Every broker will charge interest if we use their funds, its not that they have excess funds and giving us interest free. This is for sure.
But what you all noticed with zerodha is also correct as follows if my understanding is correct -
Basically what you guys are referring here is not a stock pledge with broker to get trading exposure/limits. In this system the stock pledge is done with Depository and no real funds are involved to the extent of pledge and margin used. So if I pledge shares and get 10 lakhs collateral margin, it is a margin I got against my pledge from and allowed by ndsl/cdsl but not from my broker. Here broker funds are not involved.
Now when I want to buy a Nifty future at 1.5 lakh margin, as per rule, I can use 75 k from my pledge margin and other 75 k I must bring in cash.
So for the first 75k anyway the broker cannot charge interest as it leverage allowed by depository against our stock and the second 75k we must bring in cash and if we have shortfall in this, the broker adjusts the shortfall amount and will charge interest for using that broker funds.
Zerodha is not charging interest here for first 50% because it is not the party providing that 50% margin. And it charges interest in name of late fee at 18% for any shortfall funds in the balance 50% which clients must bring. So zerodha is not doing any free interest thing here.
Also we cannot trade all position's using this pledge margin. Equity delivery and options buying cannot happen under this. This is for equity intraday, futures and options writing intraday or carry forward . Because equity bought by a party is sold by another and amount must be settled in equity delivery segment. So a exposure allowed by depository against my stock wont work as cash that has to be paid by eod. Since fno are limited period contracts and just p/l settled the product is feasible in this segment.