A few questions: Is your 12 volt deep cycle battery fully charged when it reads 12 volts? What state of charge is your battery bank when it measures 13.4 volts right after a charge cycle? When is your battery completely discharged?
There is a lot of confusion when it comes to knowing exactly the state of charge of your RV or solar batteries, or for any 12v battery. Almost every day I talk with customers who are unsure about how charged their batteries are. We don't have time to go into all aspects of batteries at this time (you can read more in the
"Safer Back-up Power" article), but we should discuss a couple of important points:
To effectively
know the condition of your battery or battery bank you need a
digital volt meter. Don't use the old style analog meter. Heck, even the $3.99 volt meter at Harbor Freight is ok for starters, though a quality meter from Sears or other hardware store would be best.
Then you need to take a reading ONE HOUR after any charging cycle. It is important to wait this hour because all batteries hold a surface charge that will give you a false reading after charging. Use this chart:
Voltmeter Reading - State of Charge
12.7 - 100%
12.5 - 75%
12.3 - 50%
12.1 - 25%
11.8 - Discharged
I've had hundreds of people tell me "my battery is ok, it measures 12 volts". Not so. As you can see by the chart, their battery is almost completely discharged at 12 volts. That's bad because a battery or battery bank should not regularly drop below 50%, with the ideal range in the 20-30% discharge rate. When the battery reaches this point it should be re-charged to full before continued use.
Lights and other appliances will start to dim when the battery gets around 90% discharged. I had one customer tell me that he charged his battery for an hour or so with a generator then his lights went dim in about an hour or two. Heck, he was operating in the 80-90% range. This is BAD for the life of the battery. I told him that he needed to operate in the full range. He never got his battery fully charged before he used it! I gave him this chart with instructions and he came back a couple of weeks later and told me I was absolutely on target. Good for him.
Anyway, if you have a battery for emergency back-up power, or for any reason, you need to know the state of charge of your batteries. There is a lot more to learn, and I challenge you to learn all that you can about battery maintenance and care, because you will save a lot of money (by not) replacing batteries.
A couple of other notes: Be careful when dealing with batteries! They are dangerous. They contain acid and they can blow up in your face if you are not careful. Wear safety goggles and other protective gear when servicing batteries. Batteries must be properly stored and vented to the outside of the structure. Explosive gas rises off all wet-cell batteries when charging. Never allow battery terminals to be shorted out. Serious explosions and fire can occur.
Check battery fluid regularly. Do hydrometer readings, especially when abnormalities in voltage readings are noticed. Battery readings will vary during extreme cold or hot temperatures.
Charging newer batteries on the cement floor is ok. It is a folk tale that the battery needs to be off the ground to charge. Actually, in hot climates the cold floor will help cool a battery, lengthening its life. A battery will not loose power by being on the floor. Electricity does not flow through plastic!
If you find that your battery is drying out and the water level is lower than the plates, don't fill up the chamber before charging. The fluid expands when charging and the liquid will overflow the cell, causing a miss-match in electrolyte. Just cover the metal plates with clean (preferably distilled) water and then charge the battery. Fill the battery level to recommended levels after charging to avoid overfilling. It is mystery to me why the liquid expands, but it does.
Batteries are the weakest link in your sytem, and they take the most care. They are invaluable for emergencies and recreational use, but they need attention and care. Be careful, get a digital meter and keep your batteries fully charged. They will serve you well.
All the best my friend, Mike
