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electrical:solar:gentle_complexity [2017/11/09 18:56]
frater_secessus [Gentle introduction, Pt 2] |
electrical:solar:gentle_complexity [2020/10/11 19:48]
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| ====== Gentle introduction, Pt 2 ====== | |
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| This is a collection of illustrations that things are more complex than can be revealed in [[electrical:solar:gentle_intro|a one-page intro]]. Again, drill down for more info. | |
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| **Solar panel output will almost never be as high as the claimed output.** In fact it will probably hover around 75% of that number under the best conditions; this is called "[[electrical:solar:output#panel_temperature|derating]]", or correcting output to actual conditions. It sounds like fraud but it's not. A panel's power is expressed as a "nominal" rating like 100W as when tested under //laboratory// conditions. Since the roof of your van is unlikely to meet those same lab conditions, if you think you need 200W of panel you probably need 250W. | |
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| **//Usable// battery capacity is about half of nominal capacity.** This is due to a quirk of lead-acid chemistry batteries ([[electrical:12v:deep_cycle_battery#flooded_lead_acid|flooded]] or [[electrical:12v:deep_cycle_battery#agm|AGM]]). You can go lower than [[electrical:depth_of_discharge|the 50% mark]] but it generally leads to early (and $$$) [[electrical:batterycide|battery death]]. So if you think you need 100Ah of battery capacity you probably need 200Ah. | |
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| **PWM charge controllers may not be able to get even the derated power out of the panels**. This is because the panel makes maximum power for conditions((Pmax)) at a particular voltage((Vmp)) which is higher than the voltage PWM controllers can use.((Vbatt)) You can work around this issue by using an MPPT controller, but they typically cost 2-3x as much as PWM. | |
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| **[[electrical:solar:charge_controller|Charge controllers]] have specific current limitations**. Regarding current they are rated by Amps of output: 10A, 20A, 40A, 60A, etc. To protect the controller from damage their nominal output is derated by .80; this assures "headroom" for power spikes due to odd solar conditions. So a 10A controller is safely used at 8A, 20A controller at 16A, etc. If you will be pumping 10A from the panels a 10A controller may not hold up. A 15A or 20A controller may be required. [[electrical:solar:charge_controller#mppt|MPPT]] controllers may be able to cope with excess current. | |
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| Controllers also have an **input voltage limit** which you exceed at your wallet's peril. Going over this limit (usually by wiring too many [[electrical:solar:series_vs_parallel|panels in series]]) will usually kill the controller and void the warranty immediately. So don't do that. | |
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| **Vigorously charging a deep-cycled house battery using only solar is $$$**. But it takes a much smaller and cheaper setup to charge in combination with [[electrical:12v:alt_and_solar|alternator]], [[electrical:shore_power|shore power]], or [[electrical:generator|generator]]. | |
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| **12v systems are not really 12v**. We call them 12v (("nominal 12v")) but deep cycle batteries are over-discharged at 12.0V; they should probably never go lower than 12.2V. During some stages of [[electrical:12v:charging|smart charging]] they might be over 15v. It is very common to hold well over 14v for hours during the Absorption statge. | |
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| **Shade will clobber your panel output.** Even a tiny amount of [[electrical:solar:output#shade|shade]] will usually have a drastic effect, losing up to 90% of what you might otherwise harvest. This means no tree shade, no antenna shade, no roof vent shade, no roof rack shade, etc. | |
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| **Flooded lead-acid (FLA) batteries need to be "watered"**. They lose water as part of their normal charging and need to be checked and [[electrical:12v:deep_cycle_battery#flooded_lead_acid|replenished with distilled water]] regularly. | |
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| **[[http://rvwiki.mousetrap.net/doku.php?id=electrical:12v:deep_cycle_battery#agm|AGM]] lead-acid batteries are not well suited to solar-only charging**. They cost about 2x that of FLA, have less capacity, require vigourous charging and cannot be charged as high in voltage. They remain popular because people think "$$$ is better" and are loathe to vent their banks. | |
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| It is possible (though uncommon) to have **one all-purpose battery for both house and starting**. This only works when [[electrical:solar:sizing#your_daily_power_needs|daily power requirements]] are quite low((40-50Ah/day)) or when [[electrical:solar:nonessential|opportunity loads]] are agressively utilized. There are two approaches to such a setup: | |
| - the premium battery deep-cycling approach as used by Sternwake((http://www.cheaprvliving.com/forums/Thread-Has-anyone-heard-of-Northstar-AGM-batteries?pid=135811#pid135811)) This requires attention and knowledge. | |
| - the "make do" [[electrical:solar:shallow_cycling|shallow-cycling]] approach. This is more suitable for car dwellers and folks on limited resources. | |
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| ===== further reading ===== | |
| * [[https://wiki.mikrotik.com/wiki/Solar_Power_HOWTO|MikroTik Solar Power HOWTO]] | |
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