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 BS EN 50342 document mandates the exact testing process of a battery, from charge voltages, current used, charge times, a 25 degrees test temperature, rest time between charges, etc. 
It even stipulates that before cyclic testing can take place, a process is gone through to determine the batteries capacity.
Once the true 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, then recharging followed by another discharge to 50% DOD, etc. until the real 'total' capacity has dropped below 80%, or in the case of a 100Ah battery below 80Ah. See our 'How a Battery Works' for more info.

It might be better to explain what is meant by battery 'failure' as the industry definition on this differs significantly from the general perception.
Each time a battery is charged and discharged, a little bit of the Plate Paste (Active material) fails to convert back so the battery slowly loses capacity. For example a 100Ah Platinum battery that has 50 cycles will lose about 0.4Ah per charge/discharge cycle. Therefore by the time the battery has performed just 25 cycles, the capacity will be down to 90Ah.
The industry regard a battery as expired when it's capacity drops below 80% due to degradation of the chemistry. 

So a battery doesn't just 'fail' suddenly, as many believe, but slowly degrades every time it is used. Eventually degrading so much, usually well beyond the 'official' 80% expiry criteria, that 'structural' failure is inevitable.
These structural failures then result in rapid voltage drop, plate shorting, etc. and it is only when they start to occur that a visible drop in battery performance is noticed by the owner, even though the battery may have actually expired some time earlier.

A battery can go on delivering cycles beyond it's "less than 80% capacity expiry" but the  80% capacity figure is not as high as it might first appear when defining a 'cycle'. 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 to the "80% capacity" figure will only give up 30Ah on each cycle if you stay within the guidelines. 
A battery down to just 70% permanent capacity only has a 20% figure to give up. 
20Ah from a 100Ah battery is not exactly useful, so while a 80% figure might look like we still have a good battery it is a bigger limitation than it looks. 

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 batteries are alleged to be shallow discharged in the 'tests' to only 25% DOD, not the 50% DOD used in the UK. 
That can inflate the Cyclic rating by up to 50% versus a battery subjected to a real BS EN 50342 European test.
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 a massive 3,000 cycles just by Deep Discharging it  :

If you look at the AGM (Blue) battery column, it isn't so good at Deep Cycling. 
It's best figure is 1500 cycles at 30% DOD
Taking it down to 50% DOD it delivers about 590 cycles, a loss of almost a 1,000 cycles just by discharging it 20% more. 
A drop to 80% DOD achieves about 390 cycles at 80% DOD.
That clearly shows that Deep Cycling isn't the most cost effective option and not what AGM batteries are about, despite the retailers claims. Most motorhome AGM batteries are 'lightweight' designs and best not discharged lower than about 30% 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 if you use that Deep cycling capability.  

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 just that little bit deeper to 90% DOD will result in almost 85% loss of life compared to shallow discharging. Dropping a Lithium below 95% DOD usually results in catastrophic damage and why the better informed guidelines are now to no more than 80% DOD.
So the marketing claims of Lithium being able to deep discharge are misleading. Just like Lead batteries, they can be Deep Discharged, but optimum lifetime won't be achieved.

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 200 amps 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 the same low 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 40% Depth of Discharge (DOD) on a typical Leisure battery, that is 60% left.

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.

You are the only web site telling us different technology batteries have different voltages, why?
We.might be the only website publishing the data but the Battery manufacturers have published it for years. see the chart below from Yuasa :

The chart shows how their old CX & YuMicron battery range had higher voltages than conventional batteries. 
Their new EFB range has slightly higher voltages again, with a resting fully charged voltage of just on 12.8v.
The Bosch L/Varta LFD ranges have resting voltages close to 13.0v, just like the best AGM/Gel/VRLA batteries. 

This is usually achieved by using stronger Acid, but it can have implications.
The Plates of a battery are subject to corrosion, this corrosion occurs more quickly when the acid is stronger, so old fashioned/conventional batteries prior to about 2001 used an acid strength that achieved about 12.5v without causing too much corrosion.
About 8 years ago Bosch/Varta developed a new Plate technology that killed corrosion at source so the acid strength could be increased. As a result their batteries had a resting voltage of almost 13.0v without the usual corrosion issues that plagued lesser technology batteries. The Varta technology was Patented so couldn't be copied, but the rest of the industry is slowing catching up, Yuasa and Exide especially.   

