Educumate Me on Batteries
 

I've been reading up and trying to understand batteries. If I'm getting this correctly, a lower mah battery would need a higher C rating in order to be able to give up the same amount of energy per time. For example, both a 2500mah 50C battery and a 5000mah 25C battery would discharge at up to 125a, but the former one would last half as long. Am I getting that right?

Second, a system will only draw as much energy as it does, right? Meaning, just because you put a higher capacity battery with a higher C rating in a car doesn't mean it's going to go any faster (given the same voltage). Right?

With that in mind, could a lower mah battery with a higher C rating give the same punch up until it starts fading?

If you want to know why I'm asking, one of the reasons is because I've developed the habit of topping off batteries before heats, but then end up at the end of the night with a battery that still has most of its juice left and takes quite a while to discharge down to storage. It also seems that once a car is set up for racing, you don't want to run it for too long per session. That makes it tempting to get batteries with smaller capacity to race with.

I'm not an expert, but this is how I see it, from a indoor clay racing view:

- mah ratings means how long the battery will last.
- I like the 3600, HV Maclan or Reedy batteries. These are about 50g lighter, and enough power for 2wd buggy stock and Mod, 13.5 Stadium Truck and 13.5 4wd Buggy, for 5 or 6 minute runs.
- I like the 4800, HV Maclan or Reedy batteries for the classes that draw more power or where the vehicles are under weight. 2wd Stock buggy, just to make weight. 13.5 Shortcourse, to make weight.
- The longer 6400 or more, HV Maclan or Reedy, for 4x4 Mod Shortcourse. Good for 10 minutes, These tanks like power.

I tend to think the C Ratings are more how fast you can charge / discharge. I know this rating is more pulled out of a hat. The Maclan HV and Reedy HV are all 120C, if I remember correctly.

Stock Classes, after the race Cycle Discharge at 30 amps, then charge at 30 amps. This tends to lower the internal Resistance and can help for more punch in Stock. I would only do this is your racing is at a point where you are not crashing and need like .2 seconds per lap to compete. Been doing this for over a year and these batteries are still strong. Race every weekend. Run the same battery in the same car for all practice/races.

Mod Classes, I just Charge at 10 amps, before I race. They don't need punch and Internal Resistance is not a factor.

Just my 2 cents. Sure everyone will have different opinions. I also here there are really only a couple of places the batteries are made. So I am convinced Reedy and Maclan are the same battery. They sure act the same.

I agree with most of what sea1swk is saying but will try to put things into perspective a little differently.

Mah readings are capacity... the larger the capacity, the longer run time you will get, but also get added punch which means your voltage will be less likely to drop under load. The C rating also effects punch, where the higher the C rating, the higher the charge rate the pack can be charged and the pack will tend to increase it's longevity before it starts to show signs of wear which can be measured as "Internal Resistance" (IR), there are some chargers which come with an IR meter to help you measure the health of your packs.

Most packs that I own start out around 1 mΩ and this value will slowly increase over time, I typically retire a pack when any of the cells go past 8 mΩ.

Here is a discharge graph for you to get an idea of C rating combined with Mah (capacity) showing punch over discharge times, sometimes a picture says 1K words :)

60C-4A is really a 2S shorty pack with 60C rating and 4000mAh, the Y axis is in volts, the X axis is in time

rA is simply multiplying the C rating times capacity as what I refer to "relative amps" in what the pack is rated to handle under max advertised load

https://c1.staticflickr.com/9/8102/8...8e2d9cc2_b.jpg


Take special note on how the lines cross between the ThrottleWorx and Built-RC packs, the lower capacity ThrottleWorx pack outperforms the Built-RC pack for the first 4-5 min, then they switch where the higher capacity Built-RC pack performs better throughout the duration of the run. This strategy did not hold true for the other 2 packs with significantly higher rA values.

What I have learned for stock racing is that it's generally best to get the largest capacity pack and C rating (highest rA) that you can fit in order to get the best possible performance. With off-road cars, the difference in added punch (acceleration) is better with a higher capacity pack than the negative effect of the added weight, in many cases the added weight can improve performance by allowing a car to jump farther based on Newton's laws of physics. A heavier car will also tend to be more stable in corners making them easier to drive. I do a face palm every time I see a driver with a 3500mAh sub shorty pack with battery weight underneath... just get a larger capacity pack in the first place ;)

As far as putting packs into storage mode, you don't really have to get them to exactly 3.8V/cell, the time you spent after the main is more than enough to reduce the voltage within reason for storage, but if you insist on getting them spot on, then simply drive your car until the LVC kicks in to save time getting your pack to storage charge.

You can't take "C" numbers seriously! They can put on any number on their batteries just to get you to believe you're getting something great.

The only brand that I know who does this is MaxAmps, they have been so bad about over stating their numbers, folks have been calling them MaxCramps for years, ha!

