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Hello. I've been looking at what might be the most cost effective way to provide power. I was wondering if i used cells that are 4.5v each (for example), do i need a special charger to charge a pack of these (a pack i will build) or does the voltage not make a difference? Obviously if the batteries are Li-Ion, Ni-Mh, Ni-Cad etc i will need the appropriate charger, but does each cell voltage effect anything at all?
Cheers,
Chris.
Are you talking about mixing battery chemistry? If so, then don't. You need a charger that can charge the chemistry of battery that you have at the cell count you have. I have yet to find any cells that give a voltage of 4.5V
You misunderstood. No mixing of chemistry. I made the voltage up.
What i'm saying is does it matter if individual cell voltage is greater than the "standard" 1.2V as far as charging is concerned?
Cheers,
Chris.
Every single nimh cell is 1.2v nominal, every single nicd cell is 1.2v nominal, every single lithium polymer cell is 3.7v nominal, every single sla cell is 6v nominal. As long as your charger can charge and is set to the cell chemistry you are using it will charge it. You can't get cells that are a different nominal voltage that the standard as far as I am aware.
Cheers Max thats exactly what i wanted to hear!
By the way, you can get obscure voltages but they tend to be made to order, or so i've been told :)
I was just curious about charging them.
So any Ni-Mh charger will charge any Ni-Mh pack, but depending on how good the charger is will determine the charge time?
Can anyone recommend any decent chargers? Ideally a multi-chemistry one, but Li-Ion is the priority at the moment as i imagine the multi-chemistry ones cost a fortune?
Thank's again.
Your not allowed to us lithium ion. The only lithium chemistry allowed is lithium polymer (LiPo) and lithium iron (LiFe). Most chargers these days are multi-chargers especially ones that charge lipos. The quality of carver makes relatively little difference to the charge time, most packs have a limit to charge rate which even cheap chargers can achieve.
A lot of people use chargers from hobbyking or giantshark I think- some thing like IMAX-B6 from memory but can't really remember as I've never bought one.
How come i cant use Lithium Ion? Surely they're safer than Lithium Polymer? Thanks for telling me though!
Arh well thats good about the chargers. Are you referring to these ones?
http://www.ebay.co.uk/itm/iMAX-B6-AC...item43bdf44715
I was already looking at these but just assumed them to be cheap Chinese crap. haha
Chris.
Now having had a proper look at the rules and at different battery types, how would the event organisers know which type of batteries i'm using if i build the packs up myself?
I assumed that li-ion and li-iron were the same thing but i now know the difference. It looks like Li-Po is the way to go now. It's going to cost me a lot more though! *angry face*
What do you mean by "build the packs up myself" ?
Weld/wire the cells together myself and apply my own heat shrink. No visible markings to indicate what i'm using.
It's starting to look like the majority of the cost is going to be on batteries. Is this the case for most builders?
Whats the cheapest way of getting 36V that will last long enough? I have 4 motors here that each draw 60+ Amps when stalled. They weigh 1.1KG (ish) each so i need to watch my weight because the chassis wont be light either.
Lipos are cheap what's wrong with using them, I assume its for a fw at only 60amp stall per motor, its what nearly everyone else uses. 36v is a high voltage is that the voltage of the motors?
Well i must admit my math skills suck. I was under the impression that the regular model kind won't cut it?
The motors are 24v but i'd like to run them at 36v if i can afford to in terms of cost and weight.
Would you mind linking me to something you think is suitable so i can get an idea of what will work?
Thanks,
Chris.
The motors scooter motors, by any chance?
Hello Ellis. If i google "scooter motor" the image results look similar, so i guess so. Why, does that matter?
Chris.
They probably have a sticker/plate on them giving some stats, are you able to see a wattage rating? It'll look something like 150w, or 300w, etc. If not, find the closest google image you can, it should be possible to gauge within a bit what size motors you have.
I ask because chances are very high that just two in the machine will be plenty of power. Even at 24v. Check out this build to get an idea what two, at nominal voltage, will get you: http://www.fightingrobots.co.uk/thre...am-gas-diary-s
Ok well they LOOK like the 100W ones but there are no stickers on mine...
http://www.screamwholesale.com/uploa...ooter_100w.jpg
That robot looks fairly powerful to be fair. If i don't use all 4 for the drive, i'll use 2 for drive and 2 on the weapon.
