Help on batteries!

[stripbuilder]

I'm just getting back into electric RC cars after a 10 year break. I'm starting to look at all the battery options for my new truck (xxx-t cr) that will be running a mamba max 5700 package. People have been telling me to go with lipos, but I think I'll stick with NiMH because I don't need to be going 60 m.p.h on a track, here are a few of my questions.(I'll refer all these questions to the setup I explained above.)

How do batteries effect torque and speed?

Does MaH have any other effects on a car besides run time?

Is there a formula to determine how long my battery will last, given a certain motor setup?

What do I look for in a battery I plan on using to race with?

Any help would be greatly appreciated. Thank You.

[Cotharyus]

Actually, if you stick with a 7.4V (2S) LiPo, you won't really go any faster if you stay geared the same, and your gearing was reasonable to start with. What you will gain is acceleration and punch for the duration of the runtime of the pack. Do yourself a favor and take a look at LiPo again. Everyone at the local track running NiMH still vs the guys running LiPo - the NiMH guys only make the A-main if one of the LiPo guys drives it into a wall. They just can't keep up over a 5 minute heat.

You might want to look at the little bit I wrote on LiPo too. I've tried them in everything I have at this point, and I'm a total convert.

http://cotharyus.net

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Cotharyus
http://cotharyus.net

[racerrandy]

You need to find out what is legal to run at the track. If lipo's are legal, you can save some money. I good race nimh pack is going to set you back about 60 dollars. You will need three of them to race with. Or you can spend 100 on a good lipo and only need one. And the lipo will last several years if you are just racing it, the nimh will need to be replaced twice a year if you want to be a front runner. Just trying to help.


Later,Randy

[SS Pede]

To answer a couple more of your questions stripbuilder:

The capacity (mAh) rating of a pack could have a slight affect on performance of the pack, not just run time. With NiMH it's not such a big thing, hard to quantify. But with LiPo, a higher capacity pack can provide more power than a similar pack with a lower capacity. When you see LiPo's that have some "C" rating, like 20C, that's related to their capacity. A 6000 mAh LiPo pack rated at 20C (20 times capacity) can provide 120 amps of current continuously (at least in theory). 20 x 6 = 120. So if you do go LiPo, pay attention to this and make sure you get a pack that can handle your setup. If you're choosing between two packs that both can handle your setup on paper, but one has higher capacity, the higher capacity pack will probably provide just a little more power (it could also be a little heavier or larger).

To calculate run time, you need to divide the capacity of the pack by the current you will be drawing from it. This is hard to do perfectly, because current draw varies immensely depending on the setup and driving conditions. Say you have the 6000 mAh LiPo mentioned above, and your motor pulls 20 amps constant on average. Your run time would be approximately 18 minutes. 6 amp-hours divided by 20 amps is .3 hours, or 18 minutes. This is just an example though, there are tons of factors that affect run time. The 18 minutes I pulled up there actually seems kind of short for a 6000 mAh pack. A lot of people get more like 25 to 30 minutes from a pack like that if they're just playing around. I just chose 20 amps continuous as a random amp draw, but your setup could be different.

[stripbuilder]

Thank you for helping me out, I understand. Although I'm not sure about spending $150.00 on 2 Lipos. I still think for now I could go ahead and get a charger that can charge both NiMH and Lipo's (Duratrax ICE Charger) and then later go ahead and upgrade to Lipo's. Right now I could make my own NiMH 5000 MaH Sub C 6 Cell batteriy packs for under $20.00 a piece.

I know the Mamba Max 5700 has an input of 12 volts. Now considering if I use a 6 Cell NiMH battery pack, the max volatge that will actually be going into my Mamba Max is 7.2 right? Because the bateries max voltage output should stay at 7.2v.

My next question is how can I measure how many amps my Mamba Max 5700 will pull on average? Or is that determined by the discharge rate? Or do I basically strap a battery into my car, run it for a measured amount of time and then strap to my battery charge to see how many MaH's are left?

Another question is about the internal resistance of a battery. How does the internal resistance effect your battery power/life?

[SS Pede]

Sounds wise to start out with NiMH and just get a charger that can do LiPo if you want to do that in the future. However, with a brushless system you'll especially want to have a nice NiMH pack...not one you can build for $20. There are no truly high-performance 5000-mAh NiMH cells out there that I know of. Maybe go for some GP4300's, IB4200's, EP4200's or EP4600's. GP, IB, and EP all make nice powerful NiMH cells that will help the performance of your brushless setup. Of course, once you start buying nice packs like that you'll spend around $100 for two packs, so LiPo starts to look less expensive.

Did you mean the Mamba Max 5700 has a maximum input of 12 cells, not 12 volts? Because that motor's maximum is 12 NiMH cells or 3s LiPo (14.4V or 11.1V, respectively). Regarding your 6 cell packs, they will actually put out over 8 volts when fully charged. 7.2 volts is just the nominal voltage of a 6 cell pack.

You can buy a fancy Eagle Tree data logger to record voltage, amperage and other factors during a run. Not sure how much they cost. The simpler (less accurate) method you describe will work OK I think. At least it will give you a rough number for average amp draw. Recharging the pack afterwards will give you the rough amount of capacity used (assuming you get a charger that shows mAh on the readout). Also, someone on this forum might have a good idea of how many amps a MM5700 will pull on average, but like I said this number can vary a lot depending on your setup.

A lower internal resistance makes the cells a bit more powerful. Your off-the-line punch will be improved if internal resistance is lower. Also, a battery will run cooler/more efficiently if internal resistance is lower. LiPo packs shine in all these areas because they have very low internal resistance.

[stripbuilder]

Sorry for the confusion on the Mamba Max input, and thanks again for the help.

When you say GP, IB, EP what does that stand for, or is it just a brand?

[evst 360]

yep, the brand. They are the best cells available, period.

[94eg]

Sorry for the confusion on the Mamba Max input, and thanks again for the help.

When you say GP, IB, EP what does that stand for, or is it just a brand?

Gold Peak
Intellect Battery
East Power