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Updated 27/05/2017

  "Hi Allan, Our motorhome is back up and running thanks to your expert help, advise and part. I cannot thank you enough in sorting out this problem and would certainly recommend you in the future. 
Regards, K.M. 4/3/2017".

                         How Does A Motorhome Battery Charger Work?

Everyone knows that a Motorhome/Caravan mains Charger starts off charging a Battery at 14.4v, then once the battery is fully charged, it drops down to 13.8v Trickle/Float, right? 

It doesn't. A Battery charger in a Motorhome or Caravan, does not always function in that way. Although the operation may be only subtly different, the implications can be major, which we will explain below.

What we are also going to try and explain is how the charging time/load can change out of all proportion when the battery bank is expanded. When a battery bank is doubled in size, charging time can go up dramatically, not by the expected factor of 2, but by a factor of 10 or even greater.

If I were to say that it is acceptable to add 20 x 100Ah batteries to a Motorhomes 10amp charger so that each battery receives just 0.5amps charge, I would be universally ridiculed. 
So almost everyone accepts that the 230v mains charger has a limit.
What isn't always clear is what the limit is and the effects of exceeding it?

In order to understand that, we need to know how the chargers in Motorhomes and Caravans work as these units can be very different in function to a Car Starter battery charger found on the High Street.

One of the most common Motorhome 18amp chargers throughout the World gives an output of 14.3v at an average rate of 12amps for 4 hours (just 1 hour on some early 18amp EBL 100 versions). 
It then drops down to a much slower 13.7v 'trickle' charge rate, even if the battery bank is still only half charged. 

So if the battery bank is big and needs 100Ah to charge up, it will get an average 50Ah before the charger drops to it's 'trickle' 13.7v mode. The remaining 50Ah will be replaced at a rate designed to be a 'Maintenance/Trickle' charge of 13.7v and very low current, often less than 1 amp.  
So where the Motorhomes standard fit single 100Ah battery will pretty much fully charge in 4 hours, if you add a second battery it might take 50 hours plus to charge both batteries. A greater than ten fold increase in charging, not the 2 fold most might expect.

Taking the battery bank upto 4 batteries will clearly magnify the effect again.
That is just a basic illustration, the real World is more complex with other factors, which we will now cover for those interested. 

There are a vast array of different types and makes of Battery chargers used in Motorhomes and Caravans around the World, often with different functionality. Here we will examine two typical units, a very basic type, then a more sophisticated unit. 

For example the Sargent Supercharge 151 (launched in the last few years) is a fixed 13.8 volt, 10amp charger where the voltage is permanantly set at 13.6v (the real value when one was tested by us), but the current varies dependent on the batteries requirement and the charger temperature.
When this charger starts up, it begins putting 13.6v into the battery at a maximum current of 10amps, more usually an average of 6 - 8 amps. 
As the battery charges, the current may drop until just a 'trickle' charge is being applied to the battery. 

Although the current changes with demand, the voltage never varies from 13.6v - 13.8v. This limits the power of the charger significantly, resulting in slower charging even on a small battery bank. 
This is how almost all chargers worked in the 1980's.

Modern chargers with a higher 14.4v voltage were introduced to speed up charging.
However, continual charging at a higher 14.4v can result in battery Plate corrosion which reduces battery capacity and shortens it's life. So it was determined that the bulk charge was best done at 14.4v, but as soon as the bulk of the battery was charged, it dropped to a 'less corrosive' 13.8v for the rest of the charge time.

A typical muli stage charger is that used inside the Schaudt Elektroblock range, such as the EBL 99 up to the EBL 220-4. 
The Schaudt Elektroblock EBL 99 is fitted in more Motorhomes World wide than any other combined Charger/Power Controller.

This one box handles Alternator charging, Fridge 12v operation, power distribution/control and Mains 230v charging of both the Habitation battery and the Starter battery. 

These EBL 18amp units start off charging a battery at 14.3v for a set timer period, such as 4 hours for a 'Wet Acid' battery.
After 4 hours the charger drops to a slow charge 'Float/Trickle' rate of 13.8v, regardless of whether the battery is fully charged or not. 

So with a 80Ah 'wet' battery discharged to 50%, the charger would need to put back about 40Ah. 
A Schaudt 18amp charger running at max charge for the '4 hour Timer' could acheive a theoretical 72Ah, so this is well within the capability of the charger which should achieve pretty much full charge well before the 4 hour timer expired. 

