CTEK Battery to battery charging : user info please

[betsy]

Hello all,

 

Does anyone out there have any first hand experience of this subject in their own motorhome? I am on the verge of purchasing a CTEK D250S battery to battery charger to "pump-up" (ie increase the level of charge over that which the normal vehicle alternator/system would produce) my twin 125 amp hr leisure batteries. The distance between my 2/3 day stopovers in southern Europe (all without mains hookup) could be as little as about 40/60 miles.

I am aware that the CTEK D250S takes its power from either the alternator (in my case) or a solar panel (I do not have one - but was going down that route before hearing about B 2 B). The helpful folk at Roadpro have recommended B 2 B as a good way to go but just wondered if anyone out there can throw further light on the subect before I take the plunge.

I am particularly interested in what voltage shows on a motorhome's control panel after such a drive (allowing, of course, a settling down period after stopping).

 

Whilst on the subject, has anyone any "user experience" of CTEK's "Smartpass" unit which seems to be something that one might think of adding (quite expensively, it seems) alongside their D25S unit?

 

As usual, thanks in advance for any help.

[Dave Newell]

Yes I have the CTEK B2B unit fitted to my camper. I previously had a Sterling unit fitted but this could draw up to 50 Amps and as my camper only has a 55Amp alternator I felt this was giving the alternator a bit of a hard time so swapped it for the more compact CTEK 20Amp unit. It works very well although a typical journey for us is more like 100 plus miles. With the engine ticking over after a good run the leisure batts will read 14.4 volts. An hour after switching off they wll typically be reading 13.8 V and after 12 hours will usually be reading somewhere between 12.8 and 13.2. All of these figures are with no load applied to the batteries. I have a bank of 3X110AH batteries.

 

D.

[Clive]

I have had a play with Ctek, Sterling and also experience with the later Sargent charging systems that do something similar with a little more technical elegance.

If your alternator is 90 / 100 amp ish then I would suggest the Sterling 50 amp jobbie, if like Dave its a lower output then follow Dave,s direction.

 

C.

 

[Brian Kirby]

But, the OP refers to moving as little as 40/60 miles between stops. 

The underlying question seemed to be: can a B2B of any kind (but specifically the CTEK) effectively re-charge her 125Ah battery given only about one hour's driving time?  Put another way, can one reasonably squirt (pardon the technical term :-))about 60Ah (assuming 50% discharged) into a 125Ah battery in one hour?

Somehow, it sounded a bit of a tall order to me.  But then, I really don't understand the physics of battery charging.

[Dave Newell]

Well the answer to that one Brian is definitely NO! The CTEK B2B unit will charge the leisure batteries at UP TO 20 Amps, if diriving for one hour that would give a maximum of 20AH into the leisure battery. to replace the assumed 60AH would require a drive of at least three hours. I answered the OP's request for personal experiences.

 

D.

[Brian Kirby]

For my part, thanks Dave.  That's a more or less what I had thought might be the case.

So, if the alternator output were higher, do you know if the rate of charge into a battery would just rise proportionately, or is there a physical limit to how fast a given battery can absorb charge?  I'm really fogged on this!

[Dave Newell]

The Sterling B2B unit will charge the leisure battery(ies) at up to 50 Amps continuously (engine running obviously) but this could well be excessive for the battery if it is a single unit. I seem to recall that real leisure batteries should not be charged at more than 20% of their total capacity or the plates can be damaged, so a 100 AH battery should never be charged at more than 20Amps. I'm sure if Brambles is reading this he can enlighten us further.

 

One of the real advantages of these B2B charging units is they are four (or more) stage chargers so will not overcharge the battery(ies) but will make the most of the battery capacity given a long enough drive.

 

D.

[Brian Kirby]

Thanks again Dave, I'll wait to see if Jon (Brambles) can give a more detailed answer on charging rates.

[Brambles]

Hi, I am having problems getting time to post a decent and accurate reply which answers everything..its quite complex. Suggest you do nottake too literally until Dave and Brian agree or not.

