Maintaining Lead Acid Batteries to Get the Most Life Out of Them

maintaining lead acid batteries

Here are two of my homemade battery packs being charged.

A friend Ed sent me a lot of information on maintaining lead acid batteries.  It is good information, but a bit technical for many people so I am going to split this post into two parts.  First, some simple guidelines to keep you from making the biggest mistakes and then the part that he sent me.  I suggest that you take the time to read both.

For an example, I will use the two types of lead acid that I store.  My homemade battery packs  contain the 12 volt sealed lead acid batteries. (Homemade Battery Packs Are Inexpensive and Easy to Make)

Several times a year I get the battery packs 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.  Remember a 12 battery should not be allowed to go below 10.5 volts at which time it is considered 100% discharged. This is the reason that 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.

My 12 volt 100 amp gel cells are kept on a smart charge all the time.  This keeps them fully charged.  To tell when your battery is fully charged, use a voltmeter to measure the battery voltage.  Look for a reading around 12.5 volts or higher 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.maintaining lead acid batteries

I use two 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.

For the battery packs, I use 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.

If the power is out here is a link to Turn Your Homemade Battery Packs into a Solar Generator.  With a little planning any battery can be charged with solar.

maintaining lead acid batteries

Duracell 2 amp charger

Now read the technical stuff that will give you more detailed information on maintaining lead acid batteries.  It explains the difference between deep cycle and starting batteries, The best batteries to use with your radios and information on NiCd and other batteries.



Battery Power 101

Batteries are only temporary power unless you have reliable, sustainable means of recharging when the AC mains are down.  Solar or photovoltaics (PV), when combined with a well-designed battery bank and appropriate charge controller provide sustainable DC power which can be used to power an AC inverter. This can serve modest emergency power needs at much lower life-cycle cost than a portable generator and fuel storage.

Attention to POLARITY is important for safety. Correct connection sequence is vital to avoid sparking or damage to system components.  An arc caused by wiring a connection in the wrong order may ignite hydrogen given off by a battery, causing an explosion!  High current flow at low voltage can still be lethal. ALWAYS disconnect all circuits before working on any power system.  Follow the correct re‑connection sequence: 

  1.    positive connection to battery
  2.    positive connection to load
  3.    negative connection to battery
  4.    negative connection to load

Typical 12‑volt lead‑acid batteries have a voltage of about 14 volts when fully charged and 11 volts if fully discharged.  Marine radio, Land Mobile Radio, aviation and amateur radio equipment doesn’t operate properly below about 11 volts.  For best battery life avoid full discharge (1.7 volts per cell or 10.2 volts for a 12V battery) Avoid any depth of discharge at which the battery voltage under working load drops below about 11 volts.  Battery systems are current-limited and their capacity is finite. Loads greatly exceeding their design capacity or excessive duty cycle cause rapid depletion of battery capacity.  Battery systems must be sized appropriately to the anticipated load.

Cranking amps tell nothing about how long a battery will run household equipment loads.  Cold Cranking Amps (CCA) represent the current an automotive “start­ing” battery provides continuously for 30 seconds at 0 degs. F before voltage is drops to 1.7 volts per cell (Vpc) or 10.2 volts for a 12 volt battery, at which it is then fully discharged.  In MCA or Marine Cranking Amps, this measurement is taken at 32 degs. F.

Reserve Capacity is the time that a starting battery can sustain a continuous 25Amp load before its cell voltage drops to 1.7vpc. This is a more useful measure, but less than perfect, because the starter in your pickup may not crank the engine enough to start in freezing weather with only 25A.

Performance measurements used for rating deep cycle batteries are amp‑hour capacity and depth of discharge (DoD).  Amp‑hour capacity is total current available over time, measured at 80 degs. F.  DoD is the percentage of capacity available during a charge‑ discharge cycle.

Amp-hour ratings of deep cycle batteries are based upon a discharge rate at 1/20 of  battery capacity. This is commonly expressed in the trade as “C over 20″.  A Battery Council International (BCI) Group 921 marine battery rated 200ah at C/20, discharged continuously at 10 amps, at 80o F., will sustain that load for 20 hrs.

Engine starting batteries are designed for 20% DoD, gel cells 25%, and “deep cycle” industrial batteries from 50% to 80%.  Starting batteries perform poorly in powering wheel chairs, computers or communications because they are designed only for short periods of  intermittent high load with periods of rest in between, not for continuous duty.

