How Does DeFi Lending Work? Collateral and Interest Rates

Lending and borrowing are among the oldest uses of decentralized finance, and they work very differently from a bank. There is no credit check, no loan officer, and no fixed repayment date. Instead a smart contract pools deposits, lends them out against collateral, and prices interest automatically from supply and demand. This guide walks through the mechanism step by step.

Why is DeFi lending over-collateralized?

A bank can lend you money unsecured because it knows your identity and can pursue you if you default. A lending protocol knows nothing about you. To lend safely to anonymous users, it requires you to post collateral worth more than you borrow. This is called over-collateralization.

Suppose you deposit 1 ETH worth 3,000 USDC as collateral. The protocol might let you borrow up to 75% of that value, or 2,250 USDC. That 75% ceiling is the loan-to-value (LTV) ratio. You now hold borrowed stablecoins while keeping exposure to your ETH, which is the main reason people borrow instead of selling.

The obvious question is why anyone would lock up more value than they receive. Common motives include staying invested in an asset while raising cash, leveraging a position, or accessing liquidity without triggering a taxable sale. The protocol does not care about your reason; it only cares that the collateral stays sufficient.

How are interest rates set without a central bank?

Interest rates in protocols like Aave and Compound are algorithmic. They move with utilization, the fraction of pooled deposits currently borrowed.

• When utilization is low, most funds sit idle, so the protocol keeps rates cheap to encourage borrowing.
• As utilization rises, rates climb to attract new deposits and discourage further borrowing.
• Past a kink or optimal point, rates spike steeply to protect the pool from being fully drained, ensuring lenders can always withdraw.

Both the deposit rate and the borrow rate float in real time. Depositors earn the borrow interest minus a protocol reserve cut, so lender yield is always somewhat below what borrowers pay. Because rates recalculate every block, a position that is cheap today can become expensive if demand surges.

What happens when collateral loses value?

The system's safety depends on collateral always exceeding debt. When a borrower's collateral value falls, or their debt grows through accrued interest, they approach a threshold called the liquidation point. Cross it and the position becomes eligible for liquidation.

Liquidation lets third parties, often bots, repay part of your debt in exchange for seizing your collateral at a discount, typically 5% to 15%. That discount, the liquidation bonus, is the incentive that keeps liquidators watching. The process protects lenders by clearing risky debt before it can go underwater, but it means a borrower can lose a chunk of collateral quickly during sharp price drops.

Two numbers govern your safety margin:

• Health factor: a ratio above 1 means your position is safe; at or below 1 it can be liquidated.
• Liquidation threshold: the maximum LTV before liquidation triggers, usually a few points above your borrowing limit.

Prudent borrowers keep a wide buffer, borrowing far below the maximum so that ordinary volatility does not push them over the edge.

What role do price oracles play?

The protocol needs to know the market value of collateral to enforce these rules. It gets this from oracles, services such as Chainlink that feed external price data on-chain. Oracles are a critical dependency: if a feed is manipulated or delayed, the protocol may liquidate healthy positions or fail to liquidate unhealthy ones. Several historical exploits targeted exactly this weak point, using flash loans to distort a price the oracle read.

Putting it together

DeFi lending is a self-contained machine. Depositors supply liquidity and earn a floating yield; borrowers post excess collateral and pay a utilization-based rate; oracles value the collateral; and liquidators enforce solvency for a bounty. No single party is trusted, which is the point, but it shifts responsibility onto you to monitor your own health factor. The main risks are collateral volatility, rate spikes during high demand, smart-contract bugs, and oracle failure. Understanding each lets you use these markets deliberately. This is educational information, not financial advice.