ContinuousWave   Whaler   Moderated Discussion Areas   ContinuousWave: Small Boat Electrical   Self Discharge of Storage Batteries: Follow Up
 Author Topic:   Self Discharge of Storage Batteries: Follow Up jimh posted 03-13-2010 11:45 AM ET (US)         In a prior discussion on the topic of self-discharge of batteries, it was determined that the principal influence on the rate of self-discharge was the purity of the lead used. SeeThis discussion included some test data which I collected myself comparing the self-discharge rate of two random batteries of different design, one a conventional flooded-cell lead-acid starting-lighting-ignition (SLI) marine battery, and the other an absorbent glass mat (AGM) valve regulated lead-acid (VRLA) battery. The data on self discharge was collected over a period of approximately 100-days.As it happens, one of the batteries in the experiment has been maintained in a disconnected state. It has been sitting dormant since being charged in December of 2008. Here are voltage measurements for the battery to date:December 22, 2008 = 12.78-volt (Day 0)April 5, 2009 = 12.70-volts (Day 104)March 13, 2010 = 12.47-volts (Day 446)The decline in terminal voltage was as followsTerminal Voltage Day 0 = 12.78Terminal Voltage Day 446 = 12.47Decline in Terminal voltage = 0.31-voltsThe rate of self discharge was thus0.31-volts / 446 days = 0.000695-volts/dayThe previously measured rates of self discharge wereAGM = 0.08 volts/104 day = 0.00076923 volts/dayFlooded Cell = 0.15 volts/ 134 days = 0.0011194 volts/dayThe observed rate of self-discharge of the AGM appears to have tapered slightly.If we project the flooded cell battery over 446 days, we would expect its terminal voltage to drop0.0011194 volts/day x 446 days = 0.4993-voltsThe flooded cell battery would thus be at a terminal voltage of approximately Flooded cell terminal voltage = 12.65 - 0.4993 = 12.15-voltsIf we projected the AGM discharge rate using the observation over the 100-day period, we would have expected the AGM terminal voltage to have decreased by0.00076923 volts/day x 446 days = 0.3431-voltsThe AGM would thus be projected to have a terminal voltage of12.78 - 0.343 = 12.437-volts We compare this to the observed voltage of 12.47 and see the self-discharge is not as deep as initially expected. This may be reasonable if one considers that as the battery terminal voltage drops, the rate of discharge may likely slow as well.Using the chart shown in my article on Battery Charge atwe would estimate the percent of discharge of the two batteries as follows:Flooded cell at 12.15-volts = 52.5-percent discharged (projected)AGM at 12.47-volts = 33.5-percent discharged (actual)AGM at 12.437-volts = 34.2-percent discharged (projected)After over one year of being allowed to self-discharge, we see that the remaining charge in the AGM battery will be greater than the remaining charge in the flooded cell battery, but the difference is not overwhelming. jimh posted 10-24-2010 12:49 AM ET (US)             Following up on the self-discharge rate of the AGM battery I have been testing, I checked its open terminal voltage tonight:Terminal Voltage Day 0 = 12.78Terminal Voltage Day 671 = 12.29Decline in Terminal voltage = 0.49-voltsThe rate of self discharge was thus0.49-volts / 671 days = 0.00073-volts/dayThe self-discharge rate has increased slightly. The battery is still above 50-percent charge, even after 671-days of sitting unattended at room temperature. Buckda posted 10-26-2010 12:52 PM ET (US)             Interesting information.I suggest you continue the experiment, with the batteries in an unheated location, such as your garage, to subject it to the same temperature fluctuations that one might expect having left a battery aboard their boat or RV over a period of prolonged storage...Realizing, of course, that a connected garage with daily driver cars in it will likely remain warmer than an unheated storage facility.... jimh posted 10-27-2010 09:07 AM ET (US)             My understanding is the process of self-discharge of a lead-acid battery will occur at a slower rate as the ambient temperature is lowered.I do not plan to continue the experiment further. I plan to re-charge the AGM battery I have been testing in order to prevent it from becoming too deeply discharged.

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