How To Choose The Right Battery

Types of batteries

Batteries handle two basic kinds of tasks, starting an engine and running electrical loads like lights, electronics and accessories for longer time periods. To choose a battery, first determine the battery’s application and then choose from one of the three battery chemistries: Flooded, Gel, AGM.

Battery application

Starting batteries

Starting batteries, which crank the starter of your  engine, are the sprinters of your electrical system. They deliver between 75 and 400 amperes or more for 5-20 seconds, and then are recharged in short order by your engine’s alternator. Like all lead-acid batteries, they are constructed with alternating layers of negative and positive plates with insulation between them.

Starting batteries have thinner and more numerous plates, providing extra surface area to generate high amperage bursts of current. The two drawbacks of this construction are that the plates are relatively fragile in high-impact environments, and that starting batteries do not tolerate deep discharges, which reduce their operating lifespan.

Deep cycle batteries

A battery bank uses deep cycle batteries, the marathon runners of the storage system. They power the electrical loads when no charge source (land power charger, engine alternator, wind generator or solar panel) is available. Consider them a kind of savings account into which energy is deposited or withdrawn.

Compared to starting batteries which deliver high bursts of energy for short periods, deep cycle batteries make possible the delivery of  amp hours over an extended period of time.  Deep cycle batteries recover fully after being heavily discharged over longer periods because their design features thicker plates with a high content of antimony. Overnight, their use might deplete 10- 80 percent of the battery capacity, depending on the load. When the batteries are recharged, energy is re-deposited into the bank, and the process, or cycle, starts over. Generally, deep cycle batteries should be sized to store three to four times the expected amount of energy to be used between recharge cycles.

Dual-purpose batteries

We generally advise that you choose either a deep cycle or starting battery for best performance and battery life, but dual-purpose batteries work well in some applications. With large, thick plates containing more antimony than starting batteries and an active lead paste chemistry, dual-purpose batteries are a good compromise.They tolerate deep discharges that would ruin a typical starting battery because they have a lower storage capacity than comparably-sized deep cycles.

Battery chemistry

Batteries are available in three chemical types for different applications: flooded, gel, AGM (Absorbed Glass Mat). Which type you choose is based on your needs (deep cycle vs. starting), the capacity and lifespan you are looking for and your budget.

Flooded Batteries

Flooded batteries, unlike other types, use a reservoir of liquid sulfuric acid, and produce hydrogen and oxygen when the battery is being charged. Vented wet cells allow the gases to escape into the atmosphere, unlike gel and AGM batteries, which recombine the gases and re-introduce them to the system. Hydrogen is an explosive gas, so battery boxes and compartments must be vented to let the gas escape safely.

Flooded batteries require maintenance—periodic inspection and topping-off with distilled water. They handle overcharging better than gel and AGM batteries, because of this hydrogen venting and because they are not sealed like the other types. They self-discharge at a higher rate (5 to 7 percent per month) and thus require off-season charging. Wet cells must be installed in an upright position and don’t tolerate high amounts of vibration. Their initial cost is lower than similarly sized AGM or gel batteries. Properly charged and maintained, wet cell deep-cycle batteries are capable of between a few hundred and over a thousand charging cycles.

Gel Batteries

Sealed, valve-regulated (SVR) gelled-electrolyte batteries offer advantages over regular flooded batteries. They self-discharge at only three percent per month, handle the highest number of lifetime charging cycles, are maintenance free, spillproof, submersble and leakproof.  A pressure release valve keeps their internal pressure at a slightly positive level, but they can release excess pressure if needed. The SVR design nearly eliminates gassing, so they are safer to install around people and sensitive electronics (but gel and AGM batteries still need to be vented). Gel batteries, because they’re sealed, are manufactured to very high quality standards. They need carefully regulated smart charging to prevent damage.

AGM Batteries

Sealed, valve-regulated AGM (Absorbed Glass Mat) batteries feature fine, highly porous microfiber glass separators compressed tightly between the battery’s positive and negative plates, which are saturated with just enough acid electrolyte to activate the battery. During charging, precision pressure valves allow oxygen produced on the positive plate to migrate to the negative plate and recombine with the hydrogen, producing water. In addition to providing equal saturation across the entire surface of the battery’s positive and negative plates, the fibers in the dense glass mats embed themselves into the plates’ surface like reinforcing rods in concrete, providing more plate support and better shock and vibration protection than in conventional batteries.

