CTEK D250S Dual alternator to battery charger

[Lorin]

After much deliberation (between a CTEK and a Sterling) I have just taken delivery of a CTEK D250S Dual alternator to battery charger for my auxillary batteries. The reasons I opted for the CTEK over a Sterling in the end were: (1) the CTEK is also a solar regulator (built in) and will automatically switch between alternator and solar power to keep the auxillary batteries topped up; (2) the CTEK is IP65 rated which can only be a good thing; and (3) the CTEK is a physically smaller unit. Like the Sterling the CTEK uses a staged charging pattern that de-sulphates the batteries and ensures that they are fully charged. It also monitors the state of the batteries to be charged and adapts the charging pattern accordingly.

However, having now got it and read the instructions there appear to be a couple of potential issues I want to check out before I install it. And I am bl**dy confused how/why an alternator doesn't cook the starters (despite reading everything google pulled up in the last 2 hours!)

Firstly, it appears the CTEK draws a current as soon as the engine is started (max output is 20amps so max draw must be just above that). It has no capability to monitor the starter batteries nor does it wait for a set time before drawing a current. Given this, am I right in thinking that there is the potential risk of the starter batteries not being sufficiently restored between stops? For example, when you are doing a a fair bit of short duration stop-start driving and the auxillary batteries are low thereby drawing the full 20amps. If this is correct, could I offset the draw of the charger by installing a bigger alternator, say a 100amp rather than the standard (I think) 80amp?

This then led to my confusion re alternators. From what I gather, an alternator can push out a max current of say 80 amps but this is regulated according to what is required (i.e., the alternator ups output in line with demand such as when turning on all the lights etc). If your starter batteries are fully charged with the engine off and you then start the engine, the alternator will replace the lost charge. And herein lies my confusion. From what I have read, the state of charge of a battery determines how much current it will draw in order to recharge itself. The alternator then regulates the current via the 'diodes' accordingly - to how much the battery will accept. However, an alternator cannot produce no current, or can it? So why doesn't it cook the batteries once they are charged?

Also, my understanding of charging batteries is that there is a max charging current above which it is damaging to the batteries and that this differs according to the state of charge. For example, when really low they can accept a high current but when nearly full they only need a low current and somewhere or other I stumpled across an equation that basically said the max current it is safe to put into a battery is approx one fifth of is capacity (for standard lead-acid). Is this correct?

If the above is correct and I have 250ah of auxillary capacity, am I right in thinking the max 20amp output of the CTEK is not really sufficient to charge the batteries - in an optimum sense? Would I be better off with the Sterling that has a max 50amp output? Related to this, whilst I cannot find confirmation, I am assuming that the 'intelligent' chargers such as the CTEK and the Sterling are able to determine the capacity of the batteries to be charged and can therefore adjust the max current they put into them rather than just bunging all 20 or 50 amps in?

I accept there is a real possibility I am just being thick and have got it all wrong, which actually I kinda hope is right. But if not, why can't one company just produce a charger that does what is needed instead of them all producing chargers that do different bits but not the whole lot.

Apologies for the essay Any help/advice greatly appreciated.

 

[fridayman]

I am watching this with interest after also spending time looking into batteries, relays and larger alternators recently.

 

[Lorin]

Right, I've just spoken to Sterling and think I have a bit more relevant info. No surprises to discover I got a few bits wrong and was lacking in a few significant details. Though I welcome any further corrections!

Just to make clear, all I want is the fastest, most efficient way of keeping my auxillary batteries topped up without any risk to my starters.

So, what I think I've found out:

How quick the aux batteries charge is determined both by voltage and current. For example, if you have two identical chargers in terms of current, say 20amps but one charges at 14.4v (i.e., CTEK) and the other charges at 14.8v (i.e., Sterling) the latter will charge quicker. So an advantage of the Sterling alternator to battery charger over the CTEK is that it will charge the batteries quicker. If my understanding is correct, the reason for this is that a battery can accept a greater current (i.e., more charge) at a higher voltage. Though there are limits.

