DC-DC Converter or charger or isolated or non isolated...? Which one to use?
Vložit
- čas přidán 28. 06. 2024
- Our solar system on our RV consists of a 24V eco-worthy lifepo4 battery bank that originally was supposed to drive the 12v systems of the RV by the way of a Victron Orion Step-Down Converter but our system evolved when we decided to bring in a small Victron Charge controller so that we could utilize the tiny solar panel Forest River gives you in their Juice Pack system which meant I needed a small 12V battery on the 12v side of the down conversion.... I have struggled through a few problems trying to get my head around this whole change and came to realize that there are a few things to consider when choosing a step down converter.
I realize my theory on what was happening with regards to current directions and electrons ect is probably flawed however my thinking on changing equipment directions was sound. I still believe an isolated converter would have maintained the voltage level at a minimum set point similar to what the charger does in power supply mode but I do live having the added control over things with the charger. At the time of this video being published the new charger has been installed and is working exactly as I hoped for some time now.
As I've mentioned before I'm no electrical engineer but we are happy to have our install working really well finally.
You've got it mostly figured out now. Note that chargers can also be isolated or non-isolated. Isolated is safer. The main safety issue has to do with negative-return paths. Since you are pushing a lot of current, you want to make sure that the returning current on the negative/ground side goes through the correct wires.
In a non-isolated system it is relatively easy to mistakenly wire in additional ground paths (for example, the ground on a data cable going between two different pieces of equipment, or an accidental ground path through the vehicle frame)... such that some of the return current on your larger power cables goes through these other ground paths in addition to the cable you provided, potentially smoking smaller cables that weren't meant to carry that current.
General rule of thumb.. isolated is safer in practice, but also harder to detect mistaken shorts because a single short won't flow any current. For DC wiring, particularly DIY, isolated is safer. Home wiring on the other-hand is not isolated due to the neutral-ground bond at the main breaker box, but GFCI and other similar safety devices will detect that the wrong current path is being taken due to a short and trip.
--
If you use diodes in the future to prevent back-flow, note that you can put several diodes in parallel to share the current and thus reduce the per-diode heat dissipation. A diode creates a voltage drop of roughly 0.7V. A schottky diode has a voltage drop more around 0.3V and is more typically used, but has significant voltage limitations so you have to be sure to get the correct kind (typically 60V schottky diodes are sufficient for use in DC battery systems). A 20A Schottky diode can handle roughly 4A in free air without a heat sink.
The power dissipation as heat through a diode is (I * Vdrop). So the (current x 0.7V) or (current x 0.3V). This is why banks of diodes are typically used for the purpose you tried to use that single diode for. You are splitting the heat dissipation across several diodes.
--
In terms of electron flow, to avoid confusion always draw current direction from positive to negative, even though the actual electrons flow the other way. The convention is that positive-to-negative is the forward direction.
The current going into a circuit is always the same as the current going out of a circuit. The current leaving a battery on the positive terminal is always the same as the current entering the battery on the negative terminal. Electrons are not created or destroyed by electronic circuits.
So basically current is just the flow of electrons. It is literally just counting the electrons that flow through a wire.
Voltage is more complex. Voltage is the measurement of the relative electric field potential between two points. Voltage is NOT the absolute electric field potential (excess or deficit of electrons relative to protons) at a single point. Voltage is always a relative measurement because that is what can cause current to flow. So even if the ground, say, has an excess of electrons, that doesn't mean it has a non-zero voltage. If your body has the same excess or deficit per unit volume that the ground has, no current flows.
Absolute electric field potential can be thought of as "static electricity", but it only matters when the excess or deficit is large enough to bridge an air gap (aka spark, static shock, etc). Thousands of volts are needed for that to happen, and unless there is something replentishing the electrons or holes the current only flows for a few microseconds before the absolute electric field potentials equalize between the two points.
Hence "isolated" circuits are safer.
-Matt
I use an mppt solar controller to do 48v to 12v. Its pretty great, it takes very little parasitic draw to keep the 12v house batteries topped off. Those step down converters are a huge parasitic drain on the 48v system, and they are built like crap. Good mppt solar controllers intelligently stop pulling juice better when the 12v gets full. I think my mppt pulls like 1 or 2 watts once the 12v bank is full. I see no reason to ever not keep those batteries charged as they are thr batteries of last resort.
Im loooking at adding another 48v 100ah bringing me to 200ah batteries and 2kw solar. Currently im losing too much potential power during the day that could run the AC or whatever at night.
Thanks to Matt for his detailed explanation!
I am building an RV system around a 48V battery too, RVs universally need 12V and I found myself with a spare Victron 100|20 MPPT charge controller. I built a test bench copy of my system and included the 100|20 as my 12V charger. This was not possible with other 48->12 conversion devices, on the Victron my wish is their command! Programmed it to charge a lead-acid battery, will change it to LiFePo charging characteristics when I move it to the RV.
I'm seeing exactly your efficiency results, 2 or 3 watts being used to trickle charge the lead-acid, don't know if it will be noticeably less for LiFePo.
Question: Do you know of any downsides to using an MPPT controller in this "ever so slightly" off spec manner?
I have looked for any disparaging info and haven't found any!
Higher voltage in and lower controlled voltage out is what Victron MPPT's do.
I considered that as an option as well
@@georgebissenden9297 we have been out off grid for over a month right now and I've had no problems with that system. I guess the only issue is if The 12 volt battery gets too low to maintain the power to the MPPT controller and it shuts off mine is a cheap one and it doesn't kick back on automatically to charge when it gets power I have to hit a little button. I've never had that happen though, I have run down my 48 volt system down a couple times but the 12 volt is always topped off so everything keeps running overnight on propane and whatnot.
@@vmlinuxz I find with the charger and presumably an mppt as well that on charger mode it will charge to 14.2 but then floats at 13.5 forever after as long as the big batteries have juice which is not exactly helpful since really all it's doing is providing a constant power supply at that point. I may lower the float voltage so the little panel can jump in a bit.