TRX4 battery hog - Voltage drop?

[OSRC]

The batteries are certainly suspect - especially my 2200 packs. Trust me, I haven't discounted them.

I have 4 of them, and all 4 have some degree of puff. I only use them in my smaller trucks usually - MST's, CC01's, etc. and as backup for my other packs when on the trail. Might retire them all and pick up 2 new ones in any case. They are all about 4-5 years old, I think I got my money's worth.

The 3800's I'm not sure about. Holding on to those for now.

[kincer]

We kinda have gotten spoiled with lipo batteries lol, my Tf2 Marlin came with a nimh battery and I just looked at it and laughed, not even sure what I'll do with it, I thought about using it but not sure I want the hassle of switching the esc to nimh every time I use it for a 15 min run time lol.

[OSRC]

Originally Posted by Voodoobrew

I ran my two months old 40c 3s 1500mAh battery dead putting around in my front yard, in my trx4 with a trailmaster sport, HW 1080, and 9tpinion. When I got it to the charger it was at 9.63v and climbing, that's an average of 3.21v per cell. My guess is medium cutoff voltage on the HW 1080 is 3.2v per cell.
The fact that your batteries read so much higher when they come off the truck makes me think you got old batteries, and by your own admission not the best perception :P

Don't need perception to read numbers off a charger.

Interesting that yours came off at a lower voltage. I would think that the low/medium/high settings would be something like 3.0/3.2/3.4. Ridiculous that the voltages are not spelled out in the manual.

Just waiting on some more battery tools to get a better picture of what the batteries are doing. Planning on playing with performance until then.

On a side note, just remembered the Bronco came with the Bluetooth module. I've never messed with it, but it does have telemetry. Should be able to view/track battery usage somewhat with it I would think. Going to put it to work with the Defender and check it out.

[kincer]

Originally Posted by OSRC

Don't need perception to read numbers off a charger.

Interesting that yours came off at a lower voltage. I would think that the low/medium/high settings would be something like 3.0/3.2/3.4. Ridiculous that the voltages are not spelled out in the manual.

Just waiting on some more battery tools to get a better picture of what the batteries are doing. Planning on playing with performance until then.

On a side note, just remembered the Bronco came with the Bluetooth module. I've never messed with it, but it does have telemetry. Should be able to view/track battery usage somewhat with it I would think. Going to put it to work with the Defender and check it out.

If I remember correctly there is something else you have to buy from Traxxas that hooks up somewhere on the truck to be able to monitor the battery on your phone with the Bluetooth. I only messed with the phone app once so not sure but I think that's the case.

[OSRC]

Originally Posted by kincer

We kinda have gotten spoiled with lipo batteries lol,

Boy, isn't that the truth. Here I am complaining about 40 minutes on the trail. Doesn't seem like all that long ago I was hoping I could make the 3 minute main on my 12t modifed and NiCad packs. Of course, it was what? 30 years ago?

Remember the chargers with the mechanical timer? Set to 15, let it charge, and then crack it back to 5-7 more minutes until the battery got warm? There's still a melted spot on the floor of my pop's house when the timer broke once and my NiCad melted down.

[kincer]

I was thinking also, first gear may be more acceptable to you on 3s, when I got out of the hobby 3 years ago all I ran was 2s and I was perfectly happy with that, I felt there was no need for 3s. When I got back into the hobby I bought my Bronco on eBay and it came with a Traxxas 4000 3s and a imax charger so I thought hey this great I'm getting a charger and a battery for less than a new Bronco. I charged it up and ran it and I've never looked back I have 6 3s batteries now and run all my rigs on them. So much more wheel speed when needed on my single speed rigs and about the only time I run high gear in the Bronco is when I'm goofing off, it's pretty fast on 3s in high gear, low on 3s to me is perfect for the trail though.

[Voodoobrew]

Originally Posted by OSRC

Interesting that yours came off at a lower voltage.

Not realy, well maybe its interesting, but its also predictable.
Kinda hard for to explain in a forum, but this is still voltage sag (probably worth googling instead of reading my golboldy gook that follows)
(google it now)

(its still not to late)

lets say you have a shiny new 100 volts battery, its just sitting there, you put a volt meter on it and its exactly 100 volts (because is hypothetical!). Now we directly connect your standard 10t 20amp (sure why not since were making up numbers anyways) hypothetical dc motor on the battery, you might believe that we are giving that motor 100 volts. Well were not, were actually giving the motor 75 volts (more made up numbers) because the amp load on the battery is more then the battery can dish out. After a minute of running the motor we disconnect it, since we already have a volt meter watching the battery we will see the voltage rise form 75 volts to 99 volts.

Now if you have a low voltage alarm set to go off at 76 volts it will trip as soon as you connect the motor, even though your battery reads 99 volts when you have removed the motor. That is voltage sag.

This time we just got 50 shiny new 100 volts batteries and we wired them all in parallel, we put a volt meter on them and we get 100 volts (hypothetically predictable). Now when we put our 10t 20amp hypothetical dc motor on the battery our voltage drops to only 95 volts instead of 75 volts. The voltage sag is much less because combining the batteries in parallel lets them pump out more amps at once, and your dividing that 20 amp draw over 50 batteries. So each battery acts like it has a 0.4 amp load.

So running batteries in parallel will help to limit the effects of voltage sag.

on an rc car the esc and terrain make the amp draw constantly change. Increasing the amp draw increases the voltage sag.

The maximum amp draw you can put on your battery is by completely binding up the motor and giving the car full throttle. (you probably don't do this)

One of the next biggest amp draws mashing the throttle from a dead stop. This can be like 10 times the amp draw of normal driving, and its high amp draw will cause extra voltage sag that will trip the low voltage alarm prematurely.

