Trickle Chargers will charge a Battery but they do it so slow it may take from 2 to 12 hours to do so depending how big or small or how weak your battery is, it may even take longer.
A Battery Tender will not charge a Battery. Tenders just keep the Battery topped off at full charge, they cycle on and off to maintain the Battery's full charge. These are great for cars or Motorcycles that are not driven a lot to keep the Battery's at full charge at all times.
Never use your vehicles Alternator to charge a dead or low Battery. The alternator is not designed to power vehicle loads and recharge the battery from a very low state of charge at the same time. Forcing the alternator to produce current at its maximum rating for a long time may overheat and damage it. The alternator may get so hot that it literally melts internal connections. Always recharge any battery that is at a very low state of charge with a battery charger, before installing it in the vehicle.
Connect the Charger to the Battery before plugging Charger in, make sure both Battery terminals are clean. When Battery is done charging unplug the Charger then you can disconnect the Charger from the Battery.
A slow charge is better for a Battery than a fast charge. Unfortunately, faster is not always better when it comes to battery charging. Charging at too high a rate can overheat and damage the battery. If you are not sure how old your Battery is charge it at the slow rate. There are two battery tests that are critical: Load test and a specific gravity test that can be found in this link.
Schumacher Electric | Applications & Education The voltage of a battery is useful but a faulty cell may allow a high voltage and no power.
Check battery temperature periodically during charging. If the battery case is hot to the t ouch (125 degrees F or more) stop charging immediately until the battery cools. Resume charging at a lower rate to prevent battery overheating. If the electrolyte bubbles, cut back the charge rate immediately. Bubbling indicates overcharging.
Battery Charge Table
12.6V or higher: 100% charge
12.4V - 12.6V: 75-100%
12.2V - 12.4V: 50-75%
12.0V - 12.2V: 25-50%
11.7V - 12.0V: 0-25%
11.7V or less: 0% (and probably not capable of being recharged).
If you have an electrical drain that keeps draining your Battery this may help you find out for sure.
It doesn't take much to discharge a battery - a few Milliamp's drain over time can do it. If you have a very small drain, you will not see it by measuring voltage.
You need to measure *amperage* being drawn, not voltage.
You could place your multimeter in current mode, disconnect the negative lead to your battery, then place the multimeter in series between the battery and the negative battery cable. You will then be able to see exactly how much current is being drawn. When the neg terminal is removed and then re-connected via an ammeter to the neg battery terminal the PCM will command all system monitors to function and there will be a fairly large draw. The PCM will check for expected actions and if none are received after 20 to 30 mins or so will command the sensors to go to sleep mode progressively. There will be a large drop in the amps when all systems are in sleep mode. Then you can start to remove fuses to check for abnormal current draw.
I think the monitored systems need over 200 milliamps to monitor for expected actions with nothing going on and sleep mode should have about 40 milliamps if all is ok. You cannot open doors or anything in this mode.
You can also have an Alternator diode failure which can quickly flatten a battery.
