ghurd). Can increase load with another FET and/or more resistors (more than 50W to hand) with proper heat sinking and dissipation. LiFePO4 battery reaching ~15.5V and SLA 14V+ nearly every day, and SLA overnight voltage mainly below 13V after ADSL load (>12W) was moved to it off-grid on the 10th.
2011-04-17: dump load operating from around solar noon but not able to hold voltage down to 15V with ~70Wp+ of PV working, so may need to add extra load. Current load making system warm but not unreasonably so; however any significant extra dump load should be sited better and heatsinked.
2011-04-22: moved 12Wp panel from LiFePO4 to SLA input so former is dumping less and latter is full more, ie better balancing.
2011-07-25: current upgraded off-grid power system circuit diagram .SHX, PDF. Simplified the system and moved input from 36Wp amorphous panels at front to main battery bank to help support the off-grid ADSL and brings the input to the main controller to just less than 180Wp nominal.
2011-09-15: note that as the LiFePO4 voltage rises and passes the SLA here, the SLA voltage apparently suddenly rises about 0.2V as the LiFePO4 takes over supplying current to the server via the diode-OR and eliminates the voltage drop in the SLA route. More generally, this tends to yoke the output voltages of the two batteries together, sharing the load, unless being driven otherwise by large charge currents.
2011-12-31: the SheevaPlug has been off-grid since the battery upgrade, with no sign of the controller's amber or red status lights even this month. I have also tweaked software settings to reduce work done in 'extreme' shortage as measured by either battery (Li or LA) not being fairly full. In December the Li battery is usually low enough to trigger 'extreme' mode. The main server now also regards any period over about 48 hours without reaching a 'good' power level as similar to 'extreme' shortage for most purposes. 14:20 UTC: voltage measured at main LA battery bank terminals 12.70V, in (astonishing) agreement with 12.7V as seen by k8055 at SheevaPlug.
2012-01-07: made it into January in spite of sunless interval from before Christmas into the New Year: see the lack of lead-acid spikes to over 14V and the sub-13V state of the LiFePO4 battery.
2012-02-02: Li battery up to 15V, balancing, for first time for a couple of months at least!
2012-08-29: Li battery has regularly gone over 15V (but below 16V), though runs of gloomy weather clearly show up as days in a row without. Generally the Li battery subsystem continues to 'just work' well.
2013-06-24: reconnected after major kitchen works, assuming C- is charge (-ve) and D- is discharge!
2014-04-27: ADSL auto-off-grid hardware finally wired up today after keeping an eye on software/w control behaviour for a while. Battery levels seem to be more stable and healthier of late. The ADSL is perhaps 50% off-grid at the moment, including in evening peak demand time. If the ADSL were off grid 100% of the time, given a measured 7.7W consumption, savings (ie extra energy not taken from the grid) would be ~67kWh/y. ~20kWh/y (ie ~30%) off-grid may be realistic, ie 1.5% of total gross household consumption (~1500kWh/y), which starts to be noticeable!
2015-05-04: MacBookAir running directly from Li battery via Mikegyver.com custom adapter lead works well and can run indefinitely off-grid given current levels of sunshine.
I'm puzzling though a possible reconfiguration to help keep more load off-grid since fitting more grid-tied PD would now be a G59 expensive and paperwork-heavy nightmare. An aspiration is to get 500Wh/d load off-grid mid-winter since grid-tied generation sags to only ~1.5kWh/d. Getting load off-grid at night or when otherwise importing is especially good, as it effectively maximises self-consumption. The main off-grid load, the RPi, is only directly responsible for ~50Wh/d, but other things around it, such as controller self-consumption, probably bring that back to ~100Wh/d. The current optional off-grid load (network connection and Loop monitor somewhat inefficiently powered) account for ~15W or ~360Wh/d. (Note: our cable TV box uses 15W on standby!) Being able to keep it off-grid most of every day including mid-winter would be a big chunk of that aspiration, but at ~1 sun-hour per day then, that implies nearly 500Wp of panels if the system is optimised for winter (and assumed able to cope the rest of the year), and with a rather optimistic take on likely storage round-trip losses. If the main target is to keep the optional load off-grid at night (~16h) then the target is 100Wh (RPi) + 240Wh, ie 360Wh + losses per day. Note that for the 4x12Vx99Ah battery bank, the efficiently-available 30% of DoD (between 50% floor and 80% absorb) is ~1400Wh, of which 360Wh is ~25%, thus reasonable.
