TRX4 battery hog - Voltage drop?

[OSRC]

Looking for a little advice from the RC experts.

My TRX4 has always been a battery hog, but this week things got out of control. In less than 4 hours, I went through over 10000 mah. Can't be right.

I use 2S 30c 3800mah packs on my trail runs.

The first lasted quite a while, and went though about 2800mah before LVC cut off the fun.

The second pack only used up 1400mah before calling it quits, mostly in first gear running.

I had a few 2200 mah packs as backup, and got less than 30 minutes out of one. Used 400mah total. Battery, motor, ESC and servos are just barely warm to the touch each time.

I think that the water running this week had gummed up the axles, which is why the later packs called it quits so early, but I've had this issue since I bought the truck. With the stock electronics, it was tripping LVC way early too, usually with using only 1400 or so mah. I've now swapped to a 1080 ESC set to cut off at 3.2v/cell and a stronger servo, still running the stock motor. Pretty much no difference today. No particular driveline issues I can find, servos all seem fine. It's a bit of a mystery.

I'm wondering... my truck is pretty heavy - about 10.5lbs. I tend to run in second gear mostly at low RPM, switching to low gear when I get to more technical terrain. I dropped down to a 9t pinion so second was a bit more trail friendly. Does running at low RPM in a tall gear like that on a heavy truck pull more amps than running in low gear at a higher RPM? Is that causing voltage drop on my batteries which is tripping LVC?

By the time I get home, the batteries have recovered somewhat so I'm not 100% sure of the voltage right when they come out of the truck, though my charger tells me the capacity put back into the pack. I'll be taking a volt checker next time so I have more info, though I don't have any reason to assume the LVC is incorrect. I'm not sure though how I can be killing the volts while leaving so much capacity.

Any thoughts would be most appreciated!

In the meantime I'm pulling apart the axles and trans for cleaning and re-greasing. Well due for some maintenance! Also debating removing the brushed setup and swapping in brushless eventually as ANY efficiency gains would be most welcome at this point.

 

[svt923]

JRH has stated in many related threads that brushed motors are most efficient when spinning as fast as possible with as much gear reduction as possible. Not being an expert in brushed motor theory and application, I would guess a motor spinning slowly with a low amount of gear reduction (such as a TRX4 in 2nd gear at partial throttle) would produce the opposite result.

 

[kincer]

You should get longer runtime in low gear, speed runs in high gear should be what kills the battery, I almost never run in high gear unless I'm just goofing off, but when I run low gear I get lots of runtime with 3s 3600 packs. I've never really measured the time because it never seemed short. Maybe running slow in higher gear with the rig being that heavy is what's doing it, maybe one of the micro servos pulling some juice maybe over or under thrown? I haven't had the issue but just guessing it's running it in high gear alot. I'd try running low gear the whole run and see if it changes. At least if it does it that way too you can rule that out. I'm running same 1080 esc with a ds3218 servo, stock mini servos with endpoints adjusted.

 

[OSRC]

I did my second pack entirely in first gear yesterday, and I got less runtime out of it. By then though, I had been in water most of the day - I'm sure there was lots of drag on the axles by then. Heavy foams, grease/water goo, etc. so i'm inclined to toss that result. I have tried just running in first before and didn't really seem to make much difference.

I never really pin it in either gear, I'm never out doing donuts or anything. Usually doing walking pace, usually in second gear at low RPM. First is for rocks, hills, anywhere that's technical. I shift a lot actually, I think it's one of the cool feature of the truck. I do think second is a bit tall compared to first, wish the gear ratios were a bit closer.

 

[Voodoobrew]

As batteries age they begin to produce what is called voltage sag. The effect you will see is when a large amp demand is placed on a battery the voltage will drop further than on a new battery. This sag will cause the low voltage alarm to trigger sooner. This does not mean the battery has less capacity for low amp draw situations.

Example with fictional numbers.
New battery 5000mAh
mAh at 1amp drain 5030
mAh at 20amp drain 4500

Old battery 5000mAh battery
mAh at 1amp drain 4950
mAh at 20amp drain 2500

The higher the draw the more obvious the voltage sag is.
Most volt meters won't place much load on a cell when testing so voltage alone is not a very accurate way to measure a battery's charge. You can charge a nicad for a minute and have it show full charge, but the second you hit it with a load it's going to be dead.

In an rc car the more resistance there is against the motor to turn, the higher the amp draw will be. If you lock your motor from spinning then mash the throttle, you will achieve maximum amp draw for that motor, and all that amp draw will cook your esc and motor in seconds. Amps create heat. This is why tall gears make motors run hot even at the same voltage.