Deep cycle batteries are better for emergency backup.  A 100w HF-SSB marine or ham radio draws 20 amps on full key-down CW or data transmit and  requires a 100ah BCI Group 27 battery to stay within a C/20 discharge rate.  At lower temperatures available capacity is reduced.  Lead-acid batteries lose 50% of their rated reserve capacity at 32oF!

More rapid rates of discharge, (such as using a too-small battery for the load) further reduce available capacity and the number of charge‑ discharge cycles the battery will provide.  A BCI Group U1 (25 lb., 31ah) gel cell is well balanced for a wheel chair, 2-meter ham FM mobile or Marine VHF radio at 25% duty cycle. A transmitter output of 25w, typical of a Marine or airband VHF radio requires 6A, approximating the C20 rate of discharge.

A land-mobile or ham 2-meter radio which transmits at 50w, draws a current load of about 10 amps at full FM carrier, a mildly oversized load approximating C/10. This is OK for intermittent “surge use,” such as 20 percent duty cycle with 30 seconds of transmit time to 2 minutes of listening, but stressing a battery frequently in long, full-power radio transmissions of over a minute without sufficient idle time of at least 4 minutes between transmissions shortens battery life. Deep-cycle, flooded lead-acid batteries tolerate C/10 loads with moderate life cycle loss. A battery pack intended for continuous C10 discharge rate as for a portable repeater should be of “deep cycle” type rated for 50% depth of discharge, such as AGM-fire resistant, valve regulated types used in portable AEDs, fire alarms and light aircraft.

A good rule for sizing battery systems for a C/20 discharge rate for landmobile, ham, marine or general aviation communications is one amp‑hour per watt of transmitter output. Estimate the amp-hour capacity required to run your station equipment for 24 hours by summing all loads: transmit current times total operating time times duty cycle, plus receive current with squelch open times standby time and repeat for each piece of equipment.

Then multiply the total loads by 150% safety factor.  If you are too lazy to run the actual current requirements numbers, use the amp-hour per watt rule for each 12 hour operational period.    

Lead‑acid batteries consist of lead alloy grid plates coated with lead oxide paste, immersed in a solution of sulfuric acid.  In manufacture the plates are subjected to a “form­ing” charge which causes the paste on the positive grid plates to convert to lead dioxide. The paste on the negative plates converts to “sponge” lead.  Both materials are highly porous, allowing electrolyte to freely penetrate the plates.  Plates are alternated in the battery, with porous, nonconduc­tive separators between them, or with each positive plate surrounded by an envelope, open at the top.  A group of negative and positive plates with their separators makes up an element. When im­mersed in electrolyte, an element comprises a battery “cell.”

In lead acid batteries each cell is nominally 2 volts.  Multiple cells are connected in series to increase voltage.  Larger or more plates increase amp‑hour capacity, but not voltage. Thicker or fewer plates per cell allow more cycles and longer life for the battery.  The lower the antimony content in the plates, the lower the internal resistance and the less resistant the battery is to charging.  Less antimony also reduces water consumption through electrolysis 

Pure lead plates may break during transportation or service operations requiring removal of the battery.   More antimony allows deeper discharge without damage to the plates and longer service lifeThe plates in most automotive batteries are 2‑3% antimony and deep cycle batteries 5‑6% Sb.  Calcium or strontium used in sealed lead-acid batteries offer the same benefits and drawbacks as antimony, but reduce self discharge when a battery is stored without being used.  Do not exceed 25% DoD with Pb-Ca batteries, (gel cells).

Cells in lead‑acid batteries are vented to permit hydrogen and oxygen to escape during charging. This also provides an opening for replacing water lost due to electrolysis.  Open caps are common in marine batteries, but modern automotive and DOT approved batteries used for commercial vehicles are of flame arrester type to prevent a flame outside the battery from entering the cell.  “Recombinant” caps contain a catalyst which causes hydrogen and oxygen liberated dur­ing charging to recombine into water, reducing the need to replace water lost from the battery.   These are recommended for stationary batteries in seasonal equipment left for long periods on a maintenance level float charge or to be used in photovoltaic systems.

