Battery medic...balance ...nice but slow

[Andje]

hey i balanced them to 3.8 before i put em all together. The only thing i didn't do was cycle them individually to find runt cells. So fair enough. But that doesn't change the overall fact that the BM balances between 200 and 400 ma with a duty cycle of 1/3. They may work for you just fine, i wasn't trying to hate, but they are not working for me because two of my cells go high. The hyperion is more expensive and more of a hassle but will work perfectly.
and the 3-4 a shunt would only be for a plug and play solution; set the bm to 4.1 and then walk away charging at around 3-400 watts. When they hit that level the bm could then shunt all the charge away, and you could walk back later without worrying.

[neptronix]

How much higher do two cells go than the others?

A little variance is actually no prob.

BTW the balance leads on these packs are about 24 gauge, i wouldn't go running multiple amps through them.
Maybe 1-1.5 amps tops.

If two cells are acting up you could always use something like a CREE flashlight connected to those 2 cells or whatever to help the drain. Light your house at the same time too

[icecube57]

You need to top balance balancing while charging does nothing cause the cell voltage isnt relative to capacity so they may all say 3.8 but all the cells could be at different level. If you have two cells that always go high... they probably have a lower capacity than the others.
I bulk charge at 4-450w and top balance with the medics with noooo problem at all. The duty cycle is only reduced if all of you channels are bleeding at the same time.

[neptronix]

Cell voltage is very relevant to capacity actually, it's just that when over 4.1v and under 3.6v, the expression of capacity is tiny in relation to the voltage difference. Literally 50 mah can be the difference of 0.5v at the bottom end of the charge for example, but right in the middle ( 3.6-4.1v ), the difference between 50mah could be as small as 0.01v, which would show up at the top of the charge or the bottom quite rapidly.

I have noted this while graphing all 12 of my lipo packs of various ages and 2 different brands ( turnigy and zippy ).

When you measure the cells during charging, you get a weird reading because the cells have artificially elevated voltages.

I think running a battery medic while charging works though, you are basically taking some of the amps that would normally go into that cell and are burning it off elsewhere.

[NeilP]

My new BM's arrived today, so now I have 4...Enough to connect to all the balance leads paralleled up on my 20s4p pack.

I went down to just under 70 volts by the time I got home this evening...and some sets of 4p cells had gone down to 2.79...where as others were still up at 3.56

Put the pack on the Meanwell for a bulk charge, with all the BM's connected up, and it is easy to see the volts of each 4p set of cells coming up...Without any balancing it is surprising how quickly the pack comes back up to balance by itself with a re charge

From a max split in cell voltages of 0.9Volt, it went to a Max split between cell volts of 0.3 within less than a couple of minutes. The low cells picked up really quickly, so that within 40 mins of charge they are all back up to within 0.03 volts This is an 8 amp charge rate...all cells currently up at 3.97 to 4.00 volts

it is a shame that the BM balance settings up go up by .1 volt. I had been charging to 4.15 per cell, so had hoped to set the BM's to 4.15 also...but it is either 4.1 or 4.2. What I reckon I will try is bulk charge and set the BM's balancing to 4.10 and just keep an eye on them as they charge. Once they all hit the 4.1...then stop the balance on the BM's just use them as monitor, and power up the bulk charging again to bring the pack up the final way ...maybe thinking of taking he packs up to 4.17

[Bazaki]

Today I did some tests with my 18s lipo set at LVC 64v at the controller.

It will stutter if the pack is 50% or lower charge at draining 80 amps, but I guess I have no other option.
I completely discharged the pack untill the bike was really dead.

I checked each cell with the cell log and all of them were very nice 3,7v without load, so that means they have about 10% of charge left which is a good thing.

If I set the LVC at 63v I have more risk at imbalance.

So I guess for a 20s lipo pack 71v is perfect.

[icekreme1002]

Just wondering if anyone ever found out if adding resistors in the empty spots on the battery medic actually increased the discharge rate?

Assuming you could use resistors with a heat sink or fan of some type, is there an ideal resistance on each channel to boost the discharge closer to 1amp per channel?

Cant really seem to find an answer to this anywhere, just hints here and there that it may work...