However, no battery is 100% efficient so the charger might need to put in 45Ah to replace the 'discharged' 40Ah. Obviously as the battery became more corroded/degraded, with lost efficiency, the number of Ah required might increase as the battery aged. Maybe requiring the charger to pump 55Ah into the battery to get it back to full charge.

Secondly, most chargers are rated at their peak output, very rarely will a Schaudt charger run at more than about 14amps for long in the real world.
Even so, the slightly more realistic figure of a max 56Ah (14amp x 4hours) is still enough to replace the 45Ah we need to put back into our 80Ah discharged battery within the 4 hour Timer period.
So to keep this simple, we will ignore the other factors and work on the still optimistic 14amps output for an 18 amp charger. 

Example  2 x 120Ah Battery Bank of 240Ah.
Lets expand the Habitation battery bank to 240Ah (2 x 120Ah wet batteries) and discharge them to 50%, so they will need 120Ah to charge up. Add on another 10Ah as our new battery bank is unlikely to be 100% efficient. The 130Ah we now need to put in, is way above the 56Ah we can squeeze in during 4 hours. 

So after 4 hours the charger will have applied 56Ah at 14.3v with still 74Ah to go, at what is effectivly designed as a short term 'trickle' rate on some chargers. In a worst case (like on some Motorhome chargers that have a lower 13.6v Float rate) this might take several days, maybe upto 60 hours?

If the batteries were Deep Discharged right down to 30% available, so 70% of the battery AH has to be recovered by the charger, the picture is obviously worse.  
If you further subtract 6amps to charge a low Starter battery during those critical first 4 hours, then possibly over 80% of the Habitation battery charge might be applied at a snails pace, 13.8v. 
In a worst case with a big battery bank it could take a week to fully charge the batteries.

That is a very, very different picture to the general perception. 
Agreed it is an extreme example, but upgrade to a battery bank of 330Ah, 110Ah x 3, and it becomes anything but extreme. 

The below extract from a Schaudt EBL 100 manual shows how the peak 14.3v initail charge is carried out for just 1 hour for a Wet battery before dropping to 13.8v. That is just 1 hour at 14 amps/14.3v with the remainder of a potential 300Ah battery bank being done at 13.8v and a few amps, potentially 140 hours charge time. 
Clearly not designed to cope with anything more than a single 80Ah wet acid battery :

Full Schaudt EBL 100 document here : 

Schaudt Elektroblock EBL 100A.pdf Schaudt Elektroblock EBL 100A.pdf
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Type : pdf

The EBL 99 has been dropped from the range, although we will go on repairing and supplying rebuilt units, and has been replaced by the new EBL 119 with a very much more effective charger. 
While still only an 18amp charger, the will charge a Lead Acid Wet/Gel battery for up to 16 hours before it drops to a slower 13.7v Float/'trickle'/maintenance or whatever term you know it as.

So in our example above, it has the potential to charge at up to 14amps for 16 hours = 224Ah.
Easily charging most supported battery banks within a day, but remember the practical limit of the unit is still a 180Ah battery bank, it just charges that 180Ah battery bank more effectively.

Note that some Solar Panel charge Regulators also behave in the same way, in that charging at 14.4v may last for 1 - 4 hours before dropping to a 13.8v float charge, regardless of the battery charge state. 
Just because the Solar display is showing 13.8v by 11:00 in the morning, does not necessarily indicate the batteries are fully charged, just that the 14.4v timer has ended and dropped into Float mode.

So the above demonstrates that a charger has a practical limit on the size of bank it can support, usually 10 times the charger current output converted to Ah. E.g. a 18 amp charger supports a 180Ah battery bank. 
In the photo below of a Calira 20amp charger the same supporting evidence is printed on the casing. It states it is suitable for a total battery bank between 70 - 200Ah only. 
Remember this is the Total battery bank and this unit, as per most good chargers, also charges the Starter battery so an allowance should be made for this.

Some people will tell you that if you add extra batteries to a charger and exceed it's capacity, all that happens is they take longer to charge. This might be true on the chargers they are used to on brand new batteries. But on the Motorhome units we repair, it isn't true in the real world, with a real set of used batteries. 
Getting the last 15% charge into an older battery is no easy task unless the charging conditions are perfect for that battery. 

Some Motorhome Habitation chargers will not have the power to get a big battery bank up to 100%, if the batteries are a few years old. That will result in a lower effective capacity for that battery bank, strain on the charger and battery deterioration.


As the Battery ages, it's load on the charger increases.
As a battery ages the situation worsens still further as a battery bank might require a lot more AmpHours to charge up. 

