# Battery over-discharge: definition and prevention



## azufelt (Jan 31, 2009)

I have heard many references to battery over-discharge. I have searched the glossary of terms here and found no reference to it. I have Googled it and read from Battery University, etc. but still don't understand the basic premis of over-discharge and specifically, how to avoid it. Can someone break it down? :thinking:


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## Marduke (Jan 31, 2009)

In simple terms, it is discharging a battery to a point that it's voltage is below some critical level which can be damaging, or even dangerous. That voltage is defendant upon the individual chemistry.

To avoid it, you simply change the cells in your device when you notice decreased performance. A few devices will have some low voltage alarm also.

Further detail depends on the chemistry in question.


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## Oddjob (Feb 1, 2009)

Using lithium ion rechargeables (probably the most popular chemistry used here) for your lights requires a little more effort than maybe what you have been used to in the past. Li-on batteries last longer if charged frequently and when their capacities are higher. Don't wait to fully discharge a cell before recharging it. Most 3.7 volt Li-on cells come off a charger at around 4.2 volts. If you get in the habit of charging them when the voltage drop somewhere below 4.0 volts you will get better longterm battery life. A decent multimeter is a must IMO when using these cells. Even if the light you are using has low voltage battery warning/shut off, it is still better for the cells to simply swap them out for new cells fairly regularly depending on how often you use your lights. Alternately you can stick to 3 volt lithium primaries and simply recycle them once depleted.


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## Mr Happy (Feb 1, 2009)

Oddjob said:


> Most 3.7 volt Li-on cells come off a charger at around 4.2 volts. If you get in the habit of charging them when the voltage drop somewhere below 4.0 volts you will get better longterm battery life.


I'm not sure this is the case? As I understand it the life of Li-ion cells is shortened the more time they spend above 4.0 volts. So if you charge them up to 4.2 V as soon as they get down to 4.0 V they will spend their whole time at an average voltage of 4.1 V, all the time above 4.0 V. On the other hand, if you delay charging them until they get down to 3.7 V their average voltage will only be 3.95 V. Lower voltage, longer life.

In fact, if you really wanted the longest life you could charge them up to 4.0 V rather than 4.2 V and discharge them down to 3.7 V. However you would not be obtaining the maximum capacity and run-time that way, and unless the cells are very expensive to buy it seems it would not be worth it.

For most people the best use of Li-ion cells is to cycle them fully between 3.7 V and 4.2 V and replace them when they become life expired.


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## tino_ale (Feb 1, 2009)

One big problem with over-discharge protection circuits is that they should take discharge current into account. Basically, the cutoff voltage should be a function of the pulled current, which is AFAIK never the case.

AFAIK, for example, AW over-discharge protection will work with relatively high discharge current, but the trigger is set too low for a low amp application (where the voltage drop, under a light load, is minimal).

When talking about batteries I find it irrelevent not to mention the load that's applied.


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