RoboSavvy Forum

Changing to LiPos

2008-10-16T19:15:50+01:00

Just got a mail from FMADirect that the Cellpro just went on sale to move stock for their upcoming charger. $50 now.

http://www.fmadirect.com/products.htm?cat=45&nid=4

Statistics: Posted by billyzelsnack — Thu Oct 16, 2008 7:15 pm

Changing to LiPos

2008-10-13T14:05:44+01:00

You use a 4s charger on a 3s pack? Is that ok? I thought you could only charge 3s packs with a 3s charger...
What do you use the connector for?
I never worked with LiPos or practically even seen them, so I'm gonna make some really dumb questions...

Depends on the charger, but the cellpro 4s can charge 1, 2, 3, or 4 cells. The extra connector is called the balance connector. It basically is just wired so each cell can be charged independently.

Statistics: Posted by billyzelsnack — Mon Oct 13, 2008 2:05 pm

Changing to LiPos

2008-10-13T14:03:39+01:00

clusher wrote:
They do seem a bit bulky... The idea is to cause minimum changes to the Bioloid Humanoid configuration. Although I am going to make a new "pack back" to replace the one with the CM5, I dont want to stray too far from the original casing... LiPos fit nicely with small changes.

You can also get smaller A123 cells out of black and decker VPX drill packs. They're often in the discount bins at home improvement stores for $15 for two cells.

http://www.rcgroups.com/forums/showthread.php?t=768729

----------
2cell pack as purchased 123 gr. / 4.34 oz.
1 cell with 2 1/2" 14 ga. leads and APP connectors 48.6 gr. / 1.71 oz.
3S pack with 6" 14 ga. leads APP connector and PQ balancing leads. 137.7 gr. / 4.86oz.
4S with 6" 14ga. leads ,Deans Ultra and PQ balancing leads 6.4oz.
Cells are .715" dia X 2.535 long , not counting tabs.
----------

Statistics: Posted by billyzelsnack — Mon Oct 13, 2008 2:03 pm

Changing to LiPos

2008-10-13T12:20:20+01:00

StuartL wrote:
I use the CellPro 4S balancing charger and FMA's own range of 3S 750mAh batteries. I've installed the Anderson PowerPole 30A connector and it all works REALLY well.

You use a 4s charger on a 3s pack? Is that ok? I thought you could only charge 3s packs with a 3s charger...
What do you use the connector for?
I never worked with LiPos or practically even seen them, so I'm gonna make some really dumb questions...

Statistics: Posted by clusher — Mon Oct 13, 2008 12:20 pm

Changing to LiPos

2008-10-13T11:09:43+01:00

They do seem a bit bulky... The idea is to cause minimum changes to the Bioloid Humanoid configuration. Although I am going to make a new "pack back" to replace the one with the CM5, I dont want to stray too far from the original casing... LiPos fit nicely with small changes.

I was considering making a kind of slots in the new casing, in order to keep the Lipos as close as possible to the Bioloid CG and at the same time to be easily removable for charging. Besides, I still have to fit in the Gumstix and a whole bunch of diferent gadgets in the new casing...

Statistics: Posted by clusher — Mon Oct 13, 2008 11:09 am

Changing to LiPos

2008-10-11T20:53:03+01:00

If you're willing to exchange a bit of size/weight for safety you could go with A123's.

http://www.flickr.com/photos/85327336@N00/2404735388/

The newer cellpro's will charge A123's as well.

A123's also mean you don't need to worry about undercharge and you can charge them insanely fast.

Statistics: Posted by billyzelsnack — Sat Oct 11, 2008 8:53 pm

Changing to LiPos

2008-10-11T18:10:18+01:00

Well, now I know who do ask when we get in problems with Gumstix

Good charger, preferably with balancer to avoid accidents. Check.

So about the maximum current... The molex connectors (I suppose those are the Ax12 connectors) are designed for 3A, but can take up to 5A peaks. So if there are 4 branches in the chain it means 12Amp stall and 20Amp peaks. So if I use a 1050mAh, it should have 15C to meet the regular usage. Right?

I'm gonna get an undercharge circuit to cut off the circuit for the AX12s.

Statistics: Posted by clusher — Sat Oct 11, 2008 6:10 pm

Changing to LiPos

2008-10-11T13:48:09+01:00

Just because the connector is spec'd at 3A doesn't mean that it can only carry 3A. It means it is only designed for 3A. I've been testing EX-106 servos and they use the same connector and I have measured peaks of up to 5A.
Over all I would agree that for a humanoid robot once should design for the average current and be mindful of the peak achieved in normal operation. It's generally very unlikely that many servos will peak at the same time.

clusher:
Welcome to the world of Gumstix robotics. We use Gumstix for our robots too. They perform very well. I'm constantly amazed at the beating they can take and the value they provide.

Statistics: Posted by Bullit — Sat Oct 11, 2008 1:48 pm

Changing to LiPos

2008-10-11T07:41:53+01:00

Robo1 wrote:
Just a comment Stuart. even if all the servos stall you wont get a stall of 33A as the molex connectors are only rated at 3A so the most would be 12A if all 4 daisy chain stall. Also my lipos are rated at 40A peak discharge in short burst.

