Which deep cycle Ah battery?

Deep cycle batteries (also often called Leisure or Marine batteries) are designed to provide a constant current over several hours compared to starter batteries which offer high power for several seconds.

Some battery types, such as lithium ion batteries, are only deep cycle and so are not labelled as such. It is lead acid batteries than can be ‘cranking’ (designed to deliver short bursts of high energy) or deep cycle. This is true of flooded lead acid and sealed lead acid batteries.

The difference is in the structure. Deep cycle batteries have much thicker lead plates to withstand long and intense discharging. Some models even have completely different plate designs to add even greater strength. Thin plates (used in starter batteries) buckle under these circumstances causing the battery to short out or limiting its ability to recharge.

There is a great deal of misunderstanding that surrounds deep cycle batteries, which often leads to extra costs as buyers follow false economies. Here we’ll cover the steps you can follow to find the right battery for your needs.

This article deals mainly with lead acid batteries and makes reference to the internal design – you can find out more about this in the article What is a lead acid battery?

(Video of What makes a Deep Cycle Battery Different with Transcript)

STEP ONE: Make sure it really is a deep cycle battery

There is very little regulation, let alone monitoring, of the deep cycle battery market. This has allowed some less reputable manufacturers to label starter batteries as deep cycle – especially in the lead acid battery arena. Starter batteries use less lead (the main cost in the production process) because the plates in the cell are thinner.

You may not notice the difference at first but after a few charges and discharges the thin plates inside will start to buckle. The active paste applied to the plates will also be dislodged at a higher rate as it is not designed to go below a 50% state of charge.

The battery may short out or it will just last for shorter and shorter periods requiring a replacement purchase long before it should be necessary.

Unfortunately, it is not possible to open up batteries and measure plate thickness, but there are two giveaways:

• Weight – lead is heavy, if there is a weight difference between two identically rated batteries the lighter one is probably skimping on lead.
• Price – if it is too good to be true, it probably isn’t the thicker plate!

When it comes to purchasing a deep cycle battery, this is one of those times when purchasing a well recognized brand is well worth the slightly higher cost.

STEP TWO: Understanding the amp hour rating

So a 100Ah battery should power a 100 Amp appliance for one hour …. right? Wrong! The amp hour rating is usually given at something called the “20 hour rate” so a 100Ah battery can power a 5 amp appliance for 20 hours ( 5 amps x 20 hours = 100).

Connected to a 100 Amp appliance, the same battery might only last a matter of minutes. The higher the rate of discharge, the higher the internal resistance inside the cells and the more energy that is wasted in heat. Conversely the lower the drain, the more efficient the battery is.

Because of this characteristic, you have to be on the lookout for under handed manufacturers who might try to pull the wool over your eyes. Instead of using the generally accepted “20 hour rate” they base their capacity on something higher like a “100 hour rate” – the battery is capable of powering a 1 amp appliance for 100 hours.

This slide of hand can have a big impact on how long a battery can power various appliances. We can see this by using a formula known as Peukert’s Law:

You can work out your own needs using our online Peukert’s Law calculator.

As you can see, two batteries, both rated 100 Ah, can perform very differently. Knowing the discharge rate used by the manufacturer is as crucial as the Amp Hour capacity itself. All reputable manufacturers will include this in their technical documentation, which should be freely available.

STEP THREE: Understanding the depth of discharge cycles

You might be thinking “I have a 5 Amp appliance that I need to power for 20 hours, so what I need is a 100 Ah battery that was defined as such using the 20 hour rate”. Not quite.

Although called “deep cycle”, these batteries don’t like complete discharge-recharge cycles and running them totally flat on a regular basis will cause internal damage that will shorten their lifespan.

To have something longer lasting, leave a generous measure of extra time. In the example above, a 120Ah rating would mean the battery rarely runs completely flat, so you will get many more cycles from your purchase.

Always remember the shallower the discharge and charge levels the longer your battery will last. If you are investing for the long term and you would like your battery to get the most from your purchase then one with an Ah rating twice that which you actually require would give you a much better service life and actually represent a cost saving. For the purposes of this article however we’ll stick with 120Ah.

STEP FOUR: The operating environment

The one issue that Peukerts Law (see above) does not take into account is temperature, it assumes temperature will be around 77°F (25°C).  Unfortunately, battery performance significantly declines in colder conditions. In leisure applications, this is often inevitable –  yachts, for example, can be cold, especially at night when far from shore and this is the exact time the battery is needed.

At 32°F (0°C), amp hours drop by 20%, at -22°F (-27°C), the decrease is 50%. Conversely, to a certain point, they will perform better in much warmer conditions. At 122°F (50°C), capacity is improved by 12%.

If your deep cycle battery is going to experience colder conditions, it is better to up the capacity in order to achieve the performance you expect. So if Peukerts Law says you need a 100 Ah battery at the 20 hour discharge rate and you have upped this to 120 Ah so as not to fully discharge it each time, up it again to take into account temperature.

Here are two worked examples using our 100 Ah battery from above (that we have increased to 120 Ah so as to reduce wear and tear from very deep cycling)

• when the surrounding temperature is regularly around freezing point, the performance drops by 20% so a 120 Ah battery is actually a 95 Ah battery – (120 Ah x 80%). To find the correct one we need to know which battery will deliver 120Ah at freezing: (120 Ah/80) * 100 = 150Ah.
• when operating regularly in an environment of around 122°F (50°C), we can reduce the Ah capacity we need by 12%, so anything rated higher than 106 Ah will fit our needs.

Summary

Throughout this article we have been considering which battery we would need to power a 5 Amp appliance for 20 hours. Here is the summary:

1. We learned how to recognize a real deep cycle battery and not a mislabeled starter battery
2. We checked the discharge rating used by the manufacturer to arrive at their Amp Hour rating, where we determined a 100 Ampere Hour battery (at the 20 hour rating) would power our 5 Amp appliance for the required 20 hours
3. In order to give us a longer operating life, we increased this to 120 Amp Hours, so it would not completely discharge on each cycle.
4. Finally, we made appropriate changes based on the environmental temperature the battery was likely to be operating in.

Buying a used deep cycle battery

It is worth knowing that the capacity of all rechargeable batteries decreases with age and deep cycle units are no exception. They will gradually last for shorter periods in each discharge.

There is no hard and fast rule to this decline as much will have depended on how the battery was used since new. If it has been completely discharged on a regular basis, it will be in a far worse condition than one that has, for example, never been below a 30% state of charge. If it has been left unused for long periods of time, there may well be sulfation on the plates, which will affect its ability to both discharge and recharge.

There is little you can do to check these issues and so buying second hand can be risky.