Lead acid battery charger and fuel gauge?
You have to build intelligence in to the charger.  A circuit can help with that, but it is not going to do it for you.  A good charger has good intelligence in its firmware. That comes from experience, not something you can buy off the shelf. 

A good battery charger will require settings for things like voltage, equalization on/off, and battery type (e.g. sealed, AGM, Gel, Flooded, etc).  

Lead Acid batteries use a CC/CV (constant current/voltage) charge method.

Lead Acid needs a slow charge, 12-18 hrs, in two stages, CC and CV.  A stationary battery even a slower charge is beneficial, around 3x slower.  

For a stationary sealed battery, 36-48 hours.

The first stage, is a constant current which lasts for about 3-4 hours until the battery is about 70% charged.

Stage two is a "topping charge" a constant voltage, as the battery starts to saturate the current gradually slows.  When the charge reaches 3–5% of the its rated capacity, stage two is over.  This usually takes about 7–10 hrs (20-30 hrs for stationary). 

The battery should not stay at the topping voltage for more than 48 hrs.  This is critical for sealed batteries. Sealed batteries are not overcharge tolerant. 

Setting the correct full charge voltage (2.30 - 2.35v per cell) is critical and debatable. Keeping the battery at topping voltage, this maximum capacity avoids sulfation on the negative plate.  On the the hand over charge saturation causes grid corrosion on the positive plate.  It's a balancing act.

For a  stationary battery under load, after stage two, the battery would then go into a float charge at about 2.26v/cell.

For a charger not under load after a full charge,  rather than a float charge a hysteresis charge is recommended.  When the battery goes to standby after the full charge, the float disconnects.  When the voltage drops due to self-discharge a CV topping-charge is used to bring it back to full charge.