# Redesigned Goodrum/Fechter BMS



## MalcolmB (Jun 10, 2008)

Just thought some of you might like to know that Richard Fechter and Gary Goodrum on the Endless Sphere forum have come up with a redesigned version of their BMS that looks as if it could be an excellent solution for many of us: http://endless-sphere.com/forums/viewtopic.php?f=14&t=5416&start=1455

(see GGoodrum's post near the bottom of the page)

Seems like they're planning to offer a surface mount version early next year.


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## dimitri (May 16, 2008)

Funny how we go from no BMSs to too many BMSs in a matter of months  , same thing happening with motor controllers ,chargers and even batteries as well. This is a great time to do an EV conversion. I bet people who built EVs 10 years ago are so jealous


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## tomofreno (Mar 3, 2009)

Hmmm...shares quite a few features with Dimitri's, but more complex. There never seems to be much discussion on how you get the charger output to be about equal to the shunt current when a shunt comes on. I am charging at 30A. If a shunt comes on and starts shunting 1A, big deal, the energy dissipated will be insignificant, the cell voltage will continue to climb and hit HVC in a couple minutes, permitting very little balancing. I can use a signal from the board to cut back the charging current to a low level, but many chargers do not have this option. 

I am not using a bms now and the cell rest voltages have gotten MORE balanced over time, all within a few mV range. There is 10x that range during charging, but I think that is mainly due to differences in internal resistance, as it is a function of charging current. I expect there are much larger differences during discharge, and ordered a couple of those little jinsu cell loggers to see. Maybe the optimum is to "balance" cells so they are all at the same dynamic voltage under a 100A load at the low end - say 3V. No idea how you could do such a thing automatically, but you should be able to do it manually by tweeking cell socs and datalogging until you get it right. The cell log data should be very interesting - I see several at the ES site are doing the same. My experience is that all this is pretty needless if you just size your pack so you can stay away from the steep slopes on the voltage versus Ah charge/discharge curves. Life is very simple then as Jack has demonstrated for a year, and I am now seeing. Of course I expect eventually something will drift with age as cells degrade.


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## Overlander23 (Jun 15, 2009)

I think the charge current runs through the BMS. Current is controlled by PWMed FETs, so direct control of the charger isn't needed since the BMS can do the throttling.

Or was that another system?


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## MalcolmB (Jun 10, 2008)

That's right, the BMS uses the high voltage cutoff (HVC) signal to modulate the charger current by PWM. The maximum shunt current is 2A. Once all the cells have reached the HVC the charger is disconnected. On this new version you can also choose to end charging as soon as the first cell reaches HVC, or after a preset balancing period.

I'm using an earlier version of this board, which had a lower shunt current and voltage limit, on my motorcycle and it's worked very well in testing so far, once I'd corrected my soldering mistakes...


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## AndyH (Jun 15, 2008)

I'll be involved with beta testing the new board after Christmas and will upgrade my BMS building service once they're 'ready for prime time' - likely in January.

The new charger control section allow control of chargers from standard power supplies up thru smart chargers - to about 50A charge input.

The new V4.x BMS has fan control to keep the BMS cool, and the boards were designed for stacking. One can also scatter the boards thruout the battery spaces. The LVC/HVC/shunt boards connect to the charge control board with one wire pair which is fully optically isolated.

The BMS is flexible enough that it can be used for different chemistries (it's being used for both LiFePO4 and LiPo now). One can set shunt voltages down so that the pack is 'top balanced' every charge, or set the shunts a bit higher than charger average so that it's more protective - only shunting and restricting charge current if one or more cells crosses a preset voltage.

I've been using the earlier version of these BMSs for a bit over a year and have assembled more than 50 so far. The're simple, they're inexpensive, they keep the shunts away from the cells, and they work.

Andy
http://www.rechargeablelithiumpower.com 



MalcolmB said:


> Just thought some of you might like to know that Richard Fechter and Gary Goodrum on the Endless Sphere forum have come up with a redesigned version of their BMS that looks as if it could be an excellent solution for many of us: http://endless-sphere.com/forums/viewtopic.php?f=14&t=5416&start=1455
> 
> (see GGoodrum's post near the bottom of the page)
> 
> Seems like they're planning to offer a surface mount version early next year.