What you said is technically correct but my advice is don't focus so much on max current. As the others mentioned the mAh is the battery capacity; almost identical to how much fuel is in the tank. C is kind of an indirect and fuzzy way of talking about the battery's internal resistance or IR. All batteries have IR. When you draw current out of a battery, you will lose voltage due to the IR. The voltage you lose is simply the current times the IR. So if you have a 10milliohm IR (0.01 ohms) and draw 100 amps, the output voltage will drop by 1 volt. The implication is that a higher C battery should have a lower IR which may or may not be actually true. If your charger has an IR feature, you can measure IR. You can't measure C. Batteries with lower IR will deliver more actual energy to your car because less is lost as heat inside the battery.


So the strategy for choosing batteries is more like this. Buy the highest capacity that fits in you car and that you are willing to pay for. You want at least a half decent C, like 40 or more. If a battery has a higher C than that and doesn't cost a lot more, then it is worth considering. A simple test is to feel your battery after a run. If it is hot, then you probably have a battery with too high and IR. You can see that a typical capacity and a decent C would suggest pretty high current that you may never actually draw. A 5000mAh battery at 50C is "technically" capable of 250 amps. You probably never draw that much so its not the right way to look at it.


These days I mostly buy GenTattu batteries because they are available and reasonable cost. Here are all their 2S 5000's for example

https://www.genstattu.com/lipo-batte...31&voltages=70

You can see that they are mostly 50 C with some rated at 100C for which you pay a little more. They probably have a lower IR but you'd have to buy one and test it or find a thread where someone has posted test results. If you drive a 50C battery and its is barely warm after a run, then a 100C battery probably won't make much difference for you.


I have been running their 6000's for longer run time.

Thanks for the responses. I don't detect any heat at all right after coming off, even if practicing for 20 minutes straight. Was just considering getting some 2500mah 70C batteries since so little is drained during a heat (especially since the qualifiers are 6 minutes), but if the voltage starts to drop that quickly, I guess that won't be a good idea.

I would run the batteries down after the race, but I race weeknights and can't hang around for that long... nor do I think the employees want to hang out that late. Maybe I just need to get a charger with a better discharge rate.

What are you running now? 5000 25C? I'd say don't go down in capacity. You might consider 5000's with more than 40 C just for better performance. While it is best to run your batteries back down to 3.85V/cell when you are done using them, it really isn't that critical. Anything less than fully charged is safer and better. So if you are down below 4.0 volts per cell then that is probably ok. Most chargers do not have a very fast discharge rate. That is because you need a physically big load resistor to have a high enough power rating. They don't fit in the compact chargers. Some chargers support using a larger external resistor. My i306 for example has this feature. But choosing the resistor and doing the required wring is a pain.

You might consider one of these dedicated dischargers
https://www.progressiverc.com/catalo.../?q=discharger

As long as you run them down when you get home that should be fine.

going with a 2500mAh pack would be a mistake... what is your voltage coming off the track right now?

I really wouldn't bother discharging my packs if the voltage is anything below 4.0V, there is a balancing act between depth of discharge and temperature where you might be fretting over something that is almost irrelevant if you are subjecting your packs to any significant amount of heat, more info here:
https://batteryuniversity.com/learn/...ased_batteries

4600mah 100C for the buggy. 5000mah 25C for the SCT.

Low 8.2's after the 6 minute heats, and mid 8.1's after a 10 minute main.

The SCT is still running sensorless and can run for about an hour on one pack!

$35 for a dedicated discharger would be a reasonable investment.

I agree that a discharger makes sense for your situation, however that linked discharger is limited to 6A. I think I spent around $10 in some halogen bulbs and made my own 30A bulb discharger using a sacrificial supply box and lamp wiring I had lying around the house... this will discharge your pack in just a few minutes rather than taking 30+ minutes with the 6A discharger... if you're interested in building your own, I can link some pics on how I made mine.

I wouldn't trust myself to be able to fully understand and successfully build one, but 2 of my brothers are electrical engineers, so I'll bet if I describe what you just did, they would be able to help me out. Thanks!

Here's a quick pictorial of how I made my no frills discharger on the cheap:
https://www.rcgroups.com/forums/show...0#post40308838

Assuming you are running 2S that works out to 4.1 and 4.05 Volts per cell. Since I deal with everything from 1S to 12S between airplanes, helicopters, and cars, it is easier for me to think in terms of volts per cell. So again assuming your batteries are 2S, 8.4V is fully charged. You are using very little of your capacity. Hard to believe really but that is consistent with you saying you can run for an hour. 25C seems a bit low but your average current must also be very low based on how little you run your batteries down. You could go to a lower capacity but only if you wanted to lower the full-up weight. That would improve acceleration but might reduce traction. Nothing is simple and everything is trade-offs. Always look up the weight of a battery when considering choices. The higher C batteries do tend to be heavier.

The problem with DIY discharger is that they have no safety features like stopping at a certain voltage or doing any balancing. Just something for the OP to keep in mind. You definitely do not want to ever discharge a lipo below 3V/cell as that can really damage a battery.