The hurdle with alternator charging is the vehicles voltage regulation is not designed to recharge a battery as fast as possible.
What max voltage is allowed, and how long it is allowed are highly platform specific. But while a battery might be charged from 50% to 100% in 6.5 hours, if brought to and held at 14.5v for that duration, when the vehicle decides that 13.7v is just fine and dandy, well 24 hours of driving at 13.7v will likely not fully charge the battery.
The Ctek and other DC to DC converters are taking whatever voltage is allowed by the vehicle and stepping it up to the Mid 14's. through a boost converter. I imagine it can also step voltage down when it believes the batteru is indeed fully charged.
I am not personally familiar with the Ctek, nor sterling DC to DC charger options nor how they actually perform/endure, or how happy they can keep the house battery in Vehicle X's usage.
My Alternator is externally regulated, and was voltage controlled by the Engine computer. I tricked the engine computer into thinking it is still attached to the field terminals on alternator with a resistor, and use an external Voltage regulator that has adjustable voltage. I put the voltage dial on my dashboard next to my voltmeter/ ammeter. Battery less than 100% fully charged, I choose 14.7v. Battery fully charged, I choose 13.6v. Sunny and fully charged, I can choose 13.5v and let my 200 watts of solar power most of my engine electrical requirements.
Not sure the ratio regarding vehicles with externally regulated alternators vs those that have VR's internal to the alternator, but choosing the voltage makes it possible to recharge as fast as possible, or recharge to as high a state of charge as possible in the time one drives.
So a house battery that is destined to only get charged when driven, is Kind of doomed to early failure, Unless the voltage regulation is addressed and mid 14's are sought and held. Perhaps some vehicles will allow 14+v. Many people will measure voltage right after starting and see 14.3v, dance a little jig, and think that is the voltage always chosen/Allowed by the voltage regulator. It is NOT. It is a matter of time before a drop to the Mid 13's is inevitable, and when that happens recharging slows by ~65%, and that is when fat cabling is used for minimal voltage drop. Mid 13's and 32 feet round trip of 10 gauge wire and you might as well take your house battery to the corner of the yard and shoot it with a scattergun.
So preventing more voltage drop by thicker copper is a big consideration, and not just because high amps at relatively low voltage require thick copper for safety. The voltage received at the battery terminals plays a HUGE part in how much the battery will ask for from the charging source, and minimizing voltage drop to house battery at both high amperages and lower amperages is important for recharging performance.
While batteries are to serve us, they cannot do so for long if not properly fed. 100% recharge need not happen every recharge, but it needs to happen every so often. Aiming for only 85%, and continuous cycling from 35 to 80% charge range will have a battery performing horribly, suffering unrecoverable capacity loss within a few dozen cycles and perhaps complete failure after as little as 150 total cycles. When the 100% recharge is then attempted on such an abused battery, It will take 12+ hours when brought to and held at 14.7v, and if the battery had 100AH capacity when new, One should expect a percentage well below this after such abuse of partial state of charge cycling.
Scoring a 90% on a test in school is damn good. Recharging a battery to only 90% time and again is Not. Without the occassional 100%, premature failure is a guarantee. Is premature failure acceptable? Well that desicision is yours. How premature will it be?...., Impossible to say without knowing the depth of discharge, how close it is returned to 100% and the number of partial state of charge cycles.
Solar and the PNW.... Well it seems that many believe that it absolutely never ever gets sunny there and therefore solar is a complete waste and therefore a possibility to be ignored. Except that rare sunny day can accomplish the true 100% state of charge that the vehicle, with its shitty voltage regulation, never could on its own. The occassional sunny day, even in the pacNW can easily double battery longevity. And while cloudy might have output at a fraction of a sunny day, the lesser output is at least mitigating some of the loads and keeping the battery from falling further that it otherwise would.
The Cheap 85 amp continuous duty solenoids, well I recommend against them for battery charging unless one Knows for sure the contacts within it have not fused together. When they do, the solenoid will still make the right noises, or very close to the right noises when triggered or let go, but there will be NO isolation with engine off and house and engine batteries will power house loads, and engine starter, and this usually kills the engine battery/both batteries quickly.
The cheap CDsolenoids should be considered a wear item and long before it completely fails, it will be causing excessive resistance in the charge path, slowing battery charging. The 200 Amp CDsolenoid with silver tungsten internal contacts might be 3 to 4x the price of the cheapest possible 85Amp CDS, but it also will last much longer, and perhaps save the user from being stranded with dead house and Engine batteries that both need to be replaced, along with the failed cheapo solenoid that killed them.
More Modern Vehicles are doing everything to get their Vehicle fleet MPG averages as high as possible, and unloading the alternator at the expense of the battery is one way they can eek out slightly higher numbers. The other way is with thinner oils, and reducing weight. And copper is heavy, and the original charging circuit is barely adequate for a fully charged engine battery and all the electrical loads possible as the vehicle left the factory. So this is the main reason Why I recommend taking power right from alternator(+) stud rather than from engine battery (+). But the vehicle's voltage regulation is still a big limiting factor.
So what to do? Raise the pressure, through a Ctek or equivalent DC to DC converter, or figure out a way to manipulate the voltage the vehicle allows its alternator to seek.
Or Shrug, say it is what it is, and just replace batteries more often.
When this gets to be annoying, especially in that period right before battery replacement, when one does not know if they will have enough power for their wants/needs, one then might decide to recharge the battery better to get more life from it, and find themselves right back at a thread like this. Especially if failure was extremely inconvenient and one had to pay a premium for a lesser battery as that was all that was available.
