battery medic mod :P fast discharge

[sn0wchyld]

Sorry for being so dumb on this, but if the OP or Fastdemise, or anyone who has done this mod, am I understanding this correctly. In the photo below, I remove the little black things in the red box, and then I solder a 6.2 ohm 5W resistor from the top yellow square to the bottom yellow square (and I do that for each line).

Is that correct?

you can remove the 'little black things' (these are surface mount resistors :wink: ) if you want, personally I found removing them to be a major pita so I left em there and added the 6.2ohm resistor in parallel.

as to where to solder on the new resistor, you have the right idea, but you can solder each leg to any mounting point along the rows you just highlighted with yellow squares. See the final 6 mounting points that have nothing on them in each set of surface mount resistors? just solder 1 leg to the left 3, and the other leg to the right 3. done. hope that makes sense.

 

[Philistine]

Thanks heaps for that Snowchyld. Just to make sure I am understaninding right (this is because I barely understand the principles of what I am doing, I realise this must be excruciating for the learned types on the board). Are you saying I can do the example on the left (ie, solder from one box to the opposite), or like on the right, ie do I need to solder the three spots into one spot. Or will either of those work?

Also, is it a 1.6 Ohm 3W or a 5W that would be best? I am interested in speed of discharge over accuracy if that is relevant.

 

[methods]

Wow... he has a QC sticker - must be a good one

You will get more aggressive balancing if you put that resistor in parallel with the existing shunt resistors but you will also have more heat. The existing resistors are 10 ohms - so you are looking at

original 10 ohms
New 6.8 ohms
New in parallel with original 4 ohms

To get your balance current take your balance voltage, multiply it by .7 and divide by the resistance

0.7 * 4V/4ohms = 700mA
700mA is what you will get if you run your new resistors in parallel with your old.

That is a reasonable value - so just put the new ones in parallel.

(the .7 accounts for the balancing duty cycle which I assume is the same on yours as it is on mine)

See my thread on the fake vs real for how I did the mods

-methods

 

[methods]

My post went up at the same time your post went up.

Look at those pads closely - you can see a trace running the entire distance top to bottom connecting them (under the silk screen). All of those resistors are in parallel - so all those pads are the same - so you can solder to one of them or all of them. Confirm this with your multimeter. ALl you are trying to do is put a resistor in parallel with the existing resistor. There are already 10 in parallel - you are simply adding an 11th.

The wattage of the resistor has nothing to do with the rate at which it will discharge - only the rate at which it will overheat.

The total parallel resistance is what decides the shunt current and that is what controls have fast you balance.

-methods

 

[Philistine]

Thanks Methods, much appreciated. I looked at your thread as well which was helpful. Now I just need to get past the electronics shop today and I can start speedier balancing of my packs. awesome.

 

[FastDemise]

If anyone removes all the smts just get a 3.9ohm resistor and you'll be right there around 700mAh balancing current. (Forgive if numbers off a tad).
Another idea is instead of using it to balance just set it to discharge to a certain voltage. I can set my PS to 24.7V(6s), 4.117V then just discharge to 4.10.

 

[Philistine]

Just did the mod then and tested it out. Works a treat, the resistors and board sure get toasty though, I don't know if I would want to use it to discharge/balance any really big differences though. But great mod to have available for fast balance.

 

[sn0wchyld]

phill, just add some sort of forced airflow over the units, and make sure you leave them face down. A desktop fan or similar should be fine.

Just to add to what methods was saying, yes, either of your options in your last pic will be fine. they are both the same as far as the BM or batteries are concerned.

And I presume you meant a 6.2 ohm resistor, rather than 1.6... otherwise that would mean youre pulling nearly 3a, or 11watts per cell!! the way to work out how many watts your resistor will need to disipate as heat is given by the formula Power = Voltage x Voltage / Resistance, in this case 11=4.2x4.2/1.6 . A 6.3 ohm resistor works better, as methods said, once paralleled with the rest gives a total of 4 ohms, or about 4 watts of heat. a tad less after duty cycle is taken into account.


edit
missed methods 2nd post, that explains it more simply than my ramblings.

 

[bigmoose]

Does the case pop open with a knife, or is it solvent ultrasonic welded closed?

 

[FastDemise]

Just using your fingers can easily seperate the 2 sides apart.

 

[methods]

Does the case pop open with a knife, or is it solvent ultrasonic welded closed?

There is a trick to it - one side has all the barbs and the other side has all the landings.
Once you get just one spot loose, by pushing in on the side with the barbs it will almost fall apart.
You can get 3 sides loose without using any force - the final side has 2 pins connecting the two sides so the "push in" trick does not work there
At that point I spread it like a clam shell and shake a little bit - the electronics will fall out
I can then go back and very softly twist the assembly until the barbs let go but the pins dont break

I found that if I dont use the push-in method I always end up breaking at least one of the barbs or the pins.

-methods

 

[methods]

If anyone removes all the smts just get a 3.9ohm resistor and you'll be right there around 700mAh balancing current. (Forgive if numbers off a tad).
Another idea is instead of using it to balance just set it to discharge to a certain voltage. I can set my PS to 24.7V(6s), 4.117V then just discharge to 4.10.

