The servo motors will survive a lot longer if they get 6 volts or less.
You could use a 7805 to provide +5 volts to them. An LM2490-5.0 would be even better.
Be sure to not run the lithium batteries down to less that 2.6volts - sudden death to them.
On the next recharge they might swell and blow up. The regulator will shut them down just about right.
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"If you want more fiber, eat the package.· Not enough?· Eat the manual."········
···················· Tropical regards,····· G. Herzog [noparse][[/noparse]·黃鶴 ]·in Taiwan
Sorry, but those regulators won't/don't just clamp off given VIN < VMIN; No, they'll continue to pass voltage (the device on the output can still draw current.)· This will be "operation" outside design guidelines.
PJ Allen,
I understand that they don't completely shut down.
But with the BasicStamp's BrownOut protection set at something like 4.5volts, the effect is just about the same. It is better than no back-up. A 7.2volt battery with a regulator outputing 5.0volts will likely get down to a 4.5volt brown out in the 'safe discharge limit'. Without the regulators, you arrive below the limit at 2.25volts.
Nonetheless, it is another reason to abandon Lithium as you cannot just leave on a run-down battery and expect to safely recharge it.
My own experience was that I bought a spare 7.2volt Lithium pack for an airplane. I used it on one robotic project for a while and then used it on a breadboarded LED project. I thought it was great. Then, I let the battery run-down and on the next charge, it swelled up like a pregnant guppy. I tossed it out because it was obvious it was ready to burst and all the literature talks of 'balls of fire'.
End result, one useless $40 battery after two weeks. The higher energy density and faster recharge rate really makes them a problem and questionable for experimental hobby electronics.
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"If you want more fiber, eat the package.· Not enough?· Eat the manual."········
···················· Tropical regards,····· G. Herzog [noparse][[/noparse]·黃鶴 ]·in Taiwan
Kramer said...
The higher energy density and faster recharge rate really makes them a problem and questionable for experimental hobby electronics.
Too right, mate -- "Questionable" to say the very least, not for the casual experiencer; I reckon there'll be a heavy price to pay for anyone taking the empirical approach here.
DEFINITELY don't use sensitive rechargeable batteries for low current applications, since before you know it they will be dead. I have had similar experiences as Kramer has described (with nimh though). However since you will have servos running you should notice when they become sluggish, and as soon as they do stop and recharge the batteries.
I think 7 volts shouldn't be too much for the servos. It's really hard to say how they will affect the performance of the servos in the long run. Also depends on the load you put the servos under.
However about the cell phone batteries, they aren't designed for high current discharge rates. Most phones are designed to discharge over the course of a few days to a week. I guess for the extreme condition, discharge rate would be the talk time (a couple of hours). I am pretty sure a boe would last shorter than that. That means you may run the risk of damaging and overheating the batteries by running them down so fast.
Overall I'd suggest against doing this, just use regular rechargeable AA's; they are low cost, more reliable, and safer. Lithium batteries only offer better energy/weight ratio, but that's not really an issue with the boe anyways.
You'd be better off, buying/making a 5 cell mihi pack. I have one I made of GP 1100 cells. The volatge sets at a safe 6.0 volts, but charges to·around 6.5-6.8 volts.·And it has current for days. You also have a less likly chance of turning your beloved bot into a fire ball.
Well, I have have damaged NiMH too, but the issue is not so daunting as the cost can be 1/20th of a Lithium cell.
The other nice thing is teh NiMH cells can be used as individual cells and damage occurs at the cell level.
Often with a pair of Lithium cells [noparse][[/noparse]a 7.2 pack], you will damage one cell but not the other. Nevertheless, you end up sacraficing the whole pack.
I just checked in the store yesterday and NiMh AAAs are rated now at 900ma/h - a few years ago that was an acceptible AA.
The AAs are now rated at nearly 3000 ma/h - a few years ago, that sparked the whole electric R/C airplane boom with lithium cells.
I will repeat myself - the only reason to be using lithium is a need for a faster charge rate.
And, it certainly doesn't hurt to have a shutdown circuitry. I have several 3.3v voltage regulators that have a 4th lead that is a sense input and intend to shut down at about 2.7volts. It is a very useful feature, but you must protect it with a diode and accept a higher shut down as a reverse hookup destroys the regulator post haste. A shottky diode as 0.5v to the threshold -- quite acceptible.
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"If you want more fiber, eat the package.· Not enough?· Eat the manual."········
···················· Tropical regards,····· G. Herzog [noparse][[/noparse]·黃鶴 ]·in Taiwan
Well, I have never used the nihi (aa ) sized cells. But have built many sub c cell packs. I use a peak charger on them. In most all cases, it never takes more than around 20mins charge time.
You can charge many batteries faster than their ideal, but you eventually shorten their useful life.
The Art of Electronics generally states that it takes about 10 hours to optimally charge a NiMh, but it can be done in an hour or two.
