People buying replacement batteries for battery backup sump pumps are often confused by the wide array of batteries available for battery backup sump pump systems and often can only compare by little more than price, without understanding there are big differences in performance and maintenance requirements between a $100 battery and a $250 battery. In a recent comment on another article here on 411Plumb I started to reply to the comment and it became rather lengthy so I decided it should become another article on its own rather than a comment reply. The information here applies specifically to the Zoeller Aquanot Battery Backup Sump Pump, but can easily be applied to other pump systems and the sizes and types of batteries used on them. The methodology of the comparison will be the same.
The Battery Group Numbers are an industry standard for the physical size of the battery and the placement of the battery terminals. In the case of the Group 27 and Group 31 the terminals are located on the top of the battery and in the same corners. The Group 27 is slightly smaller in physical size and capacity than the Group 31 option. This is not to say that other battery groups may not be used but but if you do use another battery group that meets the minimum requirements, some adaptation may be required in the battery box and in the ends of the cables and length of the cables in order to be able to connect to the different battery. Selecting a Group 921 battery for example or, building a “Bank” of batteries can yield huge amounts of storage capacity at a substantial cost as well. The choice is yours to make, and probably should be made in comparison costs to a standby backup generator system.
Next we will examine the differences between the “Wet” lead acid Deep Cycle batteries, and the “Absorbed Glass Mat” (AGM) Deep Cycle batteries. The group 27 and group 31 batteries have 6 cells which are connected in series. Each cell supplies 2 volts to the output of the battery, and a problem in any of the cells renders the entire battery useless. In the cells there are lead plates and a sulphuric acid solution which stores the electricity for later use. When needed there is electricity generated by a chemical reaction and is later recharged with a battery charger. Once you get past the simple price comparison which has similar sized batteries that are AGM batteries costing about 2 ½ times more than the Wet batteries, there are minor differences in performance, and huge differences in maintenance requirements. In the “Wet” cell type batteries when a battery is being recharged or, is receiving a trickle maintenance charge, gases are vented out of the battery cap vent. The gases being discharged through the vent are actually the loss of the electrolyte (sulfuric acid) solution so one of the maintenance requirements is replenishing the electrolyte solution by adding distilled water or, demineralized water to the cells to maintain the proper level of electrolyte solution in the cells. As a battery becomes older more frequent replenishment is required, and if the solution is ever allowed to drop to a level where the lead plates become exposed the battery will suffer immediate irreparable damage and should be replaced. The AGM batteries use less electrolyte in the cells and have a fiberglass mat which wicks the electrolyte solution up into the fiberglass mat and holds it against the lead battery plates. The AGM batteries are sealed and do not vent gases from the battery instead the gases are recombined into the electrolyte making the AGM batteries “maintenance free” in terms of electrolyte replenishment for the duration of the battery’s service life. Other maintenance is still required such as keeping the terminals free of corrosion, but electrolyte replenishment is a substantial task which is eliminated.
Next we will examine the actual performance the different sizes and types of batteries deliver. The reserve capacity figure Zoeller cites as a minimum is a 175 minute reserve capacity. Actually if they had cited “Hours at Minimum Load” or, “Amp Hours” which is a far more useful figure in comparing the performance of batteries. Reserve Capacity is actually a simplified measurement of battery performance which states the number of minutes a battery will sustain a 25 amp load before the voltage drops and the battery is “Dead.” “Hours at Minimum Load” or, “Amp-Hours” (AH) actually states the capacity at several ampere loads and the amount of time the battery is able to deliver that load amount for. The “AH” rating can be matched closer to the actual load your application uses and allow you to have a more accurate depiction of your actual results in use, at least when the battery is new. Since Zoeller does state their “Minimum“ battery will provide 4 hours of pump run time we’ll assume a 15 ampere load is the actual load of the Zoeller Aquanot Pump System. The following figures are from Interstate Batteries but may vary slightly from other battery manufacturers:
- Group 27 WET # SRM-27 5.5 Hours @ 15 Amperes
- Group 27 AGM # SC27DM 4.0 Hours @ 15 Amperes
- Group 31 WET (Not listed assume slightly higher capacity than similar sized AGM)
- Group 31 AGM # SC31DM 4.4 Hours @ 15 Amperes
It is interesting to note that as expected the larger sized battery has a larger capacity, however you will also note that the less expensive “WET” cell battery has a higher capacity than the “AGM” battery. This performance difference would be completely offset the first time you let a cell run dry rendering the battery useless. You may be able to add water, and have the battery seem to hold a charge, but the output and reliability would be severely diminished by even one time having one or more of the cells go dry.
So there you have it all spelled out for you, the determining factor in battery size, and Wet vs AGM is you. How much capacity you need, space considerations, and the level of maintenance you’ll need to perform, is all determined by you. You now have the information you need to make the intelligent choice to get the battery which best suits your specific needs.