deep cycle batteries [2] retirement data

In this second part of our deep cycle battery series, I provide detailed measurements for our 12 battery array that is due for replacement.  From these measurements, we will eventually try to reconfigure the array into a smaller set that provides better performance.  I’ll also try to use a special charging technique  to restore some of the weak cells.  The series started with, Deep Cycle Batteries [1] Living in End of Life.

“eennt!  eennt! eennt!”   The inverters warn of low battery voltage.  It still happens every day around here since our battery bank is in the end of life. Its difficult considering replacing our beloved 12 deep cycle batteries.  The details and measurements will help others in evaluating the usefulness of their older battery systems.  This once mighty battery bank could store 1000 Amp Hours of usable ‘juice’ at 12 volts!  Now, as configured, it goes from full charge, to inverter shut off with a paltry 150AH!

Lets talk about battery measurements.  There are several ways to measure the capacity and quality of a lead acid deep cycle battery like the Trojan L-16 that make up this bank.  Load testing is where you take the fully charged batteries and draw a constant current until the voltage drops to some lower level.  Well, that’s pretty much how I got the 150AH from this bank.  Another way is to measure cell, or battery voltage, but this really only indicates whether the batteries are charged, not their quality.  The third way is to measure the density, or specific gravity of the battery fluid that is called electrolyte.  This should be done when the battery bank is charged.

The electrolyte measurement can indicate the quality of the individual cells within a multi-cell battery.  The specific gravity of the fluid can range from 1.000  to 1.270 with the state of charge indicated in the table.

State of Charge Specific Gravity
100% 1.270
75% 1.230
50% 1.190
25% 1.160
discharged 1.120

To make this measurement, electrolyte is drawn up a small tube where a weighted pointer indicates the measurement on a scale.  Often the testers have a number of colored balls that float, and a table that indicates which color floating ball represents a specific gravity.  These units are very similar to ones used to measure the antifreeze in a car radiator.

The L16 batteries are a 6V unit with 3  – 2V cells.  The quality and charge state of each cell within a battery must match otherwise the weak cell will hinder the entire battery. These guidelines will help us determine the quality of our charged up battery bank.

  • Cells within a battery must all have the same specific gravity within 0.050
  • Charged up batteries with a cell measuring 1.000 are dead.

The battery bank used in this test is for 12 batteries arranged in a 2 x 6 matrix.  Each battery has 3 cells, so the table below has 6 columns for a complete 12 V source made up of 2 batteries.  There are 6 rows of 12 V sources that are wired in parallel.  The raw measurements are indicated below.

Our Deep Cycle Battery Data

While waiting and saving for a new battery bank, I’d like to reconfigure my battery bank to obtain more than the 150 AH that I currently have from this mess. Also, I want to try some “treatments”, that is, special charging techniques on some pairs of batteries to see if they can be restored somewhat.  I’ll explain more about the treatments later.  It sure would be nice to get some extended life from part of these batteries to buy more time!  From this data there are a few conclusions that can be made:

  • Batteries 1, 4, 5, 7, and 11 are pretty much worthless now since they contain a dead cell.
  • Battery number 9, appears quite weak, since all its cells have a 50% charge , and cells are within 0.050  in specific gravity.  There might be hope for this one.
  • Battery 2 has 2 good cells and one weak one that’s just a little more than 0.050 different.  This one is questionable.
  • Batteries 8 and 10 are matched to each other well and are at 75% charge.  These to batteries may be useful with a little treatment.
  • Batteries 6 and 12 are OK, and can be used further.

Note that all rows except the 5th have a dead cell in them.  This explains why these two batteries are the only ones working in this 12V array.  All other pairs really ‘want’ to be a 10V contribution and don’t add any capacity at 12V.  My theory is that the capacity of the current arrangement comes strictly from the battery pair 9 and 10. Well that’s my theory, and I’m sticking to it for now.

At 125 pounds each, we have 1500 pounds of essentially dead batteries waiting for replacement.  What can be learned from all this?  First, is that we ran through this battery bank in about 5 years instead of the 7 that was planned.  The design power budged increased significantly when I added my home office and when we switched from propane to various electric refrigerators.  Second, I hate to confess, but I became distracted somewhat a couple years and let the depth of discharge (DoD) go much lower than my design of 75%.  With deeper discharges of 50% or so, the wear out mechanism of the batteries increased.

You probably are wondering what is the mechanism that causes these deep cycle lead acid batteries to wear out?  Let me offer a few simple points that help explain why this is so.

  1. Lead acid wet cells store electrical energy through a chemical reaction that causes the electrolyte to change from a strong acid when in the charged state, to diluted, or watery acid when in a discharged state.
  2. When the cells are kept below 75% charge for any length of time, an undesirable substance can grow on the lead plates in the batteries (lead sulfate).
  3. The the whitish lead sulfate binds up some of the chemicals in the reaction and can reduce the storage capacity of the battery.
  4. Lead sulfate growing and then breaking up with charging can be like freezing water on rocks.  That is, the weathering or erosion from this process with the lead sulfate can cause the lead on the battery plates to flake off.  (that’s why deep cycle batteries have extra thick lead on their plates compared to car batteries)
  5. This should explain why you should never store a lead acid battery when it is not charged.  If you do, it will be damaged by the lead sulfate build up.  Item #2 is very important to consider when operating a solar power system.  If you design a system with a battery bank that never gets discharged more than 75%, then it will have long battery life.  Unfortunately, buying a hundred dollars worth of a product and only using twenty five dollars worth, doesn’t sit well with most budgets!

    Next time we’ll go over techniques on reducing the amount of lead sulfate in deep cycle batteries.  I hope to extract the dead batteries from my array and do some more real-world tests trying to kick start some of these batteries back into service for the summer

    [Editors note: This series is continued at,deep cycle batteries [3] Reconfigure Array ]

    Have you had experience with worn out lead acid batteries?  How long have yours lasted, or how long do you want your first set to last? Tell us below in the comments!

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  4. deep cycle batteries [3] Reconfigure Array
  5. Deep Cycle Batteries [1] Living in End of Life
2 Responses to deep cycle batteries [2] retirement data
  1. Arild Jensen
    March 17, 2010 | 7:29 am

    Marshall, it might be worthwhile taking #2 and #3 battery off line then equalizing that pair with your 12V charger. Follow Trojan’s recommendations for duration of equalization charge and monitor the temp while you are doing it.
    With just the six cells in series the equalizing current will flow through each of them. When You asked me about equalizing before I thought you meant the whole bank at once. Parallel charging paths are always trouble. Doing one pair at a time will be better. Battery #8 and # 10 also look like a suitable pair. This is a good time of year to do this with cool ambient temps. Monitor battery heating to avoid exceeding 120F internal temps. If you have any way to check temperature of individual cells so much the better. I have one of the infra red thermometers which I have used to check everything from a bad injector on a diesel engine to leaky window insulation and problems with hydronic heating systems.

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