# Split Charging a technical correction



## sallytrafic (Jan 17, 2006)

Up until recently I always said in split charging topics that the alternator on its own would not get your leisure battery fully charged, and you should regularly use a hookup to let the on board charger get the battery up to full charge.

I used to quote 85%-90% only on split charge.

I did this because I thought that alternator outputs were set to 13.8V.

DABurleigh sent me a pm (thanks Dave) stating that 14.4V was the onset of gassing and also the voltage to which alternators were set.

At 14.4V your battery will eventually be fully charged.

Of course if there is an appreciable voltage drop between the alternator and the leisure battery (fuses, relay contacts, thin wire and long runs all contribute) then you wont get 14.4V at the leisure battery terminals. 

With my battery about 70% charged I was getting 14.3V at my leisure battery. So I would expect it to become fully charged on a long run.

Sorry if I have misled anyone in the past. 

My only residual query is what happens with those with gel batteries surely 14.4V is too high for them.


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## eddievanbitz (May 1, 2005)

Even better is why is it called split charging? The batteries are in parallel when charging  

The other thing that makes me chuckle is that people with a religious fervour say that you cant mix battery types/size/wet/gel/age etc happily oblivious to the fact that their £200 plus gel battery is being charged from the wet battery supplied with the van!

The term split charging should only be applied to systems that prioritises charging, based on need (the Sterling Battery to Battery charger for example) In effect the engine starts, the engine battery gets topped up and then the output of the alternator is switched to the leisure battery (s) However as this costs more than about 30p (the cost of a cheap 4 pin relay) the manufacturers don’t do it! 

This is not aimed at any one manufacturer Hymer/Swift/Autotrail/Dethleffs all of them, inside the fancy box of tricks a simple relay closes. Dreadful really!

The other disadvantage is that when one battery is nearing the end of its life, or, has a fault, the other batter suffers the same fate as the system is only as good as the weakest battery when in parallel 

Ho hum

Cheers

Eddie


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## sallytrafic (Jan 17, 2006)

eddievanbitz said:


> Even better is why is it called split charging? The batteries are in parallel when charging  clipped....
> Eddie


The charge current coming from the alternator is split.......


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## eddievanbitz (May 1, 2005)

Which vans do that then Frank?  I agree that they should but they don't The only "split" is when the engine is turned off and the batteries are no longer in parallel

As far as I am aware there is no main stream motorhome manufacturer that "split charges"


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## sallytrafic (Jan 17, 2006)

All of them

The current flow from the alternator into each battery splits according to the internal resistance and terminal voltage of the two batteries and the other loads on the systems.

Kichoff's laws or is it Thevanin's theorem apply


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## eddievanbitz (May 1, 2005)

I know that that is the theory, however, in practice the supply for the "split charge relay" is taken from the positive of the engine battery. When the relay closes, the 2nd battery will then be connected to the engine battery. Given the distance between the batteries, the unequal cable diameter and a host of other problems, the system just cannot work as well as it should.

My point is that the batteries cannot be "split" when they are being charged in parallel.

What should happen is that both batteries are connected to a charge regulator, with cable to allow for currect drop, and the regulator control the output of the alternator, not dump it into the engine battery and then hope that sufficient gets through to the 2nd battery.

Our industry should take a leaf out of the marine industrys book when it come to true split charging of multiple battery banks


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## sallytrafic (Jan 17, 2006)

Eddie you want to use the word Split in the sense of separate I am using the word split in the sense of to diverge.

You make a living out of selling add-ons I made a living out of avionics.

Clearly we aren't going to agree. Even though the techie in me would love to see seperate regulators for all systems, cost plays a part. 

Anyway this post was about it not being 13.8V but 14.4V.

In a day or so I'm going on a trip in my van. From the SG of the battery its about 70% charged atm Ill report back after the trip.


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## eddievanbitz (May 1, 2005)

Have a great weekend Frank, and I agree that we won't agree :? 

I just think that with loads of money being spent on stupid display panels that are so complicated many people cannot use them, the people that bulid motorhomes would be better putting a decent charging system in.


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## spykal (May 9, 2005)

Hi frank

I am glad to see that you have "seen the light" as far as to whether a modern altenator can actually charge the battery to near 100%.... I dare not say 100% only near 100%. I, like you, a while back , thought that this was not possible and that was because maybe like you I have been around a long time :lol: and the early altenators ( and dynamos or even dynostarts as on my Bullnose Morris) were set at the low end of the charging voltage scale.

I had lots of discussion about this here with 12V people some who stuck to their opinions like "wotsit to a blanket" :roll: one propounded that if a fully charged battery was allowed to be charged by an altenator it would in fact be discharged to around 75-85% capacity during the "charge".

