RCM Lipo impressions

[BrianG]

I picked up a couple of Mike's new lipos (5s2p 5Ah 25c) and ran them around a little. I don't have an eagletree and the outside temps are a bit nippy, but they ran great!

After having problems with my GTP and MBX5/CORR conversion (blew ESCs), I decided to discharge the lipos for storage along with my 4s2p 5Ah Maxamps pack. But I figured I would do a little testing at the same time so I charged everything up fresh.

I use a custom CC discharge circuit (darlington transistors and power resistor setup) that will draw 10A from batteries ~10v up around 5s (the transistor and resistor setup gets quite hot even with a fan at 5s - but that's 185w too). I hook up a balancer and use a voltmeter to monitor cell voltage to get to my desired 3.5v/cell storage level.

With the MA 4s 5Ah pack hooked up, that 10A load dropped the voltage by 0.48v. When it hit my 3.5v/cell threshold, it had taken out about 2300mAh. Various cell LEDS on the balancer (LBA10) would flicker on and off during this time. Removing the load brought the no load voltage up to 3.73v/cell.

With Mike's 5s 5Ah packs hooked up, the same 10A load dropped the voltage by around 0.08v (fluctuated between 0.07 and 0.08v). When my 3.5v/cell threshold was reached, it had taken 4.36Ah out of the cell. Cell 3 on the same balancer flickered for about 30 seconds once. Removing the load brought the no-load voltage up to 3.67v.

Cell resistance is not linear so you can't really say how much voltage will drop at the rated C value simply by using the 10A load and voltage drop figures. Yeah, it's not the best test, but it certainly hints at a lot as far as performance goes.

[johnrobholmes]

Dang if that isn't a huge difference in drop! Only at 10a too!

[BrianG]

Yeah, the drop is quite a bit different, but also the amount of mAh taken out of equal capacity packs @ 3.5v/cell is huge too. Tells me the discharge curve is far flatter for more of the charge.

Eventually, I'd like to build a 100A+ constant current device to test the higher currents, but dang, the transistors and power resistor setup is kinda big. 10A takes two 2N3055 transistors, with an NPN darlington transistor driving both bases to keep the current gain high (prevents loading, and resulting v drop, of the regulator that drives everything). 100A would take 20 of those transistors, not to mention the fans and heatsink! 100A @ 18.5v is 1850w... that's a LOT of heat!

I'd like to use FETs, but not sure how to drive them in linear mode as well. They would definitely be much easier to parallel (no current-hogging due to negative temp coefficient) than transistors...

[JThiessen]

Originally Posted by BrianG

Yeah, the drop is quite a bit different, but also the amount of mAh taken out of equal capacity packs @ 3.5v/cell is huge too. Tells me the discharge curve is far flatter for more of the charge.

Eventually, I'd like to build a 100A+ constant current device to test the higher currents, but dang, the transistors and power resistor setup is kinda big. 10A takes two 2N3055 transistors, with an NPN darlington transistor driving both bases to keep the current gain high (prevents loading, and resulting v drop, of the regulator that drives everything). 100A would take 20 of those transistors, not to mention the fans and heatsink! 100A @ 18.5v is 1850w... that's a LOT of heat!

I'd like to use FETs, but not sure how to drive them in linear mode as well. They would definitely be much easier to parallel (no current-hogging due to negative temp coefficient) than transistors...

Couldnt you get an old audio amplifier and make something from that? I just threw one away that had been in my garage for some time. Wouldv'e paid that forward to ya.

[rootar]

yea mikes new lipos are BADASS! they run stone cold and have unreal punch we are running it with 1515 2.5 in a st-rr and its an awesome, i have the same setup coming for my truggy this week. plus since they are shorter than the enerland cells it leaves more room on the chassis

[brushlessboy16]

I was looking at these for my matrix, seem like they are worth it :) Mike better stock up for xmas

[MetalMan]

Couple things before I comment on your test:
1) BrianG actually blew ESCs? It's about time!
2) Most manufacturers state a storage voltage of about 3.8v/cell for the newer generation cells (older cells used 3.70-3.75v).

