Tuesday, June 22, 2010

She Cannae Take the Power Cap'n!


Automotive blade fuses... PAH!

Don't use 'em.

I had my new batteries connected to the main bank with an in-line blade fuse holder and a 30A fuse.  Not a cheap-o rubbish fuse holder but an RS one.  It was the day after the longest day today and the sun was beating down after I'd more than half drained the batteries watching movies last night.  Pulled 6.4kWh from the system :D.

Today though, the secondary bank was very greedy and spent quite some time sat at about 25-30A charging.  The poor little blade fuses couldn't hack it.  Two of them melted (the plastic holders - not the fuse wire!).

So I replaced the in-line fuse holder with an old 32 Amp MCB breaker that I got at a car boot sale a while ago.  It's better that way any way as it can disconnect the battery if there's a fault and it also serves as an isolation switch.
The rest of the afternoon's sun was soaked up without further melting or burning of plastic / rubber / fingers...

Wednesday, June 16, 2010

More on Balancing Batteries

Googling around the last couple of days I discovered that Elecsol (well known maker of leisure batteries) has released a new range of sealed VRLA AGM batteries primarily aimed at solar storage.

They make some impressive claims like that these batteries can deliver 1,100 100% discharges and 1,400 80% discharges.  They offer a 7 year "unlimited" warranty on them.

You can read the blurb on them here:  http://www.elecsolbatteries.com/literature/

The tech book / brochure did have an interesting addition to the traditional star pattern series-parallel wiring scheme.  It had not occurred to me to put balancer cables between the mid-points of the series strings to allow internal equalisation of the bank.  I'd had a problem with one of the six new batteries in my bank being low (out of balance with the other one in series) but I cured it by taking that pair out of circuit and slow charging the weak one to bring it up and then put them back in circuit.  The whole bank appears to be behaving ok now but I periodically measure the volt differences on each block. 

I might put a star equaliser network in (connecting the mid points of all three pairs to a common point so they will equalise er... equally).

The red/black lines are the power lines I have on my bank and the green ones would be the equalisation network that would allow all the weak batteries in pairs to charge up more without over charging the stronger ones.  In my "bad" pair they showed 28.2V across the pair but one was 13.8 and the other was 14.4.  With the equaliser network added, the weak "bottom side" battery might continue to charge by "finding" another weak "top side" battery to pass current through (13.8 + 13.8 is only 27.6 so that pair would continue to charge without over charging the others sitting at 14.1V).  It wouldn't help in every scenario though - if all the bottom side batteries were weak and all the top side batteries were strong then the equaliser network wouldn't help. 

Maybe it's just as well to rotate batteries in strings (like tyres on a car)?

Meanwhile, it's been a great couple of days harvesting.  In those partial cloud surges that I mentioned before, I've seen surges up to 1.98kW from my 1.8kW array (over 109% of rated power). The Sharp ND170 340Wp string stole the show though, putting out 409W (120% of rated power).  They might have managed more but the charge controller capped the output at 15 Amps!  The other charge controller was also close to capping its output to the 60 Amp limit, as it surged to over 57 Amps - the pair pumping an eye-watering 72 Amps into the battery bank.

Friday, June 11, 2010

Balancing Batteries

When connecting batteries in parallel to get more capacity, you have to be careful to balance the individual strings of batteries so that they all do the same amount of work.  Otherwise, some will become more discharged than others and suffer damage from hard lead sulphate forming on the plates that cannot be dissolved by charging.

Having batteries of different sizes is also not a good idea as the small ones will discharge to a greater extent than the bigger ones and so suffer again.

So what did I do?  I bought a bunch of different sizes and types of battery and connected them all up together... Go figure.

So, how do we bodge this so that the batteries all stand a chance of surviving?
The main block of batteries are gel types and 180Ah in size.  They have a rating of 1,000 cycles at 50% depth of discharge (the deeper you discharge a battery, the fewer times you can do it).  The Marathon ones I've just bought are AGM types and 104Ah in size.  They are intended for computer uninterruptible power supplies (UPS) and as such are not expected to be discharged every day (if ever).  They probably have a rating of 250 cycles to 50% discharge.  But if discharged by only say 25%, they might last nearly 1,000 cycles.

