SMC race Formula 7000mah packs have a really low IR

With the Nanotech they need to be cycled quite a few times before getting to their top potential. I run weekly a 6000mah Nano-Tech and it's just starting to come alive and not Fall on it's face after about 30 cycles

To cycle I use a old LRP charger that has a cutoff voltage for 1cell at 3.6 (thats as low as it will go) and it discharges at around 8A slowing down as the voltage drops. then I charge at 1C (or 6Amps) to full.


A Simple Test of IR for 1 cell packs is to use any current and Voltage meter. as long as you always use the same one the number will mean something

I use a Digital Multimeter for current and the voltage reading on the discharger.


So to see if your NanoTech Pack is getting better over time use this method. I also like to watch the voltage and the mah removed to see where the voltage will be after a run since I know I usually put back in 1500mah per run it's nice to know

Charge the pack and let it sit for min 20 min to settle the temps.

Have a pen and paper ready. Start the discharge and write down the start voltage ( usually 4.2 ) and then count to 10 and write down the voltage and current measured. the 10 seconds gives the lipo to settle from the instant current.

Now subtract the second voltage number from the First and then divide that by the current

eg. 4.2-4.12 = .08/8A = .01 Ohms or 10 milli Ohms

Don't worry about the number exactly but this gives you a baseline for that pack

my 1s packs are around 4-6 milliOhms by this method and when tested on high end equipment end up around the 3-4 as most testers only draw a small amount of current.

When you get down into the milliohm level, battery resistance becomes less important, let me explain: (wall of text warning)

1 foot of 12 gauge copper wire (solid, not stranded) has a resistance of 2 milliohms. 16 gauge wire, 4 milliohms. A 5mm plug connector to the battery would be another few milliohms as would the solder connections to your ESC/motor. So lets say you really go overboard and wire your car for LOW RESISTANCE. Big chunky wires everywhere... well, in a car that uses 6" of 12 gauge wire and two 5mm connectors, and four well-done solder joints from the battery to the ESC, then 14" (super short motor leads) of 14 gauge wire with six solder joints from the ESC to the motor, you've added up to this:

(2 milliohms from the 12 gauge) + 2(1 milliohms for each plug) + 3(3 milliohms for 14" of wire from the ESC to the Motor) + 10(0.5 milliohms per solder joint... I'm being generous here) adds up to 2+2+9+5 = 18 milliohms in your wiring at room temperature.

Seems kinda high, compared to the claimed internal resistances as low as 2 milliohms I have seen claimed a few places. Not much difference between 0.019 ohm and 0.020 ohm.

To measure resistance in batteries with your charger, it's even worse. I can discharge my packs at 35 amps, but my leads are 12 gauge and if each is 18" long with bullet plugs, there's far more resistance in my leads than my pack. As a result, my measured internal resistance can be quite high.

I think that the best packs are not the ones that fight for the lowest possible internal resistance. The best packs have the highest average voltage under load over a full run.

In my experience, low IR packs, almost always have the best avg v as well
Both test, seem to be a good indication of the packs potential

Good point @DesertRat While the rest of your wiring adds to the resistance the battery with the lowest IR will have a higher voltage vs discharge rate than a battery with a high resistance.

Which is why I discharge a pack down to 3.6 while watching the mah removed vs voltage to get a better Idea on how your pack preforms over time. If you had a v3 hobbywing esc where you can pull out come data after a run that would be a better and realistic view of the packs voltage over the 8min run. as well as Amp draw and also how you are applying the throttle and G forces but I digress.

I used on Sunday my Cheapie nano-Tech had no problem keeping up to the fast guys. Save for a crash or 12 when I was actually putting down clean laps times were about equal.


The biggest thing is to know your gear. How to use it properly and how to know when things are not working properly.

Data is your friend


Cheers

Thanks guys ill try and cycle the turnigy before I fork out for something new.

Do you have any advice on finding the sweet spot between timing and gearing? Running a trinity D3.5 maxzilla and was struggling to get it going quickly (probably the cells) warmest it came off was 40 degrees Celsius with the timing on 55degree (25 on the can) which is maxed out.

I notice on the pro driver setup sheets they always run a smaller front tire. Seems most are one to two mm smaller in the front. Why is this?

It's Called Split, More slpit means more steering to a point. you can tune the size difference to your track and driving style. I like 2mm but when on a really high bite track I go down to 1mm

Especially when traction is high, the smaller sidewall of the front tire will help keep the car from gripping too much and traction rolling. Most of all, though, 12th scales just feel dialed on small tires!

Thanks guys. I'll have to try some split. I've always cut the front and rear to the same size. (42.5 mm) I guess its time to cut the fronts on down!

It's going to depend on the goop level. Assuming an unprepped but clean track for asphalt try Ulti Hard front, Ulti Soft rear.

Carpet, well everyone else here will know more than me, but if it's a gooped up permanent track I'd go with black front yellow rear, and if it's a rollout carpet track I'd start with purple front, pink rear, and then switch to magenta front as the grip comes up.

Well, I was running the servo flat to the chassis with the horn above, which meant very little bump-toe, or maybe just a little bump toe-in. So I've now mounted it on an angle, with the horn below, and now there's a lot of bump toe-out. I'll give this a run on Friday and see how I go.

Just to clarify, by the links, did you mean the other end of the steering links at the knuckle?

Lipo Advice
 

Thanks for your help guys, I will try and cycle the turnigy to see if they come good before buying more.

Could you also give some recommendations on how to find the sweet spot between timing and gearing.

I'm using a trinity d3.5 maxzilla, with roll out of ~68mm/rev (same as others at our club), could not seem to get much speed out of it no matter what timing I'm using. It may just be the duff turnigy cells, but any ideas appreciated.

you really need to go by lap times. With the D3.5 motors seem to like a little less timing but it all depends on the track. which turn motor? sounds like 10.5?

Cheers

40.5/41.5 worked well last weekend.

What are your thoughts on the Yokomo R12C3? Wha are Pro's and Con's? I've been racing 1/12 for a year now, and looking for a little bit more advanced than my Kyosho Plasma Ra.

I'm debating between this care and an R125.2.