Bordering on Fridge Horror, there's a deeper meaning to why Chiaki wants to simply see that painting. According to Chiaki's aunt, the painting she restored was created a long time ago during a time of war and famine. It was meant to inspire peace and hope during such a time. Chiaki probably comes from a time when (as the Aunt so puts it) it seems like the world's going to end. Perhaps Chiaki wants to see it in its restored form so he can have something to keep him going in the future.
If you keep track of the timeline, or even just the set pieces, you will realize that Makoto discovered her powers, got asked out by Chiaki, and had her heartfelt goodbye at the exact same spot at the river, at the exact same sunset. In fact, the entire movie never progressed beyond that last sunset until that last goodbye, which is when everyone finally moves on and the Airplane of Love beautifully tops off the ingenious Three Parts of the Story created by Makoto's choices at this single junction in time.
After Makoto hit the nut-like shell and had hallucinations, she told this story to Kousuke and Chiaki with Chiaki laughing really loud. Chiaki was really laughing real loud because he may have found out who used his time travelling charger. As he said on another timeline to Makoto, he was really nervous that the charger might have went to a malicious-intent person but was really glad that the one using it was an idiot aka Makoto. Too bad that discussion about the hallucinations never happened due to Makoto's time-leaping to repeat that whole day.
Chiaki's wish to date Makoto, even though he wanted to return to the future from the get-go.
Plus, if you're traveling back in time, and you have a consumable device that lets you do it and might get lost/be used by someone else, bring more than one.
Strictly speaking, though, he did. Chiaki was leaping using charges that had already been transferred onto him, and had the charger as a backup in case he ran out, in much the same fashion as one might bring along spare batteries for an electronic device.
Technically, Chiaki wasn't even supposed to be there so by taking even one charge, he was going against the rules big-time. Also, because of Makoto's interventions in Chiaki's attempt to ask her out, we will never know how much longer he would have stayed.
Once Chiaki leaves into the future at the end of the film, he could easily recharge himself and return to Makoto...but for some reason he doesn't?
He's not supposed to be in Makoto's time anyway. If he returns to her time on his own, he could get in even worse trouble than he is already risking.
What happens to the "past" Makoto every time the one from the future comes back? Shouldn't there be two of them at each point when she leaps?
Rather than a Back to the Future model, it seems to be more of a rewind/reset in which her present self overwrites her past self in that part of the timeline. That's why she can undo her previous time-leaps by going back to a point before them, instead of there still being a Makoto running around doing whatever she did earlier on.
Expanding on the above: Suppose Makoto stands by the lake at 1pm, and stays there until about 3pm. At 3pm, she walks to a bus stop, and then she jumps back to 2pm. From the perspective of spectators in the "final" version of the timeline, here's what happens: Makoto waits at the lake from 1pm to 2pm. At 2pm, she teleports to the bus stop. Also, if you could measure it, you'd find that she somehow aged by a full hour in the middle of that "teleportation" process.
There's a plot hole: Auntie Witch said that many teenagers spontaneously develop time leaping-thus making this Troper think at first that Makoto did acquire this ability randomly and without a charge limit-Chiaki's appearance was an unexpected development. This Troper also wonders why Auntie Witch or Chiaki did not warn Makoto about the time leap limit at the very beginning to avoid the whole Tear Jerker plot.
Not to mention that if people in the past aren't supposed to know about time travel in the first place, how did Auntie Witch know about it in the first place? Chiaki's whole story doesn't really hold up if people (or at least someone) ALREADY KNOW about it, or at least some variant thereof. Is the future variant of time travel some sort of refined version based on the "spontaneous manifestation" version?
In the original novel, the Aunt was actually the main character, who fell in love with a man from the future. When that man left, he erased the memory of those who knew him... but apparently not perfectly, as she still remembers him and is still waiting for him. Rather tragic, really, but because of this the Aunt (Kazuko) is unaware of the specifics of this particular type of time travel, as it is different from the "drug induced" version. She's giving Makoto incorrect information, but not intentionally.