The percentage of acid in battery electrolyte is measured by its specific gravity (Sg).   Only batteries which use acid electrolyte can use specific gravity as a measurement of the state of charge.  A hydrometer is used to mea­sure how much the electrolyte weighs compared to an equal quantity of water.  The greater the state of charge, the higher the specific gravity of the electrolyte. The lower the state of charge, the weaker the acid and the lighter the electrolyte.

Differences in acid density are measured by the float in a hydrometer, which rises higher in an electrolyte sample of high Sg than in one with a lower Sg.

Measuring Sg of a wet, lead‑acid battery during discharge is a good indicator of the state of charge.  A fully charged battery has an Sg of 1.265 grams per cubic centi­meter, at  75% charge 1.225, 50% charge 1.19 and fully discharged 1.120.  During charging of a flooded battery Sg  lags the charge state because complete mixing of the electrolyte does not occur until gassing commences near the end of the charge cycle.   Because of uncertainty of mixing, this measurement on a fully charged battery is a better indicator of the health of a cell.  Therefore, Sg is not the absolute measure of capacity, but is considered in combination with load testing and open circuit voltage.

Lead‑acid batteries accept only about 1/10 of the charging current at 30 degs. F which they will accept at 80 degs. F.  

Lead-acid batteries at normal ambient temperature should be charged current from 1/10 to 1/20 of capacity. When not in service, all lead‑acid batteries self‑ discharge at rate of about 5% per month.  The rate of self-discharge increases with temperature.  If  a lead‑acid battery is left in a deeply discharged condition for a long time it becomes “sulfated” as sulphur in the acid combines with lead from the plates to form lead sulphate.

Standby auxiliary batteries require a charge controller to provide regulated, low‑level current to compensate for self discharge and protect against sulfation.  They also require regular testing, inspection and replacement of lost electrolyte.  If water is lost during charging and not replaced, the process of sulfation is accelerated in those plates which are partially exposed to air.  “Treeing” is a short circuit occurring between positive and negative plates.  This may be caused by manufacturing defect or rough handling resulting in misalignment of the plates and separators. “Mossing” caused by circulating electrolyte bringing particulates to plate tops can also cause a short.

Sealed, flooded (wet) lead‑acid batteries are also called “maintenance free” and experience less self‑discharge.  They contain lead‑calcium or lead‑strontium plates to reduce water loss and usually have catalytic recombiners to reduce water loss and sealed, valve regulated vents.  Sealed-flooded lead-acids tolerate the same temperatures as unsealed batteries, but because Sg isn’t readily measured, some sealed-wet batteries are provided with a captive float hydrometer in the electrolyte.  Sealed-wet batteries are very common for automotive starting, but should not be discharged below 25%, or their life is greatly curtailed.

Sealed lead‑acid (SLA) batteries include gel cells and absorbed glass matt (AGM), have stabilized or “starved” electrolyte, are valve‑regulated and completely sealed.  Because there is no free liquid electrolyte to spill, the battery can be used safely in any position.

SLAs are safer than flooded types for indoor use and in sensitive equipment such as computer uninterruptible power supplies, which would be damaged by exposure to acid fumes.  Any sealed battery will vent if overcharged to the point of excessive gassing, because the valves are designed to purge extreme pressure building up inside the battery case.

Automotive chargers intended for flooded batteries must not be used to charge gel cells unless they have voltage limiting circuitry to preclude their exceeding 14V during charging.  Self discharge of gel cells is minimized by storing them in moderately cool areas of 5 to 15 degs. C.

Gel  cells are NOT deep cycle.  A DoD of greater than 25% significantly reduces their life. Gel cells must not be used below ‑20 degs.C, in engine compartments of vehicles or in use subjecting them to temperatures above 50 degs. C.  Absorbed glass matt (AGM) batteries are deep cycle, can be quickly‑recharged with no current limit and provide a broad operating temperature range.  

The depth of discharge of AGMs equals industrial flooded NiCds, but with virtually no maintenance and low life cycle cost.

Marine or public safety, emergency vehicle AGMs such as Lifeline or Optima are not prohibitively expensive, have aviation type cell construction and are recommended as auxiliary power for emergency communications systems.

Dry NiCds cells used in power tools, cameras and portable radio transceivers require care to avoid deep discharge, which causes cell reversal or over­charging. This results in irreversible diminished capacity due to heat damage.  Users are overly concerned about dry NiCds developing “memory” from being stored in a charged condition for a long time.  Most dry NiCds do not fail from “memory,” but from prolonged over‑charging.   Always bring a discharged dry NiCd pack back at a slow controlled rate, but don’t charge it for more than 14 hours.  A cell which has developed memory or which has been overcharged can usually be restored by one deep discharge/recharge cycle as long as it doesn’t out-gas.

