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------------------------------------------------------------------------ BATTERY TEST - Are 'C ratings REALLY all that important with Lipos?
Tony P gives us the REAL story...an rc-mini EXCLUSIVE There has been a lot of discussion lately regarding the C rating on batteries, and how the higher the C rating the more current the ESC and motor will use which is then mean to be a huge advantage on the track. People keep clamouring for the latest battery, sure that the extra 5C will give them an advantage… But will it?
This argument already has a serious flaw as it completely dismisses that fact that the speedies we are limited to using, have a 25A and 35A continuous current ratings, with burst ratings of 90A and 190A respectively. Which implies that the most current you could sustain through the speedies would be 25/35A. Straight away there's an issue there. Looking at the most popular batteries at the recent Victorian State Titles: Harris 3200 20C 20 * 3200 = 64000mA = 64A continuous Team Orion 3200 Carbon 20C 20 * 3200 = 64000mA = 64A continuous Peak Racing 3200 Carbon 20C 20 * 3200 = 64000mA = 64A continuous Team Orion 3400 Carbon 25C 25 * 3400 = 85000mA = 85A continuous Yeah Racing 3200 20C 20 * 3200 = 64000mA = 64A continuous Looking at the speedies and the batteries we are using it's easy to see that the batteries are already capable of supplying more current then the speedies can handle continuously. Logic should then kick in and say that since everyone isn't thermalling, nor are the heatsinks on the speedies even getting warm after a 6 minute run, that we are not using anywhere near the current rating of the speedy. Even when running outside on the hottest February ever, when the temperature got to 46 degrees (this was during Black Saturday in Victoria) we ran the ESC for 1000 laps (Wodonga Mini Enduro) and although the heatsink was hot to the touch it was no where near thermalling. Now just to try to see if what we believed was really true, we decided to make absolutely sure by actually monitoring the battery usage during actual races. Below is a table showing the results of 2 race meetings, the Victorian State Titles, and the Warm up race the weekend before. As can be seen in the table below we have summarised the data from each run over the two race meetings, and we can see which pack was used for each run, plus the maximum amps recorded, the average amps used per run, maximum voltage, minimum voltage average voltage and the runtime. Before we begin interpreting the data, some background on the races. The Warm up race was a race where I was trying to find a good setup for the car and was not really happy with anything that was happening. The car was handling really badly, and was very difficult to string consistent runs together. At the Vic Titles the car was great, easy to drive and just worked, the driver a couple of times left a bit to be desired - but overall I would consider it a good race meeting. Looking at the peak amps data we can see the highest was in final 2 of the warmup race where the peak amps reached 42.3A. Looking at the graph below it can be seen that the 42.3A was only reached once, probably as a result of a coming together with another car on the track. The only other 2 peaks in the chart only reached 32/35A which is considerably lower, and more consistent with the remaining results Analysing the rest of the results it would seem values of 28-35A would seem to be the expected peak current for the ESC/Motor system chosen as the control for our mini class. Looking a bit further, the average current draw is about 1/3rd that of the peak current which would mean the constant current draw from the battery would be a lot less then the peak current. Breaking it down a bit further we can look at the graph below which shows the highlighted gray area representing a full lap of the track. We can see that most of the time the graph has short sharp peaks indicating the burst in current draw on acceleration, and then longer troughs where the acceleration has taken place, and speed is just being maintained. On a small tight track it would be that the average current draw could increase, but only marginally so. From all this information what we can see is that the Mini with the Hobbywing ESC and 13T motor combo is drawing much less current then even the 20C batteries are able to easily continuously supply. Another interesting point is that there was no real significant difference between some packs that were nearly 2 years old, as opposed to some that were brand new. Yes, there are differences, but those differences could also be attributed to any number of factors anyway, not just age. And the differences are so small as to absolutely not represent any sort of significant, provable trend in any way. Thereby also effectively destroying the 'it's better to have new packs for a big event' myth as well. So, what's the bottom line? Well, as Jamie and Adam are fond of saying on the tv – MYTH BUSTED. The whole contention that a higher C rating gives you more punch is complete rubbish. Yes, in a faster car with a lower turn motor it MAY make a difference, but in a control motor class such as Mini it makes no difference at all. There is simply no evidence to support it. At no stage does the amp draw even come close to the continuous rating of the lowest C rated battery (20C), so having a higher one will make no difference at all. Other people talk the talk – rc-mini give you the facts to back it up. Side Note - One thing that was observed with the Lipo batteries is that the voltage might be affected by the cold weather. Looking at the voltages between the warm up meeting and the actual race weekend, the main difference was the weather. The Vics was quite a bit cooler and starting much earlier meant it was significantly colder for the early runs, and the voltage was down. I guess that will be something to look at in a future article….. |
