Robot Li-Ion powersupply

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Maximilian
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Posted by Maximilian: Tue. May 19, 2009 - 05:35 PM
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For a little robot project i need a replacement powersupply, i've already read some documentation on the subject but i could really use the input.
Size and weight are limited.

The original power design:
4 lr14 c type Alcaline 1,5v batteries
Robot only needs 6v to operate
About 270 Gram

But now i need 5v(AVR/Zigbee), 6v(toy itself), 9v(addon) and i need the design to be recharchable.

I've got a new 6 cell 10,8 lion Laptop battery (300 Gram) and would like to use this as a powersupply.

The current for the 6v system is as of yet unknown, but there are about 4 little motors used in it and a small cpu, probably the peak current would be about 1A.

The current for the 9v system is about 100 mA .

The questions:
1. What would be the best way to generate the 5v/6v and the 9v from the 10,8v battery.
I would guess that a normal 780x would be a waste of current, anybody have any ideas for a LDO or something?.
2. Would the original electronics contained in the battery pack add something or could i just tear it out and use an existing external 2 lead Lion charger, do i need to take other precautions?
Does it level the batteries?
3. How can I best cut of the power completly if the battery's are almost empty?

Thanks!!

p.s. edited the title for clarification LOL

Tags:

Learning and Information, General Electronics
Last Edited: Wed. May 20, 2009 - 06:52 PM
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Kartman
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Posted by Kartman: Tue. May 19, 2009 - 11:19 PM
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1. First up a LDO is a "low drop out" regulator which means the input voltage only has to be slightly greater than the output voltage. It is still a linear regulator like the 7805 and will waste the voltage difference as heat. A switched mode regulator is suggested as this will be more efficient (~80%) - something like a national semi simple switcher.

2. The original electronics are probably a protection circuit. As to the exact nature ofthis circuit, I have no idea as I don't know what battery pack you're using.

3. You need a circuit to cut the power if the battery voltage drops below a certain threshold. Something like a TL431 and a small relay may suffice. You can also use the ADC in the AVR to measure the voltage and then turn a relay or mosfet off.

4. learn the rules of using apostrophes.

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tpappano
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Posted by tpappano: Tue. May 19, 2009 - 11:26 PM
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Quote:
1,5v batteries

I think the use of *commas* in this fashion is a common European thing.

Quote:
4. learn the rules of using apostrophes.

And capitals. 8-) 8-)

Tom Pappano
Tulsa, Oklahoma

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UNiXWHoRe
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Posted by UNiXWHoRe: Wed. May 20, 2009 - 02:16 PM
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tpappano wrote:
Quote:
1,5v batteries

I think the use of *commas* in this fashion is a common European thing.

It's a common WORLD thing. LOL...

It's called International System. Only Americans still use the English system, even England is using Metric nowadays!

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Torby
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Posted by Torby: Wed. May 20, 2009 - 03:35 PM
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Are you sure these aren't 3v cells?

If you don't know my whole story, keep your mouth shut.

If you know my whole story, you're an accomplice. Keep your mouth shut. 

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Maximilian
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Posted by Maximilian: Wed. May 20, 2009 - 04:52 PM
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Kartman wrote:
1. First up a LDO is a "low drop out" regulator.

Ofcourse, you are right(where was my head at the time?!?!?)

Kartman wrote:
A switched mode regulator is suggested as this will be more efficient (~80%) - something like a national semi simple switcher.

Parts are really hard to get around here, a simple switcher goes for something like €14,- here.
Space is also an issue.

I remember some time ago that there was a dc-dc converter "drop in" replacement for the 78/79 series without any modification nessesary, never heard anything about those again.
Will have a look around.

Kartman wrote:
2. The original electronics are probably a protection circuit. As to the exact nature of this circuit, I have no idea as I don't know what battery pack you're using.

It is a HP hstnn-d805 laptop battery.
It seems there is at least some kind of fuse and probably a mosfet in this circuit, and of course al kind of unidentifiable chips.
(p.s. corrected "ofthis" to "of this", LOL)

Kartman wrote:
3. You need a circuit to cut the power if the battery voltage drops below a certain threshold. Something like a TL431 and a small relay may suffice. You can also use the ADC in the AVR to measure the voltage and then turn a relay or mosfet off.

