A guide to deep cycle batteries

Types of Deep Cycle Batteries

There are 2 main types of deep cycle batteries.

Lead Acid Batteries

This includes Flooded, AGM (Absorbed Glass Mat), and Gel battery types. These are similar to a start battery and though they look the same on the outside, their internals are quite different.  

Lithium Batteries

Lithium Iron Phosphate (LiFePO4) These are gaining in popularity due to the increased number of times it can be recharged and a faster charging time. The downside is their higher purchase price. 

Charging a Deep Cycle Battery

There are a number of options, and some people employ all of them. It does depend on what you want to run and how important it is to you. Deep cycle batteries need a different charging profile to a start battery so getting the correct charger for them is important.

In-Vehicle

When using it as an auxiliary battery to power vehicle accessories, the simple option is parallel wiring and a manual or smart switch that charges it when activated. The more advanced solution is a DC-to-DC charger which has a lot more features and is a much better charger.

Caravan or Campervan

Similar to the in-vehicle setups with the added bonus of solar panels and a 240V charger when at a caravan park or at home in the garage or shed. A lot of serious off roaders set up their vehicles with solar (usually portable) and 240V chargers as well.

Lithium Charging

It is important NOT to charge a lithium battery with a lead acid charger. The technology is different, and you can permanently damage the battery. Make sure the battery charger you use is either designed to charge only lithium batteries or has a lithium setting on it. This goes for solar controllers as well. If the charger does not state it charges lithium batteries check the manual or ask the supplier before using it.

Battery Power (Ah)

Deep Cycle batteries are a power source or power supply designed to run portable 12 Volt DC (Direct Current) electrical equipment. Houses running off the power grid run 240 Volt AC (Alternating Current). The rating on the front of a deep cycle battery is in Ah or Amperes per Hour (Amps per Hour); this is the maximum number of amps or current the battery can flow in an hour.

So, if it has 100Ah rating it can flow up to 100 Amps of electricity in 1 hour. If there was a 100 Amp flow, then unfortunately, it would be dead flat in 1 hour. If there was a 10 Amp flow, it would be dead in 10 hours.

Usable Power

To find out how much power you need from your battery to run your portable 12V or even 240V items there are some calculations and knowledge required so you don’t run out of charge.

Discharge Limit

The type of deep cycle battery you have makes a difference as there are recommended discharge limits on both types. Discharge Limit is how much you can flatten the battery before it requires a recharge. This is recommended by manufacturers of the batteries. You can take more out of the battery and the price you pay is how long the battery lasts. 

Lead Acid — 50% discharge before recharging. If 100Ah battery this allows you to use 50Ah before recharging.

Lithium — 80% discharge before recharging. If 100Ah battery this allows you to use 80Ah before recharging.

Power Usage

Next is the amount of power the device you want to run uses in 1 hour. Power consumption is in Watt’s and the battery is rated in amps. Watts is the electrical measurement for the power required to run an electrical component, whether it is an LED light or a kettle to boil water.  Most electrical items you buy will have this data in the manual, spec sheet or written on it. E.g., 25W = 25 watts

The basic formula is P = V * I 

P = Power in Watts

V = Volts — For vehicles, rated Battery voltage 12V.

I = Amps — Rated Battery Amps

So, for a 100Ah, 12V, Deep Cycle, lead acid battery the total Watts are:

V * I = P

12V * 100Ah = 1200Watts.

Being Lead Acid, adding in the discharge rate usable power is:

12V * 50Ah = 600Watts

Calculating the Load on Your Battery

Using Watts

Now that you understand Watts you can simply add up the Watts used by each item you want to run and compare it to the total number the battery puts out. For example, you are using 3 items that have a total wattage of 120Watts. If we use our 100Ah Lead Acid battery it produces 600W. We can run these 3 items no problem as the battery puts out 600W and we only need 120W.

If left on continuously and without recharging the battery, the battery would last 5 hours before it needs recharging (600W / 120W = 5 hours).

Using Amps

We can also measure it using Amps. If you know the Watts used by each product and you have the Voltage (12V), the formula transposes to get the current flow (Amps) through each product.

P = V * I transposes to I = P / V

Using the total Watts of the 3 products as we do not have the Amps:

P / V = I

120W / 12V = 10Amps

Using our same battery that we know produces 50 Amps (Ah), we can now calculate how long the battery would last before it required recharging if the products were on continuously.

50A / 10A = 5 hours

The result in time is the same whether you use Watts or Amps. This proves the electrical theory is sound and you can use either method when calculating the load or drain on your battery.

Other Factors

There are other factors that you should be aware of when calculating load or drain. While we have accurate data on the battery and products you have plugged in and are using, there are other factors that impact load or drain on the battery. They are not large or need to be accurately calculated, it is more you need to be aware they exist and how they impact your battery.

They are:

• The health of the battery itself. If your battery is getting older it may impact the ability to sustain and hold charge.
• The length of cabling between products. The closer to the battery the better. For example, do not use a 30m cable if the fridge is 2m from the battery. Get a shorter cable.
• Make sure all joints and connections are tight and weatherproof.
• 240V power invertors. If using one or more, add this into your drain calculations as they do use power.

Selecting the Correct Deep Cycle Battery

Selecting the correct deep cycle battery for your needs is difficult, as the products you take with you changes depending on what you need on your trip. In a caravan or campervan, it is fairly straight forward, because they will come with a deep cycle battery, and you will simply need to replace the existing one.

Even then, you may want to upgrade the Ah ratings or go to a lithium battery for increased performance. We are also getting more battery powered products (tools, e-Bikes) so the need to recharge these while away is increasing.

Some options to choose from are:

Warranty — Deep Cycle can have different warranty periods so the longer it is, the higher the price of the battery.

Ah — Battery Ah can vary a lot. If buying one for the first time do your product calculations as ideally you don’t want to buy a battery that is too small for what you need/want to run.  Even if wanting to upgrade the Ah rating, make sure your charging equipment can cope with the increase.  The greater the Ah, the greater the battery cost.

Lead Acid or Lithium — Big price difference and if changing over check your charging system can do it and cope with the new battery.

Batteries are not cheap so having a choice allows you to suit your budget. When buying make sure the terminals are in the same spot as your old battery so the cabling will fit.

When it comes to replacing a deep cycle battery it is relatively straight forward compared to a start battery as there is no on-board computer involved. Not yet anyway. Basic hand tools are all you need.