Now, your results from that test indicate excellent results. I just can't get over the fact that you used a fan... At least you had an obvious reason for doing so.

Originally Posted by MetalMan

1) BrianG actually blew ESCs? It's about time!
... I just can't get over the fact that you used a fan... At least you had an obvious reason for doing so.

my sentiments exactly

I recently received one of Mikes new packs and they are indeed great!

[BrianG]

lol, the MMM V1 flamed to a crisp. The odd thing about that is it was running great with super temps of ~110*F (I checked periodically). I stopped for 10 seconds while talking with glassdoctor then it just started smoking and then flamed. The ESC is completely destroyed. It desoldered the power wires and crisped the BEC wire. The BEC still works fine once I replaced the wire. I sent that ESC in for a V3 replacement (but I did say I would take several bad V1's instead - but marked to void future warranty claims. They said they will send a V3. I tried. ).

The HV110 popped a capacitor spewing cap guts onto my GTP windshield. Must have been a weak one because the other 5 look perfect (no puffing). Sent that in too.

This sucks though because until now, I've never blown an ESC. I'm getting a Quark 125B to replace the HV and will run it on 5s. The V3 will go back into the MBX5/Corr, but I still think the Quarks are smoother. If I could get another Quark 125B instead of a MMM, I would in a heartbeat.

And yeah, I don't mind using fans, where they make sense to do so. Dissipating 185w in a stationary setup requires a fan ... or an EXTREMELY large heatsink ... and I would like to have some work space on my 8' X 3' workbench.

I set a 3.5v/cell termination simply because of the 10A load. I anticipated more voltage "rebound" once the relatively high load was removed. I should use 3.6v/cell when doing this to Mike's new cells since they drop quite a bit less, but anything from 3.6v/cell to 3.8v/cell should be just fine.

[q8-maxx]

I just received my packs (2 2s 5200mah hard case). from Mike all the cells are 3.85v reading in the balancer. its take about 40min to fully charge for each pack at 2.5amp
now all the 4 cell reading 4.20 on the charger.

Do I have worry about my new MMM (just got it from tower last week so I think its v3) or its ok with Mike lipos?

my setup e-maxx + 4.5 hv motor + 2x2s 5200mah hardcase + MMM + 12/72 gearing

but mike lipo's looks great and nice built all the cells are equal. its seems they balanced very well.

[RC-Monster Mike]

The batteries won't be the weak link. If the controller is a V3, it should say V3 on the bottom of the box/package it came in (at least my V3s do). :)

[johnrobholmes]

Yep, all v3 controllers have a big fatty v3 sticker on the bottom.

[q8-maxx]

Thanks. yes there are v3 sticker on the bottom

[BrianG]

I must have missed your response JT. Yeah, a car amp heatsink would work, but still not nearly big enough for 1850w. Even a large 1000w rms car amp only has to dissipate ~600w (class AB amps are around 60% efficient), which is one third the power that I would need to. And amps don't run at their max power constantly, unless you like listening to sine waves, so actually it is dissipating probably around 200-300w. So, I would definitely need a really big sink AND fan setup for this!

Anyway, I played around with Mike's new lipos today. I ran down a pack to LVC. Let me tell you, they dump quick! One second you have full power, the next you're done. Usually, if you let the pack sit for about 10 seconds and try to move again, it will have enough oomph to get the vehicle back to you. Not these. When they're done, they're done. Period.

I put the pack on the charger and it was right at 15.1v still after about 5 minutes. Not much rebound (good!). As soon as I started the charge, the voltage increases quickly. I only put about 400mAh before the pack was at 18.5v. The voltage increases slowly from there to CC/CV. So, the discharge/charge curve for these cells are really flat! And the cells stay within ~.08v of each other at any time so far. So, as you can see, I have nothing but praise for these so far.

[JThiessen]

And I missread yours! I didnt see the 1850W....I had actually been referring to home audio, but even those wont achieve 1850W....well, anything that I'd throw away wouldnt achieve that.