It looks like he's using Ni-Mh's?
Chris.
If it's a first robot stick with nihm packs and a normal charger. It most cases it works out really cheap and they will still give easy discharge rates. You need to know what you're doing with lipo.
edit: I'm not sure how genned up you are but get read up on charge / discharge rates in mAh, generally the higher quality the battery the quicker it will discharge. 12-24v is usually enough for most people, if you need more you could possibly look to a physically bigger motor within the voltage range.
The main reason for people moving to lipo technology is to cope with the rising popularity of brushless motors that require a crazy amount of performance from the battery. As far as I've seen, if you're staying with a conventional brushed motor setup then nihm packs will be fine for you.
Yeah i understand Li-pos can be dangerous. That's why i can't understand Li-ion being disallowed. They seem just as safe as Ni-Mh :S
I was worried about discharge rates. I don't want the batteries to bottleneck my motors.
Li-Ion has a serious disadvantage that makes it rather unsuitable for our application, high internal resistance. In other words, peak power is low. Try to power a robot with Zinc-Air batteries or even Alkaline primary cells...Quote:
Originally Posted by Chris230291
Cheap, yes, Chinese, yes, crap, no. There are even cheaper Chinese copies, and even those have a reasonable use.Quote:
Arh well thats good about the chargers. Are you referring to these ones?
Imax B6
I was already looking at these but just assumed them to be cheap Chinese crap. haha
Of course, nobody will stop you buying a Schulze charger for 10S packs or so. But I believe your wallet will object. Mine does.
We are building COMBAT ROBOTS, not Barbiemobiles.Quote:
Originally Posted by Chris230291
Primary cell batteries ain't in the rules either, just because they are unsuited for our purposes. But I don't believe, if you turn up with a suitable Li-Ion pack, you would be "disallowed" to compete.Quote:
That's why i can't understand Li-ion being disallowed. They seem just as safe as Ni-Mh :S
If you want discharge rates, you could look at the Hawker Cyclon SLA cells. Those start @2V 2.5Ah , and those little ones have a discharge rate of 500A+ that's 200CQuote:
I was worried about discharge rates. I don't want the batteries to bottleneck my motors.
You will bottleneck those motors with any NiMH pack I know of that you can get in weight. We bought two 18v custom packs for Tormenta 2, (4 drill motors) and put them in parallel, to get an estimated 60a-70a peak amp draw available from them. That figure would drive your motors and not starve them too badly, but the batteries will suffer a fair bit. And they were heavy.
I'd go LiPo. They're a bit intimidating at first, but smart enough chargers will stop before doing any damage if the setup is wrong, and once you suss it they're like any other battery technology. Difference is they weigh nothing and have incredible power figures.
Also, in truth, you'd pay a lot more for NiMH packs anything like close enough to the figures you'd need, than you would for a suitable LiPo pack.
--
Two of those motors should get a machine moving around nicely.
I agree lipos are a bit and intimidating at fist but once you've use them and got use to them you won't want to go back to NiMH
Ok cheers for the advice fella.Quote:
Originally Posted by maddox10
I know Li-Ion's are used to power things like electric bikes, so surely they are suitable in this application? I would have thought they used similar motors to the ones i have which appear to be scooter motors?
Haha yes.Quote:
Originally Posted by maddox10
Cheers again mate. I'll take a look :)
It's the maths that confuses me more than anything else. I'd rather the motors melt than the batteries reach their current draw limit. I'm not sure how i'm supposed to work it out :SQuote:
Originally Posted by plargen
Thanks for all of the replies! :)
Ok cheers mate. What numbers do i need to look for when searching for Li-Po's? Also, 60-70A? Each of my motors draw that under stall so are they really the way to go?Quote:
Originally Posted by Ellis
Chris.
Them Hawker Cyclon cells are just really expensive lead acid batteries? I guess the advantage is i can dot them wherever i have free space as opposed to having all of the battery in one lump?
I already have Lead Acids in my workshop. They're pretty much perfect for power delivery but i think the weight is the killer with those.
