DC-DC converter for batteries connected in parallel 2

[TheDamian58C]

I'm designing a range extending trailer for electric vehicles. I want to connect the battery from the trailer in parallel with the battery of the car, where the battery from the trailer would be connected on the connection between the car's battery and its load/controller. In order for the trailer's battery to match the car's battery, so the rest of electronics in the car get appropriate current and voltage parameters, I thought to put a DC-DC converter between the trailer and the car. There would also be a diode, so the car's battery doesn't charge the trailer's battery in case the car's battery is more charged, has more power etc. The trailer's battery pack would be approximately 50kW.

My questions are: is there an adjustable DC-DC converter that could do the ask described above?
is this idea even viable?

 

[waross]

Probably, if you are willing to bear the cost.
It may be simpler to use the proper voltage batteries in the trailer.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[IRstuff]

50 kWh? Have you calculated how much drag load the battery trailer imposes on the vehicle? Pulling a trailer does not come for free.

TTFN (ta ta for now)
I can do absolutely anything. I'm an expert!

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[TheDamian58C]

I forgot to mention that the input voltage would be around 69V or 207V, dependent on the configuration of the battery modules (3s3p or 9s1p). That's another thing I would like to ask, does it even matter as long as cables are thick enough? which one would you recommend?

It may be simpler to use the proper voltage batteries in the trailer.

The problem is that different EV's have different batteries with different voltages, I want this to be universal. That's why I'm asking if there's an adjustable DC-DC converter that could do the task. By asking 'if there is' I'm also implicitly asking whether you have any recommendations.

Have you calculated how much drag load the battery trailer imposes on the vehicle? Pulling a trailer does not come for free

I know. It should be approximately additional 400kg-500kg, with everything (trailer, battery, charger etc.) considered. The energy should overcompensate for the 'drag', and actually it seems to be the case that the bigger the battery the better compensation, due to the fact that more mass is responsible for energy storage. Those 50kWh are due to constraints of the trailer, and I simply don't want it to be too heavy. Anyway, I'm mainly asking here about DC-DC converter, and whether the electronic side of the project seems sound. If you want to discuss the other aspects of the project, just DM me but I don't know if it works that way on this forum, I'm sorry I'm new here.

 

[waross]

You may find better support at our sister web-site;
Makerojects

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[TheDamian58C]

You may find better support at our sister web-site;
Makerojects

It seems like that site is mostly to share your projects, I haven't found any forum on that site where you can ask engineers for help.

 

[itsmoked]

There is nothing off-the-shelf to do what you are asking. It would be a considerable electrical design effort to arrive at that too. CONSIDERABLE! You'd likely need a bunch of expensive approvals too. Tapping into the DC power side is not trivial either. Your diode isolation scheme is fairly power wasteful too.

The DC connection aspect will always be problematic due to the fact that the two battery systems, even if basically matched, will always be at different states of charge and hooking the two together with even a measly 1 volt difference could result in hundreds of amps flowing between the two.

I suggest you consider and research a different tack on this. Configure your trailer pack to instead provide "charging" of the vehicle and you completely side-step having to match anything at all.

Your trailer becomes a 240V single phase charging source. Then your "converter" is just a standard off-the-shelf inverter always putting out 240V at some amperage. All the vehicles are designed to take in 240V in their higher speed charging modes. Your pack will always be the correct voltage and you completely avoid hooking into a vehicle's proprietary DC bus.

Run a 48V pack into the inverter. Done right this also becomes a spectacular house power loss backup system.

Keith Cress
kcress -

http://www.flaminsystems.com

 

[LionelHutz]

The only DC connection point on a EV is meant for fast charging with an appropriate charger that does all the proper battery monitoring as it charges. There are standards with lots of information on how that kind of charger needs to work, which your converter would need to follow if it was to tie into that port. No half-assed DC-DC converter with a diode cuts it here.

 

[TheDamian58C]

So what you're saying is that I should rather connect the battery from the trailer in between the inlet and the charger of the car? Obviously with a DC/AC inverter between the trailer and the car. I considered that solution, but I though that would be more power wastage.

As of a DC/DC converter that could potentially do the task, I found this one:

https://www.zekalabs.com/products/isolated-power-converters/dc-dc-isolated-converter-15kw-450v

 

[waross]

Go back and reread the suggestions you have been given.
When a battery is being discharged, the internal resistance causes a drop in the terminal voltage.
A battery will have a characteristic Volts/Amps discharge curve.
When two sources are connected in parallel, they will share the load depending on the respective curves of the sources.
You may expect either the EV batteries or the inverter to "Hog" the load.
Have you factored in the pump, fan and radiator for the liquid cooled condenser?
Am I wrong?
You have dismissed Makerojects as a resource.
You want to have the advice of Engineers.
Engineers have given you good advice.
You have rejected that advice and are still advancing your pet theory.
Why are we wasting our time?
Go back to Makeroject.
This site is for professionals.
ps: RF
Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[TheDamian58C]

So many wrong assumptions here. I've asked itsmoked if I understood his suggestion correctly and I linked a DC/DC converter as an answer to the first paragraph of his reply, assuming that may be a thing that he might not have taken into consideration. I'm very grateful for his answer, as his answer is the most clarifying one. I haven't dismissed Makerojects as a resource, it just doesn't seem to be a site to ask questions regarding your project. I haven't rejected any advice and I replied to most of them. How am I advancing my 'pet theory'? by asking questions? answering them? trying to justify my decisions?
You ask why are you wasting your time, guess what? Not answering is also an option. It's your choice how you use your time.

