Originally Posted by Aer Lingus
Lithium Batteries: Portable electronic devices (PED) containing lithium metal or lithium ion cells or batteries, including medical devices such as portable oxygen concentrators (POC) and consumer electronics such as cameras, mobile phones, laptops and tablets etc when carried by passengers or crew for personal use (see 2.3.5.8). For lithium metal batteries the lithium metal content must not exceed 2 g and for lithium ion batteries the Watt-hour rating must not exceed 100 Wh. Devices in checked baggage must be completely switched off and must be protected from damage. Each person is limited to a maximum of 15 PED’s.
The regulations are all in line with those proscribed by IATA. The transport of Lithium-based batteries is fairly complicated and changes often under those IATA regulations, and I know of more than one export compliance officer who has kittens every time a query comes up. If the check-in agent couldn't determine the capacity of the battery, it's fairly understandable that they erred on the side of caution given the potential consequences if a fire developed in the hold.
The second outer for the agent is this: "Devices in checked baggage must be completely switched off and must be protected from damage". Protected from damage is likely to mean a very high bar - you'd be talking about guaranteeing that the battery couldn't be damaged or in any way pierced. That likely means a ruggedised laptop, contained within a dedicated hard case with sufficient padding around it to protect against impact. Most people wouldn't be able to comply with that.
There are a number of examples of fires within cabins where mobile phones have been damaged in seat mechanisms. It's why airlines now ask you to inform a flight attendant if yours gets stuck. A laptop battery is far, far larger and the fire would be far more serious if the battery got damaged and caught fire in an inaccessible hold. This isn't a battle you're ever going to win.