(I bet you wish you just googled it like I told you in the beginning)

So thats part of why my casual driving driving on a new battery had a lower voltage when we measured them then your more (I cant remember what word you used) extreme driving on old batteries. When the low voltage alarm tripped, both batteries were actually at the same voltage of 3.2

[OSRC]

Gotcha - I get the idea of voltage sag. Just not clear how I can test it with my limited tools. My Turnigy tester is here, and going to strap it into the truck on the next run. Should provide some better insight into the batteries as I can monitor the voltage in real time.

I bench tested the cells, and while they charge and balance fine, they are definitely getting old. I checked the resistance and here's what I got:

2200 packs:
27/32o
24/35o (!)

3800 packs:
16/39o (!)
21/20o

One of my 3800 packs is way out of whack. I don't know the initial values since I wasn't able to test previously, but a bit concerned about that first 3800 pack. I would think that IR values that are not in line between cells might cause some problems. Especially in higher draw vehicles like my Exo and Slash. I checked a newer pack I purchased for my Tamiyas (3000mah) and got 06/07o .... Hmm. Actually, I think it's time to inventory all my cells. I've got 12 lipos here in various sizes, some of them quite old. Again, I never paid too much attention to my cells unless there was an issue. Think it's time to dig in and learn some stuff. Will definitely be replacing some. I think it's time.

So, battery issues aside, got the single speed in and installed. Trans was full of factory grease and everything looked great inside. Cleaned it all out, soaked the bearings for a few days, and reassembled with my magic old grease and the single speed using the stock motor. The entire driveline is now smooth as butter, silent, and slop free just as the RC Gods intended.

I geared up with a 15t pinion (which was far happier in the 16t holes BTW) which brought the gearing from 65.6:1 (in low) up to 47.4:1 - much more in line with my other rigs. Didn't realize first gear was so low until I started digging around- 65:1 is crazy low for a trail rig. Think it's still a little low geared overall, though I'll wait to see how it drives before changing it again. Did order the smaller spur just in case I need some more options.

Also made some changes to the ESC - I had the punch and initial throttle all the way up. Softened them up a bit to lessen the load on the batteries. In fact, changed a few other rigs to the same settings while I was at it.

Buttoned up and ready for the next round on the trails - hoping to get out this weekend if the weather holds. Now armed with all this knowledge, curious to see my runtimes.

[Voodoobrew]

Originally Posted by OSRC

I geared up with a 15t pinion (which was far happier in the 16t holes BTW) which brought the gearing from 65.6:1 (in low) up to 47.4:1 - much more in line with my other rigs. Didn't realize first gear was so low until I started digging around- 65:1 is crazy low for a trail rig. Think it's still a little low geared overall, though I'll wait to see how it drives before changing it again. Did order the smaller spur just in case I need some more options.

Also made some changes to the ESC - I had the punch and initial throttle all the way up. Softened them up a bit to lessen the load on the batteries. In fact, changed a few other rigs to the same settings while I was at it.

Personally I run 80:1 to 100:1 in my rigs and get a faster motor and higher voltage.

Originally Posted by JohnRobHolmes

less waste heat at the same wheel torque and power, better startup, better drag brake, higher peak torque, more consistent vehicle during load changes and incline changes.

Originally Posted by robert@castle

Brushless motors have a minimum RPM that they will spin smoothly. We tested some 2 pole motors and most of them were 600-800RPM depending on the brand and the wind. On our slates we were able to get to spin smoothly at around 100-150RPM on V2.02. Minimum RPM doesn't change much with voltage as long as the ESC has enough PWM range and throttle resolution to handle the higher voltage.

Even with a slower spinning motor, the more you gear down the slower the truck will run with that minimum RPM. So really following Holmes' advice could actually make your truck run slower, but then give your more power when you want/need wheel speed. There really is no downsides to Volt up Gear down besides having to buy higher voltage batteries.

When people drive my rigs they are baffled at how I can get so much low end control and torque from a $20 motor on a stock axial remote. 21t 550 trailmaster, on 4s and 100:1 reduction, works like a charm for me.

I run a 9t pinion in my trx4 barely use 2nd gear. In my opinion 2nd gear is for racing or rock bouncing not crawling, why call it a crawler if its so fast?

[JohnRobHolmes]

I did spec a nicely shaped rotor and stout magnets in the TrailMaster. Your truck is set up well, but the $20 motor ain't no punk either. Combined, I'm sure it drives very well!

[OSRC]

You guys are spot on.

Got the TRX out for a nice long trail run yesterday (and met a nice group of guys from central PA). Walked it back to my truck after both 3800 packs and an older 4000 pack bit the dust before we got back to the parking lot.



I did get further than before for sure. Changing the gearing and freeing up the driveline helped, as it should. I dialed down the punch on the ESC and think I went a bit too far - the truck felt really soft on the bottom end. Might have just been the batteries too, not sure. The BOM I ran later had the same settings but felt more punchy. I'll play with the settings some more once the new batts come in. Much happier with the new gearing overall, 60:1 was just too low for my setup.

The batteries are utterly shot. Thanks to my little battery tester, I can now see what's going on. As soon as LVC kicked on, I hooked it up and saw 3.9v per cell. I held onto the rear bumper and gave it some throttle and watched the volts drop...down to 3.2 before I let off. Volt sag aplenty. Always good to have data!! Should have bought one of these a while ago.

I haven't bought batteries in years, so it's time. Went through my stash and found 7 of my 12 lipos were showing resistance at 30o and up. (the 4000 pack above showed IR at 160o on one of the cells. Ouch!) Decided to retire all 7 with honors, and today is discharge day (between making batches of cookies). Bring on the salt! Will be ordering some fresh packs today too.



Thanks guys for your input - this has been quite the learning trip.