 

The best way of looking at it is the B2B really converts your alternator into being similar to an intelligent charger, like one of the higher power Cteks. For example a 20Amp B2b would be like any good 20Amp 3 stage charger. If you get specs for a 3 stage charger then you should also be able to work out the time to charge usig a B2B

It is better than an alternator on its own because at low Alternator rpm it can boost the voltage so charge current increases and also when the battery gets near full charge (80% say) then it can maintain a higher charge voltage than an alternator would and take up to 14.8 volts pulse width modulated. Now this really makes a big difference in the time to get the final 20% or so into the battery.

Rather than being 10 hours it could be as low as a few hours.

 

There is also other characteristics which could be seen as being good or bad.If your battery is deeply discharged then an alternator can charge at a pretty hefty rate, it is not going to be 50 Amps or anything like it, but could be as high as 30 Amps for a while. This is above the recommendd charge rate, but will not last as long as the chemistry of the battery will soon limit the charge and drop it down to around 20 Amps as well as teh associated wiring ( say for a 100Ah battery). As the charge level increases this falls off with an alternator but with the B2B will be maintained for longer. The B2B will also prevent the high initial 30 Amps or at least should if it as clever as I think it is. Swings and roundabouts. Bear in mind though an alternator running slow wil not generated endless power so its low voltage can ony be boosted to a limited extent.

 

So where does this leave us. Well if you want nearly every ounce in the battery B2B will make a difference as it reduces the time for final stages of charging.

 

But there is another aspect to this. If you say only need 30Ah of power from a 100Ah, then the battery could be cycled between 50% and 80%. You do not need to take up to over 95% full. Charging up to 80% is a lot quicker, but still not feasible in 1 hour, but maybe 2 hours. Having two 100Ah batteries in parallel will let you get 60Ah between 50% and 80% full. However charging will not be quite as fast as the altrenator output volts does drop with current, but with a 150Ah allternator is not going to be much. Fitting a 20amp B2B to a bank of batteries would slow down bulk charging as it willl limit charge to its capabilties of 20Amps total, where the alternator running fast would give 40 Amps.

 

So is beginning to look like B2B is great for the person driving say 3 or 4 hours or more compared to the driver doing an 8 hour drive where it wil noty make much difference. But for the 1 hour driver....its not going to help and altrenator charging alone might be better if deeply discharged. You really cannot charge a battery more than its C/5 rate with C/10 being preferable. A lot of this charge rate is dictated by the calcium technology and to some extent is self controlling.

 

For more capacity you could use better batteries which can be discharged to 60%dod which is 40% left or even lower levels. This means you could cycle between say 40% and 70% giving again 30 Ah available and would charge a lot quicker from the alternator.

I better move on to where I am going with this. Lets say you have a 100 Ah gel capable of going down to 80%dod which is 20% left.

Ist day you start off with 100% capacity. You take 30% out. You drive for an hour and replace say 20% taking back up to 90% but 80 to 90% takes a long time so we shall say 80%. 2nd day you go from 80% to 50% and recharge to 70%. 3rd day 70% to 40%, 4th day 60 to 30%, 5th day 50% to 20%.

 

We have manage 5 days or if you prefer call it 5 stops IF we only need 30Ah of power. What you will need is a darn good gell battery and will also have to make sure you give it a really good full charge afterwards so it recovers from this abuse.

 

If you need more than 30Ah, say 60, the you can double up the batteries, but what I am trying to explain here is every user is different and needs to be thought through as to best solution and afraid unless your power usage is small then B2B for a 1 hour drive may not help you at all. The solution is the same as many and is the solar panel, Efoy, or massive bank of batteries route.

 

It also depends on dricing style, if yor ar ekeeping engine revbs uo duringteh one hours drive then yu wil be charging efficicienty, if engine is idling a lot or you are puttering aabout with ow revs and have a small alternator then B2B again could help. In dave scas with a small alternator I can really see how it helps, but for many who have B2B ther is no getting away from the fact when they say it worked well they may not have had a problem in the 1st place.

Vehicle manufactirers alomg with alternator designers an battery companies have worked hard to develop their equipment to address the problem of high power usage when vehicles are parked for long periods and need to recharge batteries quickly, so on modern vehicles we have high power alternators with low volt drop curve v current, and batteries which can absorb energy fast and recharge quickly.