Was already thinking of using the onboard AVR for battery monitoring, do you have an example of such a circuit? (worried about using current after the circuit is turned off)

I heard some horror stories about Lion batteries exploding because of 1 battery being defective so that the other batteries got a higher voltage.
Will an "auto cell detection" charger prevent this?

Kartman wrote:
4. learn the rules of using apostrophes.

Batteries ??
As you might have guessed English is not(careful here) my native language, any suggestions for improvement are welcome.

tpappano wrote:

I think the use of *commas* in this fashion is a common European thing.

Yes it is, if i use MS Excel even the "." on the numeric keyboard changes to an ","

tpappano wrote:

And capitals.

That is someone who wanted to post a question but had to leave quickly...

Torby wrote:

Are you sure these aren't 3v cells?

Yes, they are 1,5 volts.

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AndersAnd
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Posted by AndersAnd: Wed. May 20, 2009 - 05:10 PM
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Fist of all it's not named Lion but Li-ion.
When I rewad the topic title I though you were talking about a toy robot shaped like a lion, turns out you mean Li-ion batteries instead. :-)

Maximilian wrote:
2. Would the original electronics contained in the battery pack add something or could i just tear it out and use an existing external 2 lead Lion charger, do i need to take other precautions?
Does it level the batteries?

You mention a 2 lead Li-ion charger, what exactly is that? Can you give one example of one? Li-ion chargers has more than 2 wires because they need to communicate with the battery to monitor it's state.
You ask about precautions. Be aware that chraging Li-ion battteries is very dangerous if you don't know exactly what you are doing. Li-ion cells can easily explode. There's a reason it's illegal to sell bare Li-ion cells to the public as opposed to selling bare Ni-MH cells. When you buy a Li-ion battery for your laptop or cell phone, it doesn't only contain the bare cells but Li-ion batteries are alway intelligent batteries that also contain things like temperature sensors to make sure the cells doesn't overheat while charging.

Atmel has a number of AVR application notes about Li-ion chargers. You can make your own Li-ion charger: http://www.atmel.com/dyn/product...
But based on your commnets I don't know if you should even consider charging Li-ion cells.
Better stay with something safer like Ni-Mh or get a complete Li-ion battery and charger pacakge where the security is taken care of.

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dbvanhorn
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Posted by dbvanhorn: Wed. May 20, 2009 - 05:56 PM
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Some li-ion chargers have only two conductors. Those rely on a board in the battery pack to balance and provide cutoff. all-batteries.com has a nice selection, and I've used a fairly broad range of their li-poly products, from 1 up to 50AH.

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Maximilian
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Posted by Maximilian: Wed. May 20, 2009 - 06:51 PM
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AndersAnd wrote:
Fist of all it's not named Lion but Li-ion.
When I rewad the topic title I though you were talking about a toy robot shaped like a lion, turns out you mean Li-ion batteries instead. :-).

LOL, never thought of that, if i search for 2nd hand lion or Li-ion batteries here i get about the same numer of hits, hmm having a lion in your electric screwdriver could help...
And the toy is more like a big spider something..

AndersAnd wrote:
You mention a 2 lead Li-ion charger, what exactly is that? Can you give one example of one? Li-ion chargers has more than 2 wires because they need to communicate with the battery to monitor it's state.

Thats what i thought until i i saw these kind of chargers.
http://www.mscomposit.com/electronics/charger/12v/charger30.html#description
Maby the batteries themselves have all the security features inside or am i missing something?
Documentation on the chargers are hard to find, but it looks like they only have a positive and a negative terminal for charging.
They do not say anyting about a balancer, but there are ofcource lots of people who blow up their batteries.
There are also chargers with 3 leads, probably an extra wire for the temparature probe.

AndersAnd wrote:
Li-ion cells can easily explode. There's a reason it's illegal to sell bare Li-ion cells to the public as opposed to selling bare Ni-MH cells..

I already know about the exploding bit, did not know it was illegal to sell, makes sense.
On the internet you can find a lot of people who repair their own Laptop batteries by replacing the cells for new ones, how do they get a hold of these new cells?, maby it differs from country to country?

AndersAnd wrote:
Based on your commnets I don't know if you should even consider charging Li-ion cells..

I asked all of this this because i wanted to learn, it would be easy to know everything in advance.