So, to balance the different batteries lifespans, you have to try to balance the amount of work each does. To do this your main tools are size of battery and the wiring that connects them.  Thinner wires resist the flow of current and so large loads will drain batteries connected by fat wires faster than ones connected by thin ones.

In the diagram above you can see how I've wired the batteries together.  The big gel ones are directly connected to the solar chargers and the AC inverter by very heavy gauge 35mmsq cables.  This lets the gel bank do the bulk of the work when under high load. The weaker Marathon batteries are connected in 3 groups that use a star wiring pattern.  This means that each group is connected by wires that are individually quite thin (6mmsq).  But the wires are deliberately a bit long and are all the exact same length (hard to draw so take my word for it).  The other important fact is that they are all connected together at one point (I soldered them together after weaving the ends into a sort of knot. This means that, as far as possible, the wire resistance for each branch is the same and so each group of batteries will do the same work.

The whole group of six batteries is then connected to the main bank by a single pair of 6mmsq cables with a 30A fuse.  The fuse is important as 6mmsq cable can only carry about 50A and a fault could cause a few hundred Amps to flow.  If the main battery dried up or got a short circuit somewhere inside it, a very large current could flow and start a fire.  Actually, each of the groups should have a fuse but adding fuses in every leg makes it difficult to balance the resistances as the fuses and connectors introduce variances.

So, by having the Marathon battery bank 1.7 times the size of the gel one and using thin wires to connect it, I hope to keep the workload low enough on the Marathon bank to make it live for as many cycles as the gel ones.

When running loads, I've noticed that the gel bank does do more work (delivering about 2/3rds of the current).  But as the gel bank starts to run down the Marathons start to assist more.  This is because gel batteries are designed to resist deep discharge by being acid starved.  They don't have enough acid to fully convert all the lead in the plates.  As the acid runs out, the gel battery increases its internal resistance and so does less work until the output voltage falls to a point where the protection in inverters causes them to turn off.  

Because the resistance of the gel battery increases as it gets discharged, the balance of resistance between the gel bank and the Marathons will change.  At some point, the Marathons will have the lower resistance path and so assist the gel bank by doing more work.  So it's more important that before for me to stop discharging the bank before it gets much below 50% as beyond that point, the Marathons will start to be drained quite quickly.  But even then, that shouldn't be a big problem as they are 1.7 times bigger than the gel bank.

Tonight we're at 50% as the last couple of days has been very gloomy (it rained all day today).  Hopefully tomorrow will bring some sun.

I'll have to watch the charge in the morning as the reverse is true for charge balancing.  Because the gel bank is low and has high resistance, the Marathons will accept more charge current than the gel battery (at first).  The charger can deliver up to 73A and could cause the 30A fuse on the link wire to blow...

Thursday, June 10, 2010

Hello Darkness My Old Friend

Dark days have indeed returned.  The last couple of days have been a steady deterioration from the wall to wall sunshine of last weekend through the record breaking partial cloud of Monday (when I briefly saw the 1.8kW array spike to over 1.94kW) to today's wall to wall gloom.

Solar cells are temperature sensitive and when in full sun they get hot and lose some power.  But on partially cloudy and cold days, the cells get cold when there's a cloud but when the sun peeks out they put out more power for a short time before they heat up.  Also, as the sun peeks out from behind a cloud a sort of focusing effect happens to the light and for a few seconds it can be stronger than in a clear sky.  Under these circumstances it is not uncommon for solar panels to put out as much as 115% of their rated power for a few seconds.

The new larger battery bank is capable of absorbing a lot more power now and had no trouble lapping up the 73 Amps that the solar chargers blasted out.  Sadly, although these events produce very high rates of charge, they only last a few seconds so don't actually charge the batteries much.  So over the last couple of days the batteries have been slowly fighting a losing battle.

Having increased the capacity from 180Ah to 492Ah has made a big difference though, extending the time that we can survive without a decent sunny day from just one day to maybe two or three, depending on how reckless I am in the evenings.  But tonight after a particularly gloomy day where the PV output never crept above 375W and with the forecast for tomorrow to be non-stop heavy rain all day, the battery gauge is sitting at 69% tonight and likely to fall to below 50% tomorrow night.  We might have to switch the fridge freezer back to mains power at night until Saturday when the forecast is for sunny intervals again.

We'll see.