Of course, you make the battery manufacturers very happy as they sell more due to your impatience.
Too high or too low a charge rate creates chemical conditions that are irreversible. So the recharge-ability life is cut short.
Lead gel cells seem to be the most abuse tolerant of all. Lithium seem to be the least. and NiCad/NiMh are somewhere in the middle.
When I first got started with Parallax, I thought expensive batteries must be better. But after wasting a few I spent some time thinking about and reading about what batteries are and what they do. The more generic the battery, the lower the price. Specialty batteries are a marketing scam for PDAs, Computers, and iPods. In some contexts they are needed, but certainly to be avoided if one can do so with intelligence.
▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔
"If you want more fiber, eat the package.· Not enough?· Eat the manual."········
···················· Tropical regards,····· G. Herzog [noparse][[/noparse]·黃鶴 ]·in Taiwan
Comments
Dave
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Dave Andreae
Tech Support
dandreae@parallax.com
Http://www.parallax.com
·
Is there a way to limit it to 6 volts?
You could use a 7805 to provide +5 volts to them. An LM2490-5.0 would be even better.
Be sure to not run the lithium batteries down to less that 2.6volts - sudden death to them.
On the next recharge they might swell and blow up. The regulator will shut them down just about right.
▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔
"If you want more fiber, eat the package.· Not enough?· Eat the manual."········
Post Edited (Kramer) : 9/1/2006 7:19:16 PM GMT
I understand that they don't completely shut down.
But with the BasicStamp's BrownOut protection set at something like 4.5volts, the effect is just about the same. It is better than no back-up. A 7.2volt battery with a regulator outputing 5.0volts will likely get down to a 4.5volt brown out in the 'safe discharge limit'. Without the regulators, you arrive below the limit at 2.25volts.
Nonetheless, it is another reason to abandon Lithium as you cannot just leave on a run-down battery and expect to safely recharge it.
My own experience was that I bought a spare 7.2volt Lithium pack for an airplane. I used it on one robotic project for a while and then used it on a breadboarded LED project. I thought it was great. Then, I let the battery run-down and on the next charge, it swelled up like a pregnant guppy. I tossed it out because it was obvious it was ready to burst and all the literature talks of 'balls of fire'.
End result, one useless $40 battery after two weeks. The higher energy density and faster recharge rate really makes them a problem and questionable for experimental hobby electronics.
▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔
"If you want more fiber, eat the package.· Not enough?· Eat the manual."········
I think 7 volts shouldn't be too much for the servos. It's really hard to say how they will affect the performance of the servos in the long run. Also depends on the load you put the servos under.
However about the cell phone batteries, they aren't designed for high current discharge rates. Most phones are designed to discharge over the course of a few days to a week. I guess for the extreme condition, discharge rate would be the talk time (a couple of hours). I am pretty sure a boe would last shorter than that. That means you may run the risk of damaging and overheating the batteries by running them down so fast.
Overall I'd suggest against doing this, just use regular rechargeable AA's; they are low cost, more reliable, and safer. Lithium batteries only offer better energy/weight ratio, but that's not really an issue with the boe anyways.
The other nice thing is teh NiMH cells can be used as individual cells and damage occurs at the cell level.
Often with a pair of Lithium cells [noparse][[/noparse]a 7.2 pack], you will damage one cell but not the other. Nevertheless, you end up sacraficing the whole pack.
I just checked in the store yesterday and NiMh AAAs are rated now at 900ma/h - a few years ago that was an acceptible AA.
The AAs are now rated at nearly 3000 ma/h - a few years ago, that sparked the whole electric R/C airplane boom with lithium cells.
I will repeat myself - the only reason to be using lithium is a need for a faster charge rate.
And, it certainly doesn't hurt to have a shutdown circuitry. I have several 3.3v voltage regulators that have a 4th lead that is a sense input and intend to shut down at about 2.7volts. It is a very useful feature, but you must protect it with a diode and accept a higher shut down as a reverse hookup destroys the regulator post haste. A shottky diode as 0.5v to the threshold -- quite acceptible.
▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔
"If you want more fiber, eat the package.· Not enough?· Eat the manual."········
The Art of Electronics generally states that it takes about 10 hours to optimally charge a NiMh, but it can be done in an hour or two.
Of course, you make the battery manufacturers very happy as they sell more due to your impatience.
Too high or too low a charge rate creates chemical conditions that are irreversible. So the recharge-ability life is cut short.
Lead gel cells seem to be the most abuse tolerant of all. Lithium seem to be the least. and NiCad/NiMh are somewhere in the middle.
When I first got started with Parallax, I thought expensive batteries must be better. But after wasting a few I spent some time thinking about and reading about what batteries are and what they do. The more generic the battery, the lower the price. Specialty batteries are a marketing scam for PDAs, Computers, and iPods. In some contexts they are needed, but certainly to be avoided if one can do so with intelligence.
▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔
"If you want more fiber, eat the package.· Not enough?· Eat the manual."········