Frequently Asked Questions About Leisure Batteries

This FAQ section is specific to Motorhome and Caravan Leisure batteries which are used very differently to Automotive Batteries.

What is a Battery Cycle?

This first question was posted by 'Rogher' on one of the Forums, it was so well worded, I felt it worth a reply.

'How much use is it knowing that a battery could survive a stated number of cycles, and is one manufacturer’s ‘cycle’ the same as the next?
One ‘cycle’ could be from fully charged to fully depleted or simply from an initial state until recharging starts again.

We often subject our batteries to lots of ‘little cycles’, especially if enjoying solar power, so how many ‘little cycles’ make up a ‘proper cycle’? 
And what regime of use might get most out of our batteries (full cycles or smaller ones)? 
I suspect that a ‘cycle’ could be as long as a piece of string"  

You are correct, a cycle is as long as as a piece of string, it is where a battery is discharged to any degree at all and that draw replaced by a charger of some type.

However, there is also an 'official' definition of a battery cycle in the BS EN 50342 document that stipulates how batteries should be tested, etc.
The document mandates the exact testing process of a battery to meet these standards, from charge voltages, current used, charge times, a 25 degrees test temperature, etc. 
It even stipulates that before cyclic testing can take place, a process is gone through to determine the batteries capacity.
Once the battery capacity, e.g 100Ah, is determined the cyclic testing can begin. 

For the purposes of the BS EN 50342 test, a cycle is defined as - 
"Repeatedly charging/discharging the battery at C20 down to 50% Depth Of Discharge (DOD), until the battery can no longer deliver at least 80% of the rated capacity".

In other words repeatedly charging/discharging a 100Ah battery at 5amps down to a DOD of 50Ah, and continuing to do so until the battery has deteriorated so much the real 'total' capacity has dropped below 80Ah. 

Batteries lose capacity as they are used, and the BS EN 50342 standard stipulates a battery should be regarded as 'expired' when it has permanently lost more than 20% capacity. 
That is less than 80% will be available on each charge/discharge, no matter how long or many times you charge it. 
Obviously as it is used it will degrade further.

This 80% figure is important when defining a 'cycle', because if you are advised to always leave 50% charge in a wet acid battery (no more than 50% DOD recommended limit) then a battery deteriorated down to just 60% permanent capacity only has a 10% figure to give up. 10Ah from a 100Ah is not exactly useful. 
Even a battery still within the 80% capacity figure will only give up 30Ah on each cycle if you stay within the guidelines. 

So any manufacturers battery that has gone through a true BS EN 50342 test will be identical.
Problem is not all the Far Eastern manufacturers seem to have the same document as the UK.

For example, many budget starter batteries from the East, fail tests in the UK when it comes to the 'Cranking Amps' test. 
It is supposed to be carried out at zero degrees and again after the battery has been held at -18 for 24 hours.
When the Far Eastern manufactured batteries are retested in the UK, they usually pass the test at zero degrees but often fail to meet the claimed cranking amp figures at -18 degrees. 
So you can imagine other figures might be different as well?.

Should I Deep Discharge My Batteries Or Shallow Discharge them?
Typically, for all the Lead acid battery technologies, Wet, Gel, AGM you will lose roughly 30% of the cyclic ability for every 30%'ish deeper discharge.
In other words, very roughly, an average quality Leisure battery that delivers 1000 cycles at 30% DOD will deliver about 600 cycles if regularly dropped to 50% DOD and deliver about 290 cycles at 80% DOD. 

See graph below. 
If you look at the Gel Long Life, Green column you will see it achieves 4500 cycles at 30% DOD, 2500 cycles at 50% DOD and 1500 at 80% DOD.

That is a loss of 3,000 cycles just by Deep Discharging it  :

If you look at the AGM (Blue) battery column, it's a much better 'Shallow' cycler than a Deep cycler, like most Leisure AGM's. It's best figure is 1500 cycles at 30% DOD, but taking it down to 50% DOD it manages to lose almost 1000 cycles, achieving just over 500 cycles. 
From 50% to 80% it loses only about 190 cycles, achieving about 390 cycles at 80% DOD.

While lots of people harp on about Deep Cycling batteries, even those batteries specifically designed to do so, will suffer severe loss of life.  

I was recently told by a Lithium battery expert (not someone who sells them but a factories research engineer) that a similar ratio applies to Lithium batteries. But he also said regular discharges right down to 90% DOD will result in almost 85% loss of life compared to shallow discharging. 
So the marketing claims of Lithium being able to deep discharge are misleading. 

Why Are Starter Batteries and Leisure Batteries Different?
Because they are used very differently, therefore the load on the battery is different.

For example, before Stop Start Technology became the norm, a Car Starter battery did almost zero work throughout it's entire life.
Yes it can be asked to provide a big chunk of around 200amps on an engine 'Start up' for a couple of seconds, but this is provided by what is called 'Surface Charge' on the Battery Plates. 
The Plates provide the current without any real chemical reaction taking place, consequently almost zero chemical reaction takes place during the recharge resulting in almost zero real battery wear.

But when a motorhome Leisure battery is used to run the TV, lights, Satellite system, Water Pump, Heater pump/blower, etc for a few days on a Stopover, the battery can discharge quite low involving the Plates going through significant chemical reaction. 
It is not unusual for these discharges to be down to 50% Depth of Discharge (DOD) on a typical Leisure battery.

That long, slow discharge and then subsequent recharge will place quite a strain on the battery. It really will work hard so the battery has to be stronger to cope. One way of making the battery stronger to cope with regular discharging is to make the Plates thicker. 
Another is to use alternative technology, like using Silver, Antimony, Carbon, special Grids, Gels, etc.
Different manufacturers approach things differently, but the latest technology to show promise without high cost are the new  Enhanced Flooded Batteries, EFB, Leisure batteries being launched Autumn 2018. Several Car manufacturers are already adopting this new advanced technology to replace the AGM's that have failed to deliver the promised performance.