Short answer. You do. As the bailee — the person who borrowed the item for free — you are obliged to pay for the ordinary expenses needed to use and preserve the thing loaned. The lender is not expected to keep covering upkeep costs while you have possession and use of it.
What the law says
The bailee is obliged to pay for the ordinary expenses for the use and preservation of the thing loaned.
Civil Code, Article 1941 — Ordinary Expenses. Read the full provision →
The obligation falls on the borrower, not the lender
Article 1941 puts the cost of ordinary upkeep on the bailee — the person who received the item under a gratuitous loan. Because you are the one actually using the item, the law expects you to also be the one keeping it in usable condition through the ordinary expenses that come with that use, rather than passing that cost back to the lender who is receiving nothing in return.
The obligation covers use and preservation
The article ties this duty to two things specifically: expenses for the use of the thing and expenses for its preservation. This covers the routine costs of operating the item as intended and keeping it from deteriorating while it is in your hands — the ordinary running costs of having something on loan, not a one-time event you handle only when the item is finally returned.
The word 'ordinary' is doing real work here
Article 1941 speaks only of ordinary expenses. It does not say anything here about extraordinary costs — a major repair from unexpected damage, for instance, rather than routine upkeep. Whether an unusual, larger expense should fall on the borrower or the lender is not something this article resolves; it addresses the everyday cost of keeping a borrowed item in working order, not exceptional ones, and that gap should not be assumed away by either party.
Why the rule is written this way
A gratuitous loan is done as a favor, so the law does not want the lender to end up quietly absorbing the running costs of an item they are not even using. Placing ordinary upkeep on the borrower keeps the arrangement fair for someone who is lending something out of goodwill and getting nothing in exchange, while still leaving the harder question of unusual, extraordinary costs to be worked out separately.
How extraordinary expenses are actually handled
A separate provision fills the gap Article 1941 leaves open. The lender must refund extraordinary expenses for preserving the item, but only if the borrower told the lender before incurring them, unless the need was too urgent to wait for a reply. If instead the extraordinary expense arises from the borrower's actual use of the item, even without any fault on the borrower's part, the cost is split equally between borrower and lender, unless they have agreed otherwise. And any expense outside both the ordinary upkeep this article covers and that extraordinary-expense rule is simply not reimbursable at all — the borrower absorbs it.