A lot of people here trying to guess what you are asking and then giving you answers to the questions they made up in their head rather than trying to find out what you are asking so that they can give you the correct answer.
When you use a backup of your wallet file, your last balance will appear.
This will depend on the wallet he is using, and how long ago the backup was made. This could potentially be very bad advice. Why would you want to be responsible for giving advice that could cause someone to lose a lot of money?
Everything is stored on the blockchain, so it will restore only the valid balance according to the blockchain. If you were able to restore old, invalid balances it would be a major flaw in bitcoin.
You are correct that it won't restore old, invalid, balances. However, it may not restore his entire current balance either. That depends on how long ago the backup was made, and what wallet program he is using.
According to Securing your wallet on bitcoin.it
https://en.bitcoin.it/wiki/Securing_your_wallet#Securing_the_Bitcoin-QT_or_bitcoind_wallet by default there is only 100 keys in the key pool. You can manually specify a larger key pool with "-keypool" command line argument.
What this means is that after 100 transactions new keys are generated to replenish the old ones(by default).When this is done you need to have a backup of this new wallet as keys have changed. Your safest bet is to routinely back it up after creating new addresses or after sending a couple of times.
read over the securing your wallet section on the link as it will give a better explanation. Unless you regularly create new addresses or send you should be fine with backing up every few weeks/months.
Having too many backups won't hurt of course and be sure to test your backups on another machine to make sure the wallet.dat is still functioning correctly.
Where does he say he is using Bitcoin-QT or bitcoind? He says "balance I had before I lost my
phone". I'm not aware of any phones that will run Bitcoin-Qt or bitcoind. This advice would be great if it was for the wallet he is using. Unfortunately, it isn't.
Using btc-qt i like to backup eveeytime i send, not recieve. I heard the chamge adreses would be backed / known without hqving to back them up but i do it anyways. For the biger wallets i encrypt with pgp etc.
Much like the response before yours, you've decided that the OP is using btc-qt. This would appear to be a very bad assumption.
retrojka,
What wallet are you using on your phone? You need to provide this information before you can get a good answer about recovering backups.
In general, as long as a backup is recent enough and contains the private keys to all the addresses that the phone had used to receive transactions (including change), then recovering a backup will restore the entire balance that you had before the phone was lost.
As has already been pointed out, the record of the bitcoins and the addresses they were sent to are permanently recorded in the blockchain that every full node on the network has a complete copy of. All your wallet has are the private keys that it uses to create transactions to transfer control of those bitcoins to someone else's address. If you restore all the private keys, then the wallet can look up all the necessary transactions in the blockchain and you'll regain access to all the bitcoins you had when the phone was lost. Depending on the wallet, it may have generated additional addresses and private keys (and may even have kept them hidden from you) after you created the backup. It is important to understand the proper backup requirements for the specific wallet you are using.
There are 3 common types of bitcoin wallets.
Deterministic wallets use a single seed value and generate all private keys in a repeatable way from that seed. This means you only need to have a single good backup of seed, from that you'll be able to re-generate all the private keys that the wallet ever created and regain access to all the bitcoins you had when the wallet was lost.
Non-deterministic wallets generate private keys completely randomly in a non-repeatable way. In this case you need to be sure that the private keys for all the addresses that have received any unspent bitcoins are backed up. There are 2 common types of non-deterministic wallets.
Wallets like MultiBit send the change from any transaction to one of the receiving addresses that you've generated for the wallet. Since they re-use addresses in this way, you need to update your backups whenever you generate a new receiving address.
Wallets like Bitcoin Core use a new address for the change every time you send a transaction. These "change addresses" are kept hidden from you, therefore many of your bitcoins are received at addresses that you aren't even aware that you control. To prevent you from needing to create a new backup every time you send a transaction, wallets like these pre-generate many addresses. That way these pre-generated addresses are stored in the backup before the wallet gets around to using them. Wallets like this need to be backed up on a regular basis before the pool of pre-generated addresses are used up.