AndersAnd wrote:
Better stay with something safer like Ni-Mh or get a complete Li-ion battery and charger pacakge where the security is taken care of.

Ni-Mh is not only to heavy, it also would not fit inside.
The battery i have lying around would be perfect, and a complete sollution would be at least €150,- that i do not want to spend on this.

See it as a learning experience for me, and i will ofcourse take everything in consideration if i continue with this project.

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Maximilian
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Posted by Maximilian: Wed. May 20, 2009 - 07:01 PM
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dbvanhorn wrote:
Some li-ion chargers have only two conductors. Those rely on a board in the battery pack to balance and provide cutoff. all-batteries.com has a nice selection, and I've used a fairly broad range of their li-poly products, from 1 up to 50AH.

Ok, so adding a module like
http://www.batteryspace.com/index.asp?PageAction=VIEWPROD&ProdID=3228 to the batterypack and a suitable charger would clear up all problems.

Interesting, have to read more.

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AndersAnd
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Posted by AndersAnd: Wed. May 20, 2009 - 07:10 PM
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Maximilian wrote:
I asked all of this this because i wanted to learn, it would be easy to know everything in advance.

True, I just warned you because this is not for everyone and I don't know your knowledge level about electronics etc. so it best to be on the safe side and warn about the dangers. Before one starts with something like this they should have a good knowledge about electronics in general, a battery charger is not the first electronics experiments one should start with. It's up to you if you you are still up for it.
If you want to learn how to charge Li-ion cells do some googeling and reading of application notes etc. from manufactures of Li-ion charger IC or batteries. You could start by reading the Atmel AVR application notes about Li-ion chargers. I think there's schematics and example code too so you can program your own AVR based Li-ion charger.
Making your own AVR based charger could be a fun project learning experience if you're up for it. Otherwise it's also possible to get finished Li-ion charger ICs. But be very careful when you play with chragin Li-ion cells, it's not just trial and error like when you want to get a LED blinking from your AVR.

And it's not just about having an intelligent charger. You need protection circuit during the discharge of the battery too. For example to make sure you don't discharge the cells too much as this could damage the cells too.
If for example a laptop battery gets discharged too deep for example after storing it for a long time, it's impossible to charge it again even if the battery was still in good condition as the battery intelligence will prevent you from this. It is possible to get a special charger for this situation to bring deeply discharged Li-ion laptop batteries back to life. Some laptop repiar shops has these special charger to bring back life to perfectly good Li-ion batteries that have just been discharged too deep.

http://en.wikipedia.org/wiki/Lit...

Quote:
Protection circuits required

Li-ion batteries are not as durable as nickel metal hydride or nickel-cadmium designs, and can be extremely dangerous if mistreated. They may explode if overheated or if charged to an excessively high voltage. Furthermore, they may be irreversibly damaged if discharged below a certain voltage. To reduce these risks, li-ion batteries generally contain a small circuit that shuts down the battery when discharged below a certain threshold (typically 3 V) or charged above a certain limit (typically 4.2 V).

This circuit prevents deep discharge in normal use. However, when stored for long periods, the small current drawn by the protection circuitry may deeply drain the battery. Some applications attempt to recover deeply discharged cells by slow-charging them.

Furthermore, this circuit adds to the cost of lithium-ion batteries, which is usually higher than that of comparable-capacity NiMH or NiCD batteries.

Many types of Li-ion cell cannot be charged safely below 0 centigrade.

Safety features

Li-ion chemistry is not as safe as nickel metal hydride or nickel-cadmium, and a li-ion cell requires several mandatory safety devices to be built in before it can be considered safe for use outside of a laboratory. These are:[17]

* shut-down separator (for overtemperature),
* tear-away tab (for internal pressure),
* vent (pressure relief), and
* thermal interrupt (overcurrent/overcharging).

These devices occupy useful space inside the cells, and reduce their reliability; typically, they permanently and irreversibly disable the cell when activated. They are required because the anode produces heat during use, while the cathode may produce oxygen during use. Safety devices and recent, improved electrode designs greatly reduce or eliminate the risk of fire or explosion.

The safety features of lithium-ion cells can be compared with nickel metal hydride cells, which only have a hydrogen/oxygen recombination device (preventing damage due to mild overcharging) and a back-up pressure valve.