A weak NiCd must never be used without recharging, as irreversible damage occurs inside a discharged dry NiCd when a load is applied to it.

Use of adequately sized wire and appropriate connections is important in DC power systems.  DC polarity must be maintained throughout the system, as must color coding conventions of wire insulation: positive‑red, negative-black and equipment ground‑green or bare, following the wire color conventions used in automobiles.

DC Wire gages are much larger than in AC systems, because undersized wiring causes excessive voltage drop which results in loss of available power.  This causes some equipment to work poorly, or not at all.  If too small a wire gage is used between a charge controller and battery bank, the voltage drop measured during full charging rates reduces the set‑point the battery is recharged to, reducing operating capacity and life cycle.

To minimize the effects of voltage drop, keep cable runs as short as possible.  For instance, in a 12‑volt system with a 10A load, such as an LMR business mobile transceiver, the AWG #14 wire normally provided results in a 5% voltage drop over 11 ft.  AWG #10 has a 2% drop over the same  distance and 5% drop over 18 ft.  If you must extend the battery leads to reach a trunk-mounted transceiver, use wire 2 gages larger for as much of the distance as possible.  

Splice connections must be secure and able to withstand vibration, moisture and corrosion.  Splices up to AWG#8 should be overlapped for a length not less than five times their diameter, spiral wrapped at least three turns, soldered, covered with shrink wrap or electrical tape and then waterproofed.  Larger gage wires should be overlapped and connected with split bolts, soldered, covered with electrical tape and then waterproofed.

Batteries are connected in series to increase voltage or in parallel to increase their amp‑hour capacity.  These interconnected groups of batteries are called “battery banks.”

To determine the size of the battery bank needed, determine the daily power requirement in watt-hours times the number of  “days of autonomy” that the equipment must operate the loads without re-charging, plus a 30% safety factor (to ensure the batteries are not damaged from exces­sive discharge).  To convert watt-hours to amp-hours, divide by the voltage. 

Be careful around battery banks!  Even when partially charged, an interconnected battery bank can de­liver sufficient voltage and current to arc weld!   Keep sparks and other ignition sources away at all times.  Never allow tools to fall onto terminals or connections.  Never permit construction or use of shelves above the batteries.  Battery banks must always be adequately vented.

When paralleling batteries, reduce the effects of voltage drops which cause unequal resistances between parallel  branches, so that all batteries in the system operate at an equal current and voltage level.  Use the same length of cable from each battery terminal to a central junction point.  Positive and negative do not necessarily have to be of the same length.  This eliminates uneven voltage drop between batteries.

Cable size is calculated based upon the peak charging and load current demands of the system multi-plied by the resistance of the wire.  Marine grade or multi strand welding cable of AWG #8 or larger is recommended.

Most battery problems are caused by oversized loads or equipment operating at excessive duty cycle for too long. When batteries remain in a low state of charge, check the load as well as the batteries and charging system!

The four ways to determine the charge state of lead‑acid batteries, in declining order of accuracy are:

  1.    hydrometer / refractometer
  2.    actual equipment load test
  3.    artificial load test
  4.    open circuit voltage

When using a hydrometer you are working with strong acidWear eye and face protection and rubber gloves.  Have baking soda and plenty of fresh water ready to neutralize spills.  To use a hydrometer, squeeze the bulb while the inlet tube is still above the electrolyte level.  Then lower the hydrometer into the electrolyte and slowly release the bulb to draw in the electrolyte.  At the first cell being checked, fill and drain the hydrometer three times before removing a sample.  This brings the hydrometer to the same temperature as the electrolyte.  Take a sample and allow the bulb to fully expand.  The sample must be large enough to completely support the float. 

Hold the hydrometer straight up and down, so that the float does not touch the sides, top or bottom of the tube.  Look straight across the electrolyte level to read the float.  Ignore the curve of the electrolyte on the sides of the hydrometer.  Be careful not to drop the hydrometer or allow acid to drip out of it. After reading the hydrometer, to empty it slowly squeeze the bulb again with the inlet inside the cell, but just above the electrolyte level to reduce risk of spills.  Record    the Sg of each cell on a work sheet.  After use, rinse the hydrometer with fresh water at least five times to flush out any acid.  Allow it to dry completely before using it again.