Yes... but make sure the resistor power rating is up to the task. The stock resistors are already dumping 800mW for you. Makes more sense to leave them there and let them do that work. If you replace with a 3.9ohm resistor now:

0.7 * 4.1 ^2 / 3.9 = 3W

No way in hell a 3W resistor can handle that without a fan so then you will need 5W minimum - and those will heat up over what they are rated for. I would expect 150C.
The trick is distributing the power dissipation so that no one element gets too hot.

****

As far as your second suggestion about setting the power supply so that it charges just a touch over 4.1V per cell... THAT is by far the best advice given on any of these threads and exactly what I plan on doing. Dont put the energy into the cells in the first place and then you dont have to bleed it back out


****

For those making mods - please do not underestimate the power of a fan. Even a standard fan blowing randomly in the room can improve power handling by 2X. These resistors depend on airflow to take the heat away - so even the most insignificant breeze can greatly improve performance. If you have something you can set there and blast them - you will be amazed by how cool they can run.

-methods

 

[bigmoose]

Thanks for the tips methods! The case was so nice, I didn't want to ham fist it...

 

[FastDemise]

I was using the 3.9Ohm on top of the stock smts. Haha, oops. I'm sure that was my major issue with cooling as I was pulling just a bit too much for such a small package. I just set it up so the resistor sits as far away from each other and the board as possible. I use my Celllog to monitor voltage so I can leave the balancer facedown the entire time. I'm building 20Ah batteries for my bike and since its from used cells I know it'll get out of balance for more on then most quality Lipo setups

For pulling apart I pry around the balance tabs as for mine there isn't much holding on that side.

 

[bobale]

A bit of offtopic here.

I'm interested in increasing balancing current on Turnigy AccuCell 6. As you can see in the pic, there are 6 120Ohm resistors in paralell per channel, so that's 20Ohms per channel, so about 200mA of balancing current. Sure it can be doubled, but question how long it would without smoking.

So, which transistor is turning on those shunts for balancing? Is it the single IRFZ44N, which is heatsinked to the case? I'm looking to find which component is bottleneck for upping the balancing current.

 

[methods]

Your bottle neck will most likely be heat.

Also, don't assume 100% duty cycle - you should measure the average current. It could be as little as half of the V/R calculation.

-methods

 

[bobale]

Heat, but where? I always cool the charger with a fan while charging, otherwise the case gets pretty damn hot. Do you have idea which component (underboard FET?) switches on individual channels for balancing, so I can check specs for it, to see whether it would be able to cope with the current?

About discharge current: well, I've assumed it's ~200mA because it says 300mA in charger specifications.

I'll measure balancing current when I catch time, but would like to know which component will be under the most stress, so I can check its rating before upping the balancing current.

 

[steveo]

new battery medic i sorced on ebay... love to hear your thought

the good things about it..

-Very accurate withing .01v measurement... all 4 medics i bought gave me exact same voltage readings accross the board
-these medics allow you to set balance & discharge limits in .5 increments... 4.00v, 4.05v,4.10v,4.15, 4.20v etc....
-these medics chime to indicate the packs are fully balanced or discharged to the set voltage
-screens look like good quality, the processors are a big chip, not like the "blog" processors i've seen on some medics...
-the discharge board is a good distance from the lcd screen board... good thing to ensure lcd will not get to hot alow with other components on display board..


the bad...
we ... its bad depending on how you look at it...
the medic will not allow you to discharge or balance unless the cells are at a set voltage ... for example with a 3s pack...

say i want to discharge of balance my pack at 4.10v... but my cells are 4v accross the board, the medic will not allow you to put it into discharge or balance mode...

now say the cells where all rangeing from 4.11v - 4.15v... it will allow you to set it in discharge or balance mode...

long story shory, if your the person that sets the medics to discharge at 4.10v when your pack is low... these won't work well for you... you will need the cells to go about 4.10v before you can set discharge or balance mode...

despite this .. i feels these are the best medics i found in long time.. i like the sound feature when the pack is balanced or discharged to your desired voltage telling you to disconnet the medic basically...

discharge current isn't measured ... but i was able to bring a 6s pack with cell votages ranging from 4.11v - 4.17v to a even 4.10v accross the board in a hour... very good if you ask me .. this was with 10ah pack..

some pics for ya...

 

[miuan]

yup, I got the same ones but blue with 50/150W discharge accessory... the discharge voltage bug is kinda annoying but then.. THEY ARE ACCURATE, which is why we buy them. Also the .05V increments are for the good. I always charge to about 4.17 and 4.15 is a good balance voltage for me.

 

[methods]

It warms my heart to see a Fluke meter with 4 digits of resolution after the decimal point.
nice....
Too many guys around here report data (from a CA etc) without every doing a proper calibration with a reliable instrument.

-methods

 

[steveo]

It warms my heart to see a Fluke meter with 4 digits of resolution after the decimal point.
nice....
Too many guys around here report data (from a CA etc) without every doing a proper calibration with a reliable instrument.

-methods

my fluke multimeter & amp clamp are the best money ever spent.. i use them everyday in my hobbies

 

[methods]

-methods

 

[drutledge]

Steveo, miuan, which ebay seller did you go through for these balancers?

 

[steveo]

Steveo, miuan, which ebay seller did you go through for these balancers?

Fmt-model

 

[aaronlim]

discharge current isn't measured ... but i was able to bring a 6s pack with cell votages ranging from 4.11v - 4.17v to a even 4.10v accross the board in a hour... very good if you ask me .. this was with 10ah pack..

Was this after the modding the resistors or just straight from the box?