Tuesday, June 8, 2010

How to Bodge Ammeters

One of the big problems you have when putting together a battery based solar system is measuring where the juice is going.  You need to know whether it's going in the battery or to the inverter and how fast it's going.

For this you need low loss ammeters.  You need ammeters that can measure maybe 100-200A without causing much voltage loss.  You can buy shunts that you can connect in series with cables to read the current but this means more joins in the cable and each join adds resistance that causes loss.  Shunts are also mostly of the type that create a Voltage of 50mV per 500 Amps passing through them which is quite a small signal when most of the time you'll be measuring a few 10's of Amps.  They're also quite expensive...

Enter the In-Line Bodger's Shunt... :D

Solar battery systems by necessity need to be connected together by big fat cables to carry the heavy current without losing Volts ('coz we're working at maybe only 12V or 24V).  Any bit of copper cable (no matter how thick) will have a resistance and so make a voltage across it's length when a current flows.  Even the cheapest digital multimeter will read down to 0.1mV (they all have a 200mV range).  So all you need to do to measure the current in a cable is attach some sensor wires to the cable at a distance apart that causes some useful Voltage to be developed relative to the current.

I have two types of in line shunt in my system.  One uses a long spacing (about 50cm on a 10mmsq cable) to give a reading of 1mV per Amp.  On my big group of four 6V batteries, I made one of the huge 35mmsq link cables that join the blocks a bit longer than the others - just enough to make 0.1mV per Amp when I soldered the sensor wires at the lug ends. So that DMM reads 0.1 for 1A and 1.0 for 10A - you just have to ignore the decimal point on the display.
This one's a shorty as it's on a bit of 6mmsq cable.
It should go without saying (but I'll say it anyway) that you should only do this on low voltage DC power cables.  Don't try it on mains AC lines, as a) it won't work and b) you'll probably get electrocuted and die.

All you need to do is connect another DMM with a 10A current range in series with the cable in question and arrange for a few Amps to pass through it.  Then solder a sensor wire to the fat cable and using a pin, poke the cable at different places until you find the distance that gives you 0.1mV per Amp reading on the new meter.  Then you can either cut the insulation away at that point and solder the other sensor wire in there or sometimes on big 35mmsq cables you can just solder the sensor wire to the head of a drawing pin and just stick the pin into the cable at the spot.  Tape it up with electrical tape and run the sensor wires to where you want to see the meter.  It can be some distance away (like a couple of meters) as the sensor wires carry no current so won't lose any volts on the way (the DMM input is usually about 10MOhms so the fraction of one Ohm on the sensor wire makes no difference).

Here you can see my three bodged ammeters.  The top and left ones read net battery current in or out of the two main banks at 0.1mV per A (so showing 3A charge each) and the third one reads the current from the smaller Morningstar MPPT15 charge controller at 1mV per A (so showing 2.0A charge).  The big Morningstar TriStar has a built in meter so I didn't need to bodge a fourth meter. The meters are arranged so that positive readings mean charge current and negative readings means discharge current.
Ok, these shunts aren't going to be super accurate and the short (or fat) ones will only read to a resolution of 1A but mostly that's all you need to know - whether the battery is charging or discharging and by roughly how much (some Amps or lots of Amps).  

Maybe most importantly, it can tell you when there are no Amps flowing as that tells you two things: a) something is busted or b) the battery has finished absorbing power and is full.  In a system like mine with several batteries, it also tells me how well (or otherwise) the batteries are sharing the load or charge... More on this in the next post!

Monday, June 7, 2010

Attack of the Killer Bees

Woke up this morning and noticed that the end pair of panels on the patio were being partially shaded by new growth from a tall bush by the fence.
No problem, I thought.  I'll just get the loppers out and trim a bit off the top so that the morning light can reach the panel.  Snip... snip...snip.  Hmm quite a lot of bees on these flowers... You can see all the blue flowers all over the branches.  No worries, they'll just fly off.  Snip...snip...snip... what the..?  Hey! Ouch!  Wahhh!!!

Retired to a safe distance to see the top of the bush a mass of bees.  I thought one had "bumped" into my hand rather hard.  Then it started to come up in a red bump.  Oh oh...  

Luckily, we have some anti-histamine tablets in the house as I get hayfever and we had some sting neutralising liquid (ammonia solution).  If you put it on quickly after a sting the bump goes away completely and you're as good as new.

