Only in my car the battery is in with you (in fact I sit on it - but with a chair in between to make it more comfortable and less potentially painful). And it is not full of water, but instead uses jelly. Although I've no idea what flavour. This makes the battery very expensive and special order only. And so up until now the expense and the fact that when I disconnect the old battery and plug in the new one it will probably break the central locking and stuff has put me off.
But then again after over seven years I'm probably due a new one. But I will resist the temptation to open the old one and find out what flavour it really is.