Can the 10 percent rule apply?

[fireofenergy]

Especially if the battery is just a 33Ah sla hooked up to a little 30 watt panel (about just 1.5A)...

[Mike90250]

Which 10% rule ? For recharge ? Yes, still applies.

Otherwise, your battery sits at low voltage too long, and sulphates. Eventually, it will recharge, but you have lost some capacity which will never be recovered.

The charge controller prevents overcharge.

If you are just maintaining the battery, and not cycling, you could use the 5% rule?

[fireofenergy]

Sorry so late, yes I meant (in this case) about a 5% rule. I figure since the sun ain't shining all the time, lead acids (and nimh) can handle it. I would not want to let any battery discharge too low, and believe that the less discharged, the longer they last, even more so than just the total of accumulated amps in and amps out.
Thanks, I feel better about "just getting a bigger battery" instead of the "expensive charge controller". However, I will have to monitor the light... Yep, the load will be four Cree XML's in series which drop 3.2v max, and about 2.9 to 3.1v here after a 12v low drop out regulator is applied (unless drop out voltage is more than .5v). This will insure consistency and will make it last longer per charge than trying to use an inline resister.

It will also insure a gradual discharge, since, as the leds use just slightly less voltage, they dim greatly, and thus use ever less current. There should still be still enough light after a few days to still read by. Problem, though, if the user forgets the light is on (for like a month), the leds will dim to like 2.5v, obviously not good.

Now, what's nice about the lead acid, in comparison to the lithium iron (in this case), is that there is an advantage to NOT having "the flat and good" discharge curve of the lithium iron. Lead acids warn about impending darkness long before it happens (with leds). The Lithium iron "waits till the last minute" to give warning!

So, I guess I will trade in the light weight and extra cycles for the (much) greater capacity and let 'em all know that they must charge the lead acid ASAP (as they are told to do by everybody else, anyways). That's if they want the world's most efficient light.

If I can find a LDO regulating "an almost perfect" 3v each for FOUR XML's at 1.1A then the light output would be 1320 lumens with 80-CRI and warm white, 1560 lumens with neutral white and 1680 using cool white. That's less than 14 watts! Again, there's a problem. The specs say 12v +or- .3v... That little bit equates to almost a doubling in range of lumens! Here's a visual explanation

xmlgraph.jpg

http://www.cree.com/products/pdf/XLampXM-L.pdf ...

Hopefully, I will find some adjustable LDO regulator...

I could use a driver designed for leds, but that would cost $20 more, reduce the light by one whole led... AND let the lead acid discharge even faster!

[Sunking]

I meant (in this case) about a 5% rule. I figure since the sun ain't shining all the time, lead acids (and nimh) can handle it.

You got a few things mixed up and overlooking the obvious.

For NiMh and NiCd they can be charged as high as the C4 rate or 400%. For example if you have a 10 AH NiMh in some cases you can charge them at 40 amps or the 15 minute rate. So you cannot compare the nickel chemistry to lead acid.

What you are over looking is with any design of RE applications you have to replace what you use daily under worse case scenario. By that I mean accounting for system losses, highest usage, and the shortest Sun Hour days. If you design to that specification the battery charge currents take care of themselves and only leaves you with a couple of sanity checks to make sure you do not exceed the maximum and minimum charge rates of the battery.

Let's use two extreme examples. We want a system where we use 1 Kwh per day. One system in Seattle WS, and the other in Tuscon AZ. We want it to be a 12 volt battery system.

For both systems th ebattery capacity are the same of 5 Kwh or 5 Kwh / 12 volts = 416 Amp Hours.

What is going to be radically different is the panel wattage and charge controller sizes.

For Tuscon in December we receive 5.6 Sun Hours. So the panel wattage required assuming a MPPT controller is [1Kwh x 2] / 5.6 Sun Hour = 357 watts, round up to 400 watts. The max charge current using 400 watt panel and a MPPT controller = 400 watts / 12 volts = 33.3 amps. At 33.3 amps to a 416 AH battery is a 416 AH / 33.3 A = C/12 charge rate or 8.3%. C/12 rate meets the bare minimum charge rate of a FLA battery. For FLA batteries the minimum recommended charge rate is C/12, and maximum is C/8. So Tuscon pass all sanity checks and good to go.

Seattle is a problem because they only receive 1.2 Sun Hours in December, ouch this is going to hurt. Panel wattage required is [1 Kwh x 2 ] / 1.2 Sun Hours = 1666 watts round up to 1700 watts. The max charge current is 1700 watts / 12 volts = 142 amps. This gives you a significant problems.

1. it means you will have to use 2 very expensive 80 amp charge controllers with each CC having 850 watts of panel input. Essentially you would have two 850 watt systems feeding a common battery.

2. 142 amps on a 416 AH battery is a C/2.9 charge rate. That far exceeds what a 416 AH FLA battery can handle. So you have to make some changes to the battery plan. Basically you need a much larger battery than required and there are two solutions.

a. Select a FLA battery where 142 amps = the max charge rate of C/8. So that means a 8 x 142 = 1136 AH 12 volt battery minimum to handle 146 charge amps.

b. Use a AGM battery because the max charge rate on an AGM is C/4. So with 142 charge amps means you need a 4 x 142 amps = 568 AH @ 12 volts

Battery option be is the right answer for the Seattle location.

So what does this all mean? Get the hell out of Seattle and move if you want solar or go broke.

[fireofenergy]

Right now, it's snowing, off and on for days! (in the California mountains). Anyways, we all know solar doesn't work for Washington.
I'm interested in just enough to power lights at this time and therefore need a much smaller lead acid battery. I will use a single 30 watt panel to charge about a 30Ah sla. That's about a C/20 rate. I think you said that's alright, but the battery will probably come from ebay (since I don't want to waste gas going "down the hill"). So it's probably not alright.
So, to me, the obvious is more in building the MOST efficient lighting (as above)... and...

Eventually, "civilization" must either robotically mass produce more than 100,000 sq mi of (the best kind of) solar arrays AND a global network of powerlines... Or reconsider closed cycle nuclear (where water is NOT used for core cooling).

[Sunking]

I'm interested in just enough to power lights at this time and therefore need a much smaller lead acid battery. I will use a single 30 watt panel to charge about a 30Ah sla.

Minimum charge rates do not apply to SLA batteries, only flooded lead acid. With SLA you only have to worry about max charge current. More on that in a second.

I assume you have a 12 volt battery and PWM controller? If correct just look at the panel specs and see what the Imp spec is. That will be the max charge current because on a PWM controller input current = output current. So if this is a standard 12 volt panel the current should be around 1.8 amps. If using a MPPT controller around 2.5 amps.

Back to SLA batteries. Is yours AGM or Gel? If Gel you might have a problem on your hands. Max charge current on a Gel is C/20, and they use a lower charge voltage.

[fireofenergy]

I haven't bought the battery yet. The one I was thinking about just happens to be an AGM.
Thanks for the tip!

[Sunking]

I haven't bought the battery yet. The one I was thinking about just happens to be an AGM.

Then you really have nothing to worry about with AGM because the max is C/4.