I read this on the Battery University site:
Battery Fuel Gauge
One of the main tasks of the smart battery is to establish communication between the battery and user. A fuel gauge that indicates state-of-charge is part of this effort. When pressing the TEST button on a fully charged SMBus battery, all signal lights illuminate. On a partially discharged battery, half the lights illuminate, and on an empty battery all lights remain dark or a red light appears.
While the SoC (state of charge) information displayed on a battery or computer screen is helpful to the user, it does not assure sufficient runtime, because the fuel gauge resets to 100 percent on a full recharge regardless of how much capacity the battery can store. A serious miscount occurs if an aged battery shows 100 percent SoC while the battery’s ability to hold charge has dropped to 50 percent or less. We ask, “100 percent of what?” If, for example, 100 percent of a good battery results in a four-hour runtime, the same indication with a battery holding half the capacity would run for only two hours. The user should know that the fuel gauge only shows SoC and capacity is missing.
So a battery might show that all bars or lights are lit after a charge but it doesn't have any relevance to how much capacity the battery has left. I've never really trusted those battery meters on the wally world batteries after getting burned by a full set of lights on the meter several times.