AGM or GEL Batteries? Which is best?

[SunEagle]

Thanks SunEagle. This is all very educational. I'm curious about controllers as I move forward with several options. Is there a minimum and maximum range of battery Ah size and Solar panel wattage as far as the amp rating of a controller? For example will a 40a controller be just as efficient on a 300Ah battery with 400 watts of solar, as it would be with a 500Ah battery and 600 watts of solar? Or does it need to operate in a narrow range of Battery amp hours and solar wattage input?

Well a quick rule of thumb is to take the total wattage of panels and divided that number by the battery voltage. That will provide the maximum amount of amps that can be used to charge the battery as well as provide a CC amp rating. But in real life the total wattage is really less and the charging voltage is more than the battery rating. So the total amount of charging amps is less.

As long as you stay between a C/8 and C/12 charge rate for a FLA battery you should be ok. But a faster rate can cook the battery and a slower rate will cause the plates to sulfate up.

[Arlou52]

As long as you stay between a C/8 and C/12 charge rate for a FLA battery you should be ok. But a faster rate can cook the battery and a slower rate will cause the plates to sulfate up.

Can you explain what C/8 and C/12 charge rate means to us who are not familiar with it? I want to be able to calculate it since it appears important but have no idea what it means. Thanks

[Mike90250]

C = Capacity
C/10 = 10% of capacity A 200ah battery charging at a C/10 rate would be charging at 20A C/8 = 25A C/12 = 16.6A

Too low of rate, and the battery never completes the charge, and tall case batteries don't get stirred up enough from bubbling and stratify. (acid sinks to bottom, water on top)
Too high of rate, and heat starts damaging the battery

[Mike90250]

Based on everything I have heard, if someone needed 100Ah of battery usage everyday he would need a 200Ah amp AGM battery bank because he can’t go lower than 50% DOD. To fully recharge every day he would need about 500 watts of solar panels and a 40 amp charge controller to recharge the 100 Ah he used.
One guy has a different approach. He thinks you should use the 500 watts of solar panels and a 40 amp charge controller to recharge the 100 amps used every day, but double the size of the battery bank to 400Ah so your DOD would only be 25%. Would this be good or bad for the batteries? Would it give them a longer life? He says it also gives you a cushion of 100 Ah for a cloudy day when you can’t fully recharge after using 100Ah the previous day and need to use another 100Ah today. This would then be 50%DOD for the 400 Ah battery bank. Any down side to doing this if it takes an extra day or two to get back to full charge?

When you increase the battery size, you need to increase the charge source (PV panels) so that after a couple cloudy days, you are able to recharge before the batteries begin to sulfate.

From research I've seen, more than 24 hours below 80% full, and lead acid begins to sulfate.

[Sunking]

Well if using a 48 volt battery requires 1500 watts of solar (30 amps x 50 volts = 1500 watts,) then does 30 amps x 12 volts = just 360 watts of solar? The math seems like it's better to use a 12 volt battery and just have 4, 100 watt panels.

Huh?

A 1500 Watt Panel System operating into a 48 volt battery requires 1500 watts / 48 volts = 31., just call it 30 amps. Put another way a a single $250 controller and some 10 AWG wire. 1500 watts into a 12 volt battery requires 120 amps. That is two very expensive $500 60-amp controllers, much larger and a lot more expensive wiring.

As for panels if you use 12 volt battery panels are a lot more expensive than Grid Tied Panels. You are much better off using 200 to 250 watt panel sand less hardware.

12 volts system are toys, more expensive, and far more dangerous.

[Sunking]

Sageman if you make $10 per hour how many hours do you have to work to earn $100? You just did algebra and how to do all the math spoken here.

Watt Hours = Watts x Hours.

Solar has limits and here is one you have missed. About the largest controller you can buy is 80 amps. A few 100's but that is playing with fire. So lets just stick with 80 amps. This will turn on a few lights for you. A 80 amp MPPT controller today can be used with a 12 up to 96 volts in some models. Thi sdetermines the maximum panel wattage you can use based on battery voltage.