Last Edited: Wed. May 20, 2009 - 07:42 PM
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dak664
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Posted by dak664: Wed. May 20, 2009 - 07:32 PM
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10.8 volts is a typical laptop battery rating, but it is an average over a wide swing of output voltage. Depending on the type of lithium chemistry, cell voltages can range from 2-4.3 volts (the 6 cells are likely paired 2 parallel 3 series). The voltage probably changes from ~12.5 at full charge to ~8.5 at full discharge. Going beyond those limits will damage the cells and for some types can cause a fire. Tolerances are around 50 millivolts!

A DC-DC converter is needed for any kind of efficiency because of the wide voltage swing. Buck converters are a good solution if the battery voltage does not drop below Vout.

Either use a thoroughly debugged charging circuit, or choose a safe chemistry. For that I'd suggest 3 LiFePO4 cells in series, the voltage range of those is 2.0 to 3.65 volts which would allow for an efficient buck converter. The 9V@0.1A would need a 1 watt boost converter from the 6 volts, not a big deal. The AVR could be powered from an emitter follower (guessing here, I have never tried it).

And it is not necessary to charge/discharge lithium ion cells between the voltage limits, if you size the battery for the reduced capacity. They will last longer if you don't, and for some chemistries will be an 'elluvalot safer.

[edited: fixed elluvalot apostrophe]

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dbvanhorn
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Posted by dbvanhorn: Wed. May 20, 2009 - 08:10 PM
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Batteries can be way more fun than you expect.

All those safety features are nice, and you really hope that your cells, built by the lowest bidder, actually implement most of those features..

NIMH is easy to be relatively safe with, but actually pretty difficult to charge properly when outside a "shirtsleeves" environment. When they fail, it's pretty entertaining, but at least the hot boiling caustic liquid jet isn't also flammable.

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Maximilian
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Posted by Maximilian: Fri. May 22, 2009 - 08:56 AM
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If i find a way to charge them safely, and if i leave the protection circuit at the charger side(to reduce weigt/space) what would i need to replicate for safe use inside the robot?

1. Overcurrent protection(e.g. simple fuse)
2. Monitor different cells for individual deep discharge with cutoff for robot and charger.
3. Monitor temparature?

Does anyone have a working example of a cutoff circuit controlled by an AVR that wil not use any current after it "cuts" the power?

Watching the battery through the SMbus port also looks nice, but difficult to build a nice charger for it.

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UNiXWHoRe
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Posted by UNiXWHoRe: Fri. May 22, 2009 - 11:58 AM
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Maximilian wrote:
If i find a way to charge them safely, and if i leave the protection circuit at the charger side(to reduce weigt/space) what would i need to replicate for safe use inside the robot?

No! Never tamper with a Li-Ion battery pack's protection! Leave it as it is (I'm not sure what size or weight you are talking about though, all the protection circuits on battery packs I've seen are the size of a quarter and weight less than one gram...).

As for the monitoring part, just find yourself a proper charger IC and it will have all you need to monitor current and voltage and temperature; most of them have some digital feedback pins you can feed to an AVR for monitoring, or you could use the A/D on the AVR to monitor voltage. Because you are using a multi-cell pack, I highly recommend you get a cell balancing companion IC to keep your pack healty. If one of the cells become less or more charged than the ot hers, it will mess with the charger's readings, and you will end up with a mighty fire or explosion.

IMHO having all the circuitry integrated into the device (charger, protection, cell balancing) is much better, and far outweight the size or weight constraint, as I do not believe all circuitry together will weight more than 5 grams.

As to how to cut power when you detect something is wrong, a load switch device oughta do the trick, or you can build one with one N-ch and one P-ch MOSFET, with some resistors and caps.

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dbvanhorn
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Posted by dbvanhorn: Fri. May 22, 2009 - 01:14 PM
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I agree, use the protection board.

The RC Hobby guys routinely ignore this, and they also end up with fires and garages/houses destroyed.

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Maximilian
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Posted by Maximilian: Mon. May 25, 2009 - 07:34 AM
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Will try to find info on the original protection board before pursueing this road of action, or find a nice replacement for it.

Last weekend i found a good Ni-MH pack with less mAH that will fit, will have to loose some of the new features i had in mind but at least it is safe.

Will try to find a nice 9,6 Ni-MH charger circuit, any hints would be appreciated.

Thank you all for your time!!!

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