What's the best way to store NIMH Cells, charged or ...

Hello BabyDoc,

There is a difference between "consumer" use of batteries and "power" use. If your applications involve draining your batteries over several hours, you can store them anyway you want to. Just make sure they don't drop below an open circuit voltage of 1 volt.

If your applications involve pushing your cells to the limits and voltage retention under load is important, then you need to try to keep your cells "vibrant" during storage.

Keeping your cells vibrant requires more work, but you will be rewarded by much better performance from them.

To keep NiMh cells vibrant, you store them discharged and perform a charge/discharge cycle on them every 30 days. To discharge your cell, you apply a 1C load and stop when the voltage under load hits 1.0 volts. With a battery pack you stop when the voltage hits 1.0 volts per cell.

If your cells have a very high rate of self discharge, this won't work. You need to store them fully charged, and since they quickly loose their capacity, charge them every week to keep them above the 1.0 volt per cell, resting.

The problem with this is that you need to charge your cells before use, and they aren't ready to go, if you need them.

A modification to this is to discharge the cells, and battery packs, and then to just charge them a little. I use this method for my power tool battery packs. I put about 30% of their full charge back in, and that leaves enough in the pack so I can do a quick use or two of it without having to wait for the pack to charge up.

You can also do this with single cells. Discharge them down to 1 volt, then charge them for a short time. When I am charging at 1C, I go for 10 - 15 minutes. In this condition, you can actually get away with longer than 30 days between your periodic charge/discharge cycle, but to keep your cells vibrant, I would not go beyond 90 days.

I have only been using this storage method for about 4 years, but have been amazed at how well it works. I don't keep a lot of extra cells around, but I do have battery packs for my power tools. Like most people, I leave my drill unused for a few months, then need it for a job I am working on. The performance of my battery packs has held steady, using this storage procedure, and I have passed the information on to some contractor friends of mine who also have reported excellent results.

If you store your cells charged and ignore them for an extended period of time, you can sometimes bring them back by using the standard charge and doing several charge/discharge cycles on them.

Tom

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Here are some useful tips to help you get the most out of your rechargeable batteries.

Use a good charger

Intelligent or &#;smart&#; chargers are best as they will not over-charge or under-charge your batteries. Try to avoid using super-fast chargers (15 &#; 30 min chargers) as this type of charger delivers an extremely high current which heats up the rechargeable battery and can cause it to deteriorate quickly. It is estimated that a NiMH rechargeable battery might last for only 50 &#; 75 recharge cycles instead of the average 500 recharge cycles if used regularly with a super-fast charger.

Always select the right type of charger for your batteries

Different types of chargers are designed for different types of rechargeable batteries and these are usually not interchangeable. We stock NiMH rechargeable batteries and NiMH battery chargers so all our rechargeable batteries and chargers are compatible.

Always charge the same chemistry and voltage

For example, you should not mix rechargeable NiMH with NiCd or non-rechargeable alkaline batteries, or mix batteries with different voltages.

When charging batteries in pairs the two batteries must be of similar capacity and charge level to allow effective charging. For example if charging a high capacity battery that is completely drained with a lower capacity battery that is half full, the different charge times required will lead to overcharging. This should not be an issue with intelligent chargers that allow individual channel charging, i.e. the charge is controlled individually for each battery.

Avoid charging batteries with different capacities in standard chargers

As this may shorten the life of the battery. Try to recharge your batteries when they start to show as flat or if the device is starting to dim etc.

Keep battery contact surfaces and battery compartment contacts clean

By rubbing them with a clean pencil eraser or a rough cloth each time you replace the batteries.

Avoid leaving your batteries in a charger for more than 24 hours

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Whilst most intelligent chargers are set to a trickle charge once the rechargeable battery is fully charged, to avoid damage to the battery, it is still best to take the batteries out of the charger once they are charged.

Avoid leaving a rechargeable battery in a device that is turned on after the battery is completely run down

A continued drain on a discharged battery for as much as a few weeks can cause the battery to reverse polarity and stop working. Always remove the battery from a device that will not be in use for an extended period of time.

Avoid storing your rechargeable batteries in very warm places

This reduces the number of recharge cycles your batteries can achieve. Storing batteries at lower temperatures reduces the rate of self-discharge and preserves the initial energy stored in the battery.

Be patient with your new batteries

New rechargeable batteries might take 3-5 recharge cycles to acheive the stated capacity of the battery, but should work as usual once this is done.

Try Battery Reconditioning if your rechargeable batteries are not holding power as they used to.

Battery reconditioning is a method of restoring the lost performance of rechargeable batteries and can also be useful in cases where batteries appear not to be fully charging, as it can sometimes be necessary to &#;kick start&#; the chemical reaction properly so batteries can function at full capacity.

To recondition a rechargeable battery: First discharge the battery completely through normal use - using in a device such as a camera, toy or torch to the point where the device does not work.

Then, fully recharge the battery (avoiding super-fast chargers) as normal. Repeat this cycle 3-4 times to get the full capacity back.

Try to avoid discharging beyond the level of normal use by discharging in an intelligent charger, as there is a risk that a battery may be over discharged and become damaged which will limit the performance further.

Give your rechargeable batteries a full charge and discharge cycle every few months, or if they have been stored for a while. These guidelines are not essential as batteries will still perform well without them, but performance and total life span may be improved by following these guidelines; therefore giving greater capacity meaning longer time between charges as well as total number of charge cycles.

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