Desulphation Tests

A number of lead acid batteries were tested using low rate discharge tests in an attempt to identify their capacities and as a relative measure of improvement during sulphation treatments.

Of the batteries tested, four were suspected of having low electrolyte levels for some time and as such were thought to be partially sulphated, that is, with sulphation occurring at the top portion of the plates. They were observed to have a reduction in terminal voltage and capacity. The batteries were three 100AH Century N70T deep cycle batteries (labelled 1,3,4) and a 100AH Trojan 27TMX deep cycle battery (labelled 2). A new N70T (labelled 10) was also tested as a reference.

Following restoration of electrolyte levels, the batteries were tested with low rate discharges at 4A and 9A.

  • Battery 3 was treated with MgSO4 according to the recommended method, replacing the electrolyte with a MgSO4 solution, charging the battery, then replacing the solution with fresh electrolyte. The battery recovered some capacity, however it isn't clear if the fresh electrolyte added at full concentration had any role the observed recovery.
  • Battery 4 was treated with a CdSO4 additive and charged at high voltage (15.5V rather than the recommended 14.4V) over three weeks. The battery recovered almost to full capacity.
  • Battery 1 was later treated with a CdSO4 additive and charged at 15.5V.
  • Battery 2 was not treated, but recovered most of its capacity without any intervention.

These cases will be discussed in further detail below.

Load tests

Tests were carried out on all batteries after restoration of electrolyte levels and before any treatment was applied. This involved discharge at 4A followed by recharge. Graphs are provided for a 7 hour period to allow direct comparison. Early discharge tests were carried out over a three hour period, while later tests were allowed to run until the battery voltage fell to 11.15V. This point was used for a comparative capacity measure.

Capacity Measures

An attempt to measure capacity by coulomb counting of discharge current plus equilibrium terminal voltage measurements showed that the new and recovered batteries had a capacity of around 50AH to 60AH, rather less than the 100AH specified. This is not a particularly reliable way of measuring capacity. The new battery 10 was incidentally (due to equipment failure) run down to a very low charge level during the last discharge test. Coulomb counting over the entire time of its discharge also gave a capacity of 60AH and is probably reasonably accurate.


The CdSO4 treatment was effective in recovering capacity to almost that of a new battery in the case of two of the batteries. The MgSO4 treatment initially did not appear to make a significant difference, but after high voltage charging for four days the capacity also recovered markedly to almost that of a new battery. It is not possible to draw definitive conclusions without further testing.


These tests were not carried out in a controlled scientific way and the conclusions need to be considered carefully. All batteries experienced slightly different conditions. Batteries 1 and 3 were both in an operational installation for about 12 months while battery 2 was used in the same installation but for a longer period. This installation subjected the batteries to temperatures close to 50oC on occasions (although charging was suspended over 45oC). Battery 4 was in a different installation. Battery 2 is a more expensive battery with a different construction (using mysterious "technologies" promoted by the manufacturer), which may possibly have contributed to its better survival.

The MgSO4 treatment was modified to use conditions similar to the CdSO4 treatment by charging at a higher voltage for a period of time following the chemical treatment. Although the MgSO4 electrolyte was removed and replaced with fresh MgSO4 electrolyte, some of the MgSO4 may have remained behind. It is possible that the higher voltage treatment alone would have been sufficient to restore capacity given that the plates were most likely only lightly sulphated.

Battery 1 Century N70T 100AH capacity and terminal voltage low

This battery initially had no treatment applied apart from a restoration of electrolyte levels, and was used as a reference. The tests show slight but insignificant changes in voltage or capacity since the restoration of electrolyte levels. The test that was run just before the CdSO4 treatment was applied lasted for 5:50 hours before the terminal voltage fell to 11.15V. Capacity measurements showed around 30AH 5AH, about half that of the new battery 10.

A CdSO4 treatment was applied and the battery was charged at high voltage (15.50V) for 21 days as suggested in various descriptions of this treatment. The test that was done immediately following this treatment lasted for 11:10 hours before the terminal voltage fell to 11.15V. The battery voltage and capacity increased dramatically as shown in the graph below. The capacity measurements showed 47AH, close to that of battery 10.

Battery 2 Trojan 27TMX 100AH

This battery had no treatment applied apart from a restoration of electrolyte levels, but appeared to recover its capacity. The latest test lasted for 9:10 hours before the terminal voltage fell to 11.15V, very close to that of  the new battery. Capacity measurements were initially very variable, but later showed values around 50AH 10AH, also close to that of the new battery 10.

Battery 3 Century N70T 100AH capacity and terminal voltage low

This battery was treated by replacing the electrolyte with a magnesium sulphate solution, recharging and replacing the sulphuric acid electrolyte at full strength. The battery gradually recovered some capacity but not as much as might have been hoped. The test lasted for 6:45 hours before the terminal voltage fell to 11.15V, slightly more than the reference battery 1 without treatment. Capacity measurements were very variable, but after treatment showed values around 45AH 10AH.

The above plots shows some recovery of the battery voltage and capacity after electrolyte levels were restored but before treatment. These are very similar to the early discharge tests for battery 1 which is of the same type and experienced identical conditions as battery 3.

After the MgSO4 treatment (yellow 31/08/2017*) these show some recovery over time compared to the first discharge test (light blue 11/08/2017). Initially the voltage levels increased markedly, possibly as a result of replacement of fresh (and hence more concentrated) sulphuric acid electrolyte. In subsequent tests the voltage level dropped but the slope of the discharge curve also dropped allowing the battery to last longer during discharge. Unfortunately early tests were stopped after 3 hours so it isn't possible to determine the overall lifetime of the battery at those stages.

Following these tests, the battery was placed on charge at elevated voltage as recommended for CdSO4 treatments but without further chemical treatment. After four days it was removed due to overheating. The curves below show the results of load tests soon after the treatment and again after three weeks of normal use. These are compared with the original curve of the untreated battery and that for the battery immediately after the MgSO4 treatment. The terminal voltage is now significantly elevated and the load test lasted for 8:52 hours at 4A before the terminal voltage fell to 11.15V. This is now quite close to that of the new battery 10.

Battery 4 Century N70T 100AH capacity and terminal voltage low

This battery was treated by addition of cadmium sulphate and then some days later by charging at high voltage over 21 days. The battery recovered capacity quite substantially. The test lasted for 9:00 hours before the terminal voltage fell to 11.15V, close to that of  the new battery. Capacity measurements initially showed values around 25AH 5AH. After the final high voltage treatment, tests showed values of around 44AH 2AH.

These show the initial discharge curve (green 12/8/17) followed by treatment with CdSO4 (yellow 1/9/17*). There followed only very small improvement in capacity until after the battery had been placed on overcharge at 15.5V for three weeks. The final curve (brown 24/10/17**) shows a very significant improvement in capacity. 

Battery 10 Century N70T 100AH new

This battery was new and was tested as a reference to determine how well the treated batteries had recovered their capacity. The battery voltage dropped over the time of the tests, possibly a settling period as suggested by the manufacturers as being needed for the battery to gain its full capacity. In the last test the battery lasted for 9:55 hours before the terminal voltage fell to 11.15V. Capacity was about 60AH as discussed above.

First created
30 October 2017
Last Modified 30 December 2017
Ken Sarkies 2017