DC-Dc converter question

[phillipschip]

I have a quick, hopefully not stupid, question about a possible Dc-Dc converter solution for my build:
viewtopic.php?t=5590
I have a total of 10 36v modules that I have wired up with individual bms components (balancing/charge protection/low voltage alarms/temp sensors). Each module will be individually charged with separate 10a/240v chargers. (I will also have a 12v/5a charger for my 12v inverter battery). They are divided into 2 boxes, 5 modules each. Could I combine all the + and - wires in parallel for one or both boxes and use a 36v-12v Dc-Dc converter for my accessories? The total amp draw is around 7-10amps with everything on. My build is very simple and I am trying to keep weight (and cost) as low as possible.

[Aragorn]

You cannot have a battery connected in series and parallel at the same time. That would create a dead short and associated fireworks.

I presume your 10 modules are connected in series to power the main drive inverter...

It would surely be far more space efficient to use a single charger/DCDC combination unit like the MG or Outlander parts, than to somehow package multiple 10A chargers for each module?!

[phillipschip]

Yes, that makes sense. I could have figured that one out pretty easily had I drawn it out :0 I guess there would be no way to combine the draw of a 36v to 12v dc-dc across the whole pack. It might work to combine every other one in parallel? 3 out of 5? Otherwise, back to looking for an affordable 400v-12v then, or just using a larger 12v battery and charging every time I plug in and charge HV.
Does the MC or Outlander need some type of communication with BMS? I don't have that option at this point. Most of these units do and that is the reason why I have gone in the direction with the individual chargers. They take up a bit more space, but weight is only 12bs and cost was under $300 for all of them. I'm sure I'll change it at some point soon down the road.

[johu]

The Mitsubishi charger/DCDC is even less than $300 and the install will be much cleaner.

I have used a Meanwell PSU as DC/DC converter in the past (the 230V AC units run fine on 400V DC) but it wasn't all that fun.
a) it consumes energy from the 12V battery while turned off
b) So you need an extra relay to disconnect it when the car is off
c) It draws excessive current through the precharge resistor even with above relay turned off and thus cooked like 3 hefty precharge resistors
d) so you need another HV relay to only turn it on when the main DC relay is closed

it's messy. Better roll with something built for the job

[phillipschip]

That Mitsubishi unit looks like a good option but would it be possible to use with my individual 12s bms’s? I think in order to use that I would need to implement a larger much more expensive bms with Can communication etc right?

[arber333]

The Mitsubishi charger/DCDC is even less than $300 and the install will be much cleaner.
....
Better roll with something built for the job

Here you have all Mitsu CAN talk described. I use this now with my Mazda MX-3 conversion and complete system is really easy to setup with CAN. It doesnt require any CRC or crypto protection. Component reports are regular and usable for your VCU setup.
viewtopic.php?t=4307

Dont forget there is 12V DCDC converter AND HV charger in one Mitsu casing. If you would want to double on charging it has been done...
https://leafdriveblog.wordpress.com/202 ... l-part-ii/

Then there is Tesla DCDC unit if you already use the 3ph Tesla charger...
https://leafdriveblog.wordpress.com/202 ... converter/

[arber333]

That Mitsubishi unit looks like a good option but would it be possible to use with my individual 12s bms’s? I think in order to use that I would need to implement a larger much more expensive bms with Can communication etc right?

How do you intend to use your 10x 36V modules? This would imply you would need 10x 10S BMS modules? Can you elaborate on what kind of BMS you use? Do you have any communication between modules themselves? You use any central VCU or BMU unit?

In order to use Mitsu componenets You dont need any BMS in the code.
It is a good option though to have a dead mans switch setup on your CAN VCU setup. This one would trip a pin which would remove a CAN telegram ID 0x285 from the loop. This would essentially render all Mitsu components inoperable as that telegram is what Mitsu uses to protect from battery and BMS failure. Its all in my previous article.

[phillipschip]

Yes, but 12s because I am using lifepo4. Modules are more like 39v. I have access to cell info for each module via Bluetooth.
Each bms is wired for charge protection and balancing. I also have low voltage alarms on each cell group, and temp sensors for each module. My individual chargers should keep each module in balance. I know it’s pretty janky but I’m just learning and right now I’m doing it in a way I can understand and afford

[phillipschip]

With individual bms on each of my 10 modules (each one wired for full 43v cutoff and temp protection during charging), would I be able to implement the Mitsu charger converter somehow? I don’t have any way to send can messages either.

[arber333]

With individual bms on each of my 10 modules (each one wired for full 43v cutoff and temp protection during charging), would I be able to implement the Mitsu charger converter somehow? I don’t have any way to send can messages either.

