Battery technology has moved on quickly in the last few years. The right answer for your boat depends on how you use it, what charging sources you have, and how much you are willing to invest up front for a longer service life.
Flooded lead acid
Cheapest by capacity, well understood, tolerant of slow charging. Need topping up with distilled water, lose capacity below 50 percent discharge, and do not like deep cycling. A reasonable choice for a starter battery or a budget house bank on a boat used lightly.
AGM
Sealed, maintenance free, better cycle life than flooded, and tolerant of higher charge currents. The default upgrade on most Thames cruisers. Roughly 50 to 60 percent usable capacity day to day, with a sensible lifespan of five to seven years if charged properly.
Lithium iron phosphate, LiFePO4
- Around 80 to 90 percent usable capacity per cycle.
- Two to three times the cycle life of AGM.
- Far lighter for the same usable amp hours.
- Needs a compatible charger profile, a BMS, and careful integration with the alternator.
Sizing the bank
Add up the amp hours you actually use overnight, then double it. That gives you the usable capacity you want. From there you work back to the rated capacity for the chemistry you have chosen. Most Thames cruisers land between 200 and 400 usable amp hours.
Talk it through before you buy
If you are considering a bank upgrade, get a proper survey of your charging sources and consumption first. Oli offers electrical surveys and full bank installs at your mooring on the Thames.
