Remote 40W 12V PV - bypass diodes blew up - what did we do wrong?

Alternative Technology Assn Forums » Energy

Remote 40W 12V PV - bypass diodes blew up - what did we do wrong?

(20 posts) (5 voices)

Started 10 years ago by karmacoma
Latest reply from karmacoma

Tags:

No tags yet.

karmacoma
Member

Maybe this thread could be titled "Hi my name is Ian and I'm here because I stuffed up". I thought that might be a bit obscure, though.

Anyhow, I kitted my long-time partner up with a new 40W 12V PV panel and an old Plasmatronics PR1206 regulator I had, the regulator all nicely mounted with a digital voltmeter and terminal blocks for LED lights. She went to Solomon Islands, and had to buy a battery there. Bad place to buy batteries, she ended up with a 70Ah car battery, best she could do. Turned out it was flat, read 12.4V, wasn't charging much. So over the phone, I said well, just connect the panel to it direct for a few hours. She did, and immediately the two 10A10 bypass diodes in the panel burnt.

- So what did I do wrong? Why can't you connect the panel direct? I never heard about that. Or did she reverse the terminals? By the way, we aren't arguing whose fault it was, we're too sorry about it for that.

- I've googled 10A10 diodes, but can't find them in Australia -- any ideas where to get them mail order?

Posted Tuesday 13 Sep 2011 @ 9:08:36 am from IP #
rockabye
Member

Connecting a solar module direct to a battery should be fine. Only thing likely is reverse polarity has caused this.

This why a fuse is good practice when hooking things up. A 5 amp fuse may have saved it.

If the battery was reading 12.4V with no charge it wasn't flat.

Have a look here.

http://homepower.com/article/?file=HP109_pg106_Letters_5

Posted Tuesday 13 Sep 2011 @ 9:28:10 am from IP #
karmacoma
Member

Reversed the polarity - that would've been Vaelyn -- I guess you have to blow stuff up as part of the learning process. I'm sure she'll never get it wrong again.

You say it wasn't flat anyhow?? So what should we take as a guide for satisfactory voltage if the battery is just sitting there, not loaded and not being charged? I thought 13.8V?

And now I have to try and get spare-part 10A10 diodes to her. Any ideas where to get them from? (Ebay from Hong Kong a possibility but will take ages to deliver).

Posted Tuesday 13 Sep 2011 @ 9:37:09 am from IP #
rockabye
Member

A fully charged lead acid battery at rest for at least 6 hours will read about 12.5 - 12.7V. When connected to a charger most regulators switch off the charging current at around 14.2V for flooded cells with boost/equalisation charges up to 15V every week or so to improve performance.

Sealed batteries are usually kept to a range of 13.8V - 14.2V to prevent damage.

Try Jaycar or Altronics stores for the diodes AND a fuse.

Posted Tuesday 13 Sep 2011 @ 9:47:50 am from IP #
Ueee
Member

Try ebay for the diodes, you can get them out of china pretty cheap. I've never had problems with getting components from china and would recommend it. try http://www.ebay.com.au/itm/20pcs-10A10-10-Amp-1000V-10A-1KV-Axial-Rectifier-Diode-/250784740575?pt=LH_DefaultDomain_0&hash=item3a63ef74df
Jaycar, Futerlec and altronics don't sell anything that big by the looks of it, the biggest is 6amp but that would probably do.
Otherwise try searching for a 10amp 1000v power diode.

Good luck

Posted Tuesday 13 Sep 2011 @ 10:08:14 am from IP #
karmacoma
Member

Ok, so we just aim for steady-state 12.5V to 12.7V, and not expect that voltage will be higher except when charging.

But then, what do I tell her as a threshold X volts below which not use the system (its only a car battery, we don't want to deep cycle it). Like if it's been raining for 3 days, there's got to be a point where they have to say ok, no more use until it gets up to Y volts on charging. But what should X volts and Y volts be, if you see what I mean?

PS: These 10A10 diodes are really hard to find. Not Jaycar, Altronics, Element14 or rs-components. Maybe they go under a different code here. And yeah, a fuse. Hmmm.

Posted Tuesday 13 Sep 2011 @ 10:18:41 am from IP #
karmacoma
Member

Ueee, thanks yeah I was looking at those Chinese ebay sellers. I think they'd be ok to buy from, its just that their stated delivery times are very slow. Any other 10A 1000V power diode: I like that idea in a way, but if I send what turns out to be the wrong type, the other half will be very, very displeased with me.