The Corrosion inside a battery, see left hand picture above, at just two years old can be a big barrier to getting amps into a battery. This makes the charger work harder for longer. Obviously a big new battery bank, that is already giving a charger a hard time, becomes an even bigger burden after a couple of years. It is usually at this point the charger pops.
People tell us their particular failure can't be caused by the big battery bank, because "they have been on the Motorhome 2 years without issue". 
What they mean is they have not noticed an issue, didn't hear the poor charger screaming it's Heart out, running much hotter for longer than designed until it finally threw in the towel.
Nor do they take into account the ever increasing burden as the batteries age, they tend to think of a battery as a static, unchanging load, which it isn't. The burdon on a charger increases as the batteries age. 
Poor quality batteries, like almost all the 'budgets', place an even greater burden still. 

If you add a second conventional 90Ah battery, the load after 2 years is not the 180Ah you might think, but more like 270Ah because of the way some batteries behave as they age.

Lets take a theoretical example of a 110Ah budget Antimony based battery placing a load on all the charging systems as the batteries age. 
It might be a 110Ah load for the first few months, but by the time the battery is 2 years old it may behave more like a 160Ah battery in terms of the load on the charger. So if we fit 2 x 110Ah new budget batteries, by the time we reach 2 years the actual charger load might be more like 160Ah x 2 = 320Ah. That is a huge load for any Alternator, wiring, relays, connectors to try and look after, let alone the mains charger. 

How big can the load on the Charging systems get? 
Imagine a battery deteriorating to the point it is approaching a 'Dead short' load on each cell? That might be almost akin to someone putting an Iron bar across the Alternator output terminals. 

All of the theoretical 'Laboratory behaviour' of a battery and charger changes totally in the real World as battery Corrosion takes hold. The budget batteries behave massively different compared to the best. 

Ultra efficient batteries, that charge up more easily and quickly, place a lighter load on the charger over time, becoming an even more important factor if you expand the battery bank. See our Battery Technology page, the best modern batteries are only £90, yet streets ahead of the competition. 

So while we quote the 10 times rule, it is obvious from the examples above that the 5 times rule is more realistic to maintain reasonable charge times and not overload the charger. 
However, we are also very, VERY aware such advice would never be accepted by the majority who already seem to find the '10 times rule' extreme. 
One of the reasons we say an allowance should be made for the Starter Battery, which can take 6 amps charge on the EBL 220, is because this helps make the figures more realistic. Additionally a buffer is created if the Motorhome is ever in the position where both are charged at the same time. That doesn't mean adding on the full 90Ah starter battery value to the battery bank, but to make an allowance for it. 

If you upgrade the battery bank with quality, modern low corrosion batteries and then follow the 10 times rule with an allowance for the Starter battery with due regard for the extra load you are placing on the whole infrastructure, then the mains charger should be reliable. 

But that doesn't mean the rest of the unit will be. The wiring, connectors, etc might all be asked to pass much greater currents than desined to handle.
As an example see this unit below :

The Charger/Power unit above has  the split charge relay inside the unit, so all current from the Alternator to the Habitation batteries flows through these connectors which don't have a huge current rating. 
You will note that from the text/images on the front, that the Starter battery connects in through the Red, bottom left socket and the Habitation Battery through the top left, White socket. 

Adding extra habitation batteries can result in up to 40amps of Alternator charge power going in through the Red and out of the White connectors. The Alternator load to run the Fridge, average 12 amps, additionally goes in the Red socket then out to the Fridge via the Green connector.
So the combined current passing in through a single Spade connector on the Red socket (second from bottom labelled Motor 2 Positive ) can be over 50amps if extra batteries are added.
The sockets on the later unit, like the one above, have been uprated by the manufacturer to higher power RAST5 versions, but even they will struggle if a second battery is added and the discharge/recharge load not managed.  

The unit above clearly shows the complications that can arise if you take a unit outside it's design. 

Contrary to so much advice from Magazine 'technicians' and other 'experts', you can't just automatically add extra batteries to all Motorhomes without doing your homework. 
Well you can, but there might be Tears? 

This burnt out fuseboard below is typical of the type of Fire damage that can occur if things are not thought through. 

Like on the Arsilicii Power Controller above, all Alternator charge went through Fuseboard before being routed to the Fridge and Battery. 

Things You Might Want to Think About If Increasing The Battery Bank Size, Even Slightly?
See this page for advice on upgrading the battery bank size and tips on how to charge them faster if you do. 

Fiat Base Vehicle schematic :

The CBE range, like the CBE 516 works differently to most chargers, description below.

CBE 516 and 510 charger manual.pdf CBE 516 and 510 charger manual.pdf
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Type : pdf