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batery balencer

 
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Wolf



Joined: 31 Mar 2007
Posts: 27
Location: Illinois

PostPosted: Fri Jun 08, 2007 8:38 pm    Post subject: batery balencer Reply with quote

I have started a new design project. :)

I am currently working on a low cost microcontroller based battery balencer.

Of the four types of battery balencers (resistive, capacitive, inductive, and transformer based). I have desided on inductive.

Specifically a buck-boost topology, this allows every thing to be current based, and takes care of the peskey voltage drops in the mosfets and diodes. ;)

Each balencer will monitor the voltage, temperature, and balence current of each battery.

They can run on there own with no other input or control and will automaticly balence batteries... or, (and this is the cool part), they can be connected together via a 6 pin IDC ribbon cable, to an LCD screen in the cockpit. This display will show all the batteries voltage tempreture and balence current. It will also be able to log battery performance/efficency over time and predict when a battery is going to need replacing, among other things.

The balencers will be user programable: max balence current, how close to balence the batery voltage, balence only while charging, sleep modes, etc...

I am picking out parts right now, and should have a bread board version in a week or so.

Once this is finalized, I plan on doing a prototype production run of 50 (I need 33 of them for my truck), and then after a bunch of testing, I will make them avalible to anyone that wants them.

Most of the testing will be to improve the efficency, (switching frequency, continous/discontinous modes, when to change modes, go to sleep, etc...)

I don't know how much they will be yet, but the more that are made the cheaper they get.

If anyone else is intrested in them let me know. :)

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MajorWilson



Joined: 26 Sep 2005
Posts: 236
Location: Peoria, AZ

PostPosted: Mon Jun 18, 2007 6:27 am    Post subject: Reply with quote

Hey Wolf. Sounds like a really good plan. I'm very interested in them. Keep us all posted. Very Happy
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Jeff Wilson
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Wolf



Joined: 31 Mar 2007
Posts: 27
Location: Illinois

PostPosted: Sun Dec 23, 2007 12:40 am    Post subject: battery balencer update... Reply with quote

MajorWilson wrote:
Hey Wolf. Sounds like a really good plan. I'm very interested in them. Keep us all posted. :D


I have been sick for the last month (liver problems), but I am starting to get better...

Anyway I got my prototype (bread board) balencer up and running today...

I was able to balence the state of charge between two batteries using buck/boost inductor topology...

But it was only 72% efficent, I have lots of switching losses since I am running at 500Khz (to keep the size of the inductor down), So it looks like I need to get better mosfets. ;)

Or get a bigger inductor and lower the switching frequency...

I am also using the body diode in the mosfets for the return current path... which they seem to be fast enough... but I have not tried turning on the mosfet during the diode conduction cycle, which would force the current from the inductor to flow backwards thru the mosfet and bypass the diode... this would remove about 700mW of power that is lost in the diode. But this will require realy good timing, since it can only be on, while the voltage across the inductor is higher then the voltage of the target battery.

Many many options to try, thats why its in the protoype phase. :)

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MajorWilson



Joined: 26 Sep 2005
Posts: 236
Location: Peoria, AZ

PostPosted: Sun Dec 23, 2007 10:59 am    Post subject: Reply with quote

Glad to here you are feeling better. Hope all is well.

The balancer sounds more promising. I look forward to the prototype and will buy one as soon as it is ready.

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Wolf



Joined: 31 Mar 2007
Posts: 27
Location: Illinois

PostPosted: Sun Mar 30, 2008 9:35 pm    Post subject: battery balencer Reply with quote

I worked on the battery balancer some more:

Its now moved from the simulation realm to the bread board realm:
http://www.wolftronix.com/balancer/images/P3300103.jpg

Its a simple, single inductor, discontinuous, switch mode, buck boost convertor running at 250Khz.

Currenly I am using one of my 16 channel servo controller boards to measure the voltage of each battery and generate the PWM signals to drive the MOSFETs.
http://www.wolftronix.com/balancer/images/P3300106.jpg

Using the ATmega on board 10-bit ADC convertor only gives about a 15mV resolution... which is plenty for testing. :)

Atmel has a new Xmega micro that has a 12-bit ADC... that would be around 4mV resolution... which might be overkill.

Which does bring up a good question: how balanced is balanced?

Here we have the right battery at 12.10V and the left battery at 12.08V and the balencer is pumping 130mA from right to left.
http://www.wolftronix.com/balancer/images/P3300108.jpg

There is a 20mV difference in voltage, but the system has a resolution of only 15mV... so it falls in and out of being equal, which results in the 130mA current. If the balancer where to turn off now, the voltage on the right battery would rise slightly due to the balancer load being removed, and the voltage on the left battery would fall slightly due to the balancing current being removed.

The balancer will continue in this state untill both voltages are equal and there is no longer any current flow.

Now, to give it a work out, I added a battery charger to the battey on the right:
http://www.wolftronix.com/balancer/images/P3300110.jpg

Now the right battery is at 12.34V and the left is at 12.10V so there is a balancing current of 680mA.
http://www.wolftronix.com/balancer/images/P3300109.jpg

I was shooting for a 1 Amp max balance current, but the max is around 700mA when I start to saturate my inductor and start running into continuous mode. This means I need a bigger (more current) inductor. ;)

I figure that 1 Amp is pleanty since, if you had unbalanced batteries... 24 amp-hours could be transfered each day. And if your batteries are that out of balance each day, then you need to replace them. ;)

Anyway, more pics are here:
http://www.wolftronix.com/balancer

Next step is to get in some more bigger inductors to play with and replace the linear regulator powering the micro with a small switching power supply to lower the quiescent current. Chose a final (cheeper) micro controller. Lay out a circuit board. Do more testing.

Longer term stuff:
Get the CAN bus up and running. Write a nice touch screen visual interface so I can see all the voltages of all my batteries and what they are doing. :)

Sure you can buy already excisting battery balancers out there... but this is more fun! :D

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MajorWilson



Joined: 26 Sep 2005
Posts: 236
Location: Peoria, AZ

PostPosted: Tue Apr 01, 2008 9:55 pm    Post subject: Reply with quote

That's just too elegant. The board for the battery is very small and the low current draw is just plane awsome. I can't wait for the prototype. I want a set now!
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Jeff Wilson
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