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nicads or sla?


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Big AL
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Joined: 16 Oct 2004
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nicads or sla?

just a question at what people are using in there bot to power weapons and drive? i thought of using sla's but was told that they run out of juise in a bout a month? anyway what are you thoughts,what do you use and why?
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Post Tue Jan 25, 2005 11:38 pm 
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Valen
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roughly all you need to know about batteries
especially in featherweights
SLA - cheap but shitty, very low power output per mass (2.2Ah SLA is in fact ~0.8 or less) very heavy, short life time. i think glen manages to get about one event out of a set.

Nicad -- possibly the best for robot wars, high current output, low internal resistance, relativly cheap.

NiMh -- higher energy density (ie more energy per weight) in theory but higher internal resistances mean that this goes out the window when you start pulling boatloads of current.
a good (expensive) ni-mh cell will "beat" a nicad cell when the current draw isnt too high.
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Post Wed Jan 26, 2005 12:13 am 
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Waddy the phoenix



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hey look jake finished the thread... now to see who will disagree on these comments and explanations
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Post Wed Jan 26, 2005 6:29 am 
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Knightrous
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Only thing I disagree with is the short life time. We've had 4 DOSS 7amp batteries since the first RoboWars and we are still using them abusively in our robots Smile SLA's die if you don't look after them or charge them right, Glen's little trickle chargers seem to be the devil Razz
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Post Wed Jan 26, 2005 8:59 am 
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Spockie-Tech
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Nope, Jake summed it up pretty well..

The only thing I would add is that SLA's arent always shitty batteries... in a lot of applications, they are great batteries, but not Robot Wars..

They arent designed to have all their stored power sucked out of them in under 5 minutes and as a result when you try and do so, you only get about 30%-40% of their capacity from them, since their internal chemical reactions cant keep up with the demmand for current.

NiCd's dont particularly like it either, but cope a lot better, delivering 80%-90% of their stored capacity in a 5 minute discharge.

a battery's capacity is rated at a *20 hour* discharge time (ie, you take 20 hours to gradually discharge the battery to flat), so you can guess how much stress you put on them when you flatten them in 5 minutes.

The reason every starts off with SLA's is simply cost.

1 x 12volt 2.2ah SLA (Gives about 1ah of useful capacity in a bot) - $15
~ 1100 grams in weight
Battery Charger capable of charging it in 1hr - $50
Total about $65

1 x 12volt 2.4ah NiCd pack (gives about 2.2ah of useful capacity) - $~80
Approx 650 grams weight per pack
Battery Charger capable of charging in 1hr - $$150-$300
Total about $320

So the Nicad pack has more than double the capacity in nearly half the weight which means about 4x the power-weight of the SLA, but is around 5x as expensive (when the charging system is taken into account)

So if you can afford it, Nicads are definitely the way to go..
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Post Wed Jan 26, 2005 10:59 am 
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Glen
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i wouldnt say $350,

an omni charger is $50, and a 12amp PSU is $10 along with the $45 of the single 2.4ah battery pack.

but the slas are just terrible. ive had no problems with the 7ah batteries but the smaller 2.3ah ones from jaycar cobra just eats those things. not to mention super cheap ones are the worst of all. ive had 2 just instantly stop giving power and the jaycar ones slowly loose power. same with the doss batteries, the ones brett gave us at the engineering expo only have about 550 mah capacity at best.

ive got one of the powertech trickle chargers that goes at about 13.6-13.8v and jeff told me how to check it to make sure its regulated which it is.. the only thing i can assume is that slowly charging them at 1/2c kills them.

but having bought 16x 2.3ah slas in the past year for $320 odd dollars compared to the nicads which will last for a year. id say cut the crap and just buy the nicads first time out and save your money in the long run.

id like to see some tests on 2.3ah slas with discharging them at robot like levels then charging a couple at 1A and a few at 5A or whatever andrew does them at and seeing how many cycles they both last until they wont go past 12.6v. (wonder how youd go about that anyway..)
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Post Wed Jan 26, 2005 2:03 pm 
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Valen
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dont forget drill nicads do work "ok" too
Plan-B still runs "ok" on a complete set of drill nicads
2x 14.4 packs for drive and 2x 7.2 packs to boost the wep.

though we are going to an all round 24V full C system soon lol
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Post Wed Jan 26, 2005 2:49 pm 
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Spockie-Tech
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quote:
Originally posted by Glen:
i wouldnt say $350, an omni charger is $50, and a 12amp PSU is $10 along with the $45 of the single 2.4ah battery pack.


We've had some trouble with the Omni chargers, and I note that none of the larger teams use them.. they're all Triton's, Astroflights or Schulze's. If you're going to spend the $ on good cells, I wouldnt subject them to a cheap charger myself. If you want to be able to reliably crank power back into the pack without hurting it, I'd spend the extra and get a good charger.

Where do you get a 12amp regulated 12v power supply for $10 from ? Modifying PC power supplies and other hacks are fine, but not everyone has the knowledge and skill to do that. I'm comparing off-the-shelf stuff here..

And 12v Sanyo packs might only cost you $45 in cells, IF you buy 100+ of them, and IF you have the skills, time, braid, and heatshrink to make them yourself. But if you want to buy a ready-to-go battery (as the 12v SLA's are) then it will be more like $80 for a pack. You have to compare apples to apples.. comparing a turn-key battery to a pile of loose cells doesnt count.

$320 might seem high, but its a realistic cost if you want quality gear and dont want to fiddle about making packs and things yourself..
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Post Wed Jan 26, 2005 8:20 pm 
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Philip
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These guys have done a write up on batteries. http://www.teamtornado.co.uk/batt.htm
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Post Thu Jan 27, 2005 6:00 am 
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andrew



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i agree on charger quality.

makes me glad that my charger is a gws and its no triton but it gets the job done well.
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Post Thu Jan 27, 2005 7:11 am 
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DumHed
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I've actually had good success with the Omni chargers.
I was developing a battery charger here for a mobile EFTPOS product, and the Omni charger was one of our benchmarks (as recommended by Master Instruments), and gave the same final mAh figure as when the batteries were charged using a PC controlled battery analyser system.
I have seen them not want to charge a couple of batteries, but generally they either start charging after a while, so I imagine it's just not liking the batterie's initial voltage / impedance.
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Post Thu Jan 27, 2005 9:46 am 
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