High-density AGM batteries have lower internal resistance, allowing greater starting power and charge acceptance, up to 45 percent of the battery’s total capacity, and quicker recharging than other types of deep cycle batteries. Long life, a low three percent self-discharge rate and outstanding performance make AGM batteries excellent dual-purpose batteries for  those who require the fastest recharging, quick starting power and reliable deep cycle ability.

What to look for

Starting functions: the amount of power available for cranking a starter is measured several ways.

CCA vs. MCA:

The two common power measurements are CCA (Cold Cranking Amps, the number of amps a battery can deliver for 30 seconds at 0°F while maintaining its voltage above 7.2 volts) and MCA (Marine Cranking Amps, similar but measured at 32°F instead of 0°F). The reason that MCA are 20-25% higher than the CCA is because batteries work better at higher temperatures.

Reserve Minutes indicate how long a battery can sustain a load of 25 amps before it drops to 10.5 volts. A battery rated at 150 minutes can operate a 25A load for 2 1/2 hours (at 80°F). Starting batteries aren’t used to handle loads for long periods, so reserve minutes are less critical.

Size

Engine size, type, and ambient temperature determine what size cranking battery you need. High cranking power (and a larger battery) is required for cold temperatures, diesel engines, or large and high compression gas engines. The first sizing criteria is to meet the minimum CCA (if any) stated by the engine manufacturer. If a Group 31, 1000 CCA battery worked well for five years, we’d recommend replacing it with a similar model. If, however, it cranked too slowly, or failed after a season or two, we’d suggest that you look for a battery with a higher CCA or MCA rating.

Deep cycle functions

Battery capacity measurements are commonly expressed in Amp-hours  (Ah) and Reserve Minutes. Amp-hours measure the total amount of energy that a battery can deliver for 20 hours at a constant rate of discharge, before the voltage drops to 10.5 volts. This means that a 100Ah battery can run a 10A load for 10 hours. The reserve minute rating is the number of minutes that a battery can run a 25A load until dropping to 10.5V, just like with starting batteries. A Group 31 deep cycle battery with a rating of 180 reserve minutes will run a 25A load for three hours. House loads range from 5A to 25A or more. Ah is generally the more relevant measurement for house banks.

Longevity

Battery manufacturers measure longevity by discharging full batteries at a temperature of 77°F until their voltage drops to 10.5 volts. The batteries are recharged under controlled conditions, and the process is repeated until the battery fails to hold half of its rated capacity.  This measurement, called cycle life, shows how many discharge cycles a battery provides over its lifespan.  This ability to cycle repeatedly is what differentiates deep cycle batteries from starting batteries, which can’t withstand more than a few deep discharges before they begin to fail. If nothing else, cycle life provides a baseline for comparing one battery to another.

Battery tips for best performance

No matter what kind of battery chemistry you choose, follow these recommendations to get the best performance:

  • Keep in mind that whatever ampere hours are taken out of the battery , that same number of ampere hours PLUS 20% needs to be replaced in order to fully charge the battery.
  • Stay with one battery chemistry (flooded, gel or AGM) Each battery type requires specific charging voltages. Mixing battery types can result in under- or over-charging. This may mean replacing all batteries at the same time.
  • Regulate charge voltages based on battery temperature and acceptance (manually or with sensing) to maximize battery life and reduce charge time. Ensure that your charging system is capable of delivering sufficient amperage to charge battery banks efficiently. This generally means an alternator with 25% to 40% as many amperes as the capacity of your entire battery bank.
  • Keep batteries clean, cool and dry.
  • Check terminal connectors regularly to avoid loss of conductivity.
  • Add distilled water to flooded lead acid batteries when needed. Keep them charged. Leaving them in a discharged state for any length of time will damage them and lower their capacity.
  • Clean corrosion with a paste of baking soda and water.