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 Airplanes - Electric  > Batteries and Chargers
Guyo 
7/9/2004
Anybody knows a good Li-Po charger that can take its 12V DC from a car's battery?
Most of the chargers I've seen, recommend not to use the car's battery for the 12V DC input.

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 Last reply by hisham on 7/9/2004  587 Views   2 Replies 
hisham 
23/11/2004
Lipo Guidelines
---------------

Shorting any lithium battery, even for a few seconds, is enough to cause the tabs to heat enough to ruin a pack or start a fire!

http://www.pfmdistribution.com/guidelines.htm

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   551 Views   0 Replies 
hisham 
6/12/2004
Storage of your Ni-Cd R/C Packs

"How should I store my batteries at the end of the season? What should I do to them when I put them back in operation?"
The batteries should be removed from the transmitter and plane for longer term storage. Here in the south where a lot of us work out of our garage work shops I recommend putting them in the refrigerator (not the frezzer) during the off season. While not so important where your workshop rarely gets above 23 degrees C (74 F) the refrigerator is still a good bet. Why? The failure mode of Ni-Cds is separator failure, this is the material that keeps the plates from touching each other. When it fails the cell shorts. At higher temperatures it oxidizes faster. In fact, the rate doubles for every 10 degrees C increase.

"Should I store my batteries charged or discharged?" It doesn't really matter, they will self discharge in a few months stored at room temperature. If you are going to store them in the refrigerator the charge will remain for a lot longer so I would discharge them first to 1.1 volts/cell and them put them away. Good cells will just set there in the discharged condition (the voltage can vary considerably but is usually above 1 volt). In a battery with damaged "worn out" separator in the cells, the cells are apt to short if left in a discharged condition. This is actually good since it is the first indication of a cell that's going bad and it is best to replace the pack. A battery left on trickle charge will seldom short out since it is in the charged condition and any short that tries to develop will be zapped by the charge in the cell. Partial shorts (those having fairly high resistance) can be developing that can cause the cells to self discharge at a higher rate than normal and possibly leave you short in the middle of a flight after you just measured the cell when it came off charge with your ESV and everything looked OK.

The reason I recommend removing the batteries from the transmitter and plane is to protect against "black wire" disease. Should a cell short while in storage there is a high probability that there will be some leakage that can lead ultimately to the "black wire" problem.

Now when your batteries are coming out of storage, before charging, check the voltage without a load on the battery. It should read well over 1.0 volt/cell even if it has not been charged all winter. They should be essentially fully discharged, flat as we say in the business. In this condition if the battery is going bad it will probably have shorted and you will read zero volts on that cell. It may be a soft short, one that could be blown away merely by the simple action of slow charging. Don't do it! It is just lying there waiting to bite you. Replace the pack. Cut out the "good" cells if you want and use them in something less critical than your model. If you have access to a cycler running though a couple of charge/discharge cycles is a good idea just to make sure you are getting the capacity you are suppose to. Anything less than 80% of rated is suspect. Once at the field, pre-flight battery checks are in order, particularly at the beginning of the season. Since those that religiously check their flight packs with an expanded scale volt meter seem to crash less (due to battery failure) one must assume that the ritual is smiled upon by the R/C Gods.


The Black Wire Disease - What's the Cause?

The black wire syndrome is an occupance in battery packs (Ni-Cds) where the negative wire becomes corroded (turns from shinny copper to blue-black). This is the result of either a shorted cell in the pack, the normal wearout failure mode of Ni-Cds, or cell reversal when a pack is left under load for an extended period. The sealing mechanism of a Ni-Cd cell depends to some degree on maintaining a potential across the seal interface. Once this potential goes to zero the cell undergoes what is called creep leakage. With other cells in a pack at some potential above zero the leakage (electrolyte) is "driven" along the negative lead. It can travel for some distance making the wire impossible to solder and at the same time greatly reducing its ability to carry current and even worse, makes the wire somewhat brittle. A switch left on in a plane or transmitter for several months can cause this creepage to go all the way to the switch itself, destroying the battery lead as well as the switch harness. There is no cure. The effected lead, connector, switch harness must be replaced.
This leakage creep takes time so periodic inspection of the packs, making sure that there are no shorted cells insures against the problem. The cells should also be inspected for any evidence of white powder (electrolyte mixed with carbondioxide in the air to form potassium carbonate). In humid conditions this can revert back to mobile electrolyte free to creep along the negative lead. Some "salting" as this white powder is referred to, does not necessarily mean that the cell has leaked. There may have been some slight amount of residual electrolyte left on the cell during the manufacturing process. This can be removed with simple household vinegar and then washed with water after which it is dried by applying a little warmth from your heat gun..

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   480 Views   0 Replies 
Guyo 
19/12/2005
Just found something that backs up my suspicions regarding the problem i've been facing with Lipo batteries in a cold weather:

"Cold can reduce the effectiveness of any battery and Lipos seem to be especially vulnerable.
It has been suggested that you not push your batteries too much in very cold weather as they will draw more current on an already weakened battery.
Keep the batteries warm before flight. A shirt pocket inside your coat is warm enough."

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 Last reply by toto on 20/12/2005  1614 Views   1 Replies 
 
 
 
 
 
 
 
 
 

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