Spark when Plugging In

It is normal to have a spark if your battery connector does not have an anti-spark. Usually, the XT90 plug on your battery (The female plug) will have an anti-spark resistor inside, and you can identify it by seeing that it has a green color on the XT90 connector.

Nonetheless, the resistor inside over time could fail, and it will start having spark like the normal XT90 plug again. 

The spark is normal and it happens when the battery plugs in every time and charges the capacitor inside. 

To solve this either you change the plug on your battery side with a new XT90s female plug.
Or you should have a switch to switch off the battery before connecting.