We can only attempt to match this at best by using high quality high spec leisure batteries.

 

 

Ford have taken this a stage further with their intelligent power management system on some models with alternators generating much higher voltages, higher voltage required to charge batteries but regulate the volts to the 12volt sytems on board. They can really pump the amps into the batteries very fast by raising to 16 volts or so, but reguate to 12 volts (14V) for powering systems.

 

Having quickly read over what I have written, I am not sure it helps much, and I have not really given accurate figures least not what Brian and Dave were asking for which is more accurate charge current figures. C/5 to C/10 is about right.

[Brambles]

Oh heck, did I write all that tome...don't have time to read over and check it all so hope it makes some sense. Can only apologise for not abbreviating and condensing taking out what is not relevant.

[Dave Newell]

Thanks Jon for an in depth answer, for anyone who doesn't understand the C/5 C/10 bit it simply means capacity © divided by five or ten to determine charging rate. for example C=100AH divide by five gives 20Amps, divide by ten gives 10Amps.

 

FWIW I have fitted several B2B units, mainly the Sterling 50Amp unit using 25mm sq cable and testing after fitting have seen up to 30 Amps charge rate with the engine ticking over. modern vehicles have high capacity alternators that are easily capable of significant current output from low revs. Mine being a 1997 Iveco has an old technology alternator that needs to be spinning quite fast to produce meaningful charge.

 

D.

[Clive]

Or there is the KISS principle if you have a modern 90/100 amp 14 volt alternator like most 3.5 tonne vans.

 

Use a bigger battery when 75% / 80% charge will be sufficient capacity and wire your split charging system is stonking great cable with a BIG relay (contactor) and fuses. Say 25 or 35 sq mm welding cable and 50 or even100 amp fuses.

 

But weight will need to be looked at.

 

C.

 

[Brambles]

Noe teh 50Amp sterling unit Dave, that sound good with a modern alternator and your tests show this. So that may well be the answer, but I suspect with douleing up teh battreis not inmto just 1 100Ah.

 

 

Clive, stonking heavy cable, will make a difference, but you do have to be a little careful. If you were to use heavy cable then you can get much higher currents from the alternator charging, and if it goes above say 20Amps for a 100Ah battery you are not doing it much good. It is fine having the heavy cable for the engine battery, although charging is often via a seperate lighter fused cable, because rarely is the engine battery flat. With your method you would be getting constantly high peak currents charging the battery and is not a good idea, it is also not good for smaller and older designed alternators. The alternator charging system for leisure batteries needs the resistance of the connecting cable to protect the batteries from to fast a charge rate.

 

 

[Brian Kirby]

OK, slow bunny-rabbit here, so lets see if I've understood!  :-)

OP has 125Ah battery.  Lets assume over a typical 3 day stop it is discharged to 60%.  So, 125Ah x 60% = 75Ah to put back.  Maximum nominal recharge rate = C5, so 125/5 = 25 Amps per hour.  75Ah to put back at 25 Amps per hour = 3 hours driving (assuming the B2B/alternator combination can sustain that as a flat charge rate).

But, as a broad generalisation, a charge rate of 12.5 Amps per hour would be "kinder" to the battery, and would prolong its life.  (Does this imply using a B2B with a lower output rate would be preferable, albeit charging would be slower, or can they be set to charge at differing rates?)

However, if the B2B works through a 4, or more, stage, cycle as it charges, I assume the initial charge rate might be higher than 25 Amps per hour (subject to adequate alternator output), stepping down in the later stages to a lower rate, with possibly a maintenance type charge as the final stage.  I assume these stages are controlled by the increase in battery resistance as the charge progresses (or some such), and not by a "clock" function.  So:

1. What this suggest to me is that despite the higher initial charge rate, the lower rate in subsequent stages may cause a full charge to take longer to achieve than three hours. 
2. Or, is it the case that the higher initial rate is what the B2B is really all about (logical in view of the cost/claims), and the net result is the battery charges faster overall despite the falling rate in later stages. 
3. Or again, is it that the C5 charge rate governs what will happen, and will allow the B2B to achieve a full charge after 3 hours driving, whereas the alternator alone would take rather longer.

