Sunday, July 29, 2012

A Lifetime of Power? (Part 2)

Ok, so the question, "How long do these Lithium ion batteries last?" was partially answered in Part 1 a few days ago.

But there's a second aspect to a battery's longevity...  Cycle life.

Cycle life is independent of calendar life.  You can leave a cell on a shelf and never use it once and it will die.  That's its calendar life... The time it takes to die of old age from the day it was "born".

Cycle life is how many cycles of discharge and recharge the cell can do before it wears out from working hard.  Work a cell harder and it wears out faster.  Treat it to an easy life, and it will die of old age.

Batteries are somewhat akin to people in that respect.

So...  People often ask then, "Well how many cycles can this battery do?". That's also a question that has the answer, "It depends...".

All cells are quoted as living for so many cycles if you treat them right.

The biggest impact on cycle life is from how much you discharge them in each cycle.  If you discharge good gel Lead acid batteries to 50% of their capacity (50% DoD), they will last for maybe 850 cycles.  Push them harder, by discharging to 80% DoD, and the same battery may only last 500 cycles.  In each case the battery does not suddenly die, but it's ability to hold and deliver power is eroded.  When the battery capacity has dropped to 80% of it's original rating, it is considered "near dead".  This is the case for Lead acid batteries because they then rapidly get worse after that level of damage.

Lithium ion cells vary in their quoted cycle life depending on the particular chemistry.  The Winston Battery (LiFeYPO4) cells are supposed to be the longest living, with the Yttrium as the added ingredient that extends their life, even beyond the generally long life of general Lithium Iron Phosphate (LiFePO4) cells.  Claimed life for the Winston cells is up to 8,000 cycles at 70% DoD.

If used in a solar system that naturally has a daily charge and discharge behaviour, that would suggest over 22 years of daily use.  This may be longer than the calendar life of the cells though.  But it certainly suggests that, unlike Lead acid batteries, the Lithium ion cells I'm using now should die of old age before they expend their cycle life from over work.

Tests done by the department of control and telematics at the Czech Technical University in Prague have demonstrated over 13,000 actual cycles on an automated test rig that charged and discharged these cells to 10% DoD and 1.5C discharge and charge rates with no degradation of performance.

See the report here.

Friday, July 20, 2012

A Little Revamp

A bit of a change to the look of the blog.  Actually forced upon me as Google have broken the rendering of my old custom template profile widget and I couldn't figure out how to fix it.  So I've had to go to a new "off the peg" template.

A new skin but the same old bag of bolts under the hood...

Thursday, July 19, 2012

Exhibit A, M'Lud...

Right on cue, to prove my point about laptops and short calendar life induced by heat, my data logger laptop upstairs chewed up and spat out it's 6 month old lithium ion battery pack this morning.

The machine has been running hot for some time, the fan whirring like mad but not blowing out much air.  The poor machine is worked hard as it also records two CCTV cameras as well.

Then this morning, the battery LED on the machine was blinking orange.  Uh, oh.  Not good.

I pulled the DC cable out to see if the battery had any power and the machine fell over immediately.  No good.  No good.

Pulled the battery pack and it was hot in one spot, just where it was next to the heat sink assembly on the motherboard.  The heat had evidently cooked one cell, ageing it.  When things cooled down I tried again, but the pack was stone dead.

So it was screwdriver time for the laptop to find out why the thing was getting so hot.  The fan was working but just not working well.

About 20 screws later, I'd got the thing into all it's bits and could lift out the motherboard with the integral heat sink and blower.  I blew out the copper matrix exhaust with high pressure air and wads of fluff like candy floss came tumbling out of the intake.  How did all that stuff get in there???  I half expected a small mammal to come out with all the nesting material that it looked like was in there!

Anyway, with that done and the 20 screws put back where they came from, the laptop runs cool and the fan just spins slowly without making the ineffectual roaring noise it did before.  I salvaged a battery from another dead clone laptop and so we'll see how long this one lasts! 

I only put a battery in the machine as a UPS so that I can swap the AC adaptor from solar to grid power without having to reboot the machine, abusing the battery for my convenience (like I said in the previous article :D ).

There you have it, a first hand example of "accelerated ageing" testing.

Wednesday, July 18, 2012

A Lifetime of Power? (Part 1)

How long do these Lithium ion cells last?

Good question and not a straightforward answer. It depends on quite a few things...