Back in the garden, I took a closer look at what I was cutting...  Hidden inside the thickest part of the bush was a bees (or is it a wasp?  Never quite sure...) nest.  Quite beautifully crafted from paper and always has a couple of "sentries" guarding the entrance at the bottom and others making the next layer of the shell from chewed up wood.
 It's an ideal spot for them as the bush has a lot of flowers and we grow roses and all sorts of flowers plus there's a pile of rotting wood next to the bush so no shortage of building materials for them.

Oh yeah... I managed to half clear the offending branches that were shading the last pair of panels on the array so I guess I bought a few more Watt hours in the morning but I doubt I'll be doing any more hacking and slashing this summer.

Sunday, June 6, 2010

Even More Batteries!

Well, the first two Marathon batteries I bought a couple of weeks ago worked out quite well.  And flush with the money from the car boot sale of my old solar bits and bobs, I went back to the same UPS battery guy and bought up four more of the same batteries for £50 a pop.

The first pair I bought certainly hold a decent charge (although I haven't measured them on a discharge cycle) and assist the gel battery bank quite well.

Here you can see my cinema...
And nestling under and behind the sofa...
492Ah of 24V batteries! 

Ought to be "enough" for now ;)

Friday, June 4, 2010

Sowing Some Seeds

With the upgrade of all my plastic 15W amorphous panels to "proper" high power BP and Sharp solar panels complete, there remained the question of what to do with all those spare panels gathering dust in the garage.

We decided to take part in a car boot sale.

We had a load of other unwanted things to unload as well (the usual bric-a-brac) so we saddled up and headed out for the fields of Sussex.

I had the original four wet batteries and eight of the 12 solar panels to sell.  I also had a bunch of little 7Ah AGM batteries that a Hungarian dude gave to me for free when I bought a 40W panel from him last year.

Business was a bit slow at first but by the middle of the afternoon the big batteries had all gone for like £8 each and even some of the 7Ah batteries had gone for £3 each.  

Once I got my solar demonstration working (a panel plugged straight into a 12V fan) people were drawn to our pitch and were curious to know more about solar power.  By the end of the afternoon, I'd sold all eight of the panels for between £15 and £20 each and even had someone come round to my house to collect another two that I'd not had time to unscrew from the wooden frame before the sale.  I'd also sold some of the LED lights that were part of the kits and two of the charge controllers.

Some kids were amazed by solar power, asking where the electricity was coming from and exclaiming "Cool!!" when I showed them that it was coming from the sun, making the fan go slower or faster by having them stand in front of the panel so that they cast a shadow on it.

Hopefully these kids will have been as impressed as I was as a kid when I was given a solar power electronic experiment kit from Tandy (remember those "10 in1" and "30 in 1" experiment kits?).

My other secret plan was to get adults hooked on solar power.  As I've discovered, collecting solar power is addictive.  You start off with one or two small panels and before you know it you've got loads of the things and are making serious amounts of power.  By selling off these seeds at a low price, I hope that some more people will experiment with solar power and eventually take the plunge and get a full blown system.

Wednesday, June 2, 2010

Made in Japan (part 3)

Back in November 2008, while out sightseeing at the dammed lake reservoir near Miharu, we stopped at a hotel spa place that has a very good tofu cafe that sells all sorts of yummy food entirely (almost) made from tofu or derivatives (even the donuts!).

Anyway, while walking about I noticed that somehow they had also built an observatory (it gets very dark up here in the mountains away from any town).  

Next to the observatory was what at first glance appeared to be a bus shelter (err.. because it had buses parked under it).  In fact it was a sizeable solar array!

It consists of 8 rows of 14 x 167W 48 cell (23.2V MPP) Kyocera panels (I could read the info plates on the undersides).  From the date stamps on the panels it seems to have been in operation since 2002.  The nominal output is 18.7kW - maybe enough to offset most of the power used by the small hotel, observatory and cafe.

The electronics were all in a sort of phone box shaped hut next to the back row.

Travelling from the remote town of Takayama through the mountains to Matsumoto I saw quite a few new looking home based PV systems but many more old looking solar hot water systems.  These types of evacuated tube or flat plate collectors have been popular for decades with the Japanese.