A123 - Less Voltage = Less Power?

[BritCrawler]

I am setting up a AX-10 with a Sidewinder ESC, 55T Integy Lathe Motor, and a Castle BEC.

I am seriously considering using A123 rather than conventional LiPo because it's easier to manage. The problem is a 3S LiPo pack pushes out 11.1V, but a 3S A123 only pushes out 9.9V. Is this going to cause significant power loss in a crawler or is it not something to worry about?

My knowledge of Physics suggests that to get the same power at lower voltage you draw more Amps. Is the ESC or Motor capable of doing this or is it going to draw the same Amperage regardless of the Volts pushed down it? A123 is capable of throwing out the Amps big style so draw on the battery isn't a major issue

Is there any setup advice anyone can provide to get the best out of these packs with the setup I'm running?

[oberon crawler]

I think for a given motor and speedie a swap from 11.1V to 9.9 will result in lower current draw and also lower power ouput and also a lower speed and torque.
The way to combat this is running a lower turn motor. running lower turns will increase speed and current draw to make up for the lower voltage.
I'm new to brushed motors and i'm by no means an expert so if this needs correction please feel free

[Chris_The_Battery_Man]

Well first things first... I was honestly stay clear of A123. They're very over rated cells. Their power out put to volume consumption is also worse than a Nimh. Which means you get more power from a Nimh pack of the same Mah. A123 takes up alot of space. Not to mention they get their ratins in an un usable manner not real time power consumption. So 30c is a joke with those cells.

But a loss in voltage will really make the wheel speed less. And af far as power goes... that depends on what you mean by saying power. Do you mean torq or RPM? On a crawler you will probably only really notice a loss of RPM. The battery will only give what you ask of it not put out what it can... If you draw a constant of 5 amps then your pack will only be under a 5 amp load.


See this is kind of along the lines of what I get asked alot... "How many amps can that battery put out?" Well batterys dont put out amperage... Amperage is the ammount of resistance put on a battery/power delivery system while your asking for its volts. The less Amps(Resistance) there is in the power delivery process... The less voltage sag under load resulting in higher voltage and more RPM.

Now lets say you were putting a 5 amp load on your battery and it a 11.1v lipo. Thats 55.5 watts.... your motor requires 55.5 watts average. Now put in a 9.9v pack and your motor still wants 55.5 watts... That now becomes a 5.6 amp load on the battery. But this is such a close amp load that you wont really notice a defference....


Corrent me if im wrong john... your the motor guy... Im just the battery guy

[NickRummy]

You also can factor in gearing which can change the load on the motor and battery.

You can gear a 9.9V setup to keep up with an 11.1V setup but you'll draw more current out of the 9.9V battery and have less run time. The motor will also have more load on it. The higher the voltage the more efficient it'll run.

I run A123 cells but not in crawlers and love them. They are size challenged for sure.

[Ced RC]

Ohm's Law:

[sloppy]

Originally Posted by Ced RC

Ohm's Law:

that is funny I thought the same thing about 2 post's earlier


the 3 a123 pack should give you plenty of wheel speed... you will loose some but it wont be a issue.

[BritCrawler]

Originally Posted by Ced RC

Ohm's Law:

Seriously, I just printed that out. It's way more useful than the one I had in my old Physics Text Book ;)

Originally Posted by Chris_The_Battery_Man

Well first things first... I was honestly stay clear of A123. They're very over rated cells. Their power out put to volume consumption is also worse than a Nimh. Which means you get more power from a Nimh pack of the same Mah. A123 takes up alot of space.

They are significantly lighter than a stick pack, and footprint area wise a 3S is slightly smaller, from what I see, than a 8.4V 7 cell stick, it's just a different shape I think, correct me if I'm wrong.

I'm not going for A123 for uber mega power output, I'm going for it because it's light, charges fast and is much, much more durable, not to mention less dangerous, than LiPo. I mean I could cope with LiPo but I'd always be nervous about taking the packs anywhere in other people's cars, charging them in my room, and all that jazz. A123 isn't foolproof, you still need a balancer etc, but at least it's at least as hardy as NiMH, even if it doesn't last as long.

Originally Posted by Chris_The_Battery_Man

Not to mention they get their ratins in an un usable manner not real time power consumption. So 30c is a joke with those cells.

Can you expand on that? I don't quite follow you...

Originally Posted by NickRummy

You also can factor in gearing which can change the load on the motor and battery.

You can gear a 9.9V setup to keep up with an 11.1V setup but you'll draw more current out of the 9.9V battery and have less run time. The motor will also have more load on it. The higher the voltage the more efficient it'll run.

I run A123 cells but not in crawlers and love them. They are size challenged for sure.

Ahh, so the answer to my question is 'yes' you can make it draw more Amperage and compensate for the loss in Voltage. You do it by subjecting the Motor to more load, thus requireing a higher current. You would do this by gearing it with a closer pinion/spur ratio right, a ratio that for the same voltage would increase wheelspeed? Tell me if I'm way off here...

[NickRummy]

Correct. Theoretically.

A motor can only draw so much though. That's when you get overheating. Same with batteries. They can only withstand so much being drawn from them before overheating.

[EGRESSor]

http://www.elektromodellflug.de/akku-test/26650-x.htm
increased temp > decreased resistance
higher voltage at the same current

http://www.rc-raceboats.de/wbb2/atta...chmentid=18375
the cells need to heat up for high current use

[NickRummy]

I have noticed that my packs get more punchy when warm actually.

[BritCrawler]

Originally Posted by NickRummy

Correct. Theoretically.

A motor can only draw so much though. That's when you get overheating. Same with batteries. They can only withstand so much being drawn from them before overheating.

