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Author Topic:   Self Discharge of Storage Batteries: Follow Up
jimh posted 03-13-2010 11:45 AM ET (US)   Profile for jimh   Send Email to jimh  
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. See

This 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 follows

Terminal Voltage Day 0 = 12.78
Terminal Voltage Day 446 = 12.47
Decline in Terminal voltage = 0.31-volts

The rate of self discharge was thus

0.31-volts / 446 days = 0.000695-volts/day

The previously measured rates of self discharge were

AGM = 0.08 volts/104 day = 0.00076923 volts/day
Flooded Cell = 0.15 volts/ 134 days = 0.0011194 volts/day

The 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 drop

0.0011194 volts/day x 446 days = 0.4993-volts

The flooded cell battery would thus be at a terminal voltage of approximately

Flooded cell terminal voltage = 12.65 - 0.4993 = 12.15-volts

If 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 by

0.00076923 volts/day x 446 days = 0.3431-volts

The AGM would thus be projected to have a terminal voltage of

12.78 - 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 at

we 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)     Profile for jimh  Send Email to jimh     
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 = 12.78
Terminal Voltage Day 671 = 12.29
Decline in Terminal voltage = 0.49-volts

The rate of self discharge was thus

0.49-volts / 671 days = 0.00073-volts/day

The 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)     Profile for Buckda  Send Email to Buckda     
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)     Profile for jimh  Send Email to jimh     
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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