Caring for rechargeable batteries.

A bit of an introduction

First up, this not a battery tutorial – there are much better sites for that. Battery University  is but one.

This is a vexed subject with just about everybody having a different opinion concerning the “best” battery chemistry. It also  seems that everyone also has an opinion on how to look after batteries of any chemistry. The really common chemistries are NiCd, NiMh, Lead Acid and Lithium. These are the batteries that we’ll commonly find as rechargeable cells used in car cranking batteries, storage batteries for caravans and campers as well as other uses.

I’ve just had a look at the types that I use. For Amateur Radio I use LiFePo4, my phone and e-cigarette has Lithium Ion, our camper and car have Lead Acid, torches, etc. have NiMh, my old battery operated screwdriver/drill has NiCd and so it goes. You’re probably in the same situation.

To ensure the best battery longevity there are only four basic rules.

  1. Do not over discharge your battery with the exception of some chemistries that don’t mind complete discharge.
  2. Do not over charge your battery.
  3. Charge and discharge the battery’s within the manufacturers specifications.
  4. Store it correctly.

Failure to obey those simple rules will result in a shortened life span or failure. Some failures can be much more spectacular than others. Some may just leak a bit, some will puff up, some will boil over, some will quietly just fail to “hold a charge” and some will explode. Violently.

Sounds simple, yes ? No. Let’s have a look at the capacity, discharging and charging cycles of rechargeable batteries.

Before we have a look at the discharge cycle let’s have a look at how much energy is in a common lithium type. The humble 18650 cell.

These things can sure pack a punch. They can store a whole lot of energy in a very small space. To show just how much energy is contained in a single 3,000 mAh (milli Amp hour or 3Ah) cell we’ll need to do a couple of simple calculations. A single fully charged cell has a terminal voltage of around 3.7V (Volts). 3Ah means that we can draw 3 Amps for an hour (at a specified rate which we can ignore here). Power, in Watts, equals Volts multiplied by Amps which in this case works out to 11.1W which is the power we can draw for an hour. Now if we multiply that by 3600 that will give us a theoretical 39,960 watts for one second. In reality you’ll only get a fraction of that but there’ll still be enough to cause a fire or to cause the battery to fail in spectacular fashion.

So let’s have a look at capacities.

Battery capacity

This is a source of grey hair to many in the power supply industry.

When you see for example a 12V AGM or Absorbed Glass Mat, deep cycle battery of 100Ah capacity you could be forgiven for thinking that you could draw 100A (Amps) for one hour or one amp for 100 hours. You’d be wrong though. What you can actually get out of a fully charged battery is dependant on the rate at which you discharge. The measured capacity of our example battery will be a heap less if you discharge it at 100A and a heap more at one amp so some sort of standardised way of discharging is required so that we can meaningfully compare battery capacities.

With AGM batteries we seemed to have standardised on the 20 hour rate. What this means is that we can discharged a fully charged battery at a constant rate over 20 hours before it is dead flat. Our example battery can be discharged at a rate of 5A for 20 hours which equals 100Ah. So an AGM battery advertised as being of a capacity of 100Ah can be safely assumed to have twice the capacity of an AGM battery advertised as having a capacity of 50Ah.

More confusion creeps in when we look at batteries of different chemistries. Lithium batteries of whatever type, NiCd, NiMh all tend to have their capacities measured at different discharge rates. When comparing  say, AGM to Lithium, make sure that the quoted capacity is at the same rate. If all battery manufacturers specified that the capacity is measured at the 20 hour rate we’d be easily be able to compare capacities. But they don’t, so we can’t.

A few years ago I tested the capacity of a number of 18650 Lithium cells and found that although they were all marked as being 3,500mAh (3.5Ah) capacity they weren’t. Of course I was testing cells of four different brands. In the data sheet of each brand the capacity was rated at a different discharge rate. Confusing. The measured capacity at the ten hour rate varied between 1,400mAh (1.4Ah) and 4,300mAh (4.3Ah). Each cell had a measured capacity of 3.5Ah at the relevant manufacturers discharge rate.

Enough to make you pull your hair out. Anyway let’s move on to discharging batteries.

Discharging

Now this where the fun begins.

Some battery chemistries don’t care about batteries being completely discharged consistently. Others do.

Your good old AGM deep cycle battery doesn’t much like being discharged beyond about 50% state of charge or SOC consistently. Every now and again is sort of OK though. The only ill effects of deep discharge is a shortened lifespan. Maybe it’ll only last three years instead of seven.

Your car cranking battery is designed to provide very high current for a short period and to be recharged fully as soon as possible afterwards. These batteries don’t like being discharged without an immediate recharge. Leave it partially discharged or let it waste away due to internal losses and it’ll need replacing in a couple of years. In a car that’s used regularly on trips that allow the cranking battery to be fully charged the battery will last for a very long time. The battery in my wife’s car is over twenty years old and still going strong mostly because every time it is driven there is enough time for the battery to recharge completely. It also gets driven on a daily basis so it doesn’t get a chance to just sit there and waste away.

Charging

This subject will open a can of worms whenever it’s raised. Some battery types and chemistries require constant voltage, some require constant current, some require a combination. Some can tolerate very fast charging others will start getting hot and puffy whilst they engage in an orgy of destruction. In general though, battery charging is a vexed question.

To simplify matters I’d suggest using a suitable charger or a charger that can be set up for your specific battery type. For lead acid batteries you’ll commonly find that chargers have settings for AGM, GEL and flooded wet cell as each require a different charging profile. My Victron charger for example has four stages for AGM batteries. Bulk, Absorption, Float and Storage. Other chargers break the charging process into as many as seven stages.

For most battery chemistries a charger will exist that will charge it properly. It’s not good to charge NiMh cells with a charger with a LiFePo4 charging profile. Use a charger that has the correct charging profile. Of course this means using a reputable brand of charger which, of course, adds to the expense. Keep in mind, though, that using a “cheap and cheerful” charger may well result in a shorter lifespan for your batteries. A good charger will cost a heap less than having to replace expensive batteries.

The final word

Batteries are expensive. Even cheap AGM’s or flooded wet cell lead acid can cost a fair bit. The purchase price of Lithium batteries of any type can be eye watering. Please protect your investment.

To protect your investment it pays to follow the manufacturer’s suggestions with regard to discharge rates and depth of discharge. You will be well served by storing the battery as the maker suggests and by charging it using the correct charging profile.

If you look after your batteries they’ll reward you with a long and effective service life. Even that very much maligned chemistry, NiCd (or nicad) – a lot of satellites are powered by NiCd cells and their service life is immense. Why ? Because they have been cared for properly.

 

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