All of the drop in Lithiums have some sort of battery management system built in, inside the case. Many (most? all?) are of a type which shed excess current as heat. At the point they need to shed current, the batteries will already be close to being over voltage & thus getting hotter. Adding further heat, inside a sealed box is the last thing they need. Add in the fact that a major benefit of lithiums is their ability to accept high charge rates increases the possibility of more heat being shed by the BMS. The BMS will be a printed circuit board, & these too can fail if they are consistently overheated, without any access to air cooling. With a 'drop in' replacement if the BMS dies, the whole battery is cactus. The result will be a shortened lifespan of the battery. In addition, many of the cheaper battery management systems (BMS) will be just that, working only on overall voltage, rather than a cell management system (CMS) which incorporates auto cell balancing as required. Getting cells too much out of balance does occur with lithiums & can cause premature failure.

The above is a summary of my understanding of what a friend in the business of building, supplying & fitting lithium batteries has told me. He believes that for a long reliable life it is essential for a lithium battery pack to have a BMS which incorporates a CMS & that this should be external to the battery pack. I'm only repeating what I have been told, personally I have no experience of Lithiums. I do know however that were I to pay the significantly higher price for lithium over AGM (or other lead acid battery types) I would want them to last at least as long (ie. a minimum 10 year lifespan).

If the medical appliance is a CPAP machine, I run mine without a 2nd thought - average 20Ah per night or 26Ah with humidifier. If your solar/battery system is likely to struggle with similar 'extra' load. It may be a *lot* cheaper & more effective to stick with AGM battery(ies) & just add a bit more charging input (another solar panel, and/or dc to dc charging), space permitting.