On an F15, it's right next to the main battery on the right side (you cannot miss it).
I highly recommend not disconnecting the smaller battery. I've said it before and I'll say it again, only disconnect the negative terminal on the main battery, wait about 10 minutes before you do any work as some ECUs and Air Bag still hold a charge for a bit.
I've disconnected my batteries plenty of times and I've never experienced these issues I see people write about 🤷🏽
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