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Old Oct 15, 2003, 09:07 PM
J.H. (AirJer) is offline
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Be careful with LiPo Cells!


A classic case of stupidly selecting 3-cells instead of 2-cells on my charger ... and this was "just" a 2S1P 1020 pack.

Thank goodness I had set the battery on a metal surface.

The resulting fire melted the plastic housing on my bench sander and threw hot bits at least 6-feet away - there are several small burn marks on one of my airplanes to prove it.

Please double check everything when charging LiPo cells, only charge on a fire resistant surface - and don't leave them unattended!

LiPo cells are wonderful power sources, but the consequences of an error while charging are rather dramatic.
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Old Oct 15, 2003, 09:09 PM
J.H. (AirJer) is offline
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the picture
Old Oct 15, 2003, 09:25 PM
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Thanks for sharing your experience. It's just another example of the fact that we're all human, and we all make mistakes every day of our lives. That's why it always pays to follow all the safety rules, which minimizes the effects of our human errors and confines the damage to that which is replaceable.
Old Oct 15, 2003, 10:29 PM
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Speaking of confining damage... how about using a steel box to charge, transport, and store the LiPo packs in. Daniel Schübeler showed us this idea at the Equinox Electrics.

Dieter Mahlein
http://shredair.com
Old Oct 15, 2003, 11:14 PM
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Thanks for showing us the pictures. Given a few posts recently about being to paranoid about the dangers of lipos, that pic should clear things up a bit about the need for saftey. Good thing you were wise enough to charge on a non-flamable surface. The little bits that get thrown out clean up well with a little dishwashing liquid, otherwise they may be difficult to remove.

-Jim
Old Oct 16, 2003, 04:43 AM
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We really need adequate voltage protection (over and under) on both the cells and chargers and thermal protection on the packs.
If not, some day someone is going to be seriously hurt or even killed, perhaps a child.
-Dave
Old Oct 16, 2003, 06:06 AM
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Sorry to sound like a broken record, but I don't know what else to do.

If we charged in parallel, our chargers would be at 4.2 volts ONLY. There would be no possibility of this kind of accident.

No extraneous safety hardware needed.

- RD
Old Oct 16, 2003, 06:40 AM
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[From RD]

>>> Sorry to sound like a broken record, but I don't know what >>> else to do.

>>> If we charged in parallel, our chargers would be at 4.2 volts >>> ONLY. There would be no possibility of this kind of accident.

>>> No extraneous safety hardware needed.


Sound away. It makes sense, and I am surprised still that the industry apparently tries to avoid this #1 opportunity to remove what looks to be a major cause of trouble.

If a walk through the reasoning has not done it, maybe your repetition will help.

">>> No extraneous safety hardware needed."

Well, that may be going a bit far. Even with parallel charging, it seems to me that there are still some possible failure modes, and basic safety prcautions are still called for.

But I do not in any way argue with your 'broken record' point.

Chris Parent
Old Oct 16, 2003, 07:01 AM
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"Even with parallel charging, it seems to me that there are still some possible failure modes, and basic safety prcautions are still called for."

Point well taken.

- RD
Old Oct 16, 2003, 08:25 AM
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In the absence of a fullproof method to prevent adjustable voltage LiPo chargers from trying to pump too much voltage into a pack, I do like the concept of fixed voltage chargers. It's a lot simpler for those of us who fly only small aircraft limited to two LiPo cells. If we use a charger that delivers a maximum of 8.4V, we don't have to be concerned about accidently charging at 12.6V because we made a human error and forgot to change a jumper setting. But that does not resolve the issue of unbalanced cells.

Personally, I like the idea of charging individual cells one at a time. So the concept of using multiple single-cell chargers limited to 4.2V each, or a multi-port LiPo charger with several 4.2V channels, is what appeals most to me. I like this better than parallel charging, as charging one cell per 4.2V charger or 4.2V port is the ultimate in keeping it simple.

One of the main problems I see is the complexity that develops when we get away from the simple 2-cell pack and get into the multi-series, multi-parallel packs required for larger aircraft. For instance, a 3s4p pack contains 12 cells, and I doubt that anyone wants to buy 12 separate single-cell chargers. So you almost have to go to parallel charging. That seems to me to require a different thought process than the typical 2s indoor or parkflyer pack.

My guess is that simple 2-cell packs represent more than half of the LiPo R/C business, and it's the configuration I'm most interested in because it's the one I want to use. The concept of 2s packs with individual charging connectors and a pair of basic 4.2V chargers or single 2-port charger would be reasonably simple and inexpensive. In fact, the same could be said for a 3s configuration. And I think the non-parallel 2- and 3-cell packs would account for more than 90 percent of LiPo R/C use. Based on everything I've seen so far, this is the solution that most appeals to me for non-parallel LiPo packs.
Old Oct 16, 2003, 08:39 AM
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And, Dave, if we think in terms of designing our fuselages to fit the cells, rather than the other way around, we can get a C of 3300 mah in a single cell (the Kokam 3270, and a new 20C-discharge 3300 being tested now).

So, a nominal 12V, 3.3 amp-hour pack is possible with only three cells. As a matter of fact, that's the way I've been flying LiPos since FMA first started marketing them.

- RD
Last edited by RD Blakeslee; Oct 16, 2003 at 08:42 AM.
Old Oct 16, 2003, 10:27 AM
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For applications requiring higher C than can be gotten out of a single LiPo cell, or applications requiring narrower cells than the 3300, one or more pairs of identical cells can be hard-wired in parallel inside the pack. Each of these parallel arrays is treated as though it were a single cell of 2C, 3C (etc. , however many are in the array).

So, for a 3s pack only three sets of two wires each would exit the pack, and all the cells would charge in parallel but each parallel array would discharge in series.

In other words, very large packs would have no more exterior wires than smaller packs. The number of exterior wire pairs would be 2 for a 2s pack, 3 for a 3s, etc., regardless of the number of paralleled cells hardwired inside the pack.

- RD
Old Oct 16, 2003, 10:38 AM
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These threads are making me re-think my choice to go all electric/all Lipo. I've been selling off stuff to fund that idea, but I'm starting to think I should let the technology mature some more first.

I don't want to have to worry about my play things catching fire from a simple misread on my charger. I work with numbers all day every day. Occationally, I misread one or simply transpose it.

If these types of simple human errors can cause a fire then Lipos aren't for me.

Wiz
Old Oct 16, 2003, 10:38 AM
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Charging Station


Try a pyrex baking pan, deep one, for chargin lipos. Also, fire extinguisher in area, and smoke alarm. Latter two should be in every home anyway, and I finally got them.
Alphabandaid
Old Oct 16, 2003, 11:47 AM
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It would be great to see a risk analysis done on this issue. For example, what is the chance of someone getting killed/injured by LiPo cells compared to the change of getting killed/injured by driving to the flying field or by being hit by someone else's model, etc etc etc
Surely if we are able to switch our chargers to 2 cells or 3 cells or whatever and obey common sense safety, the risk is in line with normal everyday activities.

If you set your microwave to the wrong time you could surely cause a fire as well and there must be many more examples. Worries me that we see people thinking about -
Quote:
These threads are making me re-think my choice to go all electric/all Lipo. I've been selling off stuff to fund that idea, but I'm starting to think I should let the technology mature some more first.
Let's assess the risks rationally and keep things in perspective!


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