The relationship between voltage and current also explains why the alternator does not fry the starter batteries as to put it bluntly (thank you man at Sterling) - you can put a million amps in but if the voltage is below where the battery is currently at it simply wont accept the charge. However, if the voltage is too high the battery will cook. So, I assume, there must be something attached to an alternator that regulates voltage?

Another advantage I discovered is that the Sterling is connected to both your starting batteries and your auxillary whereas the CTEK is only connected to the latter. The Sterling ensures you starters are fully charged before charging the auxillaries and it also conditions them (i.e., de-sulphates) as per the auxillaries - again something the CTEK does not do. Though is this really needed as the starters do fine usually?

The bit I'm still confused about (didn't want to piss the man at Sterling off too much with my incessent idiocy), is what difference more current makes. For example, if you have two identical 100ah batteries both at 50% capacity and two identical chargers in terms of voltage, say 14.4v, what difference would it make if one charger was 50amp and the other 100amp? Would it make any difference if the battery was a 50ah or a 200ah?

Put another way, I'm trying to ascertain whether a 14.4v charger that puts out 50 amps would charge my 250ah batteries quicker than a 14.4v charger that puts out 20amps? And if so, why and by how much - I mean would the difference be significant in terms of driving times? Because if it would, then the Sterling with its 14.8v 50amp output would definitely be the better option.

 

[Jon Wildsmith]

Put another way, I'm trying to ascertain whether a 14.4v charger that puts out 50 amps would charge my 250ah batteries quicker than a 14.4v charger that puts out 20amps? And if so, why and by how much - I mean would the difference be significant in terms of driving times? Because if it would, then the Sterling with its 14.8v 50amp output would definitely be the better option.

The chargers don't 'put out' 20 or 50 amps, that's just what they're capable of supplying, but they do 'put out' i.e. set the voltage of their output. The chemistry / state of charge of the battery and charge voltage determines how much of the available current capacity is used. For 250ah of capacity I think 20a is too small. Being a bit simplistic, if you've used 50% or your 250ah capacity you're looking at 6.25 hours to put it back at 20a vs 2.5 hours at 50 amps ... Also consider that whatever loads you have connected e.g. fridge reduce the charge current available. The Stirling doesn't charge imediately, monitors the state of both batteries to make sure your starter isn't being drained, stops periodicaly to check everything is ok and has heat sensors for battery (ideal charge voltage and current depend on temp as well) and alternator. Shame it makes so much RFI

 

[Lorin]

Cheers Jon, you've confirmed what I hoped wasn't true - that the CTEK is too bl**dy small

No messing about this time I'm going to wait for the IP68 rated Sterling charger due in April.

 

[Gary Stockton]

Bump - Thread resurrection (well, Easter isn't that far away, is it?)

Lorin, what did you eventually do here? Did you go with the Sterling in the end?

I was wondering if I need to swap out my own system for a Sterling - mine is a home-bodged voltage-sensing relay hooked up to the starting batteries that then feeds my deep-cycle in the back, so no major bells and whistles (although I do have a National Lune monitor to install now ).

Does the Sterling have sooo much more capability than simply introducing a 3rd battery into my circuit in parallel, which has been working for me, that I should seriously consider one?

Cheers

 

[Lorin]

Gary, I can't say whether you really need a Sterling or not. I did opt for one in the end but got the standard 12v 50w battery to battery version. At the time the IP65 rated version wasn't available - I don't know whether it is now.

The reason I went for one over a standard split charge was the fact that they keep the battery in a higher state of charge and charge faster (I'm sure this will kick off a few comments....). They are also very easy to install. The reason I opted for the Sterling over the CTEK was as described above. It is a more 'intelligent' system and the higher max output of 50w made more sense given my 250ah aux battery bank.

So far it has performed faultlessly - at least nothing has gone wrong and my aux batteries constantly show 12.7v