Catchy title but if you use Lead Acid Batteries and use Solar to charge them, you are part of the 90% population killing your batteries. Not really your fault because you are doing what the manufactures are telling you to do. It is what they are not telling you that is killing your batteries. The Killer is Deficient Charging. The guilty party is Solar and Solar Charge Controllers.
Every Charge Controller on the market uses what is known as 3-Stage Charging algorithm, and the three stages are Bulk, Absorb, and Float. It is a good proven algorithm and works great if you use a commercial AC Powered Charger. However it does not work with Solar in 90% of the applications. The problem is TIME, and with Solar you do not have time to fully charge your batteries.
Here is the problem. To fully charge a Lead Acid battery requires you to apply a Bulk/Absorb charge of C/12 to C/8 and up to C/4 or little higher with some AGM batteries. Ideal model is C/10. So if you are using a 100 Amp battery you would apply 10 amps until the battery voltages reaches the Bulk/Absorb voltage set point of 14.4 volts. At 14.4 volts is the beginning of the Absorb stage. Absorb stage last until Charge Current tapers down to about 1 to 3% of C. On a 100 AH battery would be 1 to 3 amps. Once that point is reached, you lower the voltage to FLOAT of 13.2 to 13.8 volts depending on the Lead Acid battery type. Float keeps the battery 100% fully charged and allows the panels to supply the load with power.
So what is the problem with solar? Simple it takes 6 to 12 hours to saturate the battery in Absorb mode. There are not enough sun light hours in the day to complete the cycle fully. On Solar Charge Controllers Absorb is a timed event. Absorb is not a timed event. Absorb is finished when charge current taper off to 1 to 3% of C. When your battery reaches Absorb set point voltage is only 70 to 80% fully charged. It is going to take another 6 to 12 hours to get to 100%. No problem if you have a commercial AC battery charger.
So here is the saving grace for most of you if your charge controller allows you to set your own Bulk/Absorb/Float charge voltage. There is more than one way to charge a Lead Acid Battery or any battery for that matter except NiCd and NiMh. Turn the voltage up. It is called SPEED CHARGING. You apply a Constant Current until the battery Voltage reaches 14.8 to 16 volt set point on a 12 volt battery. As soon as the battery reaches set point, it is fully charged. There is no Absorb time involved. You force your Charge Controller to stay in Constant Current until fully charged. There is no slow time waiting for Absorb.It is as fast as physically possible.
So what is the down side? Your battery is going to use more water, and corrosion will eventually do your battery in. However you are choosing between the lessor of two evils of Sulfating or Corrosion. You have to pick which one. Sulfating comes much sooner than a Corrosion death, and Sulfating always leaves you short on power.
Of course there is another way. Any Off-Grid applications require a Generator or Alternate source of power. There will be cloudy spells, and all Lead Acid Batteries need an Equalization charge every 30 days or so no matter what the application is. Get a few cloudy days, and you go dark waiting for the sun to return plus another couple of days waiting to recharge. Equalization charges take up to 24 hours to complete and no solar system can EQ a battery or fully charge a battery in most applications.
Now some of you might be thinking why not just use more panel wattage thus a higher and faster charge rate? Good question, but there becomes a point or brick wall in which no time is gained. Sure you will get to Absorb stage sooner, but Absorb takes longer. Example if you charge at C/8 the max recommended rate you hit Absorb at roughly 80% SOC. Increase the rate to C/6 and you hit Absorb at 60% SOC and double your absorb time. That Is just the physics involved of Ohms Law and the batteries resistance you cannot change. You have to know how to work with it. On the Flip side and what utilities and emergency power operations do is use Float Chargers. They set the voltage to 13.2 to 13.8 volts and leave it there. Takes longer, up to 24 hours, but the battery sits there at 100% SOC until used with no corrosion or sulfated plates. That is why their batteries last 10 years. That option is available to some of you like week end warriors in you cabins and some other specialized applications. But you still need that alternate source of power for EQ charge and extended stays.
So what voltage is correct? You have to figure that out with your battery hydrometer. At the end of each day, check the specific gravity. Start at 14.8 volts, and if specific gravity is low, raise the voltage to 14.9 and so on until you reach full charge. If you smell a rotten egg, batteries are fizzing, and hydrometer is above full charge, lower the voltage. Very doubtful you will ever find an over charged condition on solar. You will most likely find there is no voltage high enough which tells you your system is under sized which most of you have.
Battery manufactures are now changing charge recommendations. Reason is Solar and the vast amount of warranty claims. Root cause is undercharged batteries. Companies like Trojan have done away with 3-Stage Algorithms all together. Charge to 2.47 vpc (14.82 volts) and Float at 2.25 vpc or 13.5 volts. That is where you need to start and go from there.