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Old 2nd February 2012, 10:07 PM   #1
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Default LiPo batteries, help separating facts and myths

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.
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Old 2nd February 2012, 10:30 PM   #2
djQUAN is offline djQUAN  Philippines
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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.
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Old 2nd February 2012, 10:45 PM   #3
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Sounds about right. I use 18650 batteries in various bits of hifi and for my bike light, your advice is all spot on.
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Old 2nd February 2012, 10:49 PM   #4
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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.

Last edited by hollbrow; 2nd February 2012 at 10:57 PM.
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Old 2nd February 2012, 10:53 PM   #5
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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?
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Old 2nd February 2012, 10:56 PM   #6
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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! :-)
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Old 2nd February 2012, 11:00 PM   #7
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Well I assume they are so stable when stored (voltage wise) it's not worth the risk over charging them. Some cells have protection, some don't. Protection failure would not be good.

Who want's Jiffy Pop?
Overcharging LiPo battery - YouTube
Over-Charging Li-Po cell - YouTube
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Old 2nd February 2012, 11:02 PM   #8
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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...
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Old 2nd February 2012, 11:05 PM   #9
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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.
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Old 2nd February 2012, 11:06 PM   #10
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Lithium batteries themselves have thermal fuses inside. However, things sometimes can happen.

GM Volt Fire After Crash Said to Prompt Lithium-Battery Probe - Bloomberg
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