LiPo batteries, help separating facts and myths

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I'm building a very tiny practice guitar amp which will be powered by a 1500mAh Lipo battery pack. When I first discovered LIPOs I thought they were a Godsend, offering so much energy in a small space. Discovering I needed a "balance" charger to properly charge the pack was a little bit of an annoyance, but I've found several balance chargers pretty cheap so that wasn't a major concern. But what has become a major concern is that there are so many cautions related to LIPOs that my head is spinning. So I'd really like to hear from some people with some more actual practical experience to help me separate fact from fiction here.

1. I've heard that LIPOs can catch fire when you charge them. So I was hoping that if I charged a 1500mAh battery with only 500mA balance charger, it would make the likelihood of a hazard low enough to actually build the battery into the project, without having to make provision to remove the battery for every charge (not to mention having to charge it on something like a concrete floor!) Is my assumption safe? Are LIPOs really that dangerous that they could never be charged without removal from the devices they power?

2. I'd like to think that charging at a lower rate would be both safer and good for the batteries. But then I've heard you should NOT "trickle charge" LIPOs. Well, would 500mA charge for a 1500maH be borderline "trickle"? AND... is "trickle" charging really BAD for LIPOs?

3. I've heard that a 12V LiPO pack should fully charge to about 14V. But when my my charger indicates all cells are charged and balanced, the total is maybe 12.5 at best, with no load. Does this mean I'm not fully charging?

3. I've heard that if you discharge a 12V pack down to 10 volts, you'll permanently damage the battery. Geesh!!! Do I really have to build a sensing circuit that shuts down my amplifier to protect the battery when I reach a certain voltage? How critical is this, and what would be the real "damage" point for a 12V pack?

Thanks for any and all info.
 
1 - possibly. But luckily I have never experienced this yet (hopefully, never) a close call though when my charger did not stop and made the pack bulge a little. I would still recommend atleast a separate compartment for the battery.

2 - charging at a lower rate does not matter it just takes longer. What they meant with trickle charging is that once the charging cycle is complete, the charger should stop completely. there should be no current no matter how small should flow to the battery after that.

3 - Li Poly's are 3.7V/cell. with three cells in series, that is 11.1V nominal. four cells - 14.8V nominal. charfe termination is at 4.2V/cell that makes your 3cell battery sit at 12.6V fully charged with no load. 4cell packs will sit at 16.8V. The constant voltage mode of your charger should be around +/-50mV of 4.2V per cell. Go over that and battery life (% charge and cycle life) decreases dramatically.

there are two #3's :p - Yes, discharging a battery pack too low will damage it. It is usually around 3V/cell. I've read from various sources that the point of no return is around 2.7V/cell. Cell phone and laptop batteries have built in controllers that disconnect the load when over current, over voltage (that can also set the battery on fire) over discharge and over temperature occurs.

If someone finds any mistake please correct me. This is what I've been learning for several years now and working with Li-Ion and Li-poly batts.
 
I use li-po batts a good bit with larger radio controlled planes and have also used them on an electric bicycle project- the cells they are selling these days are less prone to meltdown than the earlier ones- but it CAN happen. I would never put one on a charger and just 'shut the door and walk away'. You absolutely need automatic low voltage cut-off built into the circuitry. Once you drain a pack down past its point of no return you may as well dispose of it cause' it turns into a paperweight. If I wanted a small amp like that I'd build it- and use it- but I would be careful to adhere to the rules of li-po usage. By the way- li-po's are showing up in all sorts of things lately- the little radio control helicopters from air hogs and such use them- and those little portable "i-pod" speakers that charge from a usb port have a 1S li-po cell inside them.
Most cell phones and laptop batts are lithium or nickle metal hydride- but a lot of the same precautions apply.
 