The UniSolar (60Wp) panel + LiFePO4 as RPi power backup + resistive 30W dump load (+ optional Mac dump load) seems to work quite well and is probably safe in terms of maximum voltage and current flows in and out, so I'd like to leave it as-is. I am considering converting the LA solar string to a nominal 24V (~30Vmp) in, though probably remaining a 12V battery system for now (with the SS-MPPT-15L controller operating at 15A/200W max), and significantly over-powered on the PV side at say 250Wp--350Wp nominal, to allow the panels to be tidily vertically mounted and still provide ~250Wh/d sufficient for the RPi always, and other grid-optional loads mostly. Switching to 24V nominal will mean doing without current unmatched 12V panels, though they could be optionally connected to the LiFePO4 input when it is (say) < 14V to bolster in mid-winter.
The easily-available unobstructed south-facing wall space sections are ~1320mm+ vertically (with some ground clearance), E to W:
Set back sections are by ~105mm.
Some panel dimensions:
There are also free-standing 20Wp mono and amorphous panels connected to the LA solar controller input.
Mounting brackets, to be used on the long side of a panel, probably take 50mm.
If the current E-section-mounted 40W panel were replaced with the 60W UniSolar, and a new 250+W panel mounted horizontally to W (ignoring suitable mounting points for now), that would take 50 + 790 + 50 + 1650 + 50 = 2590mm, leaving LiFePO4 input as is now, and make a net addition for the LA system of 250+ - (100+40+20+20) = 70W. A 315Wp panel such as the MonoX NeoN brings the nominal gain to 135W. Although the system will be heavily limited by the controller when sunny (it will ignore extra available power that it cannot use without harm) a little extra solar on the other side of the house could get to nearly 360Wp, or nominally the target 360Wh/d ignoring losses. (Alternatively, the 40W E panel can be left as-is and ~20W put up elsewhere to power the LiFePO4 and ensure that ~50Wh/d can be delivered to the RPi via it.) PVGIS suggests that a vertically-mounted 315W panel would generate 330Wh/d in December which requires an average of ~80W over half an 8-hour day, well within controller spec. For a 250W panel, December gets 260Wh/d generated according to PVGIS. (It would be possible to upgrade the solar controller later, or run another in tandem, though that risks them confusing one another.)
Realistically even a 315W panel is not going to keep the small network load off-grid all night in the depths of winter, but it might get close. Saving 15W from existing grid load (from the ~40W other than the fridge-freezer) should be easier, eg upgrading a few lamps to the most efficient available.
See the Morningstar String Calculator with 315W "Neon2" 60-cell and 260W 60-cell and 1200W (4x) LG NeON 2 Black LG300N1K-G4 300W panels.
Another plan, by section, abandoning a separate feed to LiFePO4, and paralleling feed wires back in one low-power 12V nominal system, but allowing for a split to separate MPPT feeds later (eg 140W+350W):
For a nominal ~500Wp (490W + optional 20W + ~20W at front). 20160518 config is nominal 240Wp + ~20W at front. Potentially it then may be possible to, for example, use a 12V Victron Easy Solar 1600W All-in-one Solar Solution with Inverter and MPPT controller or similar (with mains charger possibly disabled to prevent extra grid imports) to take lighting circuits and select sockets off-grid when the batteries are reasonably full to trim winter grid demand. Further panel upgrades (eg 330W in place of 175W) are also possible.
2016-05-22: my current thinking is to simplify the system somewhat and remove the (+ve ground) LiFePO4 (LFP) from the system entirely, allowing rather more cycling on the 12V gel bank than previously. I am concerned that I am maltreating the LFP, discharging it to low and risking over-charging it given the undocumented BMS system. Not using my current menagerie of 12V panels feels uncomfortable even if the proposed new panels are (a) maybe 20% of the price per watt, and (b) as much as 3x the output per unit area which is important for my very limited space. I may retain one or more separate 12V-nominal strings with separate small MPPT charge controllers feeding into the one bank, eg a 24V nominal (30Vmp) and 12V nominal (~19Vmp) round the side of the house.
LiFePO4 battery removed and all PV feeding to gel LA bank. The 'test' is done after ~5 years! The observation is that the combination works well, but the complexity and partitioning of available energy may not be worthwhile.
At the same time as I contacted thousandsuns I emailed M2Power about its SLA-emulating 2-terminal batteries, eg the 12.8V 40Ah 0.3C-charge-rate device, with BMS built-in.
I'd already bought my thousandsuns battery by the time M2Power responded, but M2Power confirmed that:
In particular M2Power confirmed that An external BMS is not needed, it is integrated in our battery packs. The only requirement is that [the battery] is charged to 14.6V.
I imagine that an existing SLA controller/charger could be adjusted for an 'absorption' voltage of 14.6V and duration of ~3 hours, a little high for a flooded cell, and it might work well.