Dual Battery System with CTEK D250 not charging battery over 12 volts.


Tail-End Charlie
Also keep in mind that voltage drop is a variable and is a function of load. So it's not something that can be measured except during those times when it's actually happening.

Measuring the voltage at the input to the unit's engine battery connections won't show a dropped voltage if there is no load to pull the voltage down.

There probably won't be enough load to cause a voltage drop if A) the house battery is full, or B) the unit detected a voltage drop and stopped loading the input.

The CTEK probably responds very fast (solid state switching) to a dropped voltage on the input, so even if you do have a voltage drop issue to the unit's input, you might have a tough time actually catching it at the right moment to see it on your meter.

Edit: And it could also happen so fast that your meter's display can't keep up. So even if the meter could see it happening, you might not.
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Expedition Leader
CTEK Block Diagram ?

As far as I can guess, the D250S has two front ends and one back end.

-- Starter Battery/Alternator input - boosts voltage from a nominal 13.Xv to 14.4v. Also includes circuitry (diode of some sort) to draw the source down to 13v. This is essential to trick the alternator into providing full output, or at least more than 20A. Otherwise the alternator would bring the starter battery to its design voltage and drop the amperage. Thus the D250S must be "close enough" electrically to the starter battery/alternator to fool the voltage regulator of the alternator.

-- Solar Panel input - cuts voltage from a nominal 18v to 14.4v.

And the back end:

-- "Intelligent" lead acid charger that draws current from the two front ends and boosts/cuts the voltage in response to the amp flow through the unit. (As the battery voltage rises, the amp flow drops, when it reaches design goal, the charger drops back to float.)

There is also a diode in the circuit to provide isolation so that the camper battery cannot draw down the starter battery.

An excellent device for a smaller system when:

-- The charging voltage of the vehicle is too low to meet the needs of a camper battery, and,

-- You want to add a smaller solar panel array without the cost of a stand alone solar controller.

For a larger system:

-- The Smartpass and now the Smartpass 120 add an intelligent relay to permit faster bulk charging while still assuring that the absorb charge stage has adequately high voltage, and,

-- The Smartpass adds a passback function to bypass the diode in the S250S and allow the camper battery to maintain the starter battery.

I haven't used the CTEK system myself, but everyone that I correspond with who has installed it correctly has loved it. (And I did exchange a ton of communications until I understood what was happening.)

My free advice, worth what you paid for it, would be to disconnect everything and then reconnect with the properly size of wire, attached as laid out in the CTEK manual. (Page 20, I believe.)
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Expedition Leader
This Merits some Comment

All charge sources should be directly connected to the House bank.

The B2B charger should charge the smaller, hardly depleted Reserve/Starter, which takes hardly any current, no SmartPass needed.

A proper alt VR like Balmar MC-614 and any quality solar controller will be very programmable for voltage setpoints, how long to hold Absorb etc.

Ideally use a shunt-based BM / AH counter to allow using endAmps to determine when to call it Full, and only then go to Float.
I would submit that this suggestion misunderstands the purpose of the D250S. (Although it is discussed on page 18 of the manual.)

-- The main reason to use a D250S (or any other B2B) is that your charging system cannot provide the requisite high voltage needed to complete the charge of your camper battery. If your charging voltage is correct, then you are almost always better off with a relay based system. (Or simply using a larger battery - but, for the purposes of this discussion, we will assume that you have a good reason to use an isolating system.) If your charging voltage does not get high enough to complete the charge, then there is no point is connecting the charge source directly to the camper battery - it will never complete the charge.

-- As a practical matter it would probably require a major effort to put the camper battery into the circuit in the same electrical/logical place as the starter battery, if only because there is probably not enough room under the hood for a 200Ah+ battery bank. You could do it, but you would have to run a lot of big wiring to some other place in your vehicle to assure that the computer did not notice the change.

-- If you actually rewired the whole mess, and the voltages were correct, then there would be no purpose served by using the D250S to boost the voltages.

