My understanding is that you can pretty much use any battery charger that does not have a desulfating mode, which can damage lithium batteries. The typical older battery maintainer will recharge a lithium battery just fine, however it might not fully charge it up to its maximum rating. It has been my experience that the on-board charging systems used by BMW are very good and do a perfectly adequate job of keeping a lithium battery fully charged while the engine is running.
Current motorcycle collection: 2002 Yamaha FZ1; 2020 KTM 390 Duke; 2011 Royal Enfield Bullet 500; 2009 BMW F650GS; 2016 BMW R1200RS; 2018 Zero S 16.6 kWh