Sorry for the extra questions, which I hope are clear.  Partly I should just like to know, but also, so many questions on this topic arise I assume others may value the answers - always assuming my dumb questions are answerable!  :-)

[Brambles]

Brian Kirby - 2011-01-15 6:35 PM

OK, slow bunny-rabbit here, so lets see if I've understood!  :-)

OP has 125Ah battery.  Lets assume over a typical 3 day stop it is discharged to 60%.  So, 125Ah x 60% = 75Ah to put back.  Maximum nominal recharge rate = C5, so 125/5 = 25 Amps per hour.  75Ah to put back at 25 Amps per hour = 3 hours driving (assuming the B2B/alternator combination can sustain that as a flat charge rate).

But, as a broad generalisation, a charge rate of 12.5 Amps per hour would be "kinder" to the battery, and would prolong its life.  (Does this imply using a B2B with a lower output rate would be preferable, albeit charging would be slower, or can they be set to charge at differing rates?)

As we do not want to risk charging at too high a rate then the amp rating of an IoU charger should be matched the battery. You need to look at the actual specifications of the charger, but generally should be C/10 rate so thats 1 tenth of the capacity in Amps. The charger though can be used over a range of capacties and so for example a 20 Amp charger could be used over a range of capacities but for max life would not go below a 140Ah battery which would give a C/7 rate, a bit safer than C/5.I think most IoU chargers current (amp) specs are fixed but the more sophisticated ones can be adjusted for the maximum charge curremt. The B2B chargers I doubt can be altered. You choose the one and rating to suit your batteries.

However, if the B2B works through a 4, or more, stage, cycle as it charges, I assume the initial charge rate might be higher than 25 Amps per hour (subject to adequate alternator output), stepping down in the later stages to a lower rate, with possibly a maintenance type charge as the final stage.  I assume these stages are controlled by the increase in battery resistance as the charge progresses (or some such), and not by a "clock" function.

The charger during its bulk charging phase will limit the current to whatever the specification of the charger is, so say is 20 Amps, then 20 Amps will be fairly constant throughout the phase until the endpoint voltage is reached. 14.4 volts for flooded open batteryAs the battery gets towards 80% charged or so, then the current will fall off as to get a higher charge current would require a greater voltage than 14.4 (14.8) volts. So the chargers do taper off the controlled maximum current as end point voltage is reached.

  So:

1. What this suggest to me is that despite the higher initial charge rate, the lower rate in subsequent stages may cause a full charge to take longer to achieve than three hours. 
2. Or, is it the case that the higher initial rate is what the B2B is really all about (logical in view of the cost/claims), and the net result is the battery charges faster overall despite the falling rate in later stages. 
3. Or again, is it that the C5 charge rate governs what will happen, and will allow the B2B to achieve a full charge after 3 hours driving, whereas the alternator alone would take rather longer.

Difficult to explain this one.Not necessarily. If the alternatator is going full pelt, in other words generating 14.4 volts and capable of more than enough amps for charging then the alternator could supply what ever the battrey will take and could easily be 30 Amps, but what limits it is the chemistry of the battery (its internal resistance) and the resistance of the supplying leads. So technically it is feasible the alternator full pelt will charge faster than the B2B because the B2B is limiting to say 20 Amps (the Rating I used earlier). So to answer teh question the Alternator could be faster initially and then slows down whilst the B2B is more constant. However the alternator can charge faster than this if connecting cables are of low enough resistance and do not drop much volts. Where the B2B wins is because it will monitor the volts closer to the battery so can keep 20 Amps going in. So falling on Clives recommendation to use welding cables come into play. I do not suggest using welding cables but just heavier cables then we can boost the charge current enough the B2B is not worth while having at all..... in theory. In effect people fitting B2B and checking charge rate see an improvement with engine revved up, but could achieve the same with extra cabling between the alternator and the slave relay, slave relay and battery. When the alternator is idling, and is of low power type, then the B2B will make a big difference as it can boost the volts and hence the current but is limited as to characteristics of alternator how much power it can generate in total when idling.My guess is it will help at moderate engine speeds say 1200 up to to 2000 rpm with small older alternators, but with modern high power alternators I really have my doubts the B2B will make much difference to the bulk charging phase except when the engine is idlling or running very slow. Where it does make the real difference is because it is replacing the volt drops in the split charge relay circuit so people see an improvement. So why not just replace with heavier cable in the split charge relay circuit. You need some restance to limit the current when battery is fairly flat it as it will be like a short circuit on the alternator (an exaggeration there!!) , but just doubling up the existing cable with similar should make a big difference but needs to be tested to make sure the charge current is below the c/5 rate and preferrably around the C/10 or less for longevity of the batteries.

