Bought all my batteries home this fall for testing - start battery had been acting strange.
My tester connects the battery to a load bank through a voltage controlled power transistor. The voltage control shuts things off when the battery voltage drops to 11.5 volts. The load bank is currently sized to draw 6 amps. An hour meter is in parallel with the load bank. Check the change in the hour meter, multiply by 6, that's the battery capacity. May not be absolute, but good for comparative purposes.
Anyway, both house batteries tested fine at over 100 aHr (group 31s). House battery cut at at 0.6 hr (group 24). Curiously, after cutout the voltage of the battery jumped back to 12.5! I'm not going to try to explain it, but the battery went to the recycle. A new start battery on the list for the Defender sale in the Spring.