It's starting to look like i have to make a decision between power delivery and weight?
In combat robots the motors are slammed from forward to reverse then stalled against a wall. They repetitive ly have to accelerate drawing huge currents. On an electric bike it sees nothing like this it has to occasionally accelerate then go along at a constant speed, and it only has one of those motors, your talking about having 4! So although lithium ion are suitable for electric bikes not so much for our applications.
If each of your motors draws 60A at stall then you need a battery that can comfortably output 240A (4x60) at burst. I would go for a 5aH battery to supply those 4 motors, this can output 5A for one hour, or 10A for half an hour, or 20A for a quarter of an hour etc. so for the 3minute fight it can supply an average of 100A which should be adequate.
To find the max discharge rate of a battery you multiply the capacity (in Ah) by the discharge rate (in C). So you need a C rating of at least 48C (240/5).
To achieve this I think you need either a Hawker Oddysee SLA battery (very heavy) or an optipower Ultra 50C Lipo like this:
http://www.optipower.co.uk/Catalogue...cat=131&cnode=
This would be very light. You could if you wanted more voltage use a pair of the 4 cell optipower batteries in series.
If you only want to use 2 motors then you only need half the discharge rate in which case I think web some of the cheap hobbyking lipos could supply the current (as long as you choose very high discharge ones- they are always overrated) but I would still choose a 5Ah battery to keep the high discharge rate.
I would listen to someone else though about what capacity(Ah) you need for those motors as I have never used them.
He was saying that the 60-70A was what his Nimhs could deliver- that is not enough for your motors so you shouldn't really use NiMhs but instead lipo or Hawker Oddysee batteries.Quote:
Originally Posted by Chris230291
I doubt you would use the single cyclon cells but woudonuse buy a 12v hawker Oddysee battery pack then wire 2 or 3 in parralel to get desired voltage. Hawker Oddysee SLAs are expensive but will be able to deliver the amps needed. Those SLAs you have in your workshop would probably not be suitab as they cannot deliver anything like the same amps (they may manage 50A rather than 500A) so would nt be suitable for your robot, they were used on early low power robots but are useless and weigh a ton for no gain.Quote:
Originally Posted by Chris230291
For those motors your choie is only between good quality lipos or hawker Oddysee SLAs, I imagine they cost about the same so I would definatly go for the light weight lipos.
Cheers Max. Thats a very helpful post right there!Quote:
Originally Posted by Max
The peak stall of the motors is like 90 odd Amps, but right away it drops to 60 - 70A. Like right away.
I just popped out to the workshop and i found i already have 3 of the square 5AH 12V sealed lead acid's and 1 rectangular 7AH one. It's nice to know i'm sorted so long as the weight allows them :)
Is the "C" rating just for Li-Po's? I noticed that when i was browsing.
I just cant justify that kind of money for a battery at this stage though :( I think perhaps thats something to look into if i do well in the events, or if enjoy them for that matter! (though i'm sure i will) :)
Thanks again. Very helpful :)
Chris.
Ok. I'll forget about Ni-Mh.Quote:
Originally Posted by Max
O? So a 5AH battery doesn't mean it can cope? :SQuote:
Originally Posted by Max
Confuzzled! :S
Picture time! Here's the batteries i found. They're all "good" as in they hold a charge.
http://i159.photobucket.com/albums/t...ps0ef04ef6.jpg
Ah = capacity = how long a battery lasts
C = discharge rate= the max rate it can supply it's capacity- you multiply the C rating by the capacity to give the mac discharge current. All battery types have a C rating but they are easier to find on lipo batteries.
So your 5Ah batteries have enough capacity (will last long enough) but don't have a high enough discharge rate so won't provide high enough amps for your motors at stall- they can't use their stores energy fast enough.
But to begin with they should work particularly if you only use 2 motors and if you use 2no. 5Ah batteries in parallel to double the discharge rate and the capacity. But it will damage the batteries over time reducing their life but they are quite cheap but you won't get maximum power from your motors.
Arh ok then i think i've got all the info i need now. I see why so many people just use drill motors in their feather weights.
Thanks guys!
Chris.