A batteries capacity is measured in Amp Hours (Ah). Amp Hours is the total number of hours a battery can provide 1 amp of current, at 12 volts, before draining the battery.

Lets take a battery that is rated for 100Ah. In an ideal world, if you had a device that pulled 1 amp of current, it would last for 100 hours. However, that is not the case.

There are several different battery technologies on the market that allow you to use more of the rated amp hours than others. Lead acid batteries (most common) will typically allow you to get 50% of the capacity before it needs to be charged whereas lithium ion batteries allow you to use nearly 90% of the battery before it needs to be charged.

It is also worth noting that with lead acid batteries as the capacity drops (voltage), so does the power output. Whereas with lithium ion batteries, the power output is the same if the capacity is at 99% or 20%.

A cranking battery delivers a burst of power to start the engine. Starting an engine requires a blast of 100-200 amps. As soon as the vehicle is running the alternator will recharge those amps for the next start up.

Another important note, cranking batteries can’t be discharged more than about 25% before their lifespan is depleted. Meaning 75% of the battery must remain charged any drainage beyond that 25% will damage the charging capabilities prematurely and shorten the life span of the battery.

A typical deep-cycle battery can store energy and deliver it at lower amperages for longer periods of time. Most discharge up to 50% capacity while some ADM or Lead Acid batteries can be run down to 75% of their maximum charge without damaging the lifespan of the battery. 

By running a dual battery system the cranking battery will never drain beyond the cranking amps required to start the vehicle . The 200amps required to start the engine will be there when you need them. Allowing the deep cycle battery to power a multitude of devices:

• Lights
• Inverter
• Fans
• Fridge Freezer
• Charge Electronics
• Mattress Inflator
• Radio
• Air Compressor
• CPAP Device

It is important to pick the right size deep-cycle battery for a dual battery system based off consumption of energy. Researching how many Amps/Hour your devices use in total will determine what size battery to purchase. The chart below depicts Amps/hr required for 4 different size fridge freezers and how many hours you can expect from a 60, 80, and 100 Amp Hour deep-cycle battery.

By adding up the devices you plan to run simultaneously you can determine what size battery will fit your needs.

When running a dual battery system installing a Smart Isolator will manage both the cranking and deep-cycle battery's energy. An isolator splits the two batteries to ensure the vehicles alternator doesn't work over-time to charge both batteries. Similarly it won't allow either battery to interfier with the other causing an electrical issue.

Batteries that are fully charged at 13.2 volts, are automatically connected and are charged at the same time. When the engine is off, while using accessories such as lights, stereo, or a CB both batteries begin to drain.

After the cranking battery reaches 13.2 volts, the isolator separates the batteries, so that your cranking battery will have enough power to start the engine. The second battery continues to power your accessories.

After starting the vehicle, your main cranking battery will be charged up to 13.2 volts first. Then the isolator will begin charging your accessory battery. By only charging one battery at a time, your alternator is protected from excessive strain.

If the cranking battery somehow drained below sufficient cranking amps to start the vehicle some isolators have Start Boost button. The isolator will connect both batteries together for 1 minute, acting like built-in jumper cables. As long as the deep-cycle battery has enough power combined the batteries can start your vehicle.

Once the vehicle has been started the isolator charges both batteries for one minute. Before switching to one at a time.

As devices drain the battery, monitoring your usage or knowing how much energy is left in the deep-cycle battery is important. If you are running a single device for the weekend, chances are, your energy consumption will be low. But, on an extended trip, if you are charging devices, powering a fridge, using accessories, ect. Knowing the status of your battery is vital.

We recommend a Victron Energy Smart Battery Monitor BMV-712. Its' simple design, easy to read digital layout and Smartphone capabilities takes the stress out of monitoring battery life and consumption. Being able to view all of the information from your smartphone is an added bonus.