LiPol Saftey Questions

[pseudolobster]

Hey all,

I'm thinking of running my br off a 7.4v lithium polymer battery, but I had a couple questions before I start...

I've read a fair amount of horror stories involving LiPo fires and explosions, which I'd like to avoid if possible.

From what I hear, lithium batteries don't like being over or under charged, and they like to stay between 3v and 4.2v or so per cell... But, I'm planning on using one of BlueFusion's drivers from the gb, and as I understand it, these drivers will draw a volt, and the br diode will draw 4.5... SO... My question here is, is it safe to discharge one of these batteries to 5.5v without exploding?

The battery I'm using is one of these: https://www.dealextreme.com/details.dx/sku.7621
...which doesn't have a built-in limiting circuit afaik...
I've looked at buying one of these: http://www.all-battery.com/index.asp?PageAction=VIEWPROD&ProdID=1820
...which is a limiting circuit for a 2-cell lithium battery.. and should work fine, but it says the overdischarge cutoff voltage is 4.8v.. which seems really low, 2.4v per cell... Do I take this to mean it's safe to draw 5.5v from these cells?

Thanks!

 

[Benm]

Keep in mind that most drivers will still draw some current even below their operating voltage. Your laser might go dim or stop to lase, but some current is still used. You must swith off manually quickly in this case, otherwise it will still drain the batteries to unacceptable levels. You can rig a driver almost stop drawing any power on undervoltage, but this has to be designed in.

Over-discharge is not very likely to cause fire or explosions though, but it can damage your batteries beyond repair quickly. Over-charging is much more dangerous and should be avoided by using a proper charger, protected cells, and preferably both.

 

[pseudolobster]

Thanks for the reply!

I'm not too concerned about destroying the pack, so long as it doesn't take me with it!

It cost $7, so if it refuses to take a charge it's no big deal.. I'm mostly concerned about going up in a fiery inferno.

From what I can tell about the fusiondriver schematic it seems theres nowhere to place a pushbutton, so I'd likely be putting the power switch forward of the driver, so unless my finger's on the button it shouldn't draw anything. If these packs can safely be discharged to say, 5.7v or so, I'd just be able to stop using it once the laser starts to dim, keeping me well in the safe voltage range.

 

[wooooooolazer]

Normally the voltage you want to stop discharging is 2.75v, beyond that the batteries can develop a short in thim which is not good for lithiums.

..lazer...;D ;D ;D

 

[pseudolobster]

Normally the voltage you want to stop discharging is 2.75v, beyond that the batteries can develop a short in thim which is not good for lithiums.

Well then! That's perfect!

I'm assuming the driver won't live up to its theoretical efficiency, and the br will probably not lase below 5.6 or 5.7 or so volts, so I should probably never see it drop as low as 2.75v/cell... making this battery nigh perfect for this application!

Wish me luck, and I'll update this post from the burn ward if it doesn't pan out like I planned! ;D

 

[Gazoo]

Normally the voltage you want to stop discharging is 2.75v, beyond that the batteries can develop a short in thim which is not good for lithiums.

..lazer...;D ;D ;D

Running a lipo battery below 2.8 volts may damage it. The recommended cutoff voltage is 3 volts per cell. Most batteries explode during charging, or if drawing more current than the cell was designed for. Also being careless with them such as dropping them can cause problems. The best way to extend the life of the cell is to either buy, or build a low battery indicator.

Most of the lipo batteries now come with connectors so they can be balanced. What was happening was the batteries in a pack were not charging equally, so this too was causing explosions. Balancing the batteries corrects this problem. There is a lot more to read to understand how to properly care for lipo batteries. Search google.. ;D

 

[Benm]

I guess the whole exploding battery thing is a story that got out of hand really. There have been some cases documented, but you should keep in mind that there are probably already over a billion lithium cells in daily use right now.

Mechanical damage is more likely to cause fiery fiestas that over-discharging, and over-charging is bad news for almost any type of battery.

