Hey Guys!
For controlling two 12V motors, I'll be using an 25A sabertoooth motor driver. For powering the two motors I"m choosing an 11.1V/850mAH/3S/25C Li po battery pack. Each motor draws 9.5A(max), so totally for both the motors the current draw is 19A(max). The battery provides 21.25A. So is this battery pack wise? Or should i go for an battery pack that provides higher current?
Thanks!
The battery pack is rated at
The battery pack is rated at 850 milli-Amp Hours (mAH) and 25 A max current. So you should be OK for maximum current conditions. To know how long the battery will last, you really need to know the average current draw, not the maximum.
Converting to AH and multiplying by 60 gives you the battery’s rating in Amp-minutes.
0.85 x 60 = 51 amp-minutes
At the maximum current for your motors, that’s only about 2.7 minutes. However, your average current draw will be much lower. How long do you need your robot to run between charges, and what sort of average current do you think you will have?
As ignoblegnome mentioned,
As ignoblegnome mentioned, it’s unlikely your motors will spend much time running at max current. What will the motors be doing exactly? If they’ll be turning a bot’s wheels, how big will the wheels be, and how heavy? Do the motors have a gearbox fitted to them?
In terms of provided current that battery pack is fine, assuming nothing else on the bot draws anywhere near as much current as those motors, at least not at the same time.
I’m using the motors for
I’m using the motors for running the two wheels for an battle bot which weighs around 30KG. I’l provide the motor details
http://robokits.co.in/shop/index.php?main_page=product_info&cPath=2_3&products_id=186
I have not yet tested the
I have not yet tested the motors practically, So i Dont know about the average current draw as of now. I’m using the motors for turning the wheels of an battle bot which weighs around 30 KG
You are confusing battery
You are confusing battery capacity with battery output. 850mAh is the battery capacity, 21.25A is the max output. If you assume your motors will draw an average current of about 6A over the course of the match then 12A out of a 0.85Ah battery means your battery will be dead in a minute.
You can`t power a 30kg fighting robot with a 200g battery.
This is better. 2200mAh means you might get about 5 minutes run time.
Now I’M confused. Check my
Now I’M confused. Check my math, Ezekiel.
0.85 AH x 60 minutes/hour = 51 Amp-minutes.
With your assumption of 12A average current consumption, I get:
51 Amp-minutes / 12 Amps = 4.25 minutes.
A 2.2 AH battery x 60 minutes = 132 Amp-minutes. With the same 12A average current assumption, that’s:
132 / 12 Amps = 11minutes.
Am I missing something?
**Youre right, I hate doing**<br><p>Youre right, I hate doing math so I try to avoid it if possible! I did edit the final time from 10 minutes to 5 just after posting it because I think 12A is pretty darn conservative for a battle bot - pushing adversaries, jumping from forward to reverse, running weapons.
With a current draw that is 6 times the battery capacity you will get 1/6th of the original running time(1hr).
Hmm…So I should opt for an
Hmm…So I should opt for an high AH rated battery rather than a battery that provides High output. For example if i use an 15AH rated SLA battery,
15AH*60=900 Amp Minutes
With an average current draw about say 20A, I would get…
900/20= 45 minutes of run time…Am I correct?
But the main problem with SLA’s is that they are heavy. The reason i was opting for Lipo pack is because of their light weight. I could add up the weight in Building an sturdy chassis.
You have the calculations
You have the calculations correct. How long does a match last? You will want to make sure your bot can last through a whole match, probably with a 2-3 times factor of safety. Are the matches timed or to they just go on until one bot wins or the other cannont continue?
Design is all about balancing your requirements. In this case, weight vs battery life.
In theory, yes…
Your math seems to check out fine. But this applies to magical batteries that live in the theoretical world only.
The problem with bats is: the actual current drawn (on average or at any given moment) will greatly influence the capacity the battery will be able to give out. More current means less capacity. Or at least this time around. The data sheet for a battery will hopefully be able to tell you more about this effect.
The inverse is true as well: when you use your battery conservatively (low current), it will be able to “squeeze out” more Ah, over the course of this one charge.
Different battery chemistries behave differently in this regard. I am not sure how Lithium batteries are better than Lead or Cadmium or Manganese.
Check out this walk through for the nitty gritty.
The match lasts for a
The match lasts for a maximum of 5 mins. So i think the best option is to opt for an 4.5AH SLA battery as it weighs lesser than 15AH battery. 4.5AH battery would last for around 13.5 mins approximately. Considering the losses, it should last atleast for around 10 mins rite?
Another doubt regarding Li-Po battery packs guys!
Consider the pack I mentioned in my first post. The maximum current it can give is, 0.85*25=21.25A. While calculating, for how long the battery will work for an given amount of current draw, we are using the Ah rating of the battery. So when must the maximum current output of the battery must be considered? In this case for what purpose 21.25A must be considered?
thnx buddy:) Nice article!
thnx buddy:) Nice article!
Basically the maximum output
Basically the maximum output current of the battery should be higher than your expected peak current draw. If your robot draws 20A under stress and your batteries can deliver a max of 10A you could burn the connecting cables or electrodes and lower the life of your battery.
SLAs are probably not a good idea for a battle bot because of their weight and low discharge rate. I would instead stack up some LiPo packs.
The Li-Po pack you first
The Li-Po pack you first mentioned had these specs: 11.1V/850mAH/3S/25C
I believe the 25C at the end means it supports a peak current of 25A. So it is just within range of your peak motor load.
２５Ｃ means it has a
２５Ｃ means it has a peak discharge or max current of 25*Capacity = 21.25A. On the original pack the only problem with it was that it was too small to give a run time of more than 4-5 minutes.
Hmm…another reason I’m
Hmm…another reason I’m prefering SLA now is because of its price. They are pretty darn cheap compared to Lipo’s and for charging Lipo’s i have to get an special charger. An 4.5AH SLA weighs about 1.5KGs. That weight doesnt seem to bother me now.
The above mentioned Lipo pack provides a max current of 21.25A. How much max current will an 4.5AH SLA battery give? How should i calculate that?
And What do u mean by low discharge rate? Is that a real problem that should be looked into?