Sorry for the extra questions, which I hope are clear.  Partly I should just like to know, but also, so many questions on this topic arise I assume others may value the answers - always assuming my dumb questions are answerable!  :-)

Actually no, your questions are not dumb at all, but very difficult answer accurately as so much depends on batttery types etc. I am sure the B2B will make a big difference in some cases, but not all. People throw in extra batteries and then get longer times to recharge. They fit B2B and see faster charge times. What they did not do when fitting extra batteries is increase the size of charging cables from the alternator and split charge relay which have been specified often for just one 80 to 100 AH auxilliary battery.Dare I put my name to this reply!!!!...anon.Edit - some typos in last paragraph. You will have to excuse me if not well written, struggling to jump about the screen checking things on this heap of a computer of mine.......get my new one soon I hope..a nice new Dell All in One 23 inch screen with Core i5 processor. 1 terrabyte hard drive and 6 Gbytes of memory -- That is astoundingly vast. Better be good.Also a bit whacked out just now as been doing a lot of work recently.

[Clive]

So in summary Jon?

 

C.

[Brambles]

Clive - 2011-01-15 11:00 PM

 

So in summary Jon?

 

C.

 

The original poster is not going to charge batteries in one hour and B2B may not help. May improve a bit, but not enough.

Heavy cables are not going to work either in one hour with standard liesure batteries.

Solar panels or efoy (edit - or portable generator)should be considered, but really depends how much power they use and this we do not have a clue. So there may or may not be a problem to resolve anyway.

 

[Brian Kirby]

Thank you very much, one and all.  I've learnt a lot more, and hopefully, so has Dick (betsy).  The most interesting bit (to me) is to increase cable sizes when fitting additional leisure batteries, and then leave the alternator to do its bit.  I like the sound of Clive's big contactor, but wonder what spec should be used.  The standard automotive types I've seen the insides of do seem to have very small actual contacts (so I have always assumed these would be the weakest link, due to highish resistances), so what would be best, and where from?

[Brambles]

Brian Kirby - 2011-01-16 12:16 PM

 

Thank you very much, one and all.  I've learnt a lot more, and hopefully, so has Dick (betsy).  The most interesting bit (to me) is to increase cable sizes when fitting additional leisure batteries, and then leave the alternator to do its bit.  I like the sound of Clive's big contactor, but wonder what spec should be used.  The standard automotive types I've seen the insides of do seem to have very small actual contacts (so I have always assumed these would be the weakest link, due to highish resistances), so what would be best, and where from?

 

Just for you Brian.... http://www.boschmotorsandcontrols.co.uk/relais/relais.pdf

 

Automotive relays are pretty special really and quite amazing items.

If you look at the Bosch catalogue you can get a pretty good idea of what is available.

There is also another make which are easier to source in the aftermarket and do some hefty relays which are compact and small .... Nagares.

 

You have to watch specifications from aftermarket sellers because they may rate at 80 Amps but that could be the switching current into a motor and continuous current amost half that.

 

It is quite stagerring to see how small the contacts can be, and for low resistance usually silver tin oxide is used. For heavy switching currents though some relays use dual contacts and one contact makes and breaks before and after the other and is made of tungsten to cope with the arc, whilst the second contact copes with the high current and has lower resistance. You will see this in the likes of the Bosch 0332 002 161 ... 100Amp relay.

Not cheap for power relays, however what you can do is look at say 40 Amp relays which are readily available and fit 2 in parallel. Because each relay can cope with high switching currents they will switch say 80 amps just fine bit will not cope continuously, but with the second relay the current is shared and so is no problem. Cheaper way of achieving an 80 to 100 Amp relay circuit.

