What deep cycle battery should I use?

Deep cycle batteries are meant to be drained down and charged back up repeatedly. They have thicker lead plates inside them that can withstand this. The size and what they are made of dictate how far down you can drain them, and how often. Each time a battery is drained down and then recharged is one cycle. The deep part of deep cycle refers to them being deeply drained and then deeply charged back up.

All deep cycle batteries get stronger as they break in, so don’t be afraid to work them to get them to their peak performance.

A regular car battery is NOT a deep cycle, they are meant to be slightly discharged by starting the car, and then topped back up by the alternator. A starting battery will not last long if used as a deep cycle.

It typically starts with a 12 volt Group 24 deep cycle battery. This is about the size of a typical car battery. The group number refers to the size of the battery. All manufacturer’s have to follow the exact measurements when they make the battery for physical dimensions for it to be called a Group 24. They are all rated in amp hours, meaning how many hours will they last at a one amp draw. Some companies use reserve minutes or reserve capacity, this is the same thing, just a different rating system.

The three most common 12 volt deep cycle battery sizes are:

Group 24 is about 10 inch length x 7 inch width x 9 inch height and 80 amp hours
Group 27 is about 12 inch length x 7 inch width x 9 inch height and 90 amp hours
Group 31 is about 13 inch length x 7 inch width x 9 inch height and 100 amp hours.

Only the length is different and all have round posts as well as studs on each battery to connect to.

All lead acid batteries should only be drained down 50% for the health of the battery. This means when calculating how long the battery will last in the above examples the Group 24 has 40 available amp hours, the Group 27 has 45 available amp hours,and the Group 31 has 50 available amp hours.

If I was going camping and I wanted to know how long my battery would last, I would have to know how many amps the things I want to use will take from the battery. For example a radio might say 200 watts on it. This converts to about 2 amps. This is per hour.

So if I had a Group 24 deep cycle, I would know it would last 20 hours using that radio. (40 available hours divided by the 2 amps the radio uses per hour.) If that was the only thing I would be using, and I only listened to the radio 2 hours a day, I would know I could go 10 straight days (20 hours use) before my lead acid Group 24 deep cycle battery would be half drained and need to be charged back up.

The second part is how many times the battery can do this over it’s lifespan. This is where cost comes into play. An inexpensive Group 24 or 27 or 31 likely has somewhere around 100 cycles for it’s lifespan. If you were to use it every second weekend for that radio, it would conceivably last you about 4 years.

For the price this is actually not bad. For perspective on the other end of the spectrum is lithium 12 volt 100AH batteries with an insane 3500 cycle count. A lithium battery can be drained 100%. In theory you could use this battery for that radio for 50 straight hours, recharge it, and do this for 20 years non stop. These batteries cost up to ten times more than a regular lead acid.

So if you have a lighter amp draw, and don’t go out all that often, a 12 volt deep cycle battery works just fine.

If you have a higher amp draw, and want to stay out camping longer between charges, the most common way to go is with a pair of 6 volt batteries referred to as golf cart batteries with a 225 amp hour rating and about 350 cycles. The radio example above would have this pair of batteries last 56 hours. (225AH divided by 2 = 112 available amp hours. Divide this by the 2 amps per hour the radio is using = 56 hours of radio use)

At 2 hours a day of radio use the pair of 6 volt lead acid batteries would go 28 straight days before they reached 50% down and time to recharge.

A pair of 6 volt lead acid batteries weigh about 120 pounds. This is a lot of lead, and the more lead, the longer they can store energy.

A lot of considerations come into play. If you really are just using LED lights and a radio, just about any battery can handle it. On the other hand if you are using a fridge, microwave, heater, fan, coffee maker, toaster and so on, not only will the lighter duty 12 volt batteries not last long between charges, they can’t handle high amps being pulled out like a pair of 6 volts can, and will have a shorter lifespan.

They are made as 6 volts because it is difficult to move 120 pounds of lead around. This way it only takes one cable to join the two 60 pound 6 volts and turn them into 12 volts and is very simple to do and they are easier to manhandle.

So basically a 12 volt deep cycle lead acid is best suited for lighter loads, and a relatively shorter lifespan for less cost. A pair of 6 volt lead acid batteries are more suited for a longer time between charges, and/or a heavier load being drawn out at once.

The cost per 6 volt battery can be as little as about the same as a Group 31, or twice as much depending on how robust they are built. The two 6 volts have the edge on two 12 volts with equal amp hours, as they are built more sturdy in general.

There is another type of battery, available in 12 volts or 6 volts, and these are called AGM batteries. These have additional ingredients including a matting that absorbs the water and acid making them spill proof. They also can not produce any gas when charging making them much safer to be inside the van or camper with you. They are completely maintenance free. This type of battery can be drained down 70% and cost about twice as much as a regular lead acid battery, depending on their amp hour rating.

If you are a van dweller, or need to stretch your time between charging, or don’t want to have to check the water and top the batteries up, or need it to be spill proof, then these are the way to go. Same as the lead acid, they come in 12 volt or 6 volt and various amp hour ratings.

So in a nutshell, the 12 volt deep cycle lead acid is the least expensive and does a decent job for lighter use and shorter times between charges. The pair of lead acid deep cycle 6 volts bumps this up to heavier loads and longer times between charges. The AGM 12 volts increase available time out, and the 6 volt AGM’s even more. They both are safer and less hassle to maintain than their lead acid counterparts. The lithium’s are top of the line, and do everything as good as it gets. Now comes how to charge any of these batteries back up. Most RV’s have built in chargers. They are usually rated somewhere around 30 to 40 amps of charge. The better ones do it in stages, which is the healthiest for the battery. When you are plugged in, the charger takes care of charging the batteries, and the higher amp charging rate allows to battery or batteries to be deeply charged. If you were only to use a trickle charger of say 2 amps, while this would eventually top the batteries back up, but it isn’t that good for the health of the battery to only receive this small of a charge rate.

You can also plug a battery charger into a generator. It isn’t the most efficient but it will work. You should use a minimum of a 5 amp charger for the smaller 12 volt deep cycles, and a minimum of 15 amp charger for the pair of 6 volts.

A vehicle alternator can be used to charge them while you drive, but this is very low amperage charge and eventually will lead to early battery failure. 

Solar does work too, but generally they are best for topping up the batteries to stay out longer. It depends on how many watts of solar panels you have. If you have 100 watts and a single 12 volt Group 24 deep cycle, the 5 amps the panel puts out in the bright sun is enough of a charge rate for good health of the battery. The same panel for a pair of 6 volt batteries does not produce enough amperage for long term battery health, this would require more like 400 watts of panels. 

In an ideal world you would charge the battery up before you go camping, drain it as you use it, use the solar/genny/alternator to put some of that energy back in, then charge it back up with higher amps when you get home by plugging the RV in or using a higher amp battery charger, then do it all again.

Asking a battery specialist what is the best way to go for your particular use is always a good idea, and here at Pacific Batteries, we are always happy to help you sort it out.