Obviously all those charts that don't take battery technology into account when listing 'fixed' Acid Specific Gravities are also way out of date. 

Will a battery with more cycles always be the best option?

No there is far more to battery performance than the 'paper' cycle life.
A conventional Deep Cycle battery, like the old Trojan T104, has very thick plates and a potentially high number of cycles. However, if the technology is prone to Plate Corrosion, Sulphation, Antimony poisoning, Paste shedding, etc it is likely to suffer some 'mechanical' failure long before the Active Material in the Plates has 'expired' and it has delivered all it's 'cycles'. 

That can mean a conventional Deep Cycle battery, like the Exide ET650 battery with 360 cycles, can have shorter life than a Varta LFD90 which won't suffer corrosion, antimony poisoning, paste shedding, etc. 

As an example, conventional ultra Deep Cycle batteries tend to be high in Antimony. Such batteries generally have high fluid loss and high self discharge. A battery that starts discharging as soon as you have charged it, is far from efficient, but it quickly takes the battery into the risky area of becoming Sulphated through that discharge, leading to premature failure. 
The Varta LFD doesn't self discharge, it will stay charged in your Garage for 6 months, and it doesn't lose fluid so doesn't run the risk of premature failure through fluid loss. 
High Antimony content Deep Discharge batteries are amongst the thirstiest of batteries going. 
The Banner Energy Bull batteries with their 'drinks like a Fish' reputation, being a well publicised example. It's a thirsty battery that can consume high levels of fluid before the owner has realised the levels are lower than ideal, resulting in damage to the plates.

The Trojan/Exide style batteries are less likely to reach their designed cycle life but the 'tough as old boots' Varta LFD90 will exceed it, purely because it's advanced technology delivers maximum efficiency right to expiry of the Active material/Paste, not sidelined by some other 'mechanical/structural failure'. 

The issues noted above, amongst others, mean that a conventional technology battery loses efficiency faster than the best.

Ultra deep cycling batteries can lose efficiency very quickly. That can mean that from around 100 cycles on wards, a conventional battery can be down on efficiency by as much as 70% compared to the best.
So when charging them up they might 'waste' 30% - 50% of the power you put in. 
On a Solar charger that is a huge loss of power. 

It can mean batteries taking twice as long to charge, but drawing twice as much electricity to do it, etc. on mains, Alternator and Solar.

Deep Cycle batteries can deliver a lot of cycles, but need very special care to achieve it. Like - low discharge currents, low charge currents, fastidious maintenance, keeping the battery highly charged on a regular basis and no 'long term EHU' connection otherwise Antimony poisoning may result.
Even with all this care, the efficiency will be down on the best, even only half way into their life.

Battery efficiency really is key to a well functioning motorhome. 
We think it is far better to fit two high efficiency Varta LFD90's, one after the other, than a single Exide ET650 that will be well down on efficiency, even before 200 cycles have been used. 

Different manufacturers approach things differently, but the latest technology to show promise without high costs are the new  Enhanced Flooded Batteries (EFB) Leisure batteries being launched Summer 2019. 
These new habitation Area batteries are set to use a High Technology approach, as a result we will hopefully see some batteries that are even better than the Varta LFD/Bosch L ranges.

Several Car manufacturers are already adopting the new advanced EFB technology to replace the AGM's that have failed to deliver the promised performance.

If all that is true, why do you have the Exide ET650 is your "Best Buy Heavy Duty" battery?

We keep being asked for the most cost effective Deep Cycling battery so have had to provide some advice.

For someone living in their vehicle full time, it is an ideal choice because all of the issues like Antimony poisoning, high self discharge, etc will be much lower risk as the battery is working 24/7.

The ET650 is not an expensive battery, so even if it doesn't get the extra special TLC needed for maximum life it won't break the bank. We also felt we should provide some advice on a battery backed by European Industry Standard figures, as opposed to some of the fairy tale figures currently sprouting up, for some of the imported batteries. 

In 2018 the government department, DBEIS, sent a number of batteries to an independent test house in Germany to test that the capacities claimed on the label matched true test results.
Some had been carried out by Feb 2019, when the DBEIS stated :

"Test results I have are startling and concern me. I will be interested in the explanations of these importers/distribution/rebranders when I make contact with them". 

Clearly the test results have revealed that importers/distributors/rebranders are selling batteries that are clearly not as described.