Also if you run in water, which you do, you need to check your bearings and gears more often for grease and friction. Lock the diff servos and Remove the motor mount then spin the wheels to see if there is more resistance. They should spin freely as the motor should be the only real source of resistance in the drive train.

Sorry about all the typos, my phone keeps trying to help me spell...

 

[Hydrocarbon92]

The problem is even a worn lipo won't have much voltage sag for a brushed 550-size motor at low load.

It's possible the batteries are just getting old. A 3800 battery that only uses 1400mAh but ends at 3.5v resting means it's capacity is reduced to 1/3 of new. The stupid-low LVC in most ESC's assumes people will hit them at near full throttle/load, and forcing it well below that with constant low loads can really hurt the battery.

As far as the motor goes, peak efficiency will be between 50-75% max rpm for most. Running it a low-rpm high-load will suck considerably more juice.

 

[OSRC]

Thanks fellas, good stuff.

My batteries are quite old (about 3 years at least) and slightly puffed to boot. I'm not the best at battery maintenance either. They do run fine in my other trucks which is the kicker, though they are all far lighter than my TRX porker. I'm going to try a little experiment and drop one of those 3800mah packs in my Slash and run it out...definitely has some amp draw in that thing. Curious to see how it does.

I usually never run in water, today was a first. I'm that guy who picks up his truck and carries it over the creek. Couldn't be helped on the last trail run though. I've pulled the truck apart to clean it and found a few issues that might be contributing. Spur turns very hard in first (but not second...odd)

 

[Ditchrat]

Looking for a little advice from the RC experts.

My TRX4 has always been a battery hog, but this week things got out of control. In less than 4 hours, I went through over 10000 mah. Can't be right.

I use 2S 30c 3800mah packs on my trail runs.

.

This weekend when it was colder then a witches tit?

 

[kincer]

I totally forgot about the cold temps too, that will also affect run time.

 

[83trekker83]

my trx4 with 5000mah packs that are under 4 months old, never could get an hour with them between low and high gear use. Always thought that was not good.

What do you guys set your HW1080 for low voltage cut off?

 

[Ditchrat]

Funny thing about perception is that more often then not its wrong.


First thing I did with my lipos was keep track of how long I ran them, then how many mah they consumed. Granted I have a course in my back yard, so it was easy to run the same thing over and over again with different batteries.


However eventually I was able to establish a consumption rate of 50mah per minute. Basically a 2200mah battery should last me 45 minutes of trailing. If I don't get that then generally I look for a problem. I could recount several times where batteries low voltaged prematurely and I found frozen bearings, something binding, or extreme cold.


Even if you just walk around your block for 30 minutes then top off the battery, it isnt a bad idea to try and get a good average fuel economy.

 

[OSRC]

This weekend when it was colder then a witches tit?

it was OK here, about 50 degrees. Shouldn't be affecting batteries too much.

What do you guys set your HW1080 for low voltage cut off?

Mine's set at 3.2v

First thing I did with my lipos was keep track of how long I ran them, then how many mah they consumed.
Even if you just walk around your block for 30 minutes then top off the battery, it isnt a bad idea to try and get a good average fuel economy.

Great thinking, very scientific. I've got a pretty good "battery clock" in my head. Needed it back in the days of 12t brushed and 1500mah nicads. Having said that, don't really need it to know that 25 minutes out of a 2200 mah pack is not good though. "thumbsup" I might do a little 10 minute drive and re-top the pack and see what it eats up

 

[OSRC]

A bit of an update....

I switched out to a 9t pinion a while ago thinking it would help runtimes. Clearly not so much. When I installed it, I noticed the truck was quite loud in first gear...it was all but silent before. I chalked it up to the pinion mesh issues the TRX is known for. Seemed to be spinning ok, so I didn't investigate further.

Anyway, I tore down the entire truck over the past few days. It was time, been running this truck hard right out of the box for a while now. On pulling the motor, I noticed right away that the spur gear turned really hard. As soon as I pulled the driveshafts, the trans spun fine again. I cleaned out and re-greased the axles (the front was turning really hard as well - filled with water and grease goo - and found the driveshafts were binding terribly. I played around with them for a bit and got nowhere, so ordered some SSD's to replace them.

So clearly, I had some drivetrain binding. I assume once I bolt the motor back in, I should be much quieter, and with the drivestrain spinning free I should get some efficiency back.

