Too high of gears make batteries fry?

[RootBreaker]

Ok so my FOFF is up and running and working kinks out. I kinda threw a big sum of it up in my Car FOFF thread.. but no one answered it there as maybe just didnt see it so...

I was told for gearing on a brushless setup you want to be able to touch the motor for 5 seconds..

well I am running 32pitch 56t spur and 19t pinion.
I can touch the motor all day long (albeit it was 40 degrees on the beach)
so the motor is not even warm..

however my son broke the dean on my 1300 3s 45-90C battery so i used
2 x 500mah 3s turnigy 20c batteries in parallel.

Fried one as it puffed and dont look good but didnt catch fire.. so need to dispose of that one...
other one got extremely hot.. I didnt not recharge it yet...
I ran a 3rd one I have (same 500mah) but not in parallel.. for a bit. it got warm too.....


so my questions....

1. Can incorrect gears make the battery hot as the motor isnt hot so I was going to keep gearing it for more speed????

2. Is the correct reason for the battery issue due to the motor demanding more than what the battery can supply?
20C rating vs the one I didnt get to run 45-90C???

I am asking this as puffing one of my 500mah means I need more now.. as I am not sure I event want to use any of my 3 at this point...

the 1300C 3s 45-90C fits up front on my ax10 so thought of getting more of them then can use in the crawler or foff...

thanks guys...

p.s. the foff was fun.. when it would stay upright

[johnnyh66]

I think it's 2 things...
The low C rating and the low mah... the motor just draws too much current too fast for the batteries to handle.
I was puffing 40c 900mah lipo's every time I ran my foff. I switched to 30c 3300mah and have had no more problems.

[bigstu]

To answer your questions, yes, the wrong gearing can heat up a battery. I had my 4x4 short course truck over geared and it would heat up the batteries. Same for my crawler. When you pull the current out of the battery VERY hard, it can heat them up. What you are doing, is in essence short circuiting the battery. You pull so much current they can't keep up and make heat.

This is related to the mah of the battery and the c rating.These numbers aren't exact, jus to give you an idea of why you had a problem..
A 5000mah 2s 65c battery, when short circuited can produce over 150 amps of current. Those same cells, 65c, in a 500 mah pack will only produce about 25 amps of current. So what happens when your motor wants 40 amps of current? With the small pack they are going to get 25 with the small pack and do some damage at the same time..

[RootBreaker]

so then if I just buy 45-90c rating batteries I will be fine? and no im not asking a range..
my 1300 is a 3s 45-90c rating.. and there are more of them on hobbyking that I was looking at.. like 3000mah or 5000mah....

my esc can only handle 80a so just dont want anything to go puff....

[Old Sloppy]

You need to know your amp draw.
Taller gears will drive up your amp draw, shorter gears will drive down your amp draw.
An 80 amp speed controller can draw even more in short bursts. so get 100 amp batteries.

So buy 50c 2,000 mah batteries, or 40c 2,500 cells

My HPI 5SC truck on 6s lipo draws 170 amps.
I use Turnigy 40c 5,000 mah cells. ( 200 amp cells)
So my cells are happy and have no stress on them..

Harry

[Meatwad]

Which esc and which motor? A few more specifics could probably help others recommend the current amperage needed. Agreed on needing to know the amp draw.

[RootBreaker]

Originally Posted by Meatwad

Which esc and which motor? A few more specifics could probably help others recommend the current amperage needed. Agreed on needing to know the amp draw.

It is in mu sig... Turnigy 80a waterproof esc.. On phone and dont remember motor... 3520?

Sent from my Sprint Epic 4G using Tapatalk 2 Blue

[Meatwad]

In that case I'd also recommend 100amp packs to be safe. Easy to see how your ~10 amp packs were having trouble keeping up.

Ah x c rating = final amperage the pack can discharge. So 5000mah is 5ah... At 20c its 100amp. 2500mah/40c is also 100amp.

Have fun and respect the lipos.

[Duuuuuuuude]

Originally Posted by Meatwad

Ah x c rating = final amperage the pack can discharge. So 5000mah is 5ah... At 20c its 100amp. 2500mah/40c is also 100amp.

Have fun and respect the lipos.

The 40c pack in your example will work better in high continuous use situations (like a FOFF). More lighter duty rigs can get away with less.

In my go-fast stuff, I like my batteries to have as high a C rating as I can afford, and overall be able to supply more continuous amps than my esc can continuously pull. So if I've got a 100a esc my battery needs to at least equal that. If there is a dead short in the system I want it to take out something that won't burn my rig to the ground. Meaning if I stall a motor hard enough to do damage, I want that damage to happen in the esc, not in the battery.

ESCs have burst ratings, so do batteries, but they aren't always advertised, and they only cover a very small amount of time, so I rarely even consider them when picking a battery.

[Meatwad]

In my 4x4 sc days, I preferred the physically bigger size of the 5000mah packs for weight bias, at 30c minimum though. In FOFFing I could see preferring some weight loss.

Yeah it's hard getting solid continous ratings much less burst. 15-20% burst cushion helps me sleep at night.

[Duuuuuuuude]

I can understand the weight/size considerations of a FOFF as well, which makes the C rating that much more important.

I've got 4 5000mah batteries for my sc trucks, two are 25c, two are 50c. In my 4x4 the 25c's come off noticeably warm where the 50cs do not. Needless to say the 25cs no longer get put in the 4x4.

[Olle P]

Originally Posted by Old Sloppy

You need to know your amp draw. ...

I concur!

Originally Posted by Old Sloppy

An 80 amp speed controller can draw even more in short bursts.

I'd replace "draw" with "handle".
The ESC isn't what "draws" the current, and it's rarely a limiting factor.

It's about what the motor wants (and that has a bit to do with the gearing).

Best thing is to use an onboard logger during a run to see how much current is actually drawn from the battery, typically and during peaks.

[bigstu]

Originally Posted by Duuuuuuuude

I've got 4 5000mah batteries for my sc trucks, two are 25c, two are 50c. In my 4x4 the 25c's come off noticeably warm where the 50cs do not. Needless to say the 25cs no longer get put in the 4x4.

In my Losi SCTE, I puffed a 5300mah 65c Thunder Power 2s pack.
Of course I was also running a 4600kv motor at the time.

So, you can imagine how much power I was pulling.
Went down to a lower kv motor and geared it up a little. Worked MUCH better and didn't destroy $125 packs.
I ran the same 5300mah 65c pack in my FOFF truck and suprisingly it did very well. Even thought the weights of the truck were similar, the gearing worked to my advantage and kept temps down. Even with the 5700kv motor.

[RootBreaker]

all good info guys.. appreciate it... well my 1300 is a 45-90c...
they have bigger mah batteries with the range C ratings.. is it best to stay away from the ranged ones and go for the straight C rating?