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OK, well then for something like a guitar amp (even a practice on), I'd better at least make a simple detector circuit to light a little warning LED at least, when the voltage gets down below 10V. I ought to be able to swing that with a simple Zener diode and an op-amp comparator. But I'm still really concerned about item #1! DJ... you mention that cell phones have circuits to shutdown on overvoltage or overcurrent. But still, if fire/explosion is a real danger, how could anyone sell a Cell phone with a Lithium battery? I sure would like to know if such a slow and controlled charge makes me safe enough from a hazard to let the device charge on my kitchen table, along with my Cell! :)
 
Most lipos used for R/C use can say be charged at 1C, some maybe more but that is pushing them. 1C=equals discharge rate if I recall. Charging a battery at 1C is about the same time in theory as it takes to drain said battery, at it's safe operating limits

Charging at anything less then 1C is safer but slower.

For a hobby type battery pack you could just measure the individual cells, which is much more important than the overall voltage.

Some cell phones were just a single cell, which negates the issue of balance charging.
 
I don't understand why when charged it should stop completely. I always thought it itself stops consuming current when the current cycle is over, and can be held on constant voltage indefinitely long, no?

in an ideal world, yes. But the datasheets I've read mentioned plating of metallic lithium in the electrodes causing the unstable reaction leading to fire.

DJ... you mention that cell phones have circuits to shutdown on overvoltage or overcurrent. But still, if fire/explosion is a real danger, how could anyone sell a Cell phone with a Lithium battery? I sure would like to know if such a slow and controlled charge makes me safe enough from a hazard to let the device charge on my kitchen table, along with my Cell! :)

cell phones are charged with a circuit specifically designed for the application. The protection circuits I mention are built into the batteries themselves. If you take one apart (but I suggest you don't) you'll find a tiny pcb containing that circuitry in them. Datasheets have a max charging current listed which is usually around 1C (for a 1500mAh battery that would be 1.5A max) I would not suggest it though as it heats the battery up. I usually go 0.5C or less.

I have a lipo battery pack in my motorcycle. It charges off of the normal alternator in the bike. No problems except it doesn't like the cold. I keep it in the house in the winter. I believe these are A123 type cells. They weigh like nothing...

If I'm not mistaken, those are Li-FePo (sp?) which have a slightly lower energy density, slightly lower voltage (~3.2V) but much safer (less fire hazard and more stable) and much more charge cycle life.
 
There are lots of lithium cells out there with different chemistries.

The easiest cells to manage are the 'protected' tubular cells such as 18650, 16340 (CR123) and 10440 (AAA). They have a low power circuit built in which disconnects the output at ~3V, preventing over discharge. You get no long slow tail-off, so you don't get the option of discharging to the point of destruction, which could be a disadvantage in an absolute emergency, but for an audio device, what emergency? You can get them from dealextreme or ebay.

Most batteries (cell packs) are happiest when 'balanced', but balancing is not absolutely necessary. You can series charge 3 of these cells to 12.6V under a constant current/constant voltage regime leaving them installed and simultaneously run the device from mains power using the charger to supply power.

There's a constant current/constant voltage circuit in the LM317 datasheet.
 
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Well I'm pretty satisfied I can pull this off with some precautions. Ands the little 500mA IMax charger I bought seems to charge my 12V pack fine overnight, without even noticeably warming them (Cool = GOOD I think!).

Now, can anyone recommend a low power circuit for detecting undervoltage, so I'll know when discharge has reached the approximate 10V point where its time to stop using my batterypack? I thought this would be so easy... an OP-amp comparator with a resistor voltage divider on one input, and a similar divider with a zener diode on the other. Trouble is, at least the zeners I have on hand require a good 40mA or more to stabilize thier rated zener voltage, and that's 40mA totally wasted IMHO! I'm sure there are chips for this, but maybe there's an easier makeshift OP-amp circuit I could use?
 
