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Charging batteries via USB


#1

I have ordered a small Lipo battery http://www.hobbyking.com/hobbyking/store/uh_viewItem.asp?idProduct=6472 that will be mounted inside the Panda tinkerer box.
My question is how to charge it.
Can it be safely charged inside the box or should I remove it each time? I have also ordered this charger http://www.hobbyking.com/hobbyking/store/uh_viewItem.asp?idProduct=11060
These battery types and the whole RC battery/charging stuff is new to me.

Is it possible to build a charger that draws current from the USB port?
This is what my box looks like.


#2

This might help. There are charging and detection circuit schematics:

http://blogs.msdn.com/b/coding4fun/archive/2010/12/13/10104241.aspx


#3

I got this LiPoly charger from Sparkfun other day and their 2000mAh LiPoly batter.
http://www.sparkfun.com/products/726

  1. Batteries charge from usb or 5-7v (Max) barrel.
  2. All is left hooked up.
  3. Outputs for sys and battery.
  4. trickle charge when charge complete.
  5. defaults to barrel if both usb and barrel connector hooked up.

would be nice it they could pass-through the usb data connector.

Have not used yet. Will be interested to see what you come up with.


#4

LiPo are not as forgiving as any other battery for abuse including charging. RCG is a great resource for this and here’s a good link to start you out… http://www.rcgroups.com/forums/showthread.php?t=209187


#5

LIPO batteries are great, but they have one major problem: exploding cells.
Normally this does not happen, but in some cases (e.g. a damaged cell) it might. You do not want to have a exploding LIPO inside your tinkerer kit.

One advise, which is all over the internet, is to charge LIPO batteries outside the house or charge with special LIPO charge bags.


#6

It’s the exploding part thats scares me. I don’t want the battery catching on fire inside the tinkerer kit but hopefully it will be alright.
I have no need for fast charging and it seems that that or a faulty battery are the more obvious reasons for problems. Also batteries that have been in a RC crash might give problems but as this is a handheld controller that part should pose no problem.
The thread from rcgroups is from 2003 and hopefully batteries and chargers have become better the last 8 years.

Is there any better type of battery that would give me 1000mAh at 7-12V in a small package?


#7

Did you see this one?


#8

Is’t that more or less the the same battery I have ordered?


#9

I think so, and it looks like the one you ordered is cheaper. Sparkfun has a variety of other ones as well:

http://www.sparkfun.com/categories/54


#10

Don’t we hold them up to our face every day in our cell phones? Or they using something else?


#11

Unless you have a phone from 1995, it’s a lipo.

RC lipos may or may not be a little different. The manufactures are under a large amount of pressure to create a battery that will have an extremely high energy density and have a similarly high discharge rate.

That’s part of the reason why you see videos with RC planes bursting in the flames.


#12

Correct me if Im wrong but I thought that the problem and danger was the charging of the batteries and not the usage? As I said, Im new to the LiPo batteries and as far as I can figure out the battery I ordered should be able to handle 16.5A (1100mAh x 15C) but Im not pulling more than 500mA max so I think Ill be safe.

Still I think Ill mount the battery in a way that it can be removed for charging, just to be on the safe side. :slight_smile:


#13

You can destroy a LiPo by pulling too much current out of them just as easily as pushing too much current into them. The biggest danger comes from people doing really stupid things like trying to use a standard power supply or incorrect charger to charge them and/or not building over discharge/over-temp protection into the system.


#14

removing them from the unit when you charge is a very smart move. Careful handling along with that and specific chargers are all plusses. In your case the use of a charged battery shouldn’t be an issue, but as Jeff says over discharge is another consideration


#15

TMK, the polys at Sparkfun have:

“Battery includes built-in protection against over voltage, over current, and minimum voltage.”

I guess if you worked at, you may find a way around such protection or somehow protection could fail. Anyone here had one blow under normal conditions that was “protected”?


#16

Paging markh to the thread about lipos bursting in flames…


#17

Ok, so concerning lipo’s catching fire:

Lithium-Ion Polymer will catch fire if they get too hot. There are a few ways to do this:

  • Over charge (current) - the excess current is turned into heat. The electrolyte solution may also not be able to absorb enough of the energy, and therefore also turn the excess into heat.
  • Over discharge (current) - batteries have internal resistance, as the current increases so does the heat.
  • Over charge (voltage) - as with current, excess voltage is turned into heat.
  • Short circuit - same as over discharge.
  • Punctured cell - same as short circuit. The foil layers within the LiPo will short against other layers.

