Confused About Discharge Rates

[ciadelta16]

I have been using an 8.4v 3800mah NIMH battery with my car which is running a 5t brushless EZRUN system in it.

I enquired with the company that makes the battery as to the possibility of me purchasing some more of these batteries and I received a reply form the company stating that this battery was unsuitable and could blow up being run with this motor combi and that I should go Lipo!! The statement was that the max discharge rate of the battery and the power the motor uses can make the battery packs get too hot and explode and that the battery i am using may actuaklly be dangerous being run with this ESC/motor combo.

The battery is a 10c Discharge pack. The EZrun motor is 5t with a 65A ESC. Hope someone understands and can help me on this because i havent the foggiest!! Thanks.

Luke.

[Bluefoot]

Hi Luke, My understanding is that the mAh rating applies to the amount of energy the battery can store and discharge within one hour (3.8 Amps for one hour) and that this is the maximum storage capacity. The discharge rate is independant of this and is determined by the internal construction of the cell. I think the 10c means that the battery is rated to discharge at 10 x the mAh rating, which will mean 38 Amps in your case.

If your EZrun is sucking more than this then the battery will get hot. It will probably depend on how hard you run the car as to whether anything blows up.

I'm not an expert on these things and would happily be corrected by someone who knows more.

[ciadelta16]

Hi Luke, My understanding is that the mAh rating applies to the amount of energy the battery can store and discharge within one hour (3.8 Amps for one hour) and that this is the maximum storage capacity. The discharge rate is independant of this and is determined by the internal construction of the cell. I think the 10c means that the battery is rated to discharge at 10 x the mAh rating, which will mean 38 Amps in your case.

If your EZrun is sucking more than this then the battery will get hot. It will probably depend on how hard you run the car as to whether anything blows up.

I'm not an expert on these things and would happily be corrected by someone who knows more.

Thanks for the quick reply;) Trust me, you know more than i do!!! When i was a teenager about sixteen years ago, I never had any of these questions, I just bought a car and a battery and away I went!!! How would I know how much power my particular system is sucking then? any ideas?

I have had alook at the Esc and ot says the cont. current is 60a and the burst is 380a. Doe this mean that the battery is only able to output roughly half the needed power to the ESC? or have i got this all wrong??

[Champ85]

Hmmm... For the battery company to state simply that it is dangerous and the battery isn't sufficient without knowing more facts about how you use it tells me that they just want to sell LiPos to make a quick $$$. Maybe they are just playing it safe based on what a typical 5 turn brushless wants for power, but still...

Here is a somewhat less scientific/mathematical approach to figuring out the answer to your question:

How hot is the battery after you finish a drive? If it is warm, but not hot, you are most likely within the limits of what the battery is designed to output for current draw. On the other hand, if the battery is quite hot - say as hot as a baked potato where you cannot hold or squeeze it in your hand without wanting to drop it because of heat - then you are probably drawing more current out of the battery than it is designed to provide.

A second method, slightly more complicated (and with some basic math):

How long does the battery last before running out of juice? 30 minutes? 15 minutes? 3 minutes?

To calculate your average current draw for the entire run (which isn't perfectly accurate since there are spikes and low points while you are driving), just input your runtime (in minutes) into this equation:

Iavg = 3.8 / (runtime / 60)

For example, if you have a runtime of 10 minutes:

Iavg = 3.8 / (10 / 60) = 3.8 / (0.167) = 22.75 Amps which is less than the 10C rating of your battery pack (3.8Ah x 10 = 38 Amps)

However, if you only get 3 minutes of runtime before the pack dumps:

Iavg = 3.8 / (3 / 60) = 3.8 / (0.05) = 76 Amps which is a LOT more than the 10C rating of your battery pack!

Bottom line:

If your runtime is more than 6 minutes then you are OK (the closer to the 6 minute mark, the nearer the limit of the pack you are). If you get ANYTHING less than 6 minutes runtime, then you are drawing too much power and should probably think about upgrading.

[stu22]

The battery company is correct in what they are telling you, for the most part.

They have rated the discharge rate of the battery at 10C, or 38A, and the ESC you're running can pull 65A continuous and much higher for high load peaks. What the're worried about is the motor/ESC will "ask" the battery for more power than it was designed to provide. The cells in the battery don't know any better so they'll try to put out the power and over heat. A NiMH cell is designed to vent through the positive terminal if it over heats. I have had cells vent without any real danger, but there are those who have had them burst (most of the time because of charging at high current, not using a discharge tray, and overcharging a single cell in the pack.) The only time I have vented any cells are when I dead shorted out a pack by accident and the current caused the thing to overheat.

For the most part, you never draw the peak current for very long, although depending on your gearing the 5T could push this generality. What happens is you draw alot of power when you're accelerating from a stop, less when accelerating, much less when up to speed, and almost nothing when slowing down. It's not a constant draw on the battery and it's rarely at peak current. I've run Powerizer 30A cells on systems that draw far more than they are stated as being able to handle (9T EzRun as well as 17.5/13.5 Tekin and Novak systems) without a problem although the things to get VERY HOT. I don't think you'll have a problem with the 5T although I guess it does depend on the cells themselves and how you're using them. A short course that requires alot of acceleration all the time will be harder than a big sweeping oval.

[WillyChang]

don't worry about any current rating on NiMHs... subC cells usually only explode on (over)charging

with commercially tabbed cells, its the skinny interconnects which limit the current not the cells

if the battery cannot supply enough current the EZRUN usually starts stuttering on full throttle

with LiPO though, sucking out more than the rated max current can cause the cell to bloat - they're toast after that