After more research, I think that the LiFePO4 batteries are probably too heavy. It looks like a battery pack using 18650 lithium cells would be better.I'd like to get feedback from paddlers who have set up electric pumps in their boats, since I think it might be possible DIY a working pumps system with a LiFePO4 battery at lower cost than the EK system.
Those are the cells in almost all the power tool lithium batteries.
The battery pack is definitely the tricky part of doing a DIY setup. Using a power tool battery and charger might be a simple solution. It's too bad that most lithium packs for power tools are 18 volts or higher. The M12 Milwaukee packs are only ones I've found.
BTW, battery packs are often specified by how many cells are in series and parrlel inside. So, starting with 18650 cells which are 4.2 volts max when fully charged, we'd need 3 in series to get 12.6 volts to run a bilge pump. If each cell is rated at 2.5 amp-hours (2500 mAh) a 3s2p pack with 3 pairs of cells in series would build a 12.6 volt 5mAh pack.
In theory a 2.5 amp hour pack should run a Rule 500 pump for more than an hour, so quite a few pumpouts of an average cockpit.
Things I've learned in the past little while:
Charging lithium batteries isn't the simple task that charging a lead acid or sealed lead acid is; there needs to be a battery management system (BMS) controller in the battery pack OR an 'intelligent' controller in the charger. Some power tools have a BMS in the tool to prevent over-discharging the battery.