Follow what Mercury and BCB “recommends” …. use the 31AGM for your cranking battery. I have 2-36V Lithium Pro’s (Overkill) and a Power Pole charger, with the 31AGM. The Power Pole charger is simply the best charger I have ever owned. Simple set up, simple to monitor battery conditions and does everything it advertises it will do. Also a safety, piece of mind aspect in that you always have a emergency start button to employ if something happens on the water where your batteries become fully discharged. The likelihood of that being needed is slim to none considering how the charger works to begin with.