Storing Battery Packs

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Storage of your Ni-Cd/Ni-Mh 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 freezer) 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.

cls 12/96 rev 1/07

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