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[...] the robot i am going for uses 8 servos, a blackfin camera, and a sharp ir. Any recommendations to a good 6v battery
and for the charger i was considering this http://www.trossenrobotics.com/store/p/5914-Smart-Universal-Charger-for-2-4-7-2v-NiMH.aspx
Here is what I'd consider a cheap charger that will keep your batteries in shape and it works with all battery types up to ~12V, currents up to 5A and have build in balancer port for Lithium based batteries.
Don't confuse battery capacity, such as 3300mAh, with current rating, such as 10A. They are almost entirely unrelated. A 100mAh battery could supply 20A, for example.
I've been looking into moving to Li-po or Li-ion for 3V stuff (around 3000maH, but I know just enough about that chemistry to be afraid of it, so I'm interested in this. Do you think it would be better to buy a charger specifically for lithium based batteries, or is this good (safe)? It would be nice to have one charger for both NiMH and lithium.
I don't know what current the Blackfin needs, but with 8 servos, I'd go a little higher than 3300mAh.This is a 4200mAh battery, but it doesn't come cheap, so you should shop around a bit.
Quote from: Admin on January 30, 2012, 10:01:43 AMDon't confuse battery capacity, such as 3300mAh, with current rating, such as 10A. They are almost entirely unrelated. A 100mAh battery could supply 20A, for example.That's a very bold (but very incorrect too) claim!A 100mAh NiMH battery with a 20A discharge is simply impossible!
Capacity is very much related to max. discharge currents.
Also, don't confuse a good charger with one that lasts a long time. Charger lifetime is totally unrelated to quality, which is a matter of how well the charger takes care of the batteries.
also to admin if you don't mind me asking what was the battery you used for the erp as my robot and it are of similar builds?
This reply is more meant for Robot_Longbordr than Soeren . . .I didn't say NiMH (I meant hypothetically . . . I probably should have been more clear on that)
People often confuse a 3000mAh battery as being capable of only supplying 3A, and I'm pointing out that this is wrong. In the datasheet there is another separate current rating that must be referenced. The more modern 6V NiMHs I've seen are usually rated for ~10A. That's enough for about ~6 servos.
This is, of course, assuming you're using small servos and your battery is capable of 10A. Bigger servos can do more like 3A+ each, so you should measure them with a multimeter to avoid the guessing game. Some cheaper batteries claim 10A, but what they really mean is up to 10A under perfect laboratory conditions . . . giving you more like 5A in reality land.
QuoteCapacity is very much related to max. discharge currents.I would argue it's only a lose relation. Design, manufacturing quality, temperature, age, specific chemistry, and parallel vs serial pack designs all play a part.
The problem with quality, and that goes with anything really, is it's hard to know until after it fails on you . . .
The thread only discussed NiMHs, but even so, which chemistry would you credit with 200C?
Quote from: Admin on January 31, 2012, 12:09:40 AMQuoteCapacity is very much related to max. discharge currents.I would argue it's only a lose relation. Design, manufacturing quality, temperature, age, specific chemistry, and parallel vs serial pack designs all play a part.If it weren't closely related, max. discharge rates wouldn't be given in nC ratings.
I don't see where series and/or parallel packs enter the equation?
Quote from: Admin on January 31, 2012, 12:09:40 AMThe problem with quality, and that goes with anything really, is it's hard to know until after it fails on you . . . You mean that you don't know if your tools are good or bad until they break either? I'd assume a mechanical engineer would know how to get the right stuff by eye (and perhaps by "pinging" the steel and judge by ear)?
Brand-name manufacturer A makes a 6V battery and it has 5C, while crappy manufacturer B makes a 6V battery and it has 2.5C. Sure, it's related, but the difference is 2 fold. Hence loosely related
For safety reasons, LiPos in consumer electronics are becoming in-series only batteries (no need to balance cells when they are in series, as current is the same for all). The output voltage stays the same but the capacity and current output are affected.
