How would you understand that a lead acid accumulator is fully charged or fully discharged ?
Answers
(Note the below voltages assume a 12V Lead Acid battery, a full single cell reads around 2.2, so divide your batteries normal voltage by 2 and multiply by 2.2 for the max full voltage)
There's a few signs that are pretty telling.
If you have a smart charger then this will of course tell you, while a d**b charger should have a Ammeter which generally starts quite high (at whatever its rating is; say 6A for example) as the battery approaches full capacity you should expect the amps to gradually drop until it becomes very low/almost unreadable.
Another way is to simply check the voltage using a multimeter /voltmeter. You should expect around 12.6- 13.5 V when fully charged (and no load connected)
If it is an unsealed Lead Acid battery then you would have already have uncapped the 6 plugs, by looking inside (wear appropriate P P E!) you should see bubbles/gassing. When fully charged these bubbles will be at a fairly quick rate.
A Hydrometer is an excellent way of testing a battery's charge level as it relies solely on the chemistry of the liquid and the physics of buoyancy so is quite reliable and immune to some of the pitfalls below. (Some batteries have built in hydrometers but they only test one cell so are only good for a general indication)
Possible misreading : if a cell is shorted then your battery may approach 12v fully charged but only 5 cells are bubbling, use caution here and keep an eye on temperatures. A hydrometer will indicate which cell isn't charging. How can it happen? Any physical damage to the battery including freezing causing the plates inside to crack, warp and touch each other.
A heavily sulphated battery will appear fully charged until a load is placed on it. The charger will show very few (mili)amps as if fully charged and also the voltage may appear to be high, however you will quickly realise there is no capacity to the battery. There is a layer of insulating sulphur crystals on the lead plates preventing current/conduction and the voltage climbs quickly as what little electrolyte is left is charged but the battery is effectively useless. How can this occur? All batteries eventually suffer from this, but leaving it deep discharge below 10V for long periods as well as certain prolonged discharging/charging conditionscharging conditions. They are sometimes recoverable if youu have patience by shocking and forcing higher voltages and fast discharges to try to crack the sulphur off the plates.
These examples are unlikely to occur in a maintained/new battery however.