A123 is much more tolerant, supposedly, to high current draw than LiPo, comparably so with NiMH in fact. I understand what you mean though, also the ESC can overheat and shutdown too, which tends to be a game stopper Any system has limits, and of course you must work inside those, just nice to know I was on the right track

My only curse of the day is I bought a 55T motor not a 45T, the latter would be better on a A123 pack as it draws a higher current and goes faster per Volt, meaning I'd have compensated for a good deal of the loss of power and wheel speed. Well you live and learn I guess ;)


FWIW, NickRummy, there's another thread discussing temperature vs. pack performnce here:

Does cold affect Batt performance

According to the gurus in that thread, NiMH is quite sensitive to temperature.

[Chris_The_Battery_Man]

Originally Posted by BritCrawler

Seriously, I just printed that out. It's way more useful than the one I had in my old Physics Text Book ;)



They are significantly lighter than a stick pack, and footprint area wise a 3S is slightly smaller, from what I see, than a 8.4V 7 cell stick, it's just a different shape I think, correct me if I'm wrong.

I'm not going for A123 for uber mega power output, I'm going for it because it's light, charges fast and is much, much more durable, not to mention less dangerous, than LiPo. I mean I could cope with LiPo but I'd always be nervous about taking the packs anywhere in other people's cars, charging them in my room, and all that jazz. A123 isn't foolproof, you still need a balancer etc, but at least it's at least as hardy as NiMH, even if it doesn't last as long.



Can you expand on that? I don't quite follow you...



Ahh, so the answer to my question is 'yes' you can make it draw more Amperage and compensate for the loss in Voltage. You do it by subjecting the Motor to more load, thus requireing a higher current. You would do this by gearing it with a closer pinion/spur ratio right, a ratio that for the same voltage would increase wheelspeed? Tell me if I'm way off here...

Well basically they are getting their ratings on a discharge graph while under a certian load down to 1v per cell. Well real time that is not usable past about 2.5v considering your only using a 2 or 3 cell. When we tested them we tested them in a real world setting and they were no were near 30c... Maybe discharging down to 1v per cell but 99% of applications have already shut down WAY before 1v per cell. I cant remember our exact results but I am going to post a review soon on them.

Alot of places label their stuff by using this same loop hole or under a 1 amp load. But real world there are a few places that have real time ratings.

Not to mention Lipos under their most extreme load put out way more watts and hold a better voltage under load than the A123 do nominal.

[mattnin]

That will be an interesting review.

I have been using A123 batteries for a long time now, over a year and in applications such as 1/10 modified truck, buggy and 4WD offroad, and modified touring car asphalt, as well as 3 cell A123 in my rock crawler. As matter of fact, I have only one 2S lipo battery that I use for stock racing, I don't own any other battery except A123s. I take that back, I am using AA nimh batteries in my transmitter.

In touring car, I am definitely pulling high amperage using a Tekin R1 Pro with a Redline 362 motor using A123s and I am as fast as anyone else out there. Here is a picture of my setup. The only reason a novak motor is showing in the picture is because I was troubleshooting.

[BritCrawler]

Thanks for all this info. I am still undecided if the A123 is worth paying almost 2x the price for, and losing the voltage over a LiPo. I'm starting to think not, although the safety thing is still nagging.

Chris:
In your battery buying tips you stated you don't need stacks of power for crawling or a high 'C' rating, so it's not such a big concern regards that. It's a bummer that the ratings are a bit ambitious but in this day and age I never expect anything to work up to it's spec anyway ;)

[JohnRobHolmes]

It is a viable chemistry. I would choose the LiFe if fast charge times are a priority more than energy density.


Comparing 3s LiFe vs Lipo in crawling, you will have less amp draw in a bind just like oberon crawler stated. Less voltage on a stalled motor = less amperage= less torque and power. To get the power back (wattage wise) you would need a lower turn motor to pull more amperage. Now, the question is whether 9.9v nominal is enough for crawling. I say yes it is indeed.


The ratings for a123 are very ambitious. It takes a lot of heat to get to 30C, and the cells do drop in voltage to an unusable level for crawling. Luckily a 55t wouldn't pull more than 5C in most binds, and 13C in a full on stall. a123 will take about 40 amps (17c) at room temp before it really falls hard.

[BritCrawler]

Arrghhh... now I'm confused. I thought high-turn motors drew *less* amps than low turn ones, making the battery last longer.

Then again, more turns = more wire = more resistance & stronger mag field = higher current

Oh god, this is starting to make my head hurt, I need a lie down in a dark room! Someone make me a tea with 3 sugars! ;)

[mattnin]

Originally Posted by JohnRobHolmes

The ratings for a123 are very ambitious. It takes a lot of heat to get to 30C, and the cells do drop in voltage to an unusable level for crawling.

Under what circumstance?

I use a 3 cell A123 battery and it works just fine for crawling, and provides so much power with a Tekin R1 Pro and Holmes Hobbies Black Revolver that I break driveshafts, bevel gears, pinions, etc...

[NickRummy]

I think JRH was saying that their 30C rating doesn't help us because when 30C is loaded onto the battery the voltage drops down to an unusable level. Tops we put around 13C load on them (full on stall) which they can handle fine.

[JohnRobHolmes]

Anything over 40a and they would plummet in voltage under RC car type applications (on/off throttle use). 30C is 69 amps.


They do work just fine for crawling. Chris is just showing how even the venerable a123 has ratings that are not exactly straightforward.

[Ced RC]

Originally Posted by JohnRobHolmes

even the venerable a123 has ratings that are not exactly straightforward.

Makes me think of how the Chevy dealer told me that my Silverado 2500HD would get 22mpg on the hwy, 17mpg city, which MAY be possible with a 100mph tailwind?