With a DodgeVan , from 1988 to 2003, one can do what I did regarding tricking the engine computer and adding an External voltage regulator as described in this thread:
https://vanlivingforum.com/Thread-Your-Vehicles-voltage-regulator?highlight=voltage+regulator
With this modification, I can pretty much always charge my AGM battery at the very limits of its acceptance when driving. Idling hot is the exception, as my alternator is only good for 50 amps at idle, and 8.2 of those are required to run the engine at idle, but I will not idle just to recharge either.
My Northstar AGM battery has no issues accepting huge recharging currents, and actually is better off when charged regularly at rather extreme amperages. While most batteries can accept huge amperages when depleted, it is not so good for them to do so often. Lifeline, Northstar and Odyssey AGM's can all accept huge amperage numbers and are better off for it.
But other flooded and the less expensive AGM batteries.... is it worse to slow charge them and let them live their reduced lifespan at a lower average state of charge, or hammer them with huge amperages while the vehicles voltage regulator still allows electrical pressure up in the mid 14's? Hard to say, but I am in favor of hammering them with huge amperages, as long as their temperature is not allowed to exceed 110F.
The cheaper AGMS say to limit amps to 30 per 100AH capacity and this is also likely a good limitation for flooded batteries too, but honestly those numbers with stock vehicle voltage regulation would likely rarely be attained, and rarer to be exceeded, and if so, only briefly before the premature float voltage of 13.7 or less is initiated by a timid voltage regulator.
High amp recharging a high temperature battery should be avoided. attaching a temperature sensor to my battery and observing it recharged at 40 to 65 amps was surprising just how much temperature rise there was, and this is a low resistance AGM battery that will heat less than lesser AGMS or flooded batteries.
Well Depleted batteries in engine compartments crossing the desert in summer should be avoided, and even if the house batteries are in a 95f degree van, they will heat up surprisingly quickly when recharging, even in the later stages of absorption when they are accepting relatively few amps.
Everything is a trade off, a compromise.
My personal system is now achieving excellent battery longevity, but ALL my charging sources can have their voltage manipulated either by twisting a potentiometer, or pressing some buttons on the solar controller. I can also plug in when I have gone too many cycles without either a High amp recharge and or a full 100% recharge and this ( plugging in to achieve a true 100%) is largely responsible for me achieving excellent battery longevity. I would have to cut way back on laptop time, and drive way more in winter, for my Alternator and solar to be able to accomplish that. From Equinox to equinox over the summer my Solar alone can achieve and hold absorption voltage long enough, but my AGM needs the occassional high amp blast when most depleted, before the sun has had a chance to partially fill it. Presunrise 80 alternator amps + solar the rest of the day makes my battery very happy. Solar only slower to full daily, less so.
With my AGM battery I determine full charge when amps required to hold absorption voltage of 14.7v at 77F, taper to 0.4a or less. This would be 0.5% of the battery capacity( 20 hour rate)
The temperature compensating hydrometer is great for flooded batteries, but one can also use this amperage at absorption threshhold to accurately estimate full charge.
Generally with flooded batteries when they can accept 2 to 3% of their capacity(20 hour rate) at absorption voltage, they are in the 100% charged range, but double check with the hydrometer every so often as it will change as the batteries age. As they age they will use more water and require more time at absorption voltage. If they do not get more time, they age faster and faster.
While Voltage plays a huge part in recharging, a simple voltmeter alone is not so good at determining state of charge either when charging or discharging, or within several house of having done either.
A battery accepting 50 amps at 14.7v is nowhere near fully charged, but a battery accepting only 0.8 amps at 14.7v is, or very nearly is, fully charged.
So seeing 14.7v, without knowing how many amps are flowing, well, one is not seeing very much and could easily jump to very incorrect conclusions.
Get an Ammeter.
This one is not extremely precise with a resolution of only 0.2 amps, but requires no shunt. Slide a single battery cable through the sensor.
https://www.amazon.com/bayite-Digital-Current-Voltage-Transformer/dp/B01DDQM6Z4
Minewas initially located as to display total alternator output, but I recently moved the sensor over to a battery cable to display amps into or out of battery. Knowing how many recharging amps are flowing at a given voltage, is Extremely enlightening.
The Amp hour counting monitors are great, but the person with an Ammeter can develop a great idea of state of charge by the amps flowing at the voltage allowed/held.
The person who can control the voltage when charging with an near unlimited charging source, like an alternator, has a great level of control.
But 80% to 100% takes time. About 4 hours, no matter how powerful the charging source seeking voltages in the mid to high 14's.
Solar has the time, if not every day, then at least occassionally.
Seems the only people that complain about solar are those who park under trees always, or never set up their system correctly in the first place, or those who believe that their climate is too far north with too many ever present clouds and decide against it entirely. I can't imagine still having my batteries discharging when the sun is above the horizon. And worse, still discharging!!
Do not underestimate the joy, even of only occassionally, in getting a battery to 100% state of charge, silently, with a fridge filled with 32.5f beer.
Also while a compressor fridge might draw 3 to 4 amps when the compressor is running, the compressor does not run 100% of the time.
My 1.8 cubic foot Danfoss powered Vitrifrigo fridge, with extra insulation and a very well ventilated condenser, is averageing 0.62AH consumed each hour as I use it with low 60's overnight lows and 75F afternoon High temperatures.
My laptop consumes 3 to 4 AH each hour and more if I am streaming videos.
I only have one AGM battery, for House and engine starting duties. It has about 500 Deep cycles on it now over about 39 months of use. I will use as much as 65AH overnight, as little as 25AH, with average about 40AH. It is only 90AH total capacity, and that was when new.
Still performing well in terms of voltage held overnight, and can easily start my engine in the morning when depleted 65 of its 90AH. Frankly I find the battery to be damn impressive, and it is all because I can charge it properly. And that means high amps regularly, and to a true full often with whatever sources I have available.