 

[waross]

itsmoked's suggestion started and ended with a DC to AC inverter.

There is nothing off-the-shelf to do what you are asking

As of a DC/DC converter that could potentially do the task, I found this one:

There is nothing off-the-shelf to do what you are asking

No half-assed DC-DC converter with a diode cuts it here.

disclaimer: You may not have seen this post when you posted your response. We sometimes get overlaps when posts are close together time wise.
You can use a DC to DC inverter.
That will probably use an inverter to AC.
The AC will then may be boosted with a transformer or voltage doubler to a higher voltage AC.
The AC will then be rectified back to DC.
Now you have a DC supply at the proper open circuit voltage.
The next issue will be to modify the discharge curve so that you may draw from the source that you want. Not impossible, but you wanted simple.
It may be difficult to do that with less losses than a simple DC to AC inverter.
Wait, I forgot the liquid cooling.
You want simple:
Simplest:
Forget universal. Consult with vehicle manufacturers as to the possibility and proper procedure to connect additional batteries in parallel.
You want simple and universal: itsmoked has answered that question. As well as simplest for a universal solution it may easily be the cheapest.
Universal? The DC to DC solution with a circuit to adjust discharge curves may have to be adjusted or modified for each different model of car. There goes universal.

By the way:

I suggest you consider and research a different tack on this.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[LittleInch]

You need to find out how that dc-dc converter you found actually works.

I suspect it is a single unit which goes dc to a/c then a variable lift to a different ac voltage then back to a/c again.

Now if it can vary the ac to ac lift then it could maintain a fixed supply into the vehicle by gradually drawing down the input batteries - It's rather vague in the way it advertises itself.

SO I assume this is based on the principle that you drive into a charging place, swap the trailer for a fully charged version and then drive off in a few minutes while the discharged one is then slowly charged waiting for the next car.

Or you clip this on as a range extender when you need long distance but forget it when you don't?

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[TheDamian58C]

I know what itsmoked's suggestions said. I asked him whether the AC/DC inverter should be connected between the car's inlet and charger/rectifier. I just didn't scrap the idea with DC/DC converter at that point. I didn't say it absolutely needs to be DC/DC converter in the answer to itsmoked. I don't get why you're answering me in such a mean way, like to a dumbass. I'm just asking kindly for suggestions. Liquid cooling is a separate issue (which I think I got covered), all I'm asking about is solely the electrical aspect of things. If I come across as ignorant or mean, then I'm sorry, I don't mean to.

It seems like most of you seem to not recommend the DC/DC converter route, and I get that. I just want to explore the possibilities. If it's possible with DC/DC converter in a convenient way then I want to know how. I also want to know how to connect the AC/DC inverter to the car or rather if connecting it between the car's inlet and charger is the way to go. I'm open to explore new ideas.

SO I assume this is based on the principle that you drive into a charging place, swap the trailer for a fully charged version and then drive off in a few minutes while the discharged one is then slowly charged waiting for the next car.
Or you clip this on as a range extender when you need long distance but forget it when you don't?

Actually both, you'd swap for the next one while the other is charging, or rather rent one when you go for a longer trip and swap for another if you're out of range. This could also be bought, so you'd clip it on when going on a longer trip, otherwise it would stay in your garage.

 

[itsmoked]

I should rather connect the battery from the trailer in between the inlet and the charger of the car?

Yes. This would likely require no interaction with any of the car's systems, avoiding approvals.

The Zekalabs things may be usable in your vision. Ask them. Get a feel for the cost. I suspect it's crippling. It also requires modification of the vehicles systems - a large hurtle.

Keith Cress
kcress -

http://www.flaminsystems.com

 

[TheDamian58C]

Yes. This would likely require no interaction with any of the car's systems, avoiding approvals.

Thanks a lot! You gave me the exact answers I needed. It seems like I will go with a AC/DC inverter then.

 

[LionelHutz]

You have already posted about being a student so you didn't read or follow the rules when you signed up.

I'm curious if you've even researched the charging standards for EVs?

You have yet to explain how you expected to connect this trailer to the EV batteries. Some EVs have the charging port on the front fenders and in the center of the grill. That makes it rather difficult to get a cable routed and connected to it.