Temperature compensation is required for batteries not at 80 degs. F.   Use an accurate glass thermometer and immerse only the thermometer bulb into the acid, leave it for 5 minutes, read it and then rinse in clear water.  For every 10 degs. F above 80 degs. F a factor of 0.004 must be added.  Subtract the same factor for each 10 degs. below 80 degs. F.  As an example, if  a battery at 30 degs. F has an Sg of 1.240, the battery is 50 degs. below the standard, so the compensation is subtracted from the specific gravity.  The compensation to be subtracted is .004 x 5 = .020; so 1.24 – .020 = 1.220.  A refractometer is the most accurate way to measure electrolyte Sg, requires only a small amount of fluid and is automatically temperature corrected.  Refractometers are recommended for ongoing maintenance of large battery banks.

To test the no-load, open circuit voltage an accurate DC voltmeter is required.  Operate the loads from the batteries for five minutes to remove any surface charge the battery plates may have.  Turn off all loads and disconnect the batteries from the rest of the system.  Now measure the voltage across the terminals of every battery.  Open circuit voltage without any load is a good preliminary indicator of  the state of charge.  The open circuit voltage of a fully charged 12-volt battery is over 12.72 volts, whereas it is about 12.6 at 75%, 12.48 at 50%, and 12.12 at 25%.

To perform a load test, after recording no-load voltage, re-connect the system, but leaving float chargers, charge controllers or solar arrays disconnected.

Operate the equipment at normal duty cycle for an hour, disconnect the batteries and measure the battery voltages again.  If any battery indicates a voltage of 10 percent higher or lower than others, it should be serviced or replaced.

An automotive load tester can be used, although you must be aware that this places a large artificial load on the batteries for a short time rather than waiting for a small load to slowly discharge the batteries.

Because available capacity is reduced at faster rates of discharge, this test is not as reliable for communications use as one at operating load.

Accurate trouble shooting requires that all batteries in a bank and individual cells of unsealed, wet-type batteries be numbered.  Recording a system history identifies patterns and trends and is a great time saver for others who may service your system in an emergency, because they can focus first on the most frequent problems and can anticipate the proper tools and materials to bring.   Battery systems not used on a regular basis, such as recreational boats and RVs must be checked in spring and fall, at minimum.  Monthly is recommended.

First disconnect all loads.  If battery tops are wet or dirty, remember that fluid on top of the battery is highly acid electrolyte! Clean battery tops with a cloth or brush and a baking soda and water solution.  Rinse with clean water and dry with a clean cloth.  Remove the caps from all cells, check the electrolyte level of every cell in every battery and add distilled water to the fill line on the battery, or 2″ above the top of the plates.  Determine the battery’s state of charge with a hydrometer.  Discolored, odorous electrolyte indicates contamination caused by adding other than distilled water, which results in battery failure.  Inadequate charging without adding water can result in lead sulfate shorts between the plates, cracked partitions between cells and leakage which require the battery to be replaced.

NEVER hammer cable connections onto terminal posts! This breaks fragile spot welds between terminal posts and plates, causing shorts, which could cause a spark and ignite free hydrogen gas, causing an explosion!   Inspect all caps for sound good condition, replace and tighten securely by hand only.   Tighten battery tie-downs securely, but not so tight as to distort the case.

Batteries that will not accept a charge may have had their electrolyte boiled off from prolonged overcharging, or become sulphated by being left at a low state of charge for too long or have suffered  physical damage and should be replaced.  Inspect and repair any corroded, loose or burnt connections and blown fuses.  Cartridge fuses don’t look different when they are blown, so remove them and check continuity with an ohmmeter. A blown fuse shows an infi­nite reading, a zero reading means it is still intact. 

Always determine why a fuse blew before replacing it. Proceed logically, check the most obvious things, such as  r excessive voltage drop at the load. Knowing what failed is necessary to avoid repeating the condition causing the failure.

If the same fuse blows again, don’t consider the system operational  until everything has been checked out.


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2 Responses to Maintaining Lead Acid Batteries to Get the Most Life Out of Them

  1. uma says:

    Thanks for all the info. I would like to know if there are any calculations for estimating release of hydrogen during various discharge rates.

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