The main reason for undervoltage protection is to prevent a situation where the cells will no longer accept a charge. The best way to avoid it alltogether is charging lithiums early, there is no need to completely discharge these cells, unlike NiCds and NIMHs that suffer from a memory effect to some degree.

 

[Gazoo]

I guess the whole exploding battery thing is a story that got out of hand really. There have been some cases documented, but you should keep in mind that there are probably already over a billion lithium cells in daily use right now.

Mechanical damage is more likely to cause fiery fiestas that over-discharging, and over-charging is bad news for almost any type of battery.

The main reason for undervoltage protection is to prevent a situation where the cells will no longer accept a charge. The best way to avoid it alltogether is charging lithiums early, there is no need to completely discharge these cells, unlike NiCds and NIMHs that suffer from a memory effect to some degree.

There are quite a few documented cases in the RC forums. Anyway, it's better to be safe than sorry.

 

[Photon_Pharmer]

Hello,
I'm new to this forum, and at the beginning of my laser learning curve. I do have a lot of experience with batteries however. You should be fine with the lipo pack in this application. The danger comes from overcharging or discharging based on current not voltage. Many LiPOs can discharge at 15c continuous ($7 packs may not be rated this high). This means that an 800mah pack could discharge 12000ma continuous. Draining the pack below it's minimum voltage would only reduce the life of the pack or kill it. If your load needs 5.5volts and your battery had an 800mah capacity and 15c continuous discharge rate, you could pull 66,000mw without even getting the pack hot.

 

[pseudolobster]

re: Photon_Pharmer: Welcome to the forums! It's good to have someone who knows something about batteries around here...
I'll be pulling much less than the 1.2 amps you suggest is possible, prolly more like 50mA... So I'm glad to hear that my battery shouldn't be subject to any of the conditions that cause these thermal-runaway-catastrophic-failures you hear about sony laptops.

I'll be glad if the worst that happens is my battery no longer accepts a charge after a few cycles, that's about par for a DX product... but from what I hear these are pretty good and should last me a while anyway... basically so long as I can prevent massive heat damage to my bluray diode I'm happy.

 

[Greebo]

I can testify to the dangers of overcharging Li batteries.

I bought a MP4 player off e-bay. I'd only had it for a few weeks, had charged it a couple of times without problems.

I had it on charge in the study, but forgot about it. My daughter accidently knocked it on the floor. The next thing i know, she's screaming that it was on fire....

The amount of smoke that was being produced was amazing. There is a big scorch mark on the wooden floor, and when i opened the mp4 player up the battery looked to have expanded to about 3 times what it should have been.

 

[Benm]

There are quite a few documented cases in the RC forums. Anyway, it's better to be safe than sorry.

If its easy to get it safe, i'd do so. But there is a big difference in RC discharging with heavy loads, and a slow discharge using a current source that drops out of regulation due to lack of voltage.

As for the ebay mp3 player.. i guess its a combination of bad electronics in the charging part and mechanical shock. With lithium batteries the 'do not puncture' warning is to be taken seriously.

 

[polop]

hi i am also a beginner to lasers but i also know a littleabout li po's. safe discharge voltage varies with the use - in rc aplications you can run them to 2.7v, but in low load realy you should be loking at around 3.2v and if you want your batteries to last a realy long time you should keep them above their nominal voltage 3.7v per cell. the reason for this is that lipo's like any battery have voltage sag under high discharge (rc often require 40amps plus) then once they are discharged to 2.8 ish their voltage will increase again to around 3.6. it low demand situations like a laser this sag does not occur giving a more linier discarge voltage , making it neccesarry to stop at a higher voltage (no sag = no recovery) but in ether case there mAh rating will be similar high or low discharge.

I definately reccomend li-po in lasers because they are small, light, high capacity and high voltage (good for the ddl circuit) and now they are getting cheaper by the day. i'm going to use them with a ddl phasor diode to make a portable designe capable of long-ish duty cycles)

hope i helped