BNB staking mechanics

BNB Liquid Staking lets the user stake their funds through the corresponding smart contracts on the Binance network, accumulate rewards, and receive their stake and rewards when unstaking.

The following section explains staking requirements, fees, rewards, validators, smart contracts and function calls to interact with these smart contracts.

Requirements

The requirements when staking are:

Minimum value to stake — 0.1 BNB.
Maximum value to stake — unlimited, at the user’s discretion.

The requirements when unstaking are:

Minimum value to unstake — 0.5 BNB.
Maximum value to unstake — up to the initial stake for ankrBNB.
Unbond time (before user gets their unstaked funds) — 7–15 days.
Splitting large unstaked amounts between several unbond times due to limited nodes capacities.

Fees

Ankr takes a 10% technical service fee from the staking reward.
When unstaking, the user pays nothing, while Ankr pays:
- Unbonding transaction fee (undelegate_fee) — 0.004 BNB.
- Cross-chain transaction fee (transfer_out_fee) — 0.000075 BNB.

The user must also count in the gas price for outgoing transactions.

Rewards

Validators receive rewards every day, at midnight, UTC.

The APY is calculated from the validators' rewards.

The rewards the user gets are calculated using the exchange ratio explained later in this document.

To understand BNB Liquid Staking, you need to know the entities and understand the workflow under the hood.

The following entities are involved:

Smart contracts
Ankr addresses
Ankr validators

Smart contracts

Smart contracts and addresses involved in BNB Liquid Staking are:

BNBStakingPool (opens in a new tab) — contract on BNB Chain where the user sends their initial staking or unstaking request.
Staking (opens in a new tab) — withdraws the rewards and send them to BNBStakingPool.
aBNBb Proxy (opens in a new tab) — contract on BNB Chain is an intermediary in the ankrBNB staking and unstaking process.
ankrBNB Proxy (opens in a new tab) — contract on BNB Chain that mints or burns ankrBNB tokens for the user. All interactions go through the Proxy part.
TokenHub (opens in a new tab) — contract on BNB Chain that makes cross-chain transfers between BNB Chain and Binance Chain.
Intermediary address (opens in a new tab) — BNB backend service on Binance Chain that:
- When staking, receives the staked funds from BinancePool to send to the validators.
- When unstaking, receives the unstaked funds to later crosschain-send it to the operator address.
Operator address (opens in a new tab) — BNB backend service on BNB Chain that:
- When unstaking, receives the unstaked funds+rewards to send to stakers.

Validators

Staking workflow

User sends a request to the BNBStakingPool::stakeCerts({value:stake}) on BNB Chain. stake specifies the staked amount and should meet the requirements described above. BNBStakingPool verifies the request checking the _minimumStake and mints ankrBNB in the amount defined by the current exchange ratio, and then issues a Staked() event with the staker, amount (staked amount), shares (issued ankrBNB), false (because ankrBNB is a reward-bearing token) parameters.
BNBStakingPool accumulates minimum 100 BNB and then sends the accumulated amount via BNBStakingPool:delegate(validator, amount) to an address of one of the validators from the Ankr set.

Exchange ratio

When staking, the user receives:

ankrBNB in the amount calculated the following way: user's_stake * exchange_ratio.

When unstaking, the user receives their_stake + accumulated_rewards in BNB. It is calculated by the following formula: accumulated_amount_ankrBNB / exchange ratio.

The exchange ratio is calculated by the following formula:

uint256 totalShares = totalSharesSupply(); == ankrBNB.totalSupply();
uint256 denominator = _totalStaked + totalRewards - _totalUnbondedBonds;
_ratio = (totalShares * 1e18) / denominator;

Unstaking workflow`

User sends a request to the BNBStakingPool::unstakeCerts(shares) on BNB Chain. shares specifies the amount to unstake. You can also unstake to an arbitrarily chosen address via BNBStakingPool::unstakeCertsFor(receiverAddress, shares). BNBStakingPool verifies the request checking the _minimumUnstake and balance_of_user, to transfer ankrBNB from user to the aBNBb smart contract address (input amount) then aBNBb::burn() executes to decrease total supply of tokens, and then issues a Unstaked() event with the ownerAddress (message sender), receiverAddress (user address for receiving unstaked funds and rewards), amount (amount of BNBs to be send to the user), shares (amount of ankrBNB to unstake), false (because ankrBNB is a reward-bearing token) parameters.
BNB backend service waits until _pendingTotalUnstakes reaches minimum 100 BNB at BNBStakingPool, then calls BNBStakingPool:undelegate{value: relayerFee}(validator, amount).
After the UnbondTime, BNBStakingPool gets the rewards via Staking:claimUndelegated() and distributes unstaked funds and rewards to the users.

Additional information

Additional details To get more information about staking on Binance, read Staking (opens in a new tab), Delegating (opens in a new tab), and Binance cross-chain transfer (opens in a new tab).

To get more information about unstaking on Binance, read Unbond (opens in a new tab) and Binance cross-chain transfer (opens in a new tab).

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