Annie C
Keep it simple, for me.
Yes, the drop by 50% when solar is detected is correct
DC-DC charger (B2B) options and general discussion
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DerpsyDoodler
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I suspect they do it as a safety precaution to make sure their device isn't responsible for exceeding the charging current rating of a given battery.
DerpsyDoodler
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If you were to charge a 100AH battery at 90a you would probably be exceeding its rated charge current. BMS should protect from improper setup or error events. That doesn’t mean mfgs should make stupid devices. Anyway, what I suggested is purely speculation. Could be that they cheaped out on parts or something. I don’t know why they did it.
Annie C
Keep it simple, for me.
I am adding solar so I am counting on that drop to have less stress on the alternator/starting battery and the wires already in vehicle. Only have a 90 amp alternator.
Yeah, it's more expensive to build a device that can handle 100A than 50A. That's not really cheaping out, that's making a 50A charger instead of a 100A charger.
trevor.the.van
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There are some great resources on grounding on the forum. Check this one out here:
Grounding Made simpler - Part 4: Mobile systems.
To get the paper, click on the orange button at the top of the screen. The subject of grounding is a complex, multifaceted subject, that is often treated as an after-thought but needs to be considered from the beginning of the design and build...
diysolarforum.com
That's not how this works. Two things control power levels. Source voltage and load impedance (Ohms power law).
It's not like the battery is an decrepit dam, threatening to burst, inundating everything downstream with a huge burst of power.
I have over 7000 WH of batteries (25.V at 280AH) in my van, these would only be limited by circuit breaker or fuse if a load ever shorted out. Think serious arc welding current and the limit of concern is that appropriately sized wiring, circuit breakers and bus bars are connected to the battery. Everything on the other side of the bus bars is individually fused and wired based on that fuse size.
The problem isn't the source, its the load. So when a load claims it is rated for XXX A, what that means is you need to be sure everything feeding it (power source, wiring, fuses etc.), are sized to work with that load. Not the other way around.
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I would suggest you use an appropriate fuse (based on the size of your wiring) located as close to the source as possible. I would also add a circuit breaker or fuse to the output of the converter as well. Circuit breaker doubles as a service disconnect which is handy.
I am planning on using a 50A Short-Stop breaker for alternator power. It is just to protect against a shorted wire. It has to be something sealed since it will be exposed to the weather.
I am using a 30A DC toggle circuit breaker on the output side of the charger.
Wiring:
I have basically decided to use ABYC rated parts as much as humanly possible. Mostly Victron and Blue Sea.
Isolation for the purposes of ensuring the starter battery doesn't get drained is a totally different issue than galvanically isolation. Preventing starter battery drain is much simpler than accomplishing galvanic isolation. That is why the isolated Victron Orion TR versions cost more than the non-isolated versions. That and people will pay more.
Galvanically isolated is a nice thing to have, especially if you are charging house batteries in a trailer from the tow vehicle engine. The issue relates to how the house battery is grounded to chassis ground when the tow vehicle is disconnected. All in one RVs and vans don't have this issue (probably) so I would not expect them to need galvanic isolation.
DerpsyDoodler
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I didn’t think the batteries were a decrepit damn. I thought the alternator would pump out amps and could overload the input rating of the dc dc charger. I did not understand how the technology worked. I’ve since learned otherwise.
DerpsyDoodler
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I am performing a bus conversion (99 MCI) and am curious to learn more about this topic and how it applies to me. You say it (probably) would not apply in my situation. Can you elaborate on why?
If you are in a van or bus, then you should connect your house battery negative to the starter battery negative and let the existing negative to chassis ground connection built into the existing bus wiring take care of that for you.
The reason why is you only want to connect the house negative bus to chassis ground at one point. Otherwise you risk passing significant current through the vehicle chassis which can cause corrosion and other problems. It does mean all your wiring needs to be in pairs (positive and negative). That is why I use this stuff for all my DC wiring, except for the battery/inverter wiring.
One other reason to use galvanic isolation is if you have a positive grounded electrical system. This typically is only found in old equipment, but never say never. Hopefully nothing made in the last 50 years has a positive grounded electrical system.
DerpsyDoodler
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This is in my plan, not that I knew all the reasons why, just that circuit best practices call for it.
I understood the possibility of ground loops, but didn’t understand the implication of galvanic corrosion. Thank you!
no worries there. It is a negative ground chassis system. Are there any concerns about the chassis system being FLA and the house system being LFP? Do I need to take any other precautions?
Are there any other reasons I might want to get an isolated dc dc charger as opposed to a non-isolated one?
The DC-DC charger will take care of the battery chemistry. I will be using the ignition switch input on my DC-DC charger. That way I don't have to worry about running down my starter batteries.
For a van/bus, non-isolated is fine, just run both positive and negative wires from the starter system to the DC-DC charger and don't connect the house negative to chassis ground.
If you already have the batteries installed with the negative connected to chassis ground then you might want to go with an isolated DC-DC charger just to simplify the installation (no sense having to rewire stuff that already works).
DerpsyDoodler
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i think the redarc is the only one i‘ve seen to do 24v-24v dc, so far. i dont recall if its auto sensing or if i will need an ignition lead.
i thought house negative should be attached to the same chassis ground that the chassis battery is connected to. that was my understanding and i thought you also confirmed that in your earlier post.
i havent started wiring yet. just now working on charging and capacity testing the cells. still working out some of the research. still need to decide on some cabling. im finally putting together a system diagram, though. small steps, but forward progress.
Victron has two 24-24 chargers. The amps aren't very high though.
it is hard to find a good high amp charger for 24v to 48v
DerpsyDoodler
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Ehh. I want 50a, but would settle for something in the 30s, and I really don’t want to pay victron’s premium. If I’m going to double up in parallel I’ll get something like chargery’s ac/dc to dc converter. In dc dc mode they will only do 20 amps, but are beastly chargers when you connect them to AC. at $350 each I would not mind having two of them. Wired with a quick disconnect, I can easily remove one from the RV for use in a shop and leave the other wired to the non inverter-powered side of a distribution panel where the unused leg of a 240v 50a shore junction box will feed.
DerpsyDoodler
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Perhaps a buck boost or stepup converter with an isolator? I looked into that DIY route, but with my limited electrical and electronics engineering knowledge, didn’t think it was wise to go that route in a [not rush, but] time-constrained time-line. I feel that would require a different level of knowledge or experience that I lack; and, while I can do the studying and learn what I need to learn, it would take away from the time I have to learn about the other things I need to learn for completing this project, and I do not want to have to do this type of learning in a rush. Well that was a mouthful. Do take a breath if you read it out loud. ? anyway, I’m curtailing my scope where I can and as reasonable.
edit: when you don’t know what you don’t know, and are afraid of the time consuming exhaustive search for caveats...ooor taking on the diligent studies of rsd academy and hope the time investment results in the knowledge necessary, or do further research into what exactly will provide the knowledge necessary...ehh, that all involves times too. Soooo, KISS dictates simplify (In my case).
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