Lithium battery BMS questions

[P-hack]

I'm not entirely convinced that the thing monitoring the pack needs to be entirely isolated from the pack. Are you assuming the 12v converter is isolated (also expensive if so)? From what I gather the system needs to be isolated from the frame most importantly, so the frame doesn't act like a big antenna/ground loop, but if the controller and monitoring and charging (and lights and etc) are also referenced to the pack with no reference to frame, excepting for shielding, then no big deal.

So if your cheap non-isolating 12v converter is referenced to pack ground, and you make sure everything that depends on it does not reference frame, then the whole thing can "float". Which is probably something you should do with a non-isolated 12v converter anyway.

[mechman600]

Originally Posted by P-hack

I'm not entirely convinced that the thing monitoring the pack needs to be entirely isolated from the pack. Are you assuming the 12v converter is isolated (also expensive if so)? From what I gather the system needs to be isolated from the frame most importantly, so the frame doesn't act like a big antenna/ground loop, but if the controller and monitoring and charging (and lights and etc) are also referenced to the pack with no reference to frame, excepting for shielding, then no big deal.

So if your cheap non-isolating 12v converter is referenced to pack ground, and you make sure everything that depends on it does not reference frame, then the whole thing can "float". Which is probably something you should do with a non-isolated 12v converter anyway.

My 12V converters are indeed isolated from the car's 12V side. Isolated up to ridiculous voltages, according to the datasheet. Verified by my multimeter. Arduino boards must be isolated from each other when measuring voltages at different parts of a series string battery pack as voltage potential [obviously] increases from cell to cell, all they way up to pack voltage at the end. Each board is isolated from the car's 12V system, NOT isolated from the pack. I repeat: each board is not isolated from the cells they are monitoring (how would you measure pack voltage if they were?), which is why isolating boards from the car and from each other (via optos and DC-DC converters) is necessary.

[TurnNBurn]

I'm picking up the batteries later this week. They're being built right now. So I'm still shopping for a BMS.

As for the DIY arduino route, I think I'm willing to give it a try. I'll need some help with the programming for sure. But right now I'm gathering the shopping list. Can anyone add or critique this list please?

Arduino UNO R3 board with DIP ATmega328P
http://amazon.com/dp/B006H06TVG/ref=...I32ZH51Q6FUHPD

Amerlight TM LCD Module for Arduino 20 x 4, Black on Green, based on the popular HD44780 controller
http://amazon.com/dp/B00FMTC0KC/ref=...FB1M53TU&psc=1

Mux Shield
http://www.amazon.com/SparkFun-Mux-S...rds=mux+shield

[mechman600]

Looks good to me.
Buy a 12V actuated relay to switch your 120V charging for charger cutoff.
I came up with some charging and Ah counting/fuel gauge code. It's pretty simple, actually. Look at my thread for it.
I suggest that you spend a few weeks with your new board and go through all the Arduino exercises like I did. You really learn a lot. It is simple programming language for us DIYers. Get a small breadboard and goodies like pots, LEDs, DIP switches, momentary switches, transistors, etc. It's a lot of fun. Then start messing around with simulation code for your LCD, making it look how you want to.

[Daox]

Have you checked out my thread on my DIY open source arduino lithium BMS (dang thats a mouth full haha)? I thought I recommended it earlier, but I couldn't find a link so here it is:

http://ecomodder.com/forum/showthrea...tem-20445.html

It would work great for your setup, its very inexpensive, and I use some other premade circuits so there is less custom work and less programming.

I used the BMS on my PHEV kit that I had on my last Prius. The charging protection worked great. Unfortunately, I never got around to working all the quirks out of the discharge protection, but its really a lot less necessary if you just get an amp hour counter.

[TurnNBurn]

Originally Posted by Daox

Have you checked out my thread on my DIY open source arduino lithium BMS (dang thats a mouth full haha)? I thought I recommended it earlier, but I couldn't find a link so here it is:

http://ecomodder.com/forum/showthrea...tem-20445.html

It would work great for your setup, its very inexpensive, and I use some other premade circuits so there is less custom work and less programming.

I used the BMS on my PHEV kit that I had on my last Prius. The charging protection worked great. Unfortunately, I never got around to working all the quirks out of the discharge protection, but its really a lot less necessary if you just get an amp hour counter.

I saw the thread when you just brought it up. Looks good. But I'm second guessing myself and I don't think I'm in the mood now or have the patience to put one together. I'm losing my nerves.

I'm getting kind of frustrated at this point because I can't seem to find a BMS that will work without going the DIY route. Any other suggestions?

[P-hack]

I like the celllogs, they have a lot of features and $16 for 8 cells is pretty good (plus you get a little voltage display and alarm for each cell), and you don't *really* need an arduino in the loop, but getting it to shut off your charger or controller will take a tiny bit of research.

[TurnNBurn]

Originally Posted by P-hack

I like the celllogs, they have a lot of features and $16 for 8 cells is pretty good (plus you get a little voltage display and alarm for each cell), and you don't *really* need an arduino in the loop, but getting it to shut off your charger or controller will take a tiny bit of research.

Do you know if there's a voltage meter that can support more cells? 18?

Also, why again wont miniBMS work? Each cell is 3.8v nominal. 4.2v max.

[P-hack]

minibms is "hardcoded" for the voltages for lithium

It is also like $13/cell, so there is $234, and you need a $50 head board. And is shunted.

For $48 you can get 3 celllogs and individual cell readouts, programmable for your chemistry, and alarms, and charger/controller integration with a couple optos.

https://www.hobbyking.com/hobbyking/...uct=10952.html

I guess I should ask "what's your budget?".

[mechman600]

Would it work to use something like an op-amp (differential amplifier) for each measured cell to get a full resolution (5.0V / 1024 = 0.005V) reading for each and every cell? This would be instead of using voltage dividers to measure voltage potential at every point of the series cell string, which destroys resolution.

If so, what would this circuit look like?