12 volt battery: 1000 watts
24 volt battery: 2000 watts
48 volt battery: 4000 watts
96 volt batteries: 8000 watts.

Do the math.

Watts = Voltage x Amps. 12 volts x 80 amps = Whatt
Amps = Watts / Voltage. 1000 watts / 12 volts = ???

So you have a 500 AH battery. How many Watt Hours is that?

You cannot answer because you did not specify the Battery Voltage. Battery math is the same except now we add the TIME element, Hours.

Watt Hours = Amps Hours x Voltage.

So how many watt hours does a 500 AH battery have?

12 volts x 500 Amp Hours = 6000 watt hours
24 volts x 500 AH = 12,000 WH
48 V x 500 AH = 24,000 WH
96 V x 500 AH = 48,000 WH

So Sagerman, if you do not understand the math, guess what is going to happen. Answer. salesmen and Green Mafia will teach you their way.

[Arlou52]

C = Capacity C/10 = 10% of capacity A 200ah battery charging at a C/10 rate would be charging at 20A C/8 = 25A C/12 = 16.6A
Too low of rate, and the battery never completes the charge, and tall case batteries don't get stirred up enough from bubbling and stratify. (acid sinks to bottom, water on top)Too high of rate, and heat starts damaging the battery

So is it better to charge at C8 to charge faster and stir up the acid? Is C8 too high for a daily charging rate? Which rate from C8 to C12 is better for the battery health and long life?

[Mike90250]

If you can do c12 for enough hours to recharge, great. But there is not enough sun to accomplish that, so you have to have a higher rate, but no more than c8
It's all a balancing act !


And you can use c4 to elevate your panels (boom)

[chrisski]

So, what would be a good book to get on choosing batteries for off grid solar?

I’m picking a lot off posts like this. I wish the battery manufacturers would publish things like recommended charge rate of C/8 to C/10, but I can’t seem to find that info; I looked on trojan’s site but didn’t see that. Rules of thumb like 150 watts of panels to 100 amp hours of 12 volt batteries seem like good rules of thumb, but I’d like to get hands on something. Internet searches are leading me back to posts like this one. I’ve looked at books on Kindle unlimited, but its not quite the depth I’m looking for.

[PNjunction]

Heh, back to post #1. Stop guessing or listening to so-called preppers. 90% of the time it's just to garner "subscribers" to their blog or video to get enough hits so that manufacturers will give them product for free. The rest of the time, their advice is usually worthless, or in many cases, totally unsafe.

So - stop guessing. Before investing in batteries, panels, and gear, actually measure that fridge for 36-48 hours with a P3-International "kill-a-watt" inline meter to see how many watts are being pulled from the wall first! THEN you can make a better educated purchasing decision.

The fact that these guys are trying to run fridges of ac/inverters, rather than investing in a dc-powered fridge is beyond me. Doing that is about 30% inefficiency down the drain.

Chrisski: the rule of thumb rarely takes into account the real-world, and is generally napkin-calc. The GOAL is to do your "power budget" first (how much current you draw over time needed - or if powering solely by inverters, convert everything into watts / watthours)

Choose your battery capacity according to your need. Now you can plan your solar array appropriately. Instead of winging it, also look at the solar-insolation hours for your area, and plan your system around WINTER hours. Then add a day, maybe 2 or 3 depending on how much headroom of outage you need.

Note that planning your system requires you to know if you are totally off-grid 24/7, (daily cyclic duty), or a weekend-warrior type (intermediate), or standby/emergency. This, along with your solar insolation hours, will help determine not only the capacity of your batts, but perhaps also your chemistry.

Anyway, I'm kind of dismayed at how many throw money at the wall, without investing in the basic tools to do their power-buget themselves with current-meters and wattmeters first. Do you have a clamp-on ammeter at the very least? Get one of these before you go bigtime with major loads.