Many clients ask us to make cables to power their cameras from a multitude of sources, one of those being from the Freefly Movi M10 and M5 lithium polymer batteries. While it's physically possible to make a JST-RCY cable for Alexa Mini, Red DSMC, or other high-draw cameras, it is not advised.
While Freefly advertises their M10 battery to have a discharge rate of 7.8A, JST-RCY connectors are only rated for 3A MAX by the manufacturer. To get an idea of what that means in real world situations, the Alexa Mini is rated to have a 55 watt (3.8A at 14.4V or 4.5A at 12V) draw at normal operation (not including accessories). The Red Weapon's DC input requires cabling that can handle up to 9A at 11.5-17 volts. The current draw of any camera is almost always listed in its operation manual.
Since the Freefly (and other RC batteries) do not contain any safety circuitry, the battery cannot shutdown in case of overheat or short. If the JST-RCY housing were to melt and cause the contacts to short against one another, the battery would not shut itself off, and the batteries could catch fire or explode.
If the size and weight of your battery is a huge concern, it would be best to use a battery with connectors and wire gauges that can handle the current draw required by high-draw cameras. A possible solution would be to use the Freefly batteries that use EC5 connectors: http://store.freeflysystems.com/collections/power/products/freefly-tero-battery