@spotxuv is right, if you have battery that has been over discharged it is possible to bring the voltage back high enough that the charger will charge it by charging in a different mode for a short period. 3S or lead acid mode should both do the trick. This method is a lot safer and more controlled than your method, which won’t limit current and could potentially discharge or charge the batteries at too high a rate.
What caused the battery to be over discharged in this way? Was there an issue that you experienced? As noted in our documentation, you should avoid discharging our battery below 12V, and never below 10V.
The BlueROV2 does not have a low voltage cutoff, and voltage should be monitored to avoid over discharging. We generally call a battery discharged and pull the ROV out of the water once the voltage starts dipping below 13V. Note that these voltage guidelines are specific to our battery and may not be appropriate for other batteries, especially lithium-polymers batteries, which are more sensitive to over discharging.