Posted Tuesday 13 Sep 2011 @ 10:23:34 am from IP #
rockabye
Member

You can fit a low battery cut off device to prevent over discharge and they are usually set to disconnect the load at around 11.8V. As it is a car battery it will likely have a short life as most people drain them too much and just don't understand the consequences.

There is a lot of info on the web about this. I recommend visiting your library and grabbing a few books as well.

Battery voltage chart on this page.

http://www.freesunpower.com/meters.php

Posted Tuesday 13 Sep 2011 @ 10:40:23 am from IP #
karmacoma
Member

rockabye, I just read that Homepower letter you linked. Very pertinent. It is rather interesting that it suggests maybe just getting rid of the bypass diodes. I wonder if that is what we should do, at least in the short term -- as I thought the problem was that without them, it is possible to cook the panel. I didn't connect a blocking diode because on the 40W panel it slashes voltage too much (now that was a Homepower tip)

I am rather afraid that it wasn't just the diodes that blew, but possible some of the ribbon conductor to the cells was destroyed too. At least that's what I imagine might have happened. Vaelyn tells me there was smoke. Guess we won't know how the panel is until she can deal with the burnt-out diodes, either by replacing them or removing them. What do you think about just removing them?

Yes I had tried to find basic stuff about cut-off voltages etc, specifically by trawling the Howmepower site. I didn't find anything that really told be a figure to aim at. I am concerned too that car batteries don't last, that is why I want to get the right voltage value to tell Vaelyn, that would correspond to a 5% discharge. Any more will stuff the battery too quickly to make it a worthwhile commodity - even car batteries are over-expensive in Solomons and nobody there sells deep cycle batteries (though there are people passing off little UPS batteries at hiked prices). If she can get it right it is possible that other people will begin to learn the idea and get their batteries to last a bit longer. Problem is, I don't know theory for how to calculate out myself an appropriate cut-off voltage. Just like her, someone just needs to give me a number to parrot (at this stage we'll just go around saying "11.8 V").

Posted Tuesday 13 Sep 2011 @ 11:14:45 am from IP #
me3neuralfibrecom
Member

Does she have a multimeter?
I would test the voltage output of the panel to see if it's alive and confirm polarity.

Paul

Posted Tuesday 13 Sep 2011 @ 11:19:21 am from IP #
karmacoma
Member

Paul, yeah, she doesn't have one, but there's someone else there that does have one. I just don't really recall what is inside the junction box on the 40W panel -- it has the two bypass diodes, so I guess that it is really "two 6V panels joined together" in a manner of speaking, with three legs -- maybe a +ve on each side and a central -ve, is that right? Can't find a junction box circuit on google either, to see.

If so, they'd meter across one of the outer terminals and the central terminal?

Posted Tuesday 13 Sep 2011 @ 11:29:29 am from IP #
Lance
Key Master

For a single 12 volt panel the bypass diodes are not needed and won't do anything useful, they can simply be removed. They are only used for higher voltage series strings when one panel becomes shaded or fails so that the rest of the array can continue to produce output, but for a single panel on a 12 volt battery they are not needed.

The output of the panel will be across the two outer terminals that the diodes are connected to, the positive of the panel will be the one which has the silver line on the diode closest to it, assuming those marks are still visible.

The simplest way to test a panel to make sure it isn't cactus is to put it in the sun and connect its output terminals together via some short bits of wire by trying to strike up an arc. It sounds crude and dangerous, but because you are outside in full sun your pupils will be very small and so you will not be at risk of any arc flash etc. Just make sure she is wearing glasses or sunnies to protect from any little bits of metal that might come off. Very unlikely, but theoretically possible.

Posted Tuesday 13 Sep 2011 @ 12:17:29 pm from IP #
karmacoma
Member

Lance, that's great advice - I was just reading a Homepower article on bypass diodes too, that supports what you say. And that significant letter from Thailand that rockabye linked, really makes me wonder then if they shouldn't just be routinely removed for small PVs going into rural villages. So I'll get her to cut the diodes out and bridge the gaps with bits of terminal strip instead (she has no soldering iron).

Certainly easier to put sunnies on and short the panel to see if there's a spark, than hunting for a multimeter. Love a low-tech approach. God I just hope the panel is still ok.

Posted Tuesday 13 Sep 2011 @ 12:38:23 pm from IP #
rockabye
Member

Do not do that as shorting the gap effectively stops the cell working. Just remove the damaged diodes and test the output with a meter. In full sun you should measure around 20V when not connected to the battery.

If ok then connect the module to the battery correctly via a fuse and see if the battery voltage rises. If so it is charging. Hopefully the battery voltage will eventually exceed 14V indicating it is fully charged.

