I want to build a poor man's portable power station. There are only two things I need to run off of it: a 240watt fan to blow up my mattress and a CPAP machine all night that is rated at 12v 3 amp. I am pretty sure a mid-size 12v lead acid battery will get the job down, the only question is the recharging. I would hook it back up to my car each day and recharge as I drive around. I just don't want to overcharge the battery, so I am looking for some way to monitor the charge and stop the charge once it is topped off. How do I do that?
While the answer by wilkvolk is accurate for recharging a battery, I'm pretty sure you won't need to go through all of that to do what you need it to do. If the battery is hooked up to your vehicle and is being recharged by your on board charging system (ie: alternator), and the on board charging system is functioning correctly, you shouldn't have to worry about when to take it off the system. The system has a built in regulator which won't allow your battery to be overcharged.
If you are hooking a separate charging system to your secondary battery, then you'd want a charger which is a maintainer (ie: Battery Tender) as a maintainer will charge it to capacity, then keep it topped off as long as you're not using the battery, without overcharging.
Another consideration is you'll want to use a deep cycle battery for your needs. These provide a continuous amount of power which allow for a long duration. These will stand up to the abuses you plan on submitting it to. From this website, I deduced you'll need one of 64AH (flooded style battery) size or bigger to supply a 3 Amp load for eight hours. AGM or Gel (which would be better suited to your needs because they do not emit gasses when recharging or discharging) could be smaller.
Step 1 Wear protective gloves and goggles before you rebuild the cell structure of your 12- volt battery. You need to access the cell reservoirs that contain highly toxic sulfuric acid that immediately burns on contact with skin.
Step 2 Place your 12-volt lead-acid battery in a battery tray on a stable work surface. Remove the six battery cell caps on top of the battery. Either unscrew the caps using your fingers, or if the caps have slots, use a screwdriver. Each cell produces 2 volts and is linked in series to produce 12 volts. Put the caps to one side.
Step 3 Check the fluid level inside the six cells. It's likely you find the fluid level is below the minimum mark on the inside of the cells. This means the process of the electrons flowing from one lead plate to the other inside the cell has stopped. The fluid, called electrolyte, is sulfuric acid and enables a chemical reaction to take place between the positive and negative plates when the battery gets a flow of electricity from the car's alternator or battery charger. If the fluid level drops below the minimum marker, the plates would be exposed, and they would corrode over time.
Step 4 Rebuild the chemical cell structure by replacing the fluid. Use distilled water and top each cell up to the maximum marker inside the cell. It's important that you don't overfill the cell as during the charging process the cells get warm and the fluid expands. The distilled water will mix with any existing sulfuric acid in the cell and start to rebuild the chemical structure. However, for the process to begin the battery needs electrons to flow between the plates created by a small electric current from a battery charger.
Step 5 Replace the cell caps using your fingers or a screwdriver. Don't over-tighten.
Step 6 Put the spring clamp on the end of the black battery charger cable onto the negative battery terminal. It's clearly labeled "Neg," "-" or may have a black plastic ring around the terminal pole. Put the spring clamp on the end of the red battery charger cable onto the other battery terminal. It's clearly labeled "Pos," "+" or may have a red plastic ring around the terminal pole.
Step 7 Set your battery on the lowest setting possible; often called "trickle charge." Rebuilding the chemical structure inside the cells takes time and it's important the process is done slowly so the distilled water gets more acidic. As acidity increases so electron flow increases between the lead plates. Any corrosion on the plates caused by lack of electrolyte dissipates rebuilding the structure.
Step 8 Plug in your battery charger and turn it on. Check the setting again to make certain it's on the lowest possible. Leave the 12-volt lead-acid battery to charge slowly for 36 hours. During this time, the energy retention of the plates increases as chemical activity changes. You can check if the rebuilding process is working by touching the side of the battery after about 12 hours. You would find it's getting warm indicating that the battery is taking a charge.
Turn off the battery charger after about 36 hours. Disconnect the battery cable clamps from the battery terminals. Place your hand on the side of the 12-volt lead-acid battery, and you find it's fairly warm to the touch meaning the chemical cell structure is rebuilt, and your battery has retained a charge.
Tip If you find the battery is cold, or only part of it is warm, after the charging period you probably have to resign yourself to buying a replacement as the plates in the cells were too corroded and couldn't rebuild. Warning Only use distilled water to top up the battery cells. Regular tap water contained minerals and other impurities that prevent electron flow, reduce acidity and attach to the lead plates causing corrosion.
More on here--->https://itstillruns.com/rebuild-12-volt-leadacid-batteries-7841854.html