Old BMS General Thread

[GregsGarage]

Peter,

The slaves are working great. Next time I have the battery boxes open I will adjust the values slightly lower to match the charger, which is designed for lead acid. At the moment the lowest cells don't quite get to 3.6 volts so my balancing is probably only within around 100-125mv right after the charger switches off (although after a couple of hours the voltages are nearly level).
Current values;
CPU 250 khz

Max Cell Opto On = 3.80V
Max Cell Opto Off = 3.65V

Load On = 3.65V
Load Off = 3.60V

Min Cell Opto On = 2.40V
Min Cell Opto Off = 2.50V

Proposed changes;
Max Cell Opto On = 3.75V
Max Cell Opto Off = 3.60V

Load On = 3.60V
Load Off = 3.55V

By adjusting the voltages slightly lower I hope to find the point at which the charger decides the battery is fully charged at about the same time as the last load switches on, then all the loads should drain the cells to 3.55 volts (or what ever my final value is for "Load Off") and be perfectly balanced.

[retepsnikrep]

I think the slightly lower values are a good idea Any pics yet?

[wjdennis]

Hi everyone, I just wanted to let you know I finally got a demo board of the LTC6802 BMS

What will you be using to drive the SPI? And what will you be using as a master? And as if that weren't enough questions, how do I go about getting some samples?

Thanks.

Bill Dennis

[arnolde]

Well, the first 2 questions are really one liek I said, I'll be using a PIC microcontroller (like Peter uses too, except I'll probably go for the assembler programmable one). Maybe the PIC18F6550, which I've used before. Just so it has SPI and USB, thats all it really needs. The chips with that capability are bound to have enough other I/O lines to do anything else (display, keys, additional sensors, etc.) one could want.

To order samples, go to the Linear website and follw the instructions for ordering samples

[wjdennis]

The reason I asked about how you ordered your samples is because when I go to Linear and follow the instructinos, I end up with a page that says:

"This product is not currently available for order online. Please contact your local sales office or distributor for availability."

And there's no "order samples" link. So I thought you might have done something special.

Bill

[arnolde]

Sorry, you're right, I assumed they had updated the product page.

I went to http://www.linear.com/ad/6802.jsp?gclid ... FQodAWWTSg and used the "request more info" page. A few days later I was contacted by a local LT representative.

Ethan

[GregsGarage]

A tip when requesting samples. The company that you are requesting samples from, especially for a new product is going to be looking for large orders in the future. Not that you are going to have to commit to anything, but you will need to give the impression that your "project" could lead on to future sales or they won't be interested. Be prepared to give details of your project, including battery pack details, vehicle details and performance goals. If you don't have this info yet, get it before you contact the company. I know Ethan has prepared all of this, although that 600 volt pack still scares me!

Let us know how you get on with your BMS boards Ethan.

[retepsnikrep]

I know Ethan has prepared all of this, although that 600 volt pack still scares me!

Let us know how you get on with your BMS boards Ethan.

Ditto, what he said!! My 144v nominal 180v peak pack frightens me.

[mikep_95133]

Peter,

I have noticed on the Master schematic that LED's D3 and D4 are in series with the smoothing cap and opto that they are driving instead of in parallel. This creates a voltage drop ahead of each opto. It could also limit current to each opto instead of the limiting resistor doing it. Is this intentional?

Mike

[retepsnikrep]

Mike

They are actually in parallel with it if you look closely, so each gets the same current.

Peter