Good point, the wire is 3A too so that'd melt long before the LiPos for sustained current, however bursts are also an issue so you do need to worry about momentarily large-scale stalls...

Statistics: Posted by StuartL — Sat Oct 11, 2008 7:41 am

Changing to LiPos

2008-10-11T01:26:53+01:00

Just a comment Stuart. even if all the servos stall you wont get a stall of 33A as the molex connectors are only rated at 3A so the most would be 12A if all 4 daisy chain stall. Also my lipos are rated at 40A peak discharge in short burst.

Bren

Statistics: Posted by Robo1 — Sat Oct 11, 2008 1:26 am

Changing to LiPos

2008-10-10T20:51:05+01:00

The discharge limit on LiPos is normally expressed in 'C', where one 'C' is approximated to be how much current you can discharge from a LiPo for one hour before it's flat.

Therefore the 'C' value for the 750mAh packs I use is 750mA. My packs are 15C (I think) which means I can discharge them safely at 15 x 750mA or 11.25A. 10C pack would be 7.5A and I'm sure you could figure out other numbers...

Discharging more than the safe limit is possible but it's dangerous. The current isn't limited in LiPo packs.

Charging LiPos is also fraught with peril (ahhh, the costs of new technology) as over discharging or over charging them has similar risks. Cells should never be discharged to less than 3.2v (bearing in mind that a pack may be slightly out of balance) and should not be charged to more than 4.2v (I think it's actually 4.26v??).

Our CM-5 freezes the software and flashes the LEDs at less than 10v, the AX12 firmware does the same. This way we have a safety margin to allow us to get to the robot and disconnect the battery before getting into the danger zone.

Some packs come with circuit boards on them to limit the discharge current and to prevent over discharging. These packs are almost always more expensive but arguably worth it. The circuit is also obviously more complex for the very high current draws you want for a robot as 22 servos at 1.5A stall current each is a possible 33A draw on the pack.

Would I recommend them? Without question. If you can raise the cash for a good charger (don't bother with a cheap one) and good packs then why stay with NiMHs?

Any other questions?

Statistics: Posted by StuartL — Fri Oct 10, 2008 8:51 pm

Changing to LiPos

2008-10-10T12:06:06+01:00

Actually, I'm working with Bioloid at the university. So no problem about burning up my house! :p

And no problem about charging through CM5. One of the changes we're making will be to take the CM5 and get a Gumstix. This also means I'll be making a new "back pack" to fit a lot of gadgets in there...

I was thinking about keeping the charging pins of the Lipos available so I wouldn't have to take them out to charge them. But I suppose I should keep them at hand then...

I'll check the things you're using. My main questions are about the mAh appropriate and the undercharging circuit... I'm not sure what to use, it's the first time I look into "Lipo world":p

Statistics: Posted by clusher — Fri Oct 10, 2008 12:06 pm

Changing to LiPos

2008-10-10T09:48:31+01:00

I use the CellPro 4S balancing charger and FMA's own range of 3S 750mAh batteries. I've installed the Anderson PowerPole 30A connector and it all works REALLY well.

You don't need to balance the cells, they are usable without balancing. The problem is that unlike a NiMH pack (which, incidentally, is actually worse at becoming unbalanced) when you eventually lose the balance of a LiPo pack you end up reverse-charging one of the cells inside and it catches fire.

This is bad, mm'kay

With LiPos everything is more expensive. The charger is very expensive, the cells are very expensive, the maintenance and care required is expensive in terms of the time you'll spend doing it.

The rewards, however, are high. You'll get MUCH better run times, better servo response and a lighter battery pack.

You MUST, however, disconnect the charging circuit on the CM-5. If the CM-5 attempts to charge the LiPo Bad Things(tm) can happen. Just unsolder the large diode on the CM-5 (making a note of its orientation in case you wish to reinstall it ) and you're safe.

Never charge the LiPos in the robot, never charge them in or on something flammable and never EVER leave them unattended while charging or in use. If the casing has become punctured or damaged and free air gets to the pack it WILL spontaneously combust. You want to be there to stop your house burning down

Statistics: Posted by StuartL — Fri Oct 10, 2008 9:48 am

Changing to LiPos

2008-10-09T15:40:36+01:00

I've seen a charger that balances the cells:
http://www.batteryspace.com/index.asp?P ... rodID=3366

The cells need to have a balance connector? I suppose that should appear in the specificatons? The ones I said before don't charge in balance?

What about the discharge rate? I'm not sure what it means...

I just noticed the ones on the Store: http://robosavvy.com/store/product_info ... aff20c7f58
Are these in series? The size seems pretty good for the bioloid back case but they only seem to have one connector for both charge and discharge...

Statistics: Posted by clusher — Thu Oct 09, 2008 3:40 pm

Changing to LiPos

2008-10-09T12:21:27+01:00

You might want to look for a LiPo that has a balance connector and a charger that balances the cells. In my experience I've had a lot less problems with these type batteries once I moved to balancing the cells.

Statistics: Posted by Bullit — Thu Oct 09, 2008 12:21 pm