To easily see the state of charge accurately you should obtain and fit a lead acid battery
condition instrument such as the E meter or Curtis 901, the former is easily the best but is not cheap. The most accurate way of checking a batterys condition is with a drop tester but this is not a quick and easy method.
The cost of an E meter or Curtis may initially appear prohibitive, but you should consider how much its cost will save by preventing the premature failure of expensive batteries.
It is extremely difficult to accurately measure the state of charge of a lead acid battery and to predict the remaining capacity.
Battery capacity is not comparable to a tank full of petrol. A filled petrol tank contains a finite amount of energy which can be used either slowly or fast according to the energy required. Battery capacity is not so simple.
In a battery, the rate of which energy is drawn affects the overall amount of energy available from the battery. For example , a 100 Ah battery rated at 20 hour rate means that over 20 hours there are 100 Ah available, or to put it another way you can expect to draw up to 5 Amps per hour for up to 20 hours, 20x5=100Ah.
If you try to draw the battery down more quickly you cannot expect to get the same amount of Ah from it. For example if you draw it down over just one hour the approximate capacity will become 100 x 0.59 which =59Ah. Putting another way means that you can connect a load on the battery which will draw 59 Amps for just one hour. If you discharge over just half an hour you can only expect to get around half (47%) the capacity from your batteries.
With most uses the rate at which a battery is discharged varies enormously, you can see that any battery condition indicator has to be quite a clever piece of equipment if it is ever going to get close to giving an accurate reading.
The E meter samples the rate of discharge every 4 minutes or so (this can be varied) and recalculates the amount of time remaining before the battery will be fully discharged and also up-dates and displays a fuel gauge bar graph.
The combination of the fuel gauge and the time remaining will display along with possible displays of volts, amps. Ah and/or kWh means that the instrument is around 99% accurate and can be relied upon. Though there is one proviso, the E-meter must be correctly set up in the first place for the capacity and the type of batteries you are using. It does take a bit of fine tuning to get it set up correctly, but once it is, there is no better instrument that I have come across.
If an E-meter is not available a voltmeter giving the open circuit voltage of the lead acid battery
can give an approximate indication of battery discharge. This method cannot be relied upon as this voltage will rise if the battery is allowed to rest and then the voltmeter will in effect give a false reading. As soon as the load is reapplied to the battery the voltage will drop and the available charge will then become apparent.