DC to DC chargers, do you really need one?

[El Gringo]

Saw the arguments on the other forum, thought at the time that you were just bashing your head on a brick wall and wondered why you even bothered...
However this condensed explanation will go in my file of technical info that I keep for my own use when needed.

Thanx for taking the trouble, look forward to any/all installments.

Cheers.

[supersparky]

Keep them coming, gives me a lead on what to put in the second chapter so I know I'm heading in the right direction. No point in me rambling on about what I think you want to know and sending every one to sleep

T1, I don't think that you are putting anyone to sleep, probably the exact opposite. There are probably a few members out there going - aah so that's how it works!!
Keep the info coming.

[Wilbor]

Ditto

[T1 Terry]

OK, part 2, for those just coming out of a coma, best make yourself comfortable this time, face down on the key board is not a good look If the first episode didn't make your eyes roll back in your head, this one probably will.

A popular comparison is the water analogy, it doesn't work exactly like electricity but it can help in a generalised way, but you need to understand the different electrical terms in relation to how they would relate to water flow etc.
Volts is like pressure, the more volts the higher the pressure so the more that will flow through a cable or pipe of a certain size. Amps or current is like flow, the more current flowing the more amps moving from one place to next or water moving from one place to the other. The pipes are like the cables, if the pipe is small not a lot will flow through without a lot of pressure behind it.
So let's look at the start battery and house battery and compare it to a tank supplying water to another tank. If the top of the 2 tanks are the same height above sea level with no restrictions the water level will be the same between the 2 tanks. The same would go for the start battery and house battery, as long as there is no restriction the voltage will be the same at both the house battery terminals and the start battery terminals. Even though the voltage is the same that doesn't mean the same state of charge has balanced out between the 2 batteries like the water between the 2 tanks.
This is where the analogy starts to fall down, the battery has an internal resistance that must be over come for amps (current) to flow from one battery to the other, maybe adding a spring loaded non return valve at the bottom of the second tank and a spring loaded non return valve at the outlet of the supply tank will make a sort of comparison. Even though the water is in the first tank, unless the pressure is enough to push past the first valve against the spring and then past the second valve against the spring, no water will flow into the second tank. When some water makes it into the second tank it will add weight to the spring loaded side of the ball, so more pressure is required to push the water past the ball valve.
Now to compare this with charging from one battery to the next, the second battery or house battery is seen as a load by the start battery. As soon as amps (current) starts to flow out of the first battery to supply the load its voltage will drop, the greater the flow the greater the drop, just one of those battery things and is caused by the internal resistance of the battery. The opposite happens at the second battery, the faster the flow into the battery the greater the resistance so the higher the voltage needs to be to over come that resistance. That means the voltage going to the second battery must be higher than the voltage that is already across the terminals of that battery, like the water already in the tank holding the valve shut.
What does all this mean? Just because the voltage at the start battery is 13.6v and the voltage at the second battery is 13.6v doesn't mean the second battery is being charged, the voltage needs to be high enough to over come the internal resistance for any more charging to occur. As soon as amps (current) start to flow out of the start battery into the second battery the start battery voltage will drop, the second battery is applying a load, this new lower voltage is unlikely to be enough to keep the current flowing into the second battery so charging stops till the start battery voltage increases.
I'll try to create an example that may help.
If the second battery is deeply discharged to say 11.8v and the start battery is at 13.8v then there is a 2v differential, certainly enough for amps (current) to flow from one battery to the other...... but then the voltage is the start battery drops a bit, the alternator tries to keep it charged but there is a slight drop just the same to maybe 13.6v, the cable between the start battery and house battery starts to warm up a bit, so the resistance increases and that drops the voltage a bit by maybe 0.5v, at the same time the house battery bounces back up to 12.4v or higher because it is charging and has to over come the internal resistance, now the supply voltage is only 13.1v when it reaches the house battery so there is only a 0.7v difference between the supply and the house battery.
Here is the catch, a 12v lead acid battery there needs a 0.8v differential for charging to continue, so the terminal voltage will climb to 13.1v but without any current flowing into the battery, it just isn't charging until that 0.8v differential is reached and unless it is exceeded by quite a bit it will only charging very slowly, the trickle past the valve.

Now add the DC to DC charger, this is like a pump between the first tank and the second tank, the DC to DC charger pulls as much amps as it needs to push out the 20 amp or 40 amps the DC to DC charger is designed to pump out and hold 14.4v if it can get that high without the second battery (house battery) letting it flow in. The result is much faster charging and charging much closer to 100% full.

Tomorrow I'll explain why there needs to be more amps (current) coming into the DC to DC charger than what is coming out, it is not because the DC to DC charger is wasting all that energy, it's all to do with that evil watts word.

[El Gringo]

We need a Like button on this forum..

[Dot]

We need a Like button on this forum..

Nah cuz then there would be those that then say "there should be a dislike button" Just come out and say what you think (within reason)
Ask Terry if he likes the raw prawns

[SteveW]

My screen timed out while I was studying the content - so I re-applied voltage and now I understand Part II.

[supersparky]

T1, Your explanation is brilliant. It is almost exactly the same as the way that we were instructed many moons ago as first year apprentices.
Keep it coming.

[bagmaker]

prolly a much more important topic to caravanners as apposed to Motorhomers due to the position of the different batteries.
The explanation might go well with information about the appropriate electrical connection between tug and van, physical position of DC to DC charger, differences in wire size either side of the DC2DC and why that would be so much cheaper etc.
Education is the answer to all the problems, timing is the stopper

[bagmaker]

prolly a much more important topic to caravanners as apposed to Motorhomers due to the position of the different batteries.
The explanation might go well with information about the appropriate electrical connection between tug and van, physical position of DC to DC charger, differences in wire size either side of the DC2DC and why that would be so much cheaper etc.
Education is the answer to all the problems, timing is the stopper

Edit, duh, came into the conversation a page late sorry