In my own experience this has proved ( with modern altenators) to be a load of bunkum... my altenator gets my habitation battery to 100% ( well near ) and if following a trip I check it with a voltmeter or a hygrometer it is at 100% ( or near :roll: ).

Also if I immediatly on return attach one of my high tech multi stage chargers to the same battery to maintain it during the idle , standing on the drive period, they also register it as being 100% fully charged.

I guess I may well be wrong in all this ... if so at least I am not going to be alone or even unhappy ...12v lead acid batteries and the technology of their construction, maintenance and use surely fall into the area of "black arts"

Mike


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## DABurleigh (May 9, 2005)

Don't fret Mike; at least I recall agreeing with you at the time 

Dave


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## olley (May 1, 2005)

My alternator charges at 14.7V does that mean my batteries are at 110% capacity? :lol: 

Olley


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## sallytrafic (Jan 17, 2006)

No...

but if that sort of voltage is always getting to the batteries I expect you need to top them up often


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## EJB (Aug 25, 2007)

My reason for commenting...I am a very long term...but now 'ex' caravanner.
In a caravan the EHU charger was limited to about 13.8v to protect the caravan installations...which are connected to the battery all the time.

These are voltage readings from my MH today..after being inactive for about 7 days.

Vehicle battery (age unknown) 12.2v
Habitation battery (new) 12.7v

Voltage at battery terminals after the vehicle engine is started:-
Vehicle 14.3v.
Habitation 14.1v

After 24 hours at rest I will try the EHU tommorrow.

My initial realisation is that the Habitation battery will (nearly!) fully charge from the vehicle because the habitation equipment is isolated when the engine is running...allowing 14+v charging.

I am assuming that the EHU, tommorrow, will charge at less than 14v.

Hope I haven't put my foot in it 8O


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## spykal (May 9, 2005)

olley said:


> My alternator charges at 14.7V does that mean my batteries are at 110% capacity? :lol:
> 
> Olley


Hi Olley

Are those UK volts or USA volts? :roll:

Mike


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## asprn (Feb 10, 2006)

sallytrafic said:


> Split Charging a technical correction


Is this a Frank's Recall then? 8O

Dougie.


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## sallytrafic (Jan 17, 2006)

No use of the R word here! No this is only an adjustment what is 0.6V between friends?


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## DABurleigh (May 9, 2005)

"the alternator on its own would not get your leisure battery fully charged"

Um, well, seeing as Frank withdrew the above claim triggered by a PM from me on gassing voltage, I'll stir the pot some more by saying that the above is indeed true, depending on your definition of the word "fully". Given Eddie's interpretation of "split" this could be a thread in itself .....

[As an aside, here is a definition of "split"
http://www.thefreedictionary.com/split
note "divide and share", which seems to me what Kirchoff's law is doing to the current from the alternator in the ratio dictated by Ohms's law.....]

Our sorely missed GT was a regular claimer that the alternator would not fully charge a battery, and as everyone who appreciated him knew he was never wrong (truly! just an amazingly pedantic dog with a bone....), I shall try to explain.

The alternator maximum voltage is set at the nominal lead-acid battery gassing voltage of 14.4 explicitly to minimise the battery maintenance of having to top up the electrolyte, lost in the process of gassing. And in that choice you settle for the cumulative charge in the battery that you get. Now, if you want more charge in the battery, you can go above 14.4V, typically to a terminal voltage of 14.8, and get a very useful addition in your battery capacity. The trouble is, do this on a flooded lead acid and you'll have to top up regularly; do it on a gel/SLA battery and you'll knacker it pretty damn quick. But, your battery will then be "fully" charged.

Another downside is that the loads hanging off your battery during the time the voltage is a high 14.8 (with a multi-stage-charger the volts vary over time) will see a voltage which risks being terminal in another sense :-( Intriguingly Sargent, who supply a massive proportion of chargers to the UK motorhome manufacturers, have developed a charger (well, it does MUCH more than that) which charges the battery optionally to such higher capacities via the higher voltage while simultaneously acting as a power supply unit to deliver a normal 13.8V (yes, the voltage set in those cheap caravan single-stage chargers all those years ago) to the loads in your van. I think that aspect is only a £30 mark-up, so expect to see it.

In my van I have a Victron integrated inverter/charger which I have used for 2.5 years in its "battery-safe" mode. It keeps the terminal voltage to 14.4. I check my batteries every few months, have not needed ever to top them up, and they still deliver a capacity close to new, as I haven't regularly discharged them below 50%. Because a Victron battery monitor is also fitted, I know that after a trip home, I can plug in the EHU, and after only minutes the multi-stage charger is in float mode and often less than a single Ah has been added by the charger.

