trickle charging how long?

[Doug Huffman]

https://www.northerntool.com/shop/to...6185_200596185

NPowerXRP #60109 is what I have used on all of my batteries for some time - diesel tractor battery, sailboat battery, VW TDI diesel battery (gone to a new home) and now my X5 E70 35d N57.

I am not a battery expert. I was qualified to maintain a 126 cell TLX-39B capable of IIRC (40 years later) 1700 A 250 VDC nominal for 3 hours.

The #60109 specifies

Stage Seven - Float: Maintains the battery at 100% charge without overcharging or damaging the battery, allowing the unit to be left connected to the battery indefinitely. Ideal for batteries used intermittently such as seasonally used vehicles including classic car, motorcycles and RVs.

I cannot recommend BatteryUniversity.com too highly, particularly for non-experts.