When selecting relays there is also a massive difference between contact resistances depending on materials used and main purpose for the relay and browsing the Bosch catalogue you will see how it varies and they give new contact resistance and old resistance.

 

So for our extra battery and beefed up wiring and wanting to cope with say 80 Amps we could fit a Bosch 0332 002 161. (100A, 400 switching) or two 0332 002 192 relays in parallel for 50Amps each giving 100 Amps and individually able to switch 90 Amps peak.

 

You can switch higher currents but reduces the total number of operations.

 

You can get away with lower amp ratings because it is not often the slave relay will be switching high currents. When battery is flat, and you start the engine then alternator is running slow, but of course the engine battery is able to supply some of the curremt, but as it has just started the engine the voltage will be recovering as the slave relay closes. And when the relay opens the voltage from alternator is collapsing so the peak current for switching are not terribly important...in theory.

 

Also it is not often you will be charging at max current so you can get away with a lower rating and just occassionally overload it. They will cope with higher currents for short periods or so until they start to get pretty hot and life is also reduced but may still outlast the vehicle)

 

You are looking at between 100 to 200 millievolt drop across the contacts, at high currents which sounds a lot being up to 0.2 volts but is nothing compared to volt drop in the cables.

 

[Brian Kirby]

Thank you again Jon.  I'm afraid this has now got far too technical for my poor little head!  :-)

I get the gist, but really don't have the understanding to interpret the Bosh catalogue.  However, you have quoted your selections, so I'll happily take your opinion as gospel! 

One last thing.  With the OP's set up at 250Ah of battery capacity: if heavily discharged, I believe the resistance of the batteries is quite low.  Then you start the engine. 

What prevents the alternator "seeing" a virtual short when the fat new charging relay closes, and all those amps begin to pour down those fat new cables?  Is there no risk of damage to alternator, voltage regulator, or other electronics?  Closing that relay must surely cause a fairly significant and immediate reaction across the whole vehicle electrical system, with either voltage, or amps (or both!) suddenly plummeting, and then possibly spiking up as it all recovers?  Isn't some kind of "soft start" needed to cushion the rest of the system?  Or are we now back to the B2B!  :-D

[Brian Kirby]

Thank you again Jon.  I'm afraid this has now got far too technical for my poor little head!  :-)

I get the gist, but really don't have the understanding to interpret the Bosh catalogue.  However, you have quoted your selections, so I'll happily take your opinion as gospel! 

One last thing.  With the OP's set up at 250Ah of battery capacity: if heavily discharged, I believe the resistance of the batteries is quite low.  Then you start the engine. 

What prevents the alternator "seeing" a virtual short when the fat new charging relay closes, and all those amps begin to pour down those fat new cables?  Is there no risk of damage to alternator, voltage regulator, or other electronics?  Closing that relay must surely cause a fairly significant and immediate reaction across the whole vehicle electrical system, with either voltage, or amps (or both!) suddenly plummeting, and then possibly spiking up as it all recovers?  Isn't some kind of "soft start" needed to cushion the rest of the system?  Or are we now back to the B2B!  :-D

[Brambles]

When the batteries get really flat the impedance gets a lot higher so charge currrent is limited and acts in our favour and is not like a dead short. But yes some pretty hefty currents can flow. But as the slave relay closes as the engine is started and alternator is not generating much the current rise is fairly soft albeit can reach quite high levels. It is important after starting to let the engine idle for a while to limit charge currents and bring the battery up gently otherwise you can blow a fuse in the charging circuit if cables are too heavy.

 

 

[Brian Kirby]

Thanks again Jon.  So, if treated with reasonable care, the base vehicle electrical/electronic systems have sufficient resilience to cope with large leisure battery banks but, whatever you do, you can't re-charge discharged batteries at will, and must allow adequate time for the process to complete.  The charge rate should be between C5 and C10 (where C = total installed leisure battery capacity in Ah) and the minimum time to re-charge will be (approximately) the charge required in Ah, divided by C5, or by available alternator output in Amps where this is less than C5.