If I deselected battery safe mode, that single Ah would increase significantly, but I'd have to start topping-up the batteries.

Sorry for confusing ......

Dave


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## sallytrafic (Jan 17, 2006)

DABurleigh said:


> "the alternator on its own would not get your leisure battery fully charged"
> 
> Um, well, seeing as Frank withdrew the above claim triggered by a PM from me on gassing voltage, I'll stir the pot some more by saying that the above is indeed true, depending on your definition of the word "fully". Given Eddie's interpretation of "split" this could be a thread in itself .....
> 
> ...


so your guess would be 98-99%


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## Boff (May 10, 2005)

Hi!



DABurleigh said:


> Our sorely missed GT was a regular claimer that the alternator would not fully charge a battery, and as everyone who appreciated him knew he was never wrong (truly! just an amazingly pedantic dog with a bone....)


I am afraid that GT was even more right than that. :wink:



DABurleigh said:


> The alternator maximum voltage is set at the nominal lead-acid battery gassing voltage of 14.4 explicitly to minimise the battery maintenance of having to top up the electrolyte, lost in the process of gassing. And in that choice you settle for the cumulative charge in the battery that you get. Now, if you want more charge in the battery, you can go above 14.4V, typically to a terminal voltage of 14.8, and get a very useful addition in your battery capacity. The trouble is, do this on a flooded lead acid and you'll have to top up regularly; do it on a gel/SLA battery and you'll knacker it pretty damn quick. But, your battery will then be "fully" charged.


A battery is by definition "fully charged", when the chemical reaction that goes on during discharging, has been completely reversed. Once this point is reached, all charge that is additionally "squeezed" into the battery will only contribute to gassing. So in fact by increasing the terminal voltage above 14.4 v will not add anything to capacity.

Having said that, it must be added that this terminal voltage is battery-type and temperature dependent. So the 14.4 volts are for a lead-acid "wet" battery at a temperature of +20 degrees C. For lower temperatures the terminal voltage should be higher, up to 14.7V at about freezing, and down to 13.9 at +40 degrees C. Top-end mains chargers therefore provide at least the option to connect a temperature sensor.

To understand what happens when the battery is charged from the alternator via a split-charge-relay, we first have to look at what happens in an "ideal" charging scenario, using a top-quality mains charger:

For optimum charge, a lead-acid battery should in the beginning be charged with a constant current (I), where I should be at least 10% of the nominal capacity in Ah. So for a 100 Ah battery, I should be at least 10 A. While the battery takes in charge in this "I-phase", its voltage rises, and as the charger has to "push" in the charge against this voltage, it has to increase the charging voltage to keep I constant.

This is done until the terminal voltage (U0), as defined above, is reached. (Some chargers already slightly reduce the current I at about 0.2V below U0, but that is not important here.) Once the terminal voltage is reached, the battery is at about 75% charge level. In the subsequent phase, the voltage is kept constant, while the battery takes in further charge and therefore the charging current gradually decreases. The length of this "U0-phase" is determined by the charger. Top-quality computer-controlled chargers have monitored the battery's performance (and its temperature) during the I-phase, and compute the ideal lenght of the U0-phase based on this information. Lesser chargers just run a timer.

Typical U-phases take between 20 minutes and 2 hours. Afterwards the charger reduces the voltage to a "trickle charge" level of 13.8 volts. This is called "U-phase". The characteristic curve of such a charger is made up of the described sequence of 3 phases is I-U0-U, which is why such chargers are called IU0U chargers.

Now, what happens if we charge the leisure battery from the base vehicle's alternator:

In most MHs the leisure battery is simply connected to the engine's starter battery via a split-charge relay. Usually nothing is changed at the alternator or the alternator regulator, except maybe that a stronger alternator is installed. By design, the job of the alternator is of course only to recharge the starter battery, and to provide electricity to all the vehicles appliances (headlights, windscreen wipers etc.) while driving. In a standard motorhome the alternator does not "know" that someone has tampered with the electrics and connected a second battery (and some other appliances like a 3-way-fridge).

Operation of the alternator is controlled by the alternator regulator, an ingenious device that not only keeps the alternators output voltage (fairly) constant over a wide range of engine revvs, but also controls the charging cycle of the starter battery by applying a simplified characteristic curve.

When starting the engine, the starter draws currents of up to 150 amps out of the battery. Only for some seconds, but this - maybe together with pre-heating the glow plugs - significantly discharges the starter battery. From now on the alternator charges the battery, until it reaches its terminal voltage of 14.4 volts.

