Charging and Maintaining Sealed Lead Acid Batteries

sealed lead acid batteries

In my preps I have some sealed lead acid batteries, both 100 amp gel cells and some 7.5 amp batteries made into 12-volt battery packs.  Each pack contains two 7.5 amp 12 volt batteries.  These can be charged from the electric system in your home, vechicle or by solar panels.  See A Simple Way to Charge Your Homemade 12 Volt Battery Packs.

Several times a year I get the batteries out and check everything and make sure they are fully charged.  Sealed lead acid batteries are a good choice for many uses; they are rugged, will not leak, easily rechargeable, and offers a good amount of energy in a convenient form.  However, like most batteries they will slowly self-discharge over time.

To tell when your battery is fully charged, use a voltmeter to measure the battery voltage.  Look for a reading around 12.5 volts give or take a couple tenths of a volt depending upon the temperature of the battery.  If the battery was just removed from a charger, the reading may be high and will take a few minutes to drop to the open cell voltage.  A discharged battery will read lower than this, usually around or just below 12 volts with no load on it

sealed lead acid batteries
Duracell charger on 100 amp battery

I use three different types of chargers, my 100-amp gel cells are on a Duracell 2 amp charger which can be left on all the time.  This is a maintenance charger, when the battery is fully charged; it will then reduce the voltage it supplies to the battery.  This keeps the battery from overcharging and overheating, the charger can be left on all the time.

See also  More Good Information on Mothballing and Maintaining Small Engines.

For my battery packs, I originally started using wall warts.  The problem with these is that you can overcharge the battery packs.  So you can’t leave them on indefinitely.  Other than that, they work well and usually can be found for nothing.

sealed lead acid batteries
The charger I use for the battery packs
sealed lead acid batteries
A number of wall warts

The other charger that I have started using with the battery packs is the Delran Battery Tender Jr.  This can be left on the batteries indefinitely, since it is a maintenance charger.  As a safety precaution when I recharge the battery packs, I always make sure that the lids are open and they are vented.  No lead acid batteries should be charged in a sealed space

The lower maintaining voltage is often referred to as “float” charge, so if you are purchasing a maintenance charger make sure that it has a float setting.  A good battery charger will shift into the float mode automatically, which will not damage the battery if left connected.

Remember a battery should not be allowed to go below 1/2 its rated voltage, or 6 volts for a 12 volts battery. This is the reason that seal lead acid  batteries should be stored charged, as self discharge will eventually completely discharge the battery and damage it. Thus these batteries should always be charged after use and charged every few months if stored unused for extended periods.


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3 thoughts on “Charging and Maintaining Sealed Lead Acid Batteries”

  1. Any 12 volt, lead acid battery, regardless of its construction, flooded or AGM, all of them are considered 100% discharged when the voltage drops below 10.5 at the rated C load. Discharging them past this point, severely reduces their charge/cycle life. The fact the battery says Deep Cycle and/or Marine on the outside means almost nothing. The reason some batteries are labeled as marine, or “deep cycle” isn’t lowering the C load voltage below 10.5 volts, but is because they’re designed to sit idle for up to two years, without recharging, yet will have enough “reserve capacity” to start the engine. If we truly want a long-life battery as emergency backup power, get one with a large reserve capacity. Optima, Concorde, Hawker, Exide, and Interstate all make batteries for this type of service. The Optima high capacity batteries used in public safety vehicles are called “YellowTop”.

    While a simple open-circuit voltage check may assess suitability of a stored battery works for low-current demands, such as powering AM/FM/Weather broadcast radio receivers, devices having higher current requirements, such as emergency lighting, inverters used to run computers, power tools and appliances, or 2-way radio communications equipment, require that you check the voltage actual drop under full operating load. If the battery is unable to provide the minimum needed sustained current, you might be able to receive a radio signal, but attempts to transmit will fail, as the radio cuts-off and your signal drops out. A CB radio, handheld marine or 2-meter ham 5-watt VHF portable requires about 4 amps of current during transmit. A 25-watt Marine VHF or airband radio requires about 8 amps, a 50-watt 2-meter ham or business band mobile radio about 10 amps, and a 100-watt HF-SSB transmitter about 20 amps. A general rule which is well proven in practice is to plan for 1 amp-hour of battery capacity for each watt of transmitter output, presuming a typical 20 percent operating duty cycle, during 12-hour operational period. 24/7 continuous emergency operation requires rotation between two sets of batteries while the set not in use is being recharged.

    Thanks to Alan Applegate, K0BG, for clarifying the above comments. His web site is highly recommended as a valuable resource.

  2. I’ve got a small solar/battery setup in my shed that powers 2 small 12v gable fans when the temp gets >80 – been going well for nearly 3 years. I’ve been eyeing a desulfator as my battery is probably nearing the end of it’s normal lifespan. Do you (or anyone else reading this) have any experience with battery desulfators? If so, are they worth having?


  3. Any of the multi-phase, “smart chargers” will accomplish the desulfation, controlled discharge, recharge and maintain “float” voltage for long term storage. A well stocked marine, RV or farm supply store will have what you need.

    I use a XANTREX TrueCharge2 Battery Charger Remote Panel from West Marine, which costs about $100, which I use to maintain a bank of four L16 golfcart batteries and 2.5KW inverter as emergency backup power for alarm system, freezer, emergency lighting and ham radio. When the 1000 gallon LPG tank for the 20KW Generac runs out, there are eight Siemens SM20 solar panels and a charge controller sized approriately for the array. Technical info here:

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