615 Cycles & Still Grinnin'

Has it really been over a year since I last posted?

Well, I'm still here and so are the lithium cells.  615 cycles in and they are still working.  Ran a capacity test back at the end of June (487 cycles) and got 8.4kWh AC power out of the pack from full.  About 7% down on the 9.0kWh of the previous test at 180 cycles.  I did change the charging parameters in between though; charging to a higher Voltage but doing a very short absorption cut-off.  The net effect is less time held at a Voltage above the float level and I also reduced the float level a bit to prevent any "slow cooking" of the cells in the long sunny summer days (we finally had a summer in the UK - Yay!).

What prompted this posting was my latest toy and a return to storage deficits problems in the winter.

This bad boy is a switch mode 1-30V, 0-30A power supply.  It was about £317 delivered from Germany (but turns out it's a re-branded Chinese Manson unit).

It's survived the tests that have destroyed three other Manson sourced switch mode PSUs (branded as Maplins in the UK).  The 80W Maplins PSUs that you saw me using last year really don't like to have their DC outputs connected to a 24V battery when the output is turned off (or the AC power is off).  The spark of reverse current in-rush to the capacitors in the PSU destroys something in the Voltage control feedback loop and the next time you turn the PSU on, it immediately explodes with white smoke pouring out of the back (over-Voltage on the big capacitors, causing catastrophic dielectric failure and explosion of the caps).

Happily (luckily) the Maas HCS-3602 seems to have passed the two critical tests.  It was subjected to an AC power failure while connected to a 24V battery (the house inverter cut out as there wasn't enough solar power) and today I forgot to turn the PSU on before connecting it to the battery and a big spark of in-rush current went into its DC output.  I turned it on with trepidation, but all was well and it's sitting in the sun charging my spare Ritar lead acid AGM battery at 30 Amps as I write.

I did ask the supplier and the Manson tech support if the PSU was stable with reverse Voltage on its DC output and in-rush currents, but got stonewalled by both of them.  They'll be happy to discover my test findings but I don't see why I should be the one to test their product in the real world and have to trog back to the shop when it unexpectedly explodes.

The Ritar battery pack (formed of two 180Ah 12V blocks in series) has come in handy in the last few November gloomy days.  Just like in Star Trek, when the di-lithium crystals are depleted and they need a bit of power to get out of a pinch, Jim shouts, "Tie in the auxiliary power!".  I decided to "make it so" with these Ritars (mixing my Star Trek generations up now...).

They route emergency power through my old 1kW inverter and then via the new Maas PSU to hold the lithium battery pack at a level just above its low charge point.  When the sun comes up the next day, the solar charger only has to tickle the main battery for its Voltage to rise a bit and then the auxiliary power system cuts out.

I saunter along after breakfast and swap the PSU round to feed from the house 3kW inverter and charge the Ritar pack at a rate the solar array can cope with, using the current limiter on the PSU.  That's how I 'tested' the PSU with AC power failure.  I'd set the charge current too high and wandered off while a lonely bank of cloud came and spoiled the party.

The Ritars might be only good for 600 cycles, if I'm lucky, but as I only use them for a few odd days a week in the winter, that could be a useful way to save on just buying more massive lithium cells.  It's nice to have a "reserve tank" when you cock up your weather prediction and energy use and need to "run on fumes" for a bit.  The Ritar pack can even take a bit more abuse as I don't need to slavishly stick to the 50% DoD rule if only discharging it infrequently.  It will shorten the life, for sure, but even if I halve the life to 300 cycles; at 30 cycles a year (say) that's still 10 years use.

I'd previously tried a scheme like this but the problem was not using a 24V battery (I was tinkering with 12V batteries and a 150W inverter) and not having a high power 24V charger that could shift energy efficiently.

This Maas PSU measured over 91% efficient at 25.5V and 17A output.  It is also power factor corrected, so is inverter friendly.  I measured the DC-AC-DC conversion throughput from Ritar battery to lithium battery at about 78% efficiency (the 1kW inverter only being about 86% efficient, despite its claimed 92% rating).  The PSU barely breaks into a sweat.  It has a variable speed fan but doesn't become a fan heater in the room.

The rear has half decent binding posts to take M6 lugs, but annoyingly the screw tops are captive so I could not use the M6 ring lugs bought and had to resort to cutting off the ends to turn them into fork lugs.  I could only find M6 forks locally that would only accept 6mm2 cable.  I'm using 16mm2 cable to keep the voltage drop down at 30A, speeding up charging and keeping efficiency up (low heat loss in the cables and connections).

This power supply has some interesting features in that it has 3 memories for Voltage and current (that disable the front knobs - prevents accidental changes) and also a remote control terminal that would make it suitable as an AC charger controlled by a BMS that can remotely program the output Voltage, current and enable/disable the output.  The remote terminal takes 0-5VDC control signals and the maker kindly provided the special multi-way plug to fit the socket on the back.

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.