Obtaining a meter is essential if you want to have any chance of making your system last more than a few weeks. Jaycar are selling digital meters for $10 this month.

Posted Tuesday 13 Sep 2011 @ 8:12:48 pm from IP #
karmacoma
Member

rockabye, I am very glad you keep responding with advice, I'm all ears (eyes?). Ok so she will cut out the diodes, and NOT bridge the gaps so left.

What did you think about briefly just touching the output leads together, to see if we get a spark (life) or not (already dead).

If she can get hold of that multimeter (and its owner, who has had some basic electrical training) they can look for the 20V output: and that's just the multimeter set to VDC, not as a shunt circuit across a load, but acting as the 'load' itself, right? (See how much practical I know -- appallingly little).

And anyway, its the battery voltage rising we all want to see. I had missed your link to that table of battery voltages vs capacity earlier last night, but now have it and will treat it as holy grail.

Maybe I can get a multimeter across to her somehow, but the postal service is insecure (for those who even do have a postal address) and encumbered with leaden customs demands. The regulator panel I made for her at least has a digital voltmeter on it, which is working (soldered in though, so unable to do the open voltage check as per above). So she can, and wants to, keep an hawk eye on battery voltage.

Posted Tuesday 13 Sep 2011 @ 8:45:58 pm from IP #
rockabye
Member

Yes you can do as Lance suggested and see if there is any spark by briefly touching the cables together. If you do get access to a meter that has a 10 Amp range you can also test the output current by connecting the module leads directly to the meter and measuring the AMPs.

A 40W module will deliver 2.5AMPs in full sun. You may see slightly more as you are measuring the short circuit current. Do not connect a meter in the 10A position across the battery terminals as it will short out the battery and likely melt the meter cables.

The best solution is to connect the module back via the regulator panel as it has a meter in it. Watch to see if the voltage is rising as discussed. As it is a car battery I would stop using it as soon as the voltage falls to 12.0V worst case.

Posted Tuesday 13 Sep 2011 @ 9:19:06 pm from IP #
karmacoma
Member

Ok, here's the upshot. We just did a series of phone calls from Solomons village to me back at base in Canberra, where a lot of the thinking that evolved in this thread was applied. The bypass diodes were cut away, all the soot cleaned off, and the spark test applied. It worked! The panel was connected back to the regulator, it is is sunny morning over there and the battery charging voltage is slowly increasing. Success. The small village crowd that had gathered around this sick patient of a PV panel has all learned a new trick. Thanks fellas, that was a real contribution to overseas technology transfer.

I've actually advised them 12.6V discharge cut-off, based on that table rockabye linked, and the fact it is only a car battery, and the load is 6 LED lights with total power consumption 7.7W; daily power budget about 3Ah, or about 5% of the battery's 70Ah. capacity.

Now I'm so impressed with this forum, (also noticed everyone can spell properly = educated and smart) I'll be back at some point, with more questions.

Posted Tuesday 13 Sep 2011 @ 11:23:36 pm from IP #
rockabye
Member

Good news Ian. You may find 12.6V a little too conservative under load ie with lights on but your on the right track. With your inbuilt meter you should be able to keep the battery alive for a lot longer.

The golden rule is: If you can't measure it, you can't manage it.

Posted Wednesday 14 Sep 2011 @ 12:19:48 am from IP #
Lance
Key Master

Yes, I would say 12.6 volts to be too conservative, the terminal voltage versus % capacity varies depending on temperature, battery condition and age, and battery chemistry (some batteries have various additives in their plates that affect term voltage slightly) so I would go for something a bit lower like 12.2 under your small load. But, given good sun and the small loads mentioned, I would suspect that you will never take much capacity from the battery.

Posted Wednesday 14 Sep 2011 @ 1:38:06 am from IP #
karmacoma
Member

Nice, ok, I will not read too much of a riot act on 12.6V -- we'd say maybe it is notionally the ideal, but if it goes down to 12.4 or sometimes 12.2 then so be it.

The battery cost SBD1600 (AUD215 or so), so over its life it needs to amortise its cost in the cost of the kerosene it replaces. If it can do that, it is good; and if it can't (i.e. kerosene would have been cheaper) then it is not such good family economics and strictly speaking would not justify the expense of the equipment. Hence looking for decent battery life despite being the wrong type of battery.

Having said that, one really nice thing about solar is that Vaelyn can keep her phone charged and so can run a mobile phone 'top-up' service, as a reseller of mobile phone credit -- which is good village micro-business.

Posted Wednesday 14 Sep 2011 @ 4:04:22 am from IP #

RSS feed for this topic