Installed a 8 battery in a 8plus iPhone

I'm confused; why is it you think the 8 has a higher capacity battery than the 8 plus?

The thing is, with lithium ion batteries the capacity is pretty much a direct consequence of the size of the battery. The 8 plus has a much larger screen, which means it has a lot more room inside the case for the battery. A larger battery means more capacity. Here are the official numbers for the battery capacity of the two phones.

iPhone 8 battery mAh capacity

1821 mAh – 6.96 watt-hour

iPhone 8 Plus battery mAh capacity

2691 mAh – 10.28 watt-hour

So no, you won't get more capacity by putting in an 8 battery into an 8 plus phone; instead you'll cut your battery's storage by a third. That is, if it even works. Apple is notorious for reinventing the components for every new phone model so there's no guarantee whatsoever that the 8 battery will even plug into the 8 plus motherboard, and even if it does, you can't be sure it will work correctly; the sensors in the battery can be different enough that the battery may not work. I don't have any specific information one way or another; it's entirely possible it could work - if it will physically plug in, which is another factor at play here.

Given that the price of batteries for an older model phone like the 8 plus has dropped so much these days, there's really no point in not just buying a new 8 plus battery as your replacement.