Unfortunately it's not that easy. I'd have to open up the casing to study the electronics inside, and then study the wiring, look up the parts, and check the soldering job. I can only guess on the software. Sure, for me with electronics manufacturing experience I can identify quality. But only after buying it and opening it up. Other people without experience have to rely on reviews.
As for mechanical tools, you don't know the quality of metal it's made from until it rusts up, bends, or dulls.
And for precision tools like crimpers, it's hard to tell if it will crimp my pins right with only that 1"x1" jpeg-artifact picture on the website to judge by.
But . . . what we all see is the price.
I've learned the hard way that the only sure measurement of quality is brand-name + warranty. But seeing a price tag 3x cheaper always overrules my better judgement . . .
There's a thing you didn't grasp about balancing it seems - it's not a question of series or parallel cells (eg. two cells in parallel will not need balancing). Series connected cells need balancing to keep each cell at the same voltage as they age (and then charge) differently (even NiMH and other chemistries would benefit from balancing when charged in series, but since they aren't that much of an explosion risk, nobody throws money after that).
So, in essence, you say that you cannot judge quality by the immediate appearance (when actually holding the device in question)?
Quote from: Admin on January 31, 2012, 11:51:59 AMI've learned the hard way that the only sure measurement of quality is brand-name + warranty. But seeing a price tag 3x cheaper always overrules my better judgement . . .I have seen a lot of crap from brand names over the years
I think we've hijacked this thread . . .
If a cell dies when the batteries are in series, even if one cell shorts, the good cells are not shorted - ie a safe dead battery. If a cell dies when the batteries in parallel, and that cell shorts, you could have a fire. There was an article in IEEE Spectrum, of which I can't find on a quick search, that described this much better . . . whether this reduces battery life I can't say, but their claim was that it did maximize battery safety.
As you said, companies benefit from selling more batteries, but not from lawsuits . . .
Try judging the quality of a battery charger without being allowed to try it out - only looking at the outside casing. That's the point I'm trying to make.
We often aren't able to judge the quality until after the purchase, because the item is bought online or it's covered in packaging at a physical store. Or we just aren't an expert in that particular tool (ie first charger you've owned, etc).
In the parallel laptop battery, current is supposed to flow through the parallel paths at exactly the same rate. But slight temperature differences or tiny chemical imbalances between the two paths force more current into one of them. Over time, the current imbalance between the cells can go to an extreme that forces bits of lithium metal to adhere to the anode. When this happens, the battery is able to store less energy than it is designed to store, meaning a shorter computer run time per charge. And because lithium metal is highly reactive, those scattered bits of metal can fuel a fire if a short crops up and suddenly raises the temperature of the system.Wiring three cells in a series essentially eliminates that problem [see diagram, ” A Not-So-Simple Remodel”]. With only one path for the current to travel, it’s easier to control the flow of current, reducing the chance that lithium metal will be deposited, which would compromise the storage capability.
[...]The serial configuration has the inherent ability to dramatically reduce the risks of fire. In series, if one cell shorts, the others won't. In parallel, if one cell shorts, they all do. At least so claimed by the author of this IEEE article (I found it, finally):http://spectrum.ieee.org/energy/renewables/the-lady-and-the-liion/0
selected quote:QuoteIn the parallel laptop battery, current is supposed to flow through the parallel paths at exactly the same rate. But slight temperature differences or tiny chemical imbalances between the two paths force more current into one of them. Over time, the current imbalance between the cells can go to an extreme that forces bits of lithium metal to adhere to the anode. When this happens, the battery is able to store less energy than it is designed to store, meaning a shorter computer run time per charge. And because lithium metal is highly reactive, those scattered bits of metal can fuel a fire if a short crops up and suddenly raises the temperature of the system.Wiring three cells in a series essentially eliminates that problem [see diagram, ” A Not-So-Simple Remodel”]. With only one path for the current to travel, it’s easier to control the flow of current, reducing the chance that lithium metal will be deposited, which would compromise the storage capability.
If money is an issue, why not get this instead?http://www.onlybatterypacks.com/showitem.asp?ItemID=11502.12I plan to buy the 6V version of this in a week or so . . .