If you've been bankrupted and knocked out of a trading battle, you cannot add more funds to the battle account to re-join. Each trading battle is based on a fixed amount and the trader with the highest PnL wins.
In a trading battle, you can only trade with your initial stack. If you are bankrupted, you will have to join another battle to have a chance of winning a prize.
The rules of Interdax's trading battles regarding balances are subject to change. Any changes to Trading Battles will be posted on our Announcements page.