Lead acid batteries do not like to deal with high discharge loads such as inverters. When you apply a load larger than the 20 hour Ah rating the capacity of the bank gets smaller. Click the image to make it larger and see what I mean.


Lead acid batteries are rated at a 20 hour rating. This means a 100Ah battery can supply a 5A load for 20 hours before hitting a terminal voltage of 10.5V.


A 400Ah bank can supply a 20A load for 20 hours before hitting 10.5V. Any loads above the 20 hour rating diminish the capacity of the bank.


The 20 hour rating load is determined by; Ah rating divided by 20.


100Ah battery / 20 = 5A

125Ah battery / 20 = 6.25

225Ah battery /2- = 11.25A


LFP batteries are not rated at at 20 hour rate like a lead acid battery is. How they are rated for Ah capacity can cause lots of headaches trying to actually figure it out.


Some prismatic cells are rated at a .5C load or 50% of the Ah rating at 25 Celsius / 77F yet others are rated at 100% of capacity or a 1C Load at 77F. While these batteries do not have much capacity loss between high discharge, such as a 1C load, and mid discharge at a 0.5C load, there are small differences.


What this means a 400Ah LFP battery rated at .5C can deliver all 400Ah's at a 200A continuous load. At 1C it might deliver slightly less. CALB cells for example are rated at 1C / 77F so a 400Ah bank should deliver 400Ah's with 400A load at 77F.


For off grid fractional "C" use I would not fret over the rated capacity as you will rarely if ever be drawing at anywhere even close to 0.3C let alone a full 1C. You would be wise however to capacity test your bank at somewhere slightly above your "average" DC load for your vessel, perhaps 15-20A for manyt cruising boats.. This will give you a real usable Ah capacity to work with.


Conversely a 400Ah lead acid battery with a Peukert of 1.27 will only deliver 215Ah's at a 200A load and just 178 Ah's into a 1C load. Big, big difference....