Odd that the motor and ESC were completely cool to the touch. Motor had to be working hard...Not only is the Beard heavy with the Defender body, but I've got knuckle weights and was running weighted wheels last weekend. It's a lot of heft to be moving around and churning though lots of thick mud to boot. I've never had anything get hot, even over the summer months so the drivetrain binding is kind of a surprise. Usually a hot motor is the first clue. I'm sure those center shafts have been locking up for a while as the truck has been getting louder over time.

I'm still not sure how I can be tripping LVC with so much capacity left, but common sense tells me that freeing up the drivetrain should make a massive improvement.

 

[kincer]

Please fill us in to if this corrects the issue, I'm running the 1080 with 3 cell one is a 4000 and the other two are 3600 and it seems to run forever, I have weighted wheels as well but no brass as of yet. Just interested in case I have the problem in the future, Thanks

 

[OSRC]

I certainly will! hoping to have a chance to test it out this weekend. 61 on Sunday? Nice!!

After tearing down the truck, I'm pretty sure the binding in the drivetrain is the root cause. If the spur is hard to turn by hand, for sure it's going to create all kinds of problems. What I did was:

- Replaced the center shafts with SSD's - the stock ones were locked up solid.
- Pulled apart the axles, cleaned and regreased all the gear surfaces with a lighter grease. Soaked the bearings as well to loosen them up. Some of them were turning quite hard, sealed or not.
- Note that adding the spool really loosened up the axle as well. The diff is complicated with all those bearings and what not - even when clean it turned a bit harder than I'd like. With the spool installed, it spins noticeably smoother.
- Played with gear mesh - went back to an 11t pinion (purely for performance purposes) in the D hole. Where it's much happier. I might add. There was some metal dust inside the motor cover, so it's been working hard for sure.

I put it all back together and wow, what a difference. Before i put the motor back in, I checked out the spur and it was like warm butter. Everything is so much smoother and the truck dives almost silent now.

Packs are charged and ready to go. Curious to see how much difference all this work makes.

 

[discgolferlee]

combo of older batteries and binding drivetrain...I am surprised you were getting even the run time s you were. keeping that drivetrain smooth and bind free will be your biggest gain. Even though it's not really safe to keep running older/puffed batteries, you should notice a big difference just by getting rid of that resistance being provided by drivetrain issues you found.

 

[OSRC]

Ran the same loop as last weekend today, with slightly warmer temps outside.

I made it 41 minutes on my first 3800 pack, using 1387mah before LVC kicked in.
Second 3800 pack made it back to the truck, about another 40 minutes but it was limping back there - down on power, LVC was about to kick in anytime. 1164mah used.

So freeing up the drivetrain didn't make any difference (other than my truck being much quieter!!) on battery consumption.

Really odd.... Still not sure what to make of it. I do notice possible volt drop through the run...truck seems to have noticeably less power after about 15-20 minutes.

I'm starting to lean toward it being a battery issue. I run these 2 hard pack 3800's pretty much only in this truck since they fit perfect - wondering if they are on the way out. But then, had this issue with a 2200 pack that I know is good, so again I'm a bit confused.

I'll swap around a few packs between trucks and see what happens.

 

[Voodoobrew]

did you try those 3800's in other cars?

 

[OSRC]

I have in the past, though not lately. I generally run them in trucks that need the wires coming out of the top as opposed to the side due to the battery tray configuration. Ascender and CR01's also use them exclusively as do my Slash, Exo, etc. since they are hard case - never had an issue before. Do use them quite a bit.

I plan to run them in the Slash here shortly. Plenty of amp draw there. If there's a problem with the pack, I should notice it pretty quick. It is way cold here this week, just waiting for a slightly warmer day.

Next time out with the Defender, I'm going to run my 5000mah pack as a comparison. Doesn't quite fit in the tray, but I'll be able to rig up something for a single run.

 

[JohnRobHolmes]

At this point, it is either battery condition or your LVC isn't actually at 3.2v per cell. Slight chance you might have a bad solder joint on the ESC plug, but it would be smoking hot to cause LVC so soon.


I typically burn through 3000ish mah per hour though, with a brushed setup. It doesn't sound like you are too far off. I would recommend you run in first gear more though. The ratio difference is about 2.5:1 between first and second, which means you will eat up motor brushes 2.5 times faster by running in second gear instead of first for any given pace of driving. It's not as big deal for brushless, but running in first will still be more efficient and net longer runtimes and cooler running electronics.