Well then you should get one VCU controler that would be able to at least send CAN telegrams on one channel.
You could use one of digital inputs to sense for the BMS signal if needed as well as for the EVSE signal for charging.

There are several possibilities. Let me check if i have any of ESP32 VCU left and working.
https://leafdriveblog.wordpress.com/202 ... esp32-vcu/

Then there is Teensy VCU from aot93
viewtopic.php?t=2167

And sort of VCU with Arduino DUE
https://leafdriveblog.wordpress.com/202 ... it-module/

[phillipschip]

Ok thanks! It’s way over my head but I could learn right?
Which unit would be most affordable and user friendly?

[phillipschip]

I don’t see how I could implement a charger like the Mitsu with the bms setup I currently have. I have the individual bms’s wired for charge protection only-not discharge. (Negative on batt connected to bms/other wire (c minus)connected to negative on charger/positive of charger connected to module positive. I could send the 12a of charge power through the HV, but the bms’s would be bypassed.

[arber333]

I don’t see how I could implement a charger like the Mitsu with the bms setup I currently have. I have the individual bms’s wired for charge protection only-not discharge. (Negative on batt connected to bms/other wire (c minus)connected to negative on charger/positive of charger connected to module positive. I could send the 12a of charge power through the HV, but the bms’s would be bypassed.

I dont follow... can you draw a schematic of your battery? And put a contactor, charger and inverter in it.
Then think on the fact you either need to trigger main contactor before charge or you keep charger/DCDC connected to HVDC always...

Edit: Mitsu charger will start to work from 240Vdc-ish up and will stop after 396Vdc.

[phillipschip]

Ah, ok. If it turns itself off it would work. There would be two problems though. First is I’m using 10 modules 43v fully charged so 396v wouldn’t fully charge the pack. Second problem would be keeping all 10 modules in balance with each other. Each has a bms but there isn’t any communication between those. I could occasionally individually fully charge each module if needed I guess. I’ll work up a diagram if still needed. Thanks for the help!

[arber333]

....I’m using 10 modules 43v fully charged so 396v wouldn’t fully charge the pack.....

Well any time you go out of the normal EV envelope, which is from 80S to 96S things get expensive. Or unobtanium as chargers and DCDCs go.
You need to think of spare parts for your EV too. In case anything dies on you you need to find replacement.
Using 10x 10S = 100S and i think it could work up to 400V. There your cells would get up to 4V and 95% SOC.

I dont think having a BMS disconnect inside the pack is a good idea. You could get in trouble. Maybe you could simply put GND line through all BMS wires and sense for it with VCU. If one BMS would close the loop would be cut and VCU would loose connection and trip alarm which is different than just opening HVDC. Or You could simply wire BMSs to observe cells and wire ONLY charger line through BMS in series.

[phillipschip]

Even more problematic, my system is 120p ( Lifepo4 cells)

[phillipschip]

delete

[tom91]

I strongly recommend getting a propper BMS setup instead of relying on the cheap Chinese garbage. They wont last and will result in a fire.

You need to consider safety and reliability above all else. Especially since you are new to this and lack the knowledge to be able to cut corners.

DO NOT post things twice, it wont get you more attention. There is reasons people do not post on your questions or want to get involved.

[phillipschip]

Thanks for the response. I didn't intend to post that twice. I'm not really sure why that is there.

There is reasons people do not post on your questions or want to get involved.

Did this happen before? If so it was unintended.

[uhi22]

I think what Tom means is, that we have seen a lack of basic electronics knowledge, so it is quite dangerous to guide into one or the other direction, because it is difficult to consider all basic things, and the chance is there that a small misunderstanding leads to a big bang.

[phillipschip]

I strongly recommend getting a propper BMS setup instead of relying on the cheap Chinese garbage. They wont last and will result in a fire.

I totally agree with this. My build will be in the testing stage for a very long time, and this is what I have for now due to budget. Theses little bms's have been working flawlessly for me for a couple of years on various batteries though. Before hitting the road for real my build will look much different

I think what Tom means is, that we have seen a lack of basic electronics knowledge, so it is quite dangerous to guide into one or the other direction, because it is difficult to consider all basic things, and the chance is there that a small misunderstanding leads to a big bang.

Ok, I understand and point taken. I do feel like I have some basic electronics knowledge. I did build my son an 80v 60mph go kart last winter :0 I fully understand that a car is whole different thing, and that is why I have been asking so many questions. I figured it never hurts to ask right? Maybe so. I'll hold back, do more research, and filter out the questions from now on.