Cheers for any help

[Arlo1]

Just wondering if anyone ever found out if adding resistors in the empty spots on the battery medic actually increased the discharge rate?

Assuming you could use resistors with a heat sink or fan of some type, is there an ideal resistance on each channel to boost the discharge closer to 1amp per channel?

Cant really seem to find an answer to this anywhere, just hints here and there that it may work...

Cheers for any help

I did it and it doesnt seem to help I think they moniter the amp draw and limit them selfs!

[icekreme1002]

Thanks Arlo1,
I pulled the thing apart and had a play myself. I am very new to the electronics worls so didnt really know where to start but heres what i have done.

Each channel is roughly 10.9ohms.
I added a 3w 6.2ohm resistor on a few channels hoping to increase the rate through higher wattage, or by 50% change in resistance.

Unfortunately I cant work out how to use the multimeter to check the difference between channels properly (newb!). When i cut a balance wire and put the meter in series, the medic doesn't seem to work at all.

The new 3w resistors get extremely hot. And the stock resistors in the modded channels seem to get just as hot to the touch as the stock channels. So maybe the extra generated heat is burning more power....? Wish i could check somehow...

Anyways, i plan on boxing and fanning them, and hooking in a cell-log for the hvc.
Something like this...




But if i cant get more power, the fan etc prolly isnt worth it, and ill just worry about the HVC.

[NeilP]

Well, if it is working...it should be easy to tell..the voltage on the modded channels will drop faster...set it to discharge and monitor the voltage of the cells on the BM screen

[icekreme1002]

yeah cant test that until i get my $1 40mm fans from China... haha.

The 3w resistors heat up really quick and get a little to hot for my comfort.

If i get time tonight i will get a 120mm fan out of my computer and see how it goes and report back

[icekreme1002]

Neilp I took your advice.
I found a 80mm fan and hooked it all up to a 5000mah 6s zippy.
Here are my results.



Thats about as far as i care to take my cell out of balance for now, but I guess it confirms its working .

So just to confirm, each channel before mod measured 10.9ohm, i added 6.2ohm 3w metal oxide resistors to channels 3-6 and ended up with 5.2ohm on those 4 channels.
I figured this would give me 1/2 the resistance and about twice the watts, so if something could happen it would. haha.
And I have ended up with about 40-50% more powerful discharge rate.

Can someone explain the electronics behind this? is it the increase i wattage that allows for extra power to be burnt?
If so, is it as simple as working out the watts on each channel then using a percentage to work out how much your willing to push the thing? Or does resistance come into play too?
If someone could explain this to an electronics newb, i would appreciate it.

[bigmoose]

The power being dissipated in each of your balance or "load" resistors can be calculated as Watts=Voltage^2/Ohms.

There are two ways to approach this, let's see if we get the same answer. You used a 10.9 ohm resistor in parallel with your added 6.2 ohms. Let's say the battery voltage is 4 volts.

Power dissipated in 10.9 ohm resistor = 4^2/10.9=16/10.9=1.468 watts
Power dissipated in 6.2 ohm resistor = 4^2/6.2 = 16/6.2 = 2.581 watts
Total dissipation = 4.049 Watts/channel

We could look at the parallel equivalent resistance 1/r = 1/10.9 + 1/6.2 Rparallel = 3.952 ohms

Total power dissipation Watts = 4^2/3.952 = 4.049 watts!

So paralleling the internal resistors does not change the power dissipated by the internal resistors.

Be careful adding lower and lower external resistances. There is likely a P channel FET switch in the resistor chain. At some point you will exceed it's ratings. Keep an eye on the temperataure of these FETs. They will likely be the 3 lead parts near the surface mount resistors, and equal in number to the resistors.

[icekreme1002]

Thank you very much big moose.

I'm not sure if i was quite clear enough on what I have done but i get your explanation.
Cheers.

Each channel stock measured at 10.9 ohms.
I wanted to 1/2 resistance so added what i had, a 6.2ohm and ended up with 5.2ohm. (realise that does't add up, cheap mutimeter).

So I guess this means to start with power dissipated was 1.468 watts. (10.9ohm)

But after the mod its up around 4 watts.

I am far more than happy with that increase, I guess time will tell if it lasts long term.
I will also look to directing some of the airflow toward the FET's.
Cheers again bigmoose!