So, for the OP, with 250Ah installed, and if 60% discharged, the required charge would be 150Ah.  C5 would give a required charge rate of 50A, which should be within the capability of a modern alternator.  The (approximate) charge time is 150/50 = 3 hours driving at sufficient revs for full alternator output so, assuming an average speed around 40MPH, the distance between stops to reinstate full charge would be about 120 miles.  Since that is 2-3 times the distance the OP envisages travelling, their batteries would become progressively more deeply discharged over time.  Their options seem to be to limit discharge to around 50Ah, which makes a bit of a mockery out of the 250Ah installed, but can be reinstated within about 40-60 miles of travel or, as you summarised above, to supplement battery charging via solar panel, Efoy, generator, (or occasional mains hook up for about 24 hours).  B2B would help somewhat, but insufficient improvement to solve problem as stated so, overall, money probably better spent elsewhere.

[Dave Newell]

The B2B will help and will certainly do more than the standard alternator and split charge setup. for a start the existing split charge wiring is likely to be fairly light gauge so will cause resistance to high charging rates. The alternator of a modeern motorhome is quite capable of producing a sustained 30 Amps at tickover ( I have personally measured this and proved it is so) so there is no real need to have the engine spinning at high rpm, normal driving will be more than adequate.

 

The other main advantage of the B2B is that it doesn't kick in as soon as the engine is running but gives a delay to allow the engine battery to be replenished, a simple relay wil not give this delay but energise as soon as the alternator output is sufficient to energise the coil, in other words virtually immediately the engine is running.

 

The standard alternator and split charge system will initially give a high current to the leisure batteries but it will very soon tail off to a mere trickle whereas the B2B will give a sustained higher current to the leisure batteries.

 

My advice would be to go for the CTEK D250S (which is a B2B charger with solar input. A major advantage of this is that when a solar panel is connected it will split the charge to both leisure and engine batteries) and a decent sized solar panel, say 80 Watts or better.

 

D.

[80overland]

Please note, I am no expert at all but having just bought a CTEK D250S there are a number of points I thought worth raising.

 

1. It is not a B2B charger - only Sterling make one of these. It is an alternator to battery charger. I do not believe the CTEK can be connected to your starter battery as the Sterling can.

 

2. I have seen no information to suggest that the CTEK waits for the starter batteries to be charged before starting to draw current. The instructions specifically state that it starts drawing current as soon as you start the engine. My understanding is that this could be a major issue if you are doing a fair bit of stop-start driving as your starter batteries are not prioritised and could therefore be being under-charged. The CTEK has no function to monitor your starter batteries. I am of the opinion that this is a major design flaw and as such I am considering returning the CTEK and buying a Sterling (genuine B2B - which specifically monitors and prioritises your starter batteries ensuring they are fully charged before drawing a current).

 

3. My understanding of the Sterling is that the internal electronics ensure that it will never damage your batteries by putting in too much charge. It would only put in 50amps if it determined that it was safe to do so for the specific battery type and capacity. Therefore, the Sterling would appear to be the better option for bigger battery banks but if 100ah or less it would be no better than the CTEK.

 

4. Whereas the CTEK is IP65 rated the Sterling is not and therefore needs to be protected. The Sterling is also a much bigger unit.

 

5. Only the CTEK D250S Dual can accept both solar and alternator input. The standard D250S does not accept solar - is worth checking as some suppliers only have the latter.

 

6. The CTEK has no ability to split the charge between the starter and auxillary batteries. It simply charges the auxillary batteries and has no connection at all to the starters. In order to do that you would need to also buy the CTEK Smartpass which doubles the price! Or, I guess there would be a way to connect the CTEK to both the starters and auxillary using relays or something to make it switch between them but it certainly has no built in function that allows it to do that.

 

7. Having emailed CTEK numerous times, I can attest that their customer service is appalling. They often don't reply and when they do their answers are often lacking the requested detail. This is something thst is also reflected in the instructions they provide and the information on their website - both of which are severely lacking in required information. In contrast, Sterling got back to me asap with a detailed answer.

 

I specifically bought the CTEK because I wanted the IP65 rating for durability and I wanted the ability to add solar. However, I am now wondering whether I should return it and get a Sterling which is a real shame.

 

 

Just to reiterate I really am no expert and stand to be corrected but is my two bits worth based on having just bought one and been left pretty pi**ed off by what it appears to not do!