under what conditions should you charge interest on money loaned...

[jackdaniels]

I've been in a similar situation. The solution is to simply explain to your friends that while you do not have the CASH handy, your credit is good and you would be happy to lend them the money charging them only what the bank charges you. That way you can still be a good friend and lend out some cash, while charging some minimal interest that is fair and reasonable - 5%-7% per year, compounded, just like the bank would - money costs money after all.