The D250S excels in allowing you to properly charge deep cycle batteries, which want a charging voltage of 14v+, in a vehicle, like some Toyotas and Mercedes, that has a charging voltage below 14v. If, as suggested, you add your own regulator, or boost the overall voltage of the vehicle, then there is no longer any reason to use a D250S.

I would submit that this suggestion misunderstands the purpose of the D250S. (Although it is discussed on page 18 of the manual.)
My comment were addressing the primary/big picture issue, not any specific device chosen, which to me comes after the fundamentals are covered.

I completely agree that many situations are addressed without resorting to a B2B charger.
Adding to this existing thread. My CTEK was really great until recently.

I had some odd flashing light patterns on my CTEK and the solar charging was not functioning correctly. My 250 is out of warranty so I opened it myself. One screw of the six has a little read plastic cap that has to be pried up. The circuit board is pretty complex with numerous discrete parts as well as what appears to be two processors and the circuitry for the DC to DC conversion. Apparently they load firmware into them and a sticker on the bottom of the unit will tell you what revision yours is at. Mine is Rev 50 which apparently had improved MPPT functionality.

I found that the unit has three replaceable blade type 30 amp fuses in line with each 12 volt connector: alternator, solar panel and the "service" battery. The fuse for the solar input was blown and the battery fuse looked suspect so I replaced them both.

The unit's lights appeared to be sequencing normally but battery charging by solar panel seemed slow. It turns out less than an amp of current was flowing from the panel to the CTEK even though the panel could source 3 Amps in bright sun. For those interested the solar input cuts in around 12 volts - this is not specified in the literature for the unit which only mentions the cut in for the Alternator input and the maximum limit of 22V for the inputs.

I opened up the unit again to map out a little of the circuitry. There are high power diodes in series with all three DC connectors and those tested good. Across the service battery positive output and ground is a reverse polarity protection diode Schottky diode (MBR1035 10A 35V) that would short and blow out the inline fuse.

The four power MOSFETs in the DC-DC converter seemed okay. For the solar and service battery connections there appears to be two in-line power MOSEFETs (IRFB3004) that act as switches to select those ports depending on the charging sequence at play.

Both of those FETs appear to be damaged which may explain the low charging current. I've found replacements through Mouser Electronics for about $6 each including shipping. I'll post up again once repairs are complete.
Great ! Looking forward to a repair report.
Surprised to see fuses like that inside. How adequately do you think its mosfets are heatsinked ?
I seen lots of dead stuff with what I consider crap or marginal heatsinking...
Great ! Looking forward to a repair report.
Surprised to see fuses like that inside. How adequately do you think its mosfets are heatsinked ?
I seen lots of dead stuff with what I consider crap or marginal heatsinking...
I find it hard to pass judgement on the heat sink as I don't know all of the OEM parameters. Here are some observations that lead me to guess that it may be okay:

The DC-DC conversion circuits should be very efficient (95-97%) so I would not expect much heat to be generated
(Worst case 20 Amps at 13.6v out is 272 W so power in would be 287 watts for 95% so 15 Watts to dissipate?)
The heat sink is a simple 1/8" aluminum plate cut to fit under the PCB, maybe 36" square
Only 6 MOSFETs share the heatsink; 4 for DC conversion and two as switches
The FET's Rds on is only 1.4 milli-Ohms for the switches so miniscule heat dissipated
There is no airflow and the plastic case presents a thermal barrier to dissipation but still only 15 watts worst case for a limited time, not continuous
Successfully finished the repairs and tested the CTEK and all is good!

I learned more detail about how it works. The main assembly is incredibly complex both electrically and mechanically and it was pretty time consuming to disassemble. I tested with two car batteries and a solar panel and saw up to 2 Amps flowing out of the 50W panel with late afternoon sun. The secondary battery was charged at 14.55 Volts for a while even when the solar panel dipped to 13 volts
The two MOSFETs I replaced are from International Rectifier, Part No. IRFB3004. $4 each. They function as switches for selecting either the alternator input or the solar panel input.
There is temp sensor glued to the aluminum plate that serves as a heat sink for protecting the six MOSFETs; four FETs are for the DC-DC converter and two act as input switches
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