Firstly it depends on what type of Lithium chemistry you are talking about.  Some types of cell are optimised for power delivery (highest power from the smallest, lightest cell).  Some are optimised for energy delivery (the most energy stored per cell).  Some are optimised for safety (no metallic Lithium or dangerous oxides that promote incendiary-bomb-like fires). 

Many of these optmisations have an impact on the lifespan of the cells, both in their "calendar life" and their "cycle life".  Today, let's start with the issue of calendar life.

Calendar life is a measure of how long it takes for a cell to self-destruct due to chemical impurities that cause side reactions to the desired ones.  These side reactions gradually cause the cell to die and they start happening the day the cell is made.  Nothing you can do to stop it, but you can slow it down.  That's why AA cells and so on in the shops have a "best before" date stamped on them.  Alkaline cells have a calendar life of about 4 years.
The very best industrial Lead acid batteries have a calendar life of up to 13 years, but only if you keep them cool (under 25C).  For every 5C warmer you store them, you HALVE their calendar life!

Lithium ion cells generally have quite long calendar life of over 10 years.  But it depends on the chemistry and how they are stored.  Lead acid batteries like to be stored in a cool place, fully charged and topped up regularly.  Lithium ion cells like to be stored in a cool place and at a low state of charge (30-50%).  But you have to be careful that they do not become over discharged (by attached monitoring chips or other circuits).  They don't like being stored on charge all the time and in a hot place.

Unfortunately, that's often the kind of treatment they get.  Laptop batteries are notorious for short calendar life of just 2-3 years.  This is due to poor storage conditions.  They are installed in the machine when used on the mains all the time.  The cheap chargers keep the battery on charge the whole time (even when the laptop is turned off) and the heat from the motherboard can be extreme (45-55C locally inside the unit, near one or more cells of the battery pack).  It would be better to half discharge the battery, take it out and keep it somewhere cool until needed.  But then who does that with a laptop?  It's very inconvenient to put the battery back in and charge it before needing to use the machine away from the mains.  So people "abuse" the battery for convenience sake and complain when it dies even though they haven't used it for many real discharge cycles.

Some Lithium ion cells are more sensitive to storage conditions (especially temperature) than others.  Lithium Iron Phosphate cells (like the Winston ones I'm using) are relatively "ok" with warmer temperatures.  It does not have as big an impact on their life as for some Lithium metal oxide cells, which degrade more severely with heat.  These metallic oxide cells are historically the types of cells used in laptops, so not a good combination...  Cells that don't like heat, stored permanently in a warm / hot place.

It's also why the sealed lead acid batteries in my computer UPS (uninterruptable power supply) used to die every two years... It gets hot (35C) in the box where they are housed.  If I keep the batteries outside the box and extend the wires into the box, the batteries last for at least 4-5 years.  The culprit was temperature.  After all, the batteries never discharged as they were for emergencies only.

So... My Lithium home energy battery bank is kept in a cool part of the house and I keep it shaded from the sun.  The inverter is positioned so that it does not blow warm air at it.  At temperatures ranging from 17-23C there (recorded by the charge controller) the battery pack should last for well over 10 years.  Nobody knows for sure, because they've not been in production long enough to find out but I'll be one of the first to know!

Most testing of calendar life is done by deliberately running the battery at higher temperatures in "accelerated testing" and then estimating the life at lower temperatures, assuming that all the chemical reactions slow down in a linear way with temperature.

The other aspect to calendar life is storage charge level.  As mentioned, different types of cells like to be stored at different charge levels.  Lead acid likes to be stored fully charged and Lithium likes to be stored at 30-50% charge.  This makes Lithium ion cells more suited to renewable energy storage, especially solar power.

Solar batteries are by nature only charged during the day and then discharged at night.  The ideal off grid solar power system has a battery that is never fully empty but also never kept fully charged.  If it did reach full charge, it would mean no more energy could be stored and so sunlight would be wasted.  In that case, either the solar array is too big for the battery or the battery is too small for the array.  In Winter, most solar batteries struggle to maintain anything close to full charge and may spend days or even weeks at low to middle charge levels.

As you can see, Lithium ion cells fit this model much better than Lead acid cells.  Lead acid cells are quickly damaged by not being kept fully charged 24 hours a day.  Lithium ion cells PREFER not to be kept fully charged for longest calendar life.

Next time I'll discuss cycle lifespan.

Or why not talk to us at Sustainables4U about your energy storage needs!