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## tomofreno (Mar 3, 2009)

Andy,

How does this system control the charging current? Is the modulated FET downstream of the charger, or does it supply a PWM signal to the charger to control its output?

Tom


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## Tesseract (Sep 27, 2008)

tomofreno said:


> ...How does this system control the charging current?...


Funny, I was wondering something similar... mainly, what sort of dumb (literally, unintelligent) charger this is supposed to control?

I mean, who spends $10k-$15k on a LFP battery pack and then charges it with a "Bad Boy" (i.e. - a variac and a bridge rectifier)???

Looks like a solution in search of a problem, but maybe I'm missing something?


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## tomofreno (Mar 3, 2009)

> I mean, who spends $10k-$15k on a LFP battery pack and then charges it with a "Bad Boy" (i.e. - a variac and a bridge rectifier)???


 I think the main users of these are people with electric bicycles, scooters, and motorcycles ( I believe Goodrum has a bicycle), so they aren't spending nearly that much on cells. It was only later that they started increasing number of cells, hence voltage, that could be regulated, first for motorcycles and now even for cars.


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## MalcolmB (Jun 10, 2008)

tomofreno said:


> How does this system control the charging current?Tom


I know your question was aimed at Andy, but someone just asked the same question on Endless Sphere and Gary gave a detailed answer (near the bottom of the page): http://endless-sphere.com/forums/viewtopic.php?f=14&t=5416&start=1470


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## AndyH (Jun 15, 2008)

Hi Tom,

The BMS's charge control section has a single FET (V2.x) and a pair (V4.x) that 'throttle' the charge input. There is no need to communicate with the charger at all.

Not saying it's 'the' way, but it's proven to be 'a' way to get the job done.

Andy



tomofreno said:


> Andy,
> 
> How does this system control the charging current? Is the modulated FET downstream of the charger, or does it supply a PWM signal to the charger to control its output?
> 
> Tom


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## AndyH (Jun 15, 2008)

It's not as absurd as one might think.

The early V2.x BMS was designed to control a number of cerebrally-challenged devices - from fixed power supplies to lead acid chargers. The design has evolved up thru V2.6c to work with more advanced chargers.

One example of charging flexibility is for my 21S pack fo 60Ah TS cells. I can use the 15A Thunder Sky charger (either directly on the pack or thru the BMS), or I can charge with my <$200 0-100V 0-5A bench power supply. I can also use both in parallel. The BMS doesn't care - and neither do the cells.

Users of this BMS don't have to worry about interfacing with a CAN, decoding the red or green charger wires, reprogramming the charger, or selling the kids to buy a Brusa. 

Andy



Tesseract said:


> Funny, I was wondering something similar... mainly, what sort of dumb (literally, unintelligent) charger this is supposed to control?
> 
> I mean, who spends $10k-$15k on a LFP battery pack and then charges it with a "Bad Boy" (i.e. - a variac and a bridge rectifier)???
> 
> Looks like a solution in search of a problem, but maybe I'm missing something?


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## tomofreno (Mar 3, 2009)

> It's not as absurd as one might think...


 I suppose if the FETs never completely stop current this might work...but the Manzanita chargers are severely damaged if operated unconnected to a load. I don't know how large the impedance of the load can be without causing damage. I don't think I would want to try it without consulting Rich.


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## AndyH (Jun 15, 2008)

tomofreno said:


> I suppose if the FETs never completely stop current this might work...


The FET(s) PWM once any cell starts to 'shunt', then shut off when the charge is complete. It's been working rather well since at least Feb 2008.



tomofreno said:


> ...but the Manzanita chargers are severely damaged if operated unconnected to a load. I don't know how large the impedance of the load can be without causing damage. I don't think I would want to try it without consulting Rich.


That's fine as well - flexibility is key. One can still use the HVC opto to tell the charger to shut down either via a comms interface or via a relay on the mains.

You can also set the Manzanita to shut down just before the shunts come on - say 3.63V per cell - and charge the pack directly without connecting the charger thru the BMS. The shunts will still be there to keep cell voltage under control. This is how I'm using my V2.3 BMS with the Thunder Sky TSL60-15 charger, which ends the 'bulk' portion of the charge at 3.62V per cell.

The beauty of this BMS is that one can choose to series/parallel a couple of industrial power supplies for a LOT less money than they might spend on one of the high dollar chargers.


