DIY Electric Car Forums banner

1 - 10 of 10 Posts

·
Registered
Joined
·
258 Posts
Discussion Starter #1
Hello everyone,


I have two builds in my driveway right now. One, a 1996 Toyota Celica GT Convertible with 82 Powin 100ah lifepo4 cells wired in a 2P41S configuration. The second, a 1974 VW Super Beetle with 80 Xalt Energy 75ah HP lithium NMC cells. Both packs have their advantages, but this is a discussion just about BMS.


The Celica, has used a Manzanita Micro 12 channel regulator BMS board for years. The reg boards have never been able to read voltages right under load. They show some cells at like 0.2V or even NEGATIVE voltages which is obviously impossible because I have manually checked with a meter and they never sag below like 2.7V under very high loads. Nonetheless, there are always red lights stating low cells on the reg boards, and for years Ive ignored them because I know it's a false reading. All of my connections are fine, I have checked over and over to figure this issue out. My pack is in perfect balance under that BMS, but I never discharge below 20% SOC, I want my pack to last. Aside from the annoying red lights and false readings under load, it's been very reliable and survived two accidents!


Unfortunately, last night, it looks like one of my boards has finally failed. :-( Thankfully, it failed such that it wouldn't let my charger charge, so good to know.


The Beetle however has an Orion 2 BMS in it. I have not been too happy with it in all honesty. All of my connections are good, but this BMS likes to throw an error code when the wind blows the wrong direction. I do like the remote monitoring and supposedly you can use the BMS to control CHAdeMO chargers. The J1772 functionality wouldn't work with my application because I am using a manzanita micro non isolated charger, so had to make my own J1772 control circuit. The SOC with the BMS was supposed to be super accurate, but after watching it and testing, I feel like I could throw random numbers in a hat and pick some and it would be way more accurate...so no, I haven't been experiencing the accuracy everyone else has been experience supposedly.



So here is the topic for discussion:

There are some features I like on both systems, i.e. Manzanita micro BMS has just worked and kept balance, just not very user friendly and doesn't easily give out info. Orion has lots of features, CHAdeMO support, wifi monitoring support, uses CAN etc.



I personally prefer a BMS module setup like the Manz. Micro boards because with the Orion I didn't like being restrained on where I can put my BMS unit because it is all one unit.

My question to everyone is, what would be the perfect BMS for you? What kinds of features do you want in it?


Here's mine:


I monitor my pack from the pack level as well, count ah, and never discharge below 20% SOC as I have stated. I don't really worry about a cell being under discharged as if your pack is well balanced with a BMS, my belief has always been focus on the upper end.


I would want to have a BMS system in 12 cell modules or so with:

-Cell balancing (shunting at least 250mA, Orion does 10mA and Manz.Micro does 60mA)
-Cell over voltage protection (obviously)
-Remote monitoring
-Relay to shut off charger

- J1772 connectivity that works whether your charger is isolated or not
-CHAdeMO support
-Affordable (I want it to be simple, and JUST WORK! I shouldn't have to break the bank just to get a simple, quality BMS system that is scale-able!)


Let me know what you guys think. I am thinking about designing my own that is simple and just works without an issue, and most importantly, affordable. Then feeds all of the critical info to a mobile app on your phone. Please feel free to share all ideas for your DREAM BMS! I might also add, would be cool if it had a current meter built in to track AH and SOC.



Cheers,


Adam
 

·
Registered
Joined
·
742 Posts
I consider the term "BMS" to not be just one gadget, but a collection of functionality.

I would not expect charge regulation to be included,

other than failsafes for when the main hardware fails - true for almost all functions, multiple redundant layers of defense

Handling overtemp and under is critical.

Current rates as well as voltage.

Pack level vs whole bank, vs individual cells
 

·
Registered
Joined
·
311 Posts
Ahh the holy grail of battery management systems.

At its most basic level, it needs to prevent all cells in a battery from going below their minimum, above their maximum, and maintains a state of balance (consistent voltages) across the whole pack. In addition to this, it should monitor the temperature of the battery in a multitude of places and prevent overheating.

HOW it does these things, and WHAT IT DOES when these points are met, is what makes the BMS good. In the case of a race bike, all I want the BMS to do it let me know there is a problem. Then I can come into the pits and assess. In the case of a road going vehicle, I want it to manage everything, at all times because reliability is king.

Also, do these monitoring systems work at all times? Or only when the vehicle is powered on? If it works at all times, is the parasitic draw substantial? Should the BMS have a throttle input so it can limit power draw when a cell is low? Or just cut you off completely? Do you want the charger to change its charging power according to the status of the highest cell in the pack? Or will it go on a bulk voltage and throttle back anyway? Should the BMS interact with the J1772 charge inlet? Or should the charger do this and the BMS is independent of it?

All of these situations will depend on what you want to achieve wit the BMS. Personally, I want the BMS to do the most basic duties at all times and under all circumstances, but with minimal parasitic draw.

Oh, it needs to be easy to install and immune to errors and false alarms.
 

·
Registered
Joined
·
258 Posts
Discussion Starter #4
Thank you both for your replies, great points! I completely forgot about the temp, that's really important for measuring as well. Maybe have a relay output for over and under temp? Under temp really would be great to turn on some heating pads for some cells in cold weather!


The parasitic draw is an important point as well that I will have to consider. I think personally, anything under 10mA for a BMS board is not anything to worry about. As long as the parasitic draw is pulled evenly from all cells on the BMS board...


I think I might begin my dev soon on my BMS system. I want it to be simple, cheap, and just get the job done and keep things safe.
 

·
Registered
Joined
·
742 Posts
Be able to

user-adjust all setpoints, nothing hard coded

isolate the cells from the BMS when "pack level only mode" is desired

completely shut it down for long-term storage
 

·
Registered
Joined
·
258 Posts
Discussion Starter #6
Be able to

user-adjust all setpoints, nothing hard coded

isolate the cells from the BMS when "pack level only mode" is desired

completely shut it down for long-term storage
Great ideas! I really like these thoughts as well, I will try to put this into the design. Have spent the weekend brainstorming and will begin working on it soon.
 

·
Registered
Joined
·
742 Posts
Ability to scale the relay/contactors up and down separately, so same sensor / controller board with setups between 20A vs 500A
 

·
Registered
Joined
·
78 Posts
A couple of additional points to consider :

1) Support for both Hall and Shunt current sensing with programmable values
2) Sturdy connectors for both cell leads and all other cabling
 

·
Registered
Joined
·
742 Posts
I don’t really understand what you mean here, can you elaborate a bit more?
The actual current usually passes through a monolithic controller / sensors board integrated with the cutoffs, hence a "20A" version so much cheaper than - or completely different from - the 200A one.

When the board is designed so low-power relays are triggering open / close of external, maybe third-party standard contactors, it can be used for a large range of amperages.
 
1 - 10 of 10 Posts
Top