plays a part in these ratings and this is why it is usually recommended that you: 1. Only use batteries of the same voltage and amp hour capacity from the same manufacturer...See more on batteryguy Reddit
Can I safely connect two 18v Milwaukee batteries in series to
Probably fine, these 18v power tool batteries typically are pretty friendly for this sort of series connections since their BMS tends to be pretty rudimentary.