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## ahambone (Jan 13, 2009)

> ...but the Manzanita chargers are severely damaged...


I suspect that this can be a problem with some smart chargers working with the G/F BMS. My G/F BMS is connected to a (5p) x 24s pack of headways that I charge with a reprogrammed Zivan NG3. The NG3 is setup for a 24s LiFePO4 profile and outputs up to 15 amps during bulk-charge phase.

When I first tried the Zivan with the BMS everything went fine until the the first high-voltage shunt turned on and the FET on the BMS cut the current to the pack by interrupting the negative lead from the charger. The Zivan didn't like this at all. It blinked its status LED red and played a pair of tones to indicate that something was wrong. 

After sitting around with the problem for a few minutes I decided to try an experiment. I needed some kind of buffer between the Zivan and the BMS so that the Zivan didn't "see" the pulses where the G/F BMS cut off the charging current to let the shunts burn off energy. I ended up placing a 3300mF capacitor in parallel between the charger and the pack. This approach worked perfectly.  No more angry zivan and charging runs to completion. I suspect that users who connect the G/F BMS kit to other "smart" chargers that are LiFePO4 aware are going to need to do something similar.

I've got some footage of the large capacitor wired into the BMS and charger about a minute into my last "conversion status" clip. I won't clutter this thread with the video, but if you want to see it all connected you can watch it here: http://www.youtube.com/watch?v=966ETjc-cFQ


Cheers,
--Adam


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## AndyH (Jun 15, 2008)

Hi Adam,

I've seen your photos and video - excellent work!

I have a similar challenge with my V2.3 BMS and the "too smart for it's britches" Thunder Sky TSL charger. It doesn't like to be disconnected from the pack - not because it will be damaged but because it's routinely pausing to sense the pack. If it loses touch, it enters some sort of undocumented 'error' condition where it shuts down and flashes it's lights. I feel gypped, though - no music from the TSL.  

I look forward to trying the V4.x control section with my TS chargers, but I'm not going to lose any sleep if the charger still gags a bit because the TSL wasn't designed to work with a BMS.

Andy


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## tomofreno (Mar 3, 2009)

> I ended up placing a 3300mF capacitor in parallel between the charger and the pack.


 Good idea! I expect this would work for the Manzanita provided the RC time constant is large enough to limit voltage rise at 30A output current.


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## lapwing (May 8, 2009)

tomofreno said:


> I suppose if the FETs never completely stop current this might work...but the Manzanita chargers are severely damaged if operated unconnected to a load. I don't know how large the impedance of the load can be without causing damage. I don't think I would want to try it without consulting Rich.


I have to wonder about Manzanita chargers. I have very dumb days when I know I could hook up and toast a Manzanita charger for this very reason.

The other key thing I worry about is a non isolated charger. Under ABYC I'm not allowed to install one on a boat. I just can't get past that. 

This Goodrum/Fletcher BMS is just getting better and better.

The only thing I would love to see added to it, is temperature compensation in the control section. With the improved modularity an upgrade would certainly be possible. I love the jumper idea for the variable constant voltage on time. Inspired and simple. 

Maybe a microprocessor section to do temperature comp. on the same board?

Whatever you guys think of Rickard's video on charging, I got one critical thing from it, that ties in with every marine battery charger install I have done in the last 10 years. I insist on temperature compensation whether AGM or flooded lead. 

As Rickard points out several times in his charging video, there is no magic cutoff voltage number - it depends! A key parameter is temperature. LVC and HVC are temperature dependent. Also I got from the video is that LiFePO have a Peukerts number, which implies Ah in/out is rate of amp dependent. Though it is much less marked than lead/acid, it's a factor.

Admittedly this is only part of the story. Ideally some sort of "smart" Ah / Coulomb counter could be good to but having used and installed a whole bunch of Heart interface/Xantrex e-meters etc. - as state of charge meters, they are for folks who like to fiddle, and are only as good as the last reset and calibration. They can be very misleading and frustrating, unless tweaked _ad nauseum._

Some kind of temp compensated LVC is the most useful thing I could add.

The option to balance, or not is awesome. Combined with some sort of Cell Log monitor, strikes me as the best solution. 

I can be a smart fiddler on my smart days, and leave well alone and looky looky only, when I feel battery challenged. 

Awesome!


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