Friday, June 29, 2012

Still No Trouble at t'Mill

131 daily cycles completed on my lithium battery bank.  No drama, no problem.  Not very interesting subject matter for a blog.

After tinkering with various settings of charging that only seemed to provoke one cell to want to wander off towards "higher ground", I reverted back to the original scheme of charging to 3.50V per cell (28.0V for the pack) but only for 30 minutes instead of up to 60 minutes.  All charging for longer seemed to achieve was to make one cell go high and cause the rest of the pack to drop (as the charger wound back the power).

Charging to the original Voltage and keeping the time short seems to be the best way.  It also has the side benefit that the water heater comes on sooner.

I'm now going into business making these things!

I'm working with a friend who runs Sustainables4U and we're packaging these storage systems for stationary (not moving from the coal shed) and mobile (on a trailer or in the back of a van) applications.

We've taken delivery of a batch of 200Ah Winston cells and will make a prototype portable generator.  Something you can use at a building site or at a festival or even an eco show in a field that will make mains electricity to use without the noise, smoke and smell of a petrol generator - the sort that are always burbling behind burger vans at car boot sales...

We'll have a sort of flight case on wheels that will hold either 4x 200Ah cells or 4x 400Ah cells to give 2kWh or 4kWh of usable energy storage and a 1kW or 3kW pure sine inverter respectively.

I've even invested in some PCB CAD software to turn out a proper version of the inverter interface board so it won't even be bodged together with stripboard and bits of old string.


  1. I hope to be retired by the time these cells pack up!

    1. Luckily these have fallen quite a bit in price. Along with PV of course. At this point in time I fail to see why there is still valid opposition to having the bulk of the energy come from PV/battery combinations.

      I would imagine by the time you retire there may in fact be some really exciting things to be had. :)

      Did you get your London 2012 front row tickets? ;)

  2. Can you shed some light :

    In your production page it notes the 2012 for say June or so is 185KWH. You mention having 3.2KW Array. Dividing these numbers by 30.5 days comes out to little over 6kwh a day on average. 2 some hours of solar insolation? I would imagine you to have at least double this amount. Let alone the amounts you gain from the other sunlight hours, not simply peak.

    What am I possibly missing here? :/

    1. Being off grid the "generation" figures are actually usage figures. The meter is a normal OFGEM kWh meter on the output of the inverter. If a load isn't turned on then no energy is "generated".

      6kWh per day average is about as much as I use in the house. The other 1-2kWh per day is used by the electric cooker, a couple of minutes of the electric shower and a couple of minutes of the hair drier. The 6kWh is everything else in the house.

      You'll notice that the chart has been flat since March at around 180kWh per month. That's as much as we need to run the house.

      The immersion heater absorbs some of the energy that the battery can't when it gets to be full but the immersion heater is limited to a 650W load by a 110V transformer, so there's only so much it can absorb. Still, on a good day, it's enough to fully heat a tank of hot water and so spare us from using gas to heat the water. On days when the immersion heater gets a good run, the total solar energy consumed can be as much as 11 to 12kWh.

      Another factor is that the battery chargers max out at 2kW output. The array is over-sized so that I still get usable amounts of power on bad weather days and in Winter. The peak output ratings of PV modules are very rare to see.

    2. This is actually what I figured after writing it out and reading further postings. It falls to not what is possible to be produced a day based on your array but a lack of a place to put the energy thus the excess just 'sits' there.

      In other words, as you put it, if you had a grid tied system, then you would always be able to put more PV input into the mains, thus max out your production numbers. However, an off grid system only has to power all YOUR loads. If it meets that then nothing else is needed.

      :) We recently had a several day power outage. Over the hours after the first loss the neighborhood was awash in generator noise. Made me think on what really you need in an outage. Lights (something to do with the human psyche and comfort) and possibly the freezer. In our case the cable internet box as well. I think a portable 'small' sized solar generator would fill this adequately.

    3. Grid tied PV with battery backup is the next step that people can take. This is especially useful in areas where the mains is a bit unreliable.

      Normal grid tied systems shut down in a power cut to protect the linesman working in the street from micro-generators. But this renders the solar power that is available during the day unusable in the home.

      A battery backup system can be built in from the start or retrofitted to an existing grid tied system to isolate the house (or rather some protected circuits in a house). During a power cut the battery power plus any available solar power can be used without danger to power workers in the street.

      But backup is only the start.

      Lithium batteries allow daily use of the stored energy in a grid tied system. Night time load can run from stored energy while day time excess (after recharging the battery) can be exported.

  3. Hi Outta, just seen you're going into business and have sent a message to Sustainables4U.

    Cheers, Paul