Thursday, November 7, 2013

Fun With Generators

So the nights drew in, and so did the clouds in the days.

We had a massive storm the other week in the UK.  A depression barrelled across the Atlantic, whipped up by the jet stream, and smashed into the southern counties of the UK and Wales in the night.

Around 600,000 houses lost power due to trees falling on power lines.

I was prepared.  Batteries fully charged; petrol generator tested and loaded with fuel.  Thankfully, we didn't lose power here but it got me to thinking that I'd never tried to charge the battery from the generator.  The generator would have been just used to provide AC power directly.
With the storm passed and the new Maas PSU capable of delivering up to 765W to the house battery, I decided to test out how well they'd work together.

Now this generator isn't brilliant.  It's a 2-stroke engine and that means you need to add oil to the petrol and all of it is burnt in the engine.  You have to balance having enough oil to lubricate the engine against the volume of smoke the engine produces from burning the oil.  Let's just say I need to work on my fuel mixing skills, as the garden was enveloped in a thin blue mist while this thing was running at 'full power'.

And that brings me to the power output... It's rated at 650W continuous and 720W peak.  But generator manufacturers are notoriously, errr... 'generous' when it comes to their product labelling.

I got a plug-in power meter and used that to measure the generator output and load performance.

The first thing I discovered was that generators can't start large step loads.  I rigged up the Maas charger, dialed in 17A on the limiter and started up the generator with no load.  After letting it warm up a bit, I clamped the positive battery cable on and the load stepped from near nothing (10W) to 450W; or it would have if the generator engine hadn't simultaneously stalled.

So then I tried again, with the current limit dialled down to 5A and then ramped it up.  The generator Voltage started to drop off.  From an essentially no load Voltage of 240V, it dropped off to 195V at 520W.  I didn't ramp up the power any more as the PSU states that the line Voltage should be a minimum of 220V -10% = 198V.  Switch mode power supplies can often fail from under-Voltage supply.  I wasn't going to push it.

At the same time as the Voltage sag, the engine laboured and slowed, reducing the output frequency to 46Hz from 50Hz.  It also didn't like holding the load, with the engine cycling (revving) a bit on a one second-ish cycle.

So I dialled in 450W on the Maas PSU and the generator could handle that (just); cranking out 200-205V and 46-47Hz with the engine droning steadily and less blue smoke from the exhaust.

I let the thing run for about an hour and a half and then tried cranking up the power again.  The line Voltage still sagged to 195V, but the engine seemed happier with no instability (revving).  Even the smoke seemed to have abated somewhat.  I let it run for another 30 minutes or so and no unexpected smoke or outright fire issued :D.

Test over, and the missus complaining about the smell of 2-stroke engine in the garden, I shut it down and dug out my Brother labelling machine to amend the power output labelling on the side of the generator.
Given that I picked this thing up for £30 at a car boot sale (plus a bit more for repairing the broken fuel tap), I'm not unhappy.

But even with a power factor corrected load (it measured 0.98 under load), the generator wasn't able to deliver the charger's full power to the battery.  The generator couldn't manage more than about 70% of it's rated power without unacceptable de-rating of the line Voltage and instability.

So I guess I'm on the look-out for a more chunky machine with at least a 1kW output (claimed) and a 4 stroke engine (to spare the roses another fumigation).

Monday, November 4, 2013

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.