Low voltage detector for LIPOs

Well on that last topic, here's a useful circuit I've found as a low voltage detector. It's a starting point. The author apparently is using some very high brightness LEDs, presumably to be able to see the low voltage indication from the ground when flying an RC plane. For me, a little 2mA LED was sufficient for my Guitar Amp (low voltage warning). So in my case, R5, D2,D3 and D4 were eliminated, and R4 was anywhere around 4.7K worked fine. If you only have a 20MA LED, you may have to go as low as 470 ohms. I also used 2N2222 transistors, since I have so many of them. Finally I used 10K instead of 1K for R3, because I AM using such a low current LED and I just don't need very much current amplification. Plus, the 10K resistor results in only about 1mA wasted to run the circuit in its usual non-alarm state vs over 10mA. The other values shown for R1 seem "about" right, but I used 10K for a my 3 cell monitor. With the other changes I mentioned, this will cause the alarm LED to come on when the 12V pack gets to about 9.2 volts.
 

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I was thinking something along the same lines.

Some of the ones for R/C planes etc have a "home" type feature that reduces your speed down to half, giving you enough time to get the model back safely.

These circuits are built into the speed controller, although I have seen a stand alone version similar to the one you are describing.
 
How can they sell something that when it malfunctions, it is a Class D fire so you had better run? Because everyone else does.

Although quite entertaining to blow up, generally speaking they are fairly safe if they have overvoltage/undervoltage cut off for each cell (each one!) They also should have vents in them that rupture if the pressure inside gets too high from overcharging/over discharge or thermal runaway. The reason those Sony cells started on fire was due to an internal short as there was metal shavings in the electrolyte which would cut through the plastic after expansion/contraction from cycling the batteries. If it has an internal short, no protection devices will work (called Battery Management Systems or BMS) The cells internally short to themselves, it creates heat rapidly goes into thermal runaway and boom. They don't call it "explosion", the term is "violent venting with flame" :rolleyes:

Much safer is Lithium-Iron Phosphate (LiFePO4) VS Lithium-Cobalt Oxide. You can shoot a hole in a LiFePO4, crush them, short the terminals together and it might vent a bit--won't blow though. The cell phone type have a flammable metal and and oxidizer mixed together--they used to call those solid fuel rockets.

The Volt used LiMn batteries, better than LiCoO but can burn if you do something stupid and use electrically conductive coolant (when dried) and give it to the government who would crush it and turn it upside down for 3 weeks. Chevy is switching to A123 Systems LiFePO4 so the LG Chems are heading to the sunset.

I like LiFePO4 batteries, I've been using the A123 versions for the last 5 years plus with my DeWalt 36V tools. They throw out the current, light weight, as long as you charged them in the past 6 months or so--they hold their charge and I've had no issues.

So if LiFePO4 batteries can take 3,000 cycles, heat does not bother them, they won't explode, nothing toxic in them, should last 7 to 10 years at least and (in bulk) cost less to make than LiMn/LiCoO/LiPo.... they should of taken over the market quickly. Not so fast...here are the downsides

They are 3.2V and not 3.6V like regular Li cells. It was easy to change from NiCd/NiMH cells since they are 3.6V for three of them. It costs money to change the design for the different voltage.

They don't have the same watt hours by weight/size as regular lithium cells, it is about 25% lower. Most consumers would rather risk blowing their body parts off than have a slightly thicker phone or a little less runtime.

The Tesla roadster used over 6,900 Sony laptop cells--heck of a battery management system to keep the pack alive. One cell goes into thermal runaway which causes the cells around it to go...and so on. The new models are going to run LiFePO4 cells, much better performance, longevity, cycle life and it won't explode.

To remember how to take care of batteries, remember it this way. Only car batteries or lead-acid likes to be trickle charged or charged to 100%. ALL other batteries should never be trickle charged or kept at 100% charge. For long term storage, lithium chemistry prefers to be kept at a 40 to 60% charge level.

One last tip, don't use regular lithium batteries in a sealed metal device--like a flashlight for instance. They make a really big mess if they ever vent violently with flame and will take chunks out of cement etc.

If you have an afternoon to blow and would like to learn more about fun with batteries, here is the site.

Basic to Advanced Battery Information from Battery University

It is owned by Cadex which makes battery chargers/conditioners/analyzers for all sorts of cells. :whacko:
 
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