When & When Not to Fully Discharge Your Batteries?

March 21, 2010 | By Rakhitha | Filed in: Tech.

I wanted to write this post after reading “Battery Myths” by Foxhound. It was a very nicely written post on the myth of not recharging the battery until it is fully discharged. It made me want to write a post on maintaining rechargeable batteries. But it took me some time to finalize it. This post is based on knowledge I gained over time when I was trying to look after batteries of mobile phones, digital cameras, laptop and few power backup systems.

In my opinion the biggest culprits of spreading battery myths is not the ‘wise old man’ as Foxhound said. It’s those nice looking sales girls (and guys) working at mobile phone shops. Every time that I have been to one of those shops to buy a phone, I have received exactly the wrong advice on how to charge the phone. Most of those batteries clearly has “Li-Ion” marked on it. But they advice not to charge it until it is fully discharged. Most of my friends have received the same advice. More often than not, people don’t believe it when some one is trying to convince them that they are actually damaging their batteries.

I think that those who buy laptops use their batteries relatively better. Because most of the laptop manuals come with good instructions on how to take care of your batteries. Also those users often find out type of their batteries and maintenance guides from the web.

Why Fully Discharge Batteries Before Charging

Before completely rejecting the myth, it’s probably good to know the source of the myth. Nope I am not going to talk about the ‘old wise man’. But a little bit about Ni-Cd batteries. Some types of these batteries has this thing called memory effect. They lose capacity if they are repeatedly charged after partial discharges. Battery university has a very good article on it, so I am not going to write more about it. If you want a less technical article you can read Wikipedia page on memory effect. Sometime back, Ni-Cd batteries were the most commonly used rechargeable battery, apart from those Lead Acid batteries. IMO that created the myth “All batteries must be fully discharged before recharging”.

There are many other reasons for reduced capacity. This includes overcharging, battery aging, incorrect battery gauges, …. Often users mistake these issues as memory effect too.

Well that’s about the myth. Most batteries that you find today in consumer devices are Li-Ion batteries. They don’t have memory effect. But there are legitimate reasons for fully discharging the batteries.

Most modern devices/batteries come with a built-in battery gauge. When you keep recharging the batteries after partial use, these gauges start to show incorrect battery levels. This lead user (and device) to think that capacity of the battery is lower. Also since the gauge show an incorrect battery level, device will cut off due to low battery even when there is more juice left in the battery. To fix this problem, you have to recalibrate the battery gauge. Normally this involves fully discharging the battery and then fully charge it continuously. But this is not discharging until it hit zero. You normally run the equipment until it cut off. Some equipment/chargers come with a built-in function to do this. If you check your laptop manual you will find more instructions.

Also as Foxhound explained, Li-Ion batteries has a permanent capacity loss problem. Rate of capacity loss is proportionate to the temperature and charge level. If you have to store the battery for a long time, it is advised to discharge the batteries and store them in a cold place. But frequent discharges and recharges are also not good. For more info you can read wiki page on Li-Ion batteries.

You might wonder why most mobile devices are using Li-Ion batteries even though they are loosing capacity when kept at full charge and high temperatures. Thing is, Li-Ion batteries can deliver a higher capacity in a small/light package, which is a critical need for most mobile devices. We got to live with it until we invent a better battery with high energy-to-weight and energy-to-volume ratios.

Why Fully Discharging is Bad

Fully discharging  and recharging is not good for most types of batteries. It put too much strain on them. Rechargeable batteries are manufactured to last only a limited number of full charge cycles and partial charge cycles. Every charge cycle reduce its life.

Also most of those things that you call batteries are not really batteries. They are battery packs. Inside them there are more than one serially connected cells. These cells are not perfectly identical. Some contain less charge than others. When the battery pack if fully discharged, some cells will reach zero state before others. Then those zero state cells will be reverse charged by rest of the cells. Reverse charging can ruin your battery. Some devices/battery packs has built-in safety mechanisms to cut-off before this happen. Therefore when some one say that you have to discharge your battery, don’t go all the way to zero. Most of the time, what they tell you is to discharge it to a level below 40%. Therefore always follow specific instructions.

Few Words on Charging

Most times people damage batteries them self by incorrectly charging. You can’t charge your battery using any charger that you have at home. You have to make sure that the  charger is made for your type of batteries. We often make this mistake with those standard size rechargeable batteries (AA, AAA, C, D). Since different types of those batteries has same size and shape they all fit in same charger. But their internal chemistry is different. Chargers for different types of batteries use different methods to detect full charge state of a battery to avoid overcharging. This get messed up if you use the charger on a wrong type of battery.

Some cheap chargers often overcharge batteries reducing their life time. It’s best to use a regular slow chargers as much as possible. Fast charging can reduce the life of batteries.

Don’t Invent a New Myth

Even-though Li-Ion is the most common thing available these days, you may encounter different batteries in your day to day life. They have different qualities and need to be maintained in different ways. Ni-Mh batteries used in some devices such as laptops. These batteries may be used more in the future thanks to their high energy density and low-cost. Don’t get them mixed up with Li-Ion. They don’t have a permanent capacity loss like Li-Ion. But they do have relatively smaller memory effect compared to Ni-Cd and a high self discharge rate. Most of the backup systems including UPSs are using Lead Acid type batteries. They are cheaper and live longer than Li-Ion even when you keep them at full capacity.

This is just few thoughts on the topic. If you have done all the wrong things over the years, it may be too late for your batteries already. Whenever you buy an equipment, make sure to find out exact type of battery it uses. And read up on how to take care of them. Also when you buy Li-Ion batteries, check the manufactured date. You don’t want to buy one that was sitting there too long in a shop or a storage facility. Because Li-Ion batteries has the permanent capacity loss issue. It start from their manufactured time. You do want to buy a battery with most of its capacity intact.

If you love your batteries and want to take best possible care of them, you can read more on Battery University website. This website mostly cover Li-Ion batteries. But it’s not that difficult to find information on other types on the web. Please let me know your comments on this post. Specially if I have got any facts wrong.


Related Links

Ni-Cd batteries on Wikipedia

Ni-Mh batteries on Wikipedia

Li-Ion batteries on Wikipedia

Lead-Acid batteries on Wikipedia

Memory effect on Wikipedia

Memory effect on Battery University

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