Flooded Lead Acid
20 hour Discharge Capacity Test.
One of the surest ways to test the capacity of your Flooded Lead Acid
batteries is to charge them up once a year, following the manufacturer recommendations.
1. Full Charge - During absorption test and watch the trailing amps drop to below .5% of the 20ah Capacity (for T105, 20hr rating is 225ah, 225 x .005= 11.25amps) to be sure they are fully charged. For T105 I do the final charge separately rather than in series (12vdc nominal).
2. Equalize following the manufacturer specs (again it is best to equalize T105 separately). If the
batteries have had partial state of charge PSOC for large
parts of the summer, particularly for boats on moorings without at least some SolarPV, it is better to equalize for 2-3 hours, otherwise 30 minutes would probably be acceptable, depending on how much they have sulfated (uneven or lowered SG readings and lower charged voltage are signs of this)
3. Let the batteries sit 24 hours and then put a small 2-3 amp load on the
battery for 5 minutes to take the surface charge off.
4. 20hr Discharge Capacity Test Calculation:
- 20 hr Rating: From manufacturer's literature. For T105 20hr =225ah
- Continuous Load required: AH rating/20 hours. For T105 225ah/20hr=11.25amp
5. So the goal is to maintain a 11.25 amp discharge on the batteries for 20 hours, at which point the
battery should be fully discharged and at 10.5vdc (5.25vdc for a
single 6vdc T106). During the discharge draw down the amperage will vary due to changes in chemistry and the fact that the voltage will be declining. Thus the resistance will need to change over the 20 hours to have an accurate test. We can discuss how it is best to do this in later posts. Using a good Digital Multimeter you should
record voltage and amps every hour, and make any necessary adjustments to meet the calculated amps.
6. An accurate Coulomb counter or SOC Meter wired up with a shunt to negative is useful. Once the battery is fully charged, reset the meter. Note the conditions and try to take readings every hour for voltage and ah discharged.
I think we've covered enough territory for this post.