Once the temperature gets to a certain point, the battery will go “critical” - this is a self sustaining chemical reaction which cannot be stopped. The cell decomposes rapidly, first puffing up, then rupturing to jet very toxic (and flammable) smoke, the internal temperature then reaches a point where that jet of smoke catches fire. At this point you no longer have a battery, but a rocket motor. You have violent eruptions of smoke and flame, and additional cells that may not have been initially affected by the heat are also now over heated. This increases the fire jet effect, as the flammable smoke from the other cells is also set on fire by the first. All in all, this can be either a very good evening, or a very very bad one depending on your intentions.

LiPo’s that have decomposed can fly several metres under their own thrust, spewing fire and some all the way, happily setting fire to everything in sight. This is an excellent way to burn down your garage, house, or car depending on where you were charging it!

Additionally, a cell discharged below 2.7V per cell may not be able to accept charge again, one below 2.2v/cell almost certainly will not. At this point, charging it is the same as pushing current into a fully charged lipo - your LiPo transmutes into a rocket motor!

To avoid all of the above, use a charger designed for the chemistry of cell you are using. DO NOT use a Lithium Ion (like what’s in older electronics) for Lithium Ion Polymer (very bad idea). Do not use a Lithium Ion Polymer or Lithium Iron on a Lithium Iron Phosphate (LiFePo4) battery. Above all, do not use a lead acid or NiMH/NiCd battery charge on a lipo cell unless you’re looking for fireworks.

As a fun fact, a 2200mAh 3 cell lipo contains a similar amount of energy density to a stick of dynamite. It releases it over 2-3 seconds rather than 0.1s, but it still a violent decomposition.

Basically: Use a charge circuit or IC designed specifically for the battery you are using - they are perfectly safe if you treat them properly and with the respect they deserve. Shooting them as in the video of mine Chris linked does disappointingly little, however i wouldn’t recommend charging a cell after doing so.


#18

Thanks for all your information.

My other concern with using the LiPo inside my Tinkerer kit is this minimum voltage. I thought that the Panda would shut down somewhere above 6V but with my power supply it still runs beneath 6V. Is there some sort of circuit (off the shelf) that I can add to avoid running the batteries to low?


#19

The betteries in phones are generally Li-Ion, not Li-Po. The battery in phones concist of a hard metal case, instead of the pouch of Li-Po. This makes Li-Ion more rugged and less prone to venting.

In my robot I took the following direction:
[ulist]I used one cell Li-Po, thus my battery is 3.7V.
On the battery I placed a LiPo cell monitor chip that protects the battery when Over/Under vontage and Over/Under current. The chip also does coulomb counting for battery remaining estimation.
Usually I use a USB powered LiPo charging chip, but as this battery is 4Ah I opted for a 1A 5V LiPo charging chip.
[/ulist]

To power my Panda from the 3.7V I used a boosting module from TI that will boost 3.7V to anywhere from 5V to 15V, adjustable by resistor. I set it to 5V and power directly into the 5V pin of my Panda, thus skipping the 5V analog regulator.

The reason for using single cell is that there are more charging/monitoring circuits for single cells than for multi cells. Also, in multi cell packs it is important to balance the cell voltages during charge(No single cell voltage must rise above 4.2V), and to ensure that no single cell falls below ±3V during discharge. NiCd and NiMH batteries where much more forgiving with cell balancing. If one cell reach full before the others then it will just heat up and disipate the extra charge as heat while the other cells catch up. With LiPo, if you over charge by 0.1V then the cell becomes dangerous/dead.

If there is any interest then I can post my circuits.

[quote]This might help. There are charging and detection circuit schematics:
http://blogs.msdn.com/b/coding4fun/archive/2010/12/13/10104241.aspx
[/quote]
I would not use that circuit. As far as I know, LiPo is supposed to stop charging when the charging current falls to 10% of the initial charging current. Also, that circuit doesn’t do cell balancing. Anyway, there are chips, for single cell packs, that require one chip and 2 caps. Nothing else.

Thanks,
Errol


#20

Thanks for all the good info MarkH and Errol.
I have question. AFAICT, all the Li-Po at SparkFun http://www.sparkfun.com/products/8483 seem to have protection chip in them already. If you get these batteries, would you still need to go farther with the other circuits you talk about? I suppose even with max care, you can have variables outside the control loop like defects and such.