1. AE6PM Rechargeable_Battery_Basics 1 Rechargeable Battery Basics More than you ever wanted to know. Don Steinbach AE6PM Saratoga Amateur Radio Association April 6, 2011

2. AE6PM2 Typical Batteries Non-rechargeable (primary cells) Alkaline Carbon-Zinc Rechargeable (secondary cells) Ni-Cd (Nickel-Cadmium) Ni-MH (Nickel-Metal Hydride) Lithium-ion Lead-Acid –Flooded –Gel –AGM

3. AE6PM3 Lead-Acid Batteries Oldest type of rechargeable battery. –Invented in 1859. Consists of lead plates in the presence of an electrolyte. –Plates are lead and lead oxide. May be alloyed with antimony, tin, calcium or selenium. –Electrolyte is approximately 35% sulfuric acid and 65% water. Each lead-acid “cell” produces ~2.14 volts. As the battery discharges, the electrolyte reacts with the plates, changing their surface to lead sulfate.

4. AE6PM4 Lead-Acid Batteries When the battery is recharged, the lead sulfate reforms back into lead and lead oxide, and the specific gravity of the electrolyte is restored. Over time, the lead sulfate converts to a crystalline form that no longer dissolves on recharging. –This process is accelerated if the battery is left in a discharged condition. –Sulfation can be avoided if the battery is fully recharged immediately after a discharge cycle. Excessive charging electrolyzes some of the water, producing hydrogen and oxygen (called “gassing”). –Mixture is highly explosive.

5. AE6PM5 Lead-Acid Battery Types Flooded (Wet Cells) –Electrolyte is a liquid. –Battery is usually not sealed and not spill-proof. –Usually vent hydrogen gas during charging. Valve-Regulated Lead-Acid (VRLA) –Gelled Electrolyte (Gel Cell) Electrolyte is immobilized by a thickening agent. –Absorbed Glass Mat (AGM) Electrolyte is absorbed in separators of matted glass fibers. –VRLA batteries are hermetically sealed and designed to produce little or no hydrogen gas during charging. All use the same chemistry. Construction varies.

6. AE6PM6 Battery Service Types The three service types are Starting, Deep Cycle, and Marine: Starting –Sometimes called SLI, for starting, lighting, ignition. Typical automobile battery. –Designed to produce very large starting current for a very short time. –May last for thousands of cycles in normal starting use (2% - 5% discharge). –Automotive batteries will generally fail after 30 – 150 deep cycles. –Plates are composed of a lead “sponge’. Gives very large surface area, but is quickly consumed if the battery is deep-cycled.

7. AE6PM7 Battery Service Types Deep Cycle –Designed to be discharged as much as 80% many times. –Maximum life if average discharge is kept at about 50%. –Plates are usually solid lead, not sponge. Less surface area, hence less “instant” power like starting batteries provide. –It will not hurt a deep cycle battery to be used as a starting battery, but cannot supply as much cranking amps. Marine –Hybrid. –Compromise between starting and deep-cycle.

8. AE6PM8 Typical Specifications CCA – Cold cranking amps –The number of amps a battery can deliver at 0° F for 30 seconds and not drop below 7.2 volts for a 12-volt battery. CA – Cranking amps –Same as CCA except tested at 32° F. –Also called MCA – Marine cranking amps. RC – Reserve Capacity –The number of minutes a fully charged battery at 80° F will discharge at a 25-amp rate until the voltage drops below 10.5 volts. AH – Amp hour –The number of amps the battery can deliver for a number of hours until the voltage drops to 10.5 volts. Typically for 20 hours. See Peukert’s Law.

9. AE6PM9 Size Codes and Capacities For large batteries: Six-volt golf cart batteries are typically 180 – 225 AH capacity BCI = Battery Council International BCI CodeCapacityVoltage U134 – 40 AH12 V Group 2470 – 85 AH12 V Group 2785-105 AH12V Group 3195-125 AH12 V

10. AE6PM10 Battery State-of-Charge Open-circuit voltage vs. SOC for 12V lead-acid battery: % ChargeFloodedGelAGM 10012.60 or higher 12.85 or higher 12.80 or higher 7512.4012.6512.60 5012.2012.3512.30 2512.00 011.80 Note: Voltage under load at full discharge (0% charge) is 10.5V.

11. AE6PM11 Charging Algorithm “Smart Charger” uses a three-stage charging algorithm. For a 6-cell/12-volt flooded battery: –Bulk charge Constant current charge until battery voltage reaches 14.4 volts at which time the battery is ~80% charged. –Absorption charge Constant voltage of 14.4 volts. Current decreases as battery charges to ~98%. –Float charge Regulated voltage of not more than 13.4 volts and usually less than 1 amp. Battery eventually reaches ~100% SOC.

12. AE6PM12 Charging – AGM & Gel AGM and Gel batteries have different charge voltage limits. –14.4 to 14.6 volts maximum at 68° F for AGM. –13.8 to 14.1 volts maximum at 68° F for Gel. Varies somewhat with manufacturer. –See table on next slide. –Check the specs for your battery. Automatic temperature-sensing voltage-regulated chargers are highly recommended. –Voids or bubbles can form. –Water cannot be replaced if continually overcharged.

13. AE6PM13 Charging – AGM & Gel AGM and Gel batteries have different charge voltages. AGM Gel Temp °FOptimum Charge Maximum Charge Optimum Float Maximum Float 70-7913.7014.0013.4013.70 60-6913.8514.1513.5513.85 Temp °FOptimum Charge Maximum Charge Optimum Float Maximum Float 70-8014.3014.6013.4013.70 60-7014.4514.7513.5513.85

14. AE6PM14 Charging – Flooded Flooded batteries are less critical because they are vented and water can be replaced. –Automobiles typically charge at 13.8 to 14.4 volts. For a 6-cell automotive lead-acid battery at room temperature: –Open circuit voltage at full charge: 12.6 V –Fully discharged: 11.8 V –Charge with: 13.2 – 14.4 V –Gassing voltage: 14.4 V –Continuous float voltage: 13.2 V Note: These voltages are for 20°C and must be adjusted -22 mV/°C for other temperatures.

15. AE6PM15 Notes Battery capacity is reduced as temperature goes down, and is increased as temperature goes up. –At freezing, capacity is reduced by 20%. Battery life is reduced at higher temperatures. –Battery life is cut 50% for every 15° F above 77° F. Deep-cycle battery life is directly related to how deeply the battery is cycled each time. –Life at 50% DOD = 2X 80% DOD. –Life at 10% DOD = 5X 50% DOD. Acid spills can be neutralized with ammonia or sodium bicarbonate (baking soda).

16. AE6PM16 Economics AGM and Gel batteries are more expensive than flooded batteries. AGM is a newer technology. –Low self-discharge. –Accepts higher charge rates than Gel. –Costs 2 to 3 times as much as an equivalent flooded battery. Flooded deep-cycle battery vs. flooded SLI battery –Additional cost probably not warranted for ham radio emergency power or Field Day application. –Need to consider how many “deep” cycles the battery will encounter. Maybe 20 cycles over 10 years for Field Day. Maybe 10 cycles over 10 years for home use. –Batteries will probably die of old age at 5 – 7 years, long before cycle life limit is reached.

17. AE6PM17 The End