If your wallet has outgoing transactions, the balance displayed will not be correct. To get a correct balance in a view-onlywallet, you have to import the accompanying key images of each output of the wallet.
Next, create your view-onlywallet by typing `monero-wallet-cli --generate-from-view-key wallet-name`. The last argument will be your new wallet's file name. You will be prompted for `Standard address` and `View key` by the wallet. Paste in your original wallet's address and private (secret) view key. Next, enter and confirm a password for your new wallet.
Open an existing wallet and type `address` and `viewkey` commands to display the wallet's address and its private (secret) view key. Type `exit` to close the wallet.