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Also interesting. I have had a 10W panel connected to a 7Ah battery with the identical charge controller and phone set up out in a farm paddock in Australia for 3 months now and it has been working like a champion. Even though it was over the winter with lots of overcast conditions. Here is some photos. So your assessment of a 25W panel2012-07-16-592.jpg2012-07-16-594.jpg is interesting given my experience.
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Because I am familar with that battery, or I should say the series of AGM battery an dit is recommended to charge at C/4 rate where C = the AH capacity so 5 AH / 4 H = 1.25 Amps. You can go a bit lower than that but I would shoot for 1 amp which will take a 20 watt panel.
You say this is used to charge a phone? What is the setup? Are you using an inverter or DC converter?MSEE, PEComment
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Ah. Thanks. Didn't know that. That is the kind of knowledge I would like to have had when selecting a battery. Where can you find the C values for a particular battery?
BTW, the battery that is out there now is a SLA-12V7-F2 - 7.2Ah. If that is also a C/4, same thing. Interesting that the 10W panel is holding up.
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When the battery says 12V7.5Ah/20HR, what does the 20HR mean?Comment
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I'm back. I went out and bought a new 10W panel to replace the 5W one that did not seem to be doing much. This is what I have had out in the paddock for months without a problem. The vendor also sent me a new Morningstar Sunsaver Charge controller to replace the suspect unit, although I had the 6L model and they sent me the 6 model, the difference being that the latter doesn't have the low voltage disconnect. Not to worry. Hopefully I will never have that problem if the panel and battery are doing their job.
So I hooked it all up, the difference being this time that I also bought a Watts Up meter to measure the amount of power the load is actually using over a period.
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This is a great device. I fully charged both the 5Ah lead acid battery and the phone's Li-Ion battery and put the whole thing up outside for 2 days to stabilize. Then I hooked up the Watts Up meter last night to see what the phone was using over time. It is a little early - I probably need to let this run for a few days - but the readings after 16.3 hours early in the morning were as follows: 0A, 12.68W, 0W. 4.8Wh, 0.24Ap, 0Vm, 3.0Wp, 0.409Ah. So basically:
- The phone is using next to no power at the moment
- The battery seems okay (I will check the voltage again once the sun hits the panel)
- Peak draw was 240mA / 3 Watts. This was when I first connected the load.
- Energy used was 4.8Wh. I thought it interesting that if you divide this by the 0.409Ah you only get 11.75V.
I guess this means that over 24 hours I might see around 7.1Wh, but I will only know once I average it over a few days. If that's the case, the 10W panel should be more than enough, providing a days worth of power usage in an hour of full sun. Or at least, that's the theory right? I have measured the short circuit current of the panel and it is around 550mA. I will be interested to see what the voltage gets up to in full sun today, which was where I started.
Ultimately I need to hook the Watts Up between the panel and battery over time as well and compare the two.Comment
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Less than an hour later at 10:30am, full sun had hit the panel. The first thing I wanted to do was check the voltage across the load, which is where I had problems with the 5W panel. This time it had jumped up to 13.64V (Unlike the 12.4V of the 5W panel). So we are back in business. Other elapsed readings for the load rose to 5.1Wh and 0.431Ah.
Of course, the old charge controller was suspected to be bad, so I still need to isolate that factor out with the 5W panel, just to be sure. In "theory", the 5W panel should have been enough...Comment
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Remember that even totally honestly rated panels use Standard conditions (STC) which are usually not met except on ideal days is clear weather. A rule of thumb to convert that to more realistic numbers (PTC) as used on PVWatts is to multiply by .8. Then look at whether you need to add in additional corrections for not aiming the panel close enough to the sun angle, etc. and you 5 watt panel could end up being a 3 watt panel or less.
Also, the 5 watt nominal output is the product of Imp and Vmp. But when charging a battery with a PWM controller you will usually be pulling the panel output voltage down well below Vmp, reducing the power output accordingly.
"In theory, there is no difference between theory and practice. In practice, however...."SunnyBoy 3000 US, 18 BP Solar 175B panels.Comment
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Exactly.
The assumption I made, which was probably invalid, was that if the phone drew X amount of energy over 24 hours and the battery stored 10X of energy and the panel on average would supply 3X of energy in a day, then I should be okay with that panel. What I didn't account for was current. When the phone was drawing current (280mA), it was basically the same current that the panel would supply, so the SLA battery was not being charged. And that was in full sun. If the phone was not connected, over time, the SLA battery would probably charge, but in any kind of cloud obscured sun, the panel current would drop to 50mA or less. Hardly a charge. My mistake was multiplying poor sun condition current by time and then figuring this would result in enough watt hours over a day to meet the phone requirements. Invalid assumption. Going up to the 10W panel meant that even when the phone was drawing power, there was still a good excess amount of current going into the battery. Plus the battery voltage was not sagging. That was another big problem with a marginal charging setup. I am fairly confident that the 10W setup is going to work with the 5Ah battery, as it has been working for 100+ days non stop on the current prototype (7.2Ah battery) I have installed.
Part of the learning experience was that mobile phones have a variable charging system that is hard to predict. One minute it is nothing, the next 280mA, then 140, then back up etc. I found if the panel can supply well in excess of the peak current, then it is more likely to be a trouble free system.Comment
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It is a very nice instrument - however mine invariably reads 0.3 or 0.6v lower than my Fluke 87V and a cheaper voltmeter at the battery terminals. So if you are making critical voltage adjustments based solely on the Watts Up, you may want to doublecheck with another meter. Either my unit is faulty, or behind internal diodes that are dropping voltage by .3 or .6 depending, or I have some lossy connections.
I guess what I'm trying to say is allow for some error-tolerance in the overall readings. Still a great unit that I use a lot for many projects.Comment
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This morning the Watts Up meter was measuring 13.58V across the load. My Mastech digital multimeter measured 13.66V across the same load. So I am only seeing 0.08V difference.Comment
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Preliminary results are in
I fully charged the 5Ah SLA battery as well as the mobile phone's Li-Ion battery. I then connected up the panel, charge controller, SLA battery and phone and left them all running for 2 days. I then used the Watts Up meter to measure the energy used by the phone over a 2 day period, then measured the energy coming from the panel over another 24 hour period. Results:
Average energy used by phone in 24 hours: 7Wh
Average energy provided by the 10W panel in 24 hours (Sunny weather): 15Wh
Conclusions: 1) The 5W panel was never going to cut it. 2) The 10W panel is sufficient in good weather.
Questions to be answered: 1) What will the 10W panel provide on a dark, cloudy day. 2) What will be the energy provided by the panel in full sun with a depleted battery.Comment
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Found my problem with the watts up - cold solder joint on an external switch I was using.
On a dark, cloudy day when you can just barely discern the edge of a shadow on anything, expect about 1 watt / 1 percent. Other panel-killers are things like wet-leaves, bird droppings, shadows of course and general dirt buildup. Even something as small as a tree limb shadow can be a concern.
On a perfect day into your discharged battery, the panel will put out 10 watts until the the system goes into absorb. From your 2-day data, looks like you are getting about 7.5 w daily, and that looks to be around a reasonable 70% efficiency when overall losses are taken into account.
Things are good on a daily basis, but if you want more days of autonomy to deal with bad weather, the system will start to grow...Comment
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