In theory, the leisure battery would also receive the full 14.4 volts as soon as the starter battery is full. However, in practice, voltage drops occur in the (longish) cables between the two batteries, in the split-charge relay and in the fuses. In addition, some regulators then even slightly reduce the voltage once the starter battery is full. Typical voltages at the leisure battery while the engine is running are around 13.8 to 14.1 volts. So not enough to run a fully-fledged U0-phase.

Now, theoretically it is still possible to fully charge a battery even without reaching the full terminal voltage. However, the time to achieve this is much, much longer. So a full 100% charge of the leisure battery from the alternator can only be expected if a) the battery was already relatively full and b) the engine is running for a very long period. Which means that in practice we will still end up with the leisure battery only charged up to 85 to, maybe, 95%, from the alternator.

There are however special alternator regulators, as well as so-called "B2B" chargers ("battery-to-battery") on the market, which ensure that a full IU0U curve is run on the leisure battery while driving. But hardly any motorhome builder fits such devices as a standard.

Best Regards,
Gerhard


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## loddy (Feb 12, 2007)

Boff

Where do you get the terminal voltage of 14.4 volts from ?

Having hands on experience of measuring same I was taught that it should be 14.2 volts'

Loddy


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## 104477 (May 13, 2007)

This has been very interesting reading, as I have little knowledge of battery complexities, until now.
In the boats I had a hand in building we used Victron charger units but also had a switch to enable power to engine from starter bank alone or both starter and leisure bank or neither for when moored up. This gave the option of using the leisure batteries to start the engine should the engine one fail at sea or middle of lake etc. An option which appears to be missing on the m/h front. The Vetus engines also had two alternators fitted ( not surprising when the leisure bank is six 100 odd amp hour gel units) but then weight was not such an issue as it is for us motorhomers :roll: 
Eddievanbitz and Spykal are right in that mnfr's could take a leaf out of the marine industry's book and that it is a black art , rather like tuning a propeller!


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## Scotjimland (May 23, 2005)

rapidorob said:


> .
> In the boats I had a hand in building we used Victron charger units but also had a switch to enable power to engine from starter bank alone or both starter and leisure bank or neither for when moored up. This gave the option of using the leisure batteries to start the engine should the engine one fail at sea or middle of lake etc. An option which appears to be missing on the m/h front.


I recently fitted a Victron charger/invertor, great bit of kit.. it also has a separate 4 amp output to float charge the starter battery.

An emergency start switch is fitted to RVs connected as you describe..


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## 104477 (May 13, 2007)

Maybe we should have got an R.V then Jim   
But then the weight issue is not so great on them is it? Ours is rated at 3400kg max, sufficient for part timing but not for the fulltimers, it is one for the future I think! The Victron is good unit, the only bad situation we had was when a "Marine Electrician" wired one up wrong!!!!


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## Boff (May 10, 2005)

loddy said:


> Having hands on experience of measuring same I was taught that it should be 14.2 volts'


Well, I always learned 14.4 volts at 20 degrees C. 14.2 volts would then be the terminal voltage at about 30 degrees C. This is, at least, for "classical" wet batteries and gel batteries. For AGM batteries it would be 14.8 volts at 20 degrees C.



rapidorob said:


> This gave the option of using the leisure batteries to start the engine should the engine one fail at sea or middle of lake etc. An option which appears to be missing on the m/h front.


I know that there are some vans around which have this option, too. Probably self-made by their owners, but it works. However there is a difference:

If the engine battery in a MH fails, then this is a mere nuisance, at worst a loss of some money to pay the recovery man. If the engine fails to start on a vessel at sea, or even a river navigation, then this may end up in a life-threatening scenario.

Best Regards,
Gerhard


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## spykal (May 9, 2005)

Boff said:


> snip: If the engine battery in a MH fails, then this is a mere nuisance, at worst a loss of some money to pay the recovery man. If the engine fails to start on a vessel at sea, or even a river navigation, then this may end up in a life-threatening scenario.


Agreed Boff but if we ended up stranded in the middle of nowhere because I let the engine battery go flat it could well be a life or death situation.... you don't know my wife do you :lol:

I still enjoy reading these "battery" threads even though we have chewed over the detail so many times...... I wonder why it is such a fascination for some of us? Maybe because the behaviour of the lead acid battery is not finite?

Mike

P.S. Rapidorob...we must compare notes on prop tuning :wink: ...I did a few years of powerboat boatracing in the 60s and used to fettle away modifying and cupping my own props ... as you say another black art.


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