"A DC Fester in an AC spaceship?"
Figured a couple basic lessons already:
Lesson one: You need a bigger solar cell, than you think you do.
Lesson two: Your bigger solar cell than you think you need, will cost you an astonishing amount of money more than you originally even considered likely.
But the real clear lesson I'm learning is - battery capacity is actually rule number one.
I started out with a simple 8 amp-hour 12v battery. It charges slowly, but it charges, from a basic solar battery tender. So far so good.
However 8 amp-hours it turns out, really is only large enough to run some 12v lights full time with in a backup situation, or maybe an hour of browsing FR. Solar charged or any other way. After which, you're stranded in the dark, without any FR (which is the worst part!) until well into the following morning earliest.
Granted if the power's out, that's useful. Though, only just.
Lesson learned. So I got a larger capacity battery. Same manufacturer. Same voltage. 18 amp-hours.
The larger battery is actually useful. But I've also now got that perfectly good, yet smaller, too limited to do anything with, except run emergency lighting with, battery as well.
Rather than set up two backup systems as I go, one for lights (off the smaller battery) and a larger one for some more useful amount of power - I am wondering whether the two equivalent though different-capacity 12v batteries can be combined, for one additional step forward?
So: my greenhorn electrician question is: Can a 12v 8 amp-hour lead acid battery, be connected parallel to a 12v 18 amp-hour lead acid battery (safely) to in effect create one virtual single, larger 12v battery?
(both batteries, sealed lead acid)
Or am I likely to blow up both batteries, perhaps even blinding myself with battery acid in the process? Which would be inconvenient.
Thanks for any feedback! Cautionary posts will be given proportionally more weight when considering possible options. :)
Nothing to do with your charging system, but if limited on battery size for emergency DC lightning go with LED stuff. At an automobile parts retailer the other day I saw some nice (low current) LED sets, for cheap. Strips, buttons and individual lamps.
Do it. There is a lot of good info on solar power systems and battery banks. Be sure to check out youtube videos. Around these parts, they are used at fishing and hunting camps in place of noisey generators.
You can make a DC dimmer that helps prolong battery life.
"A DC Fester in an AC spaceship?"
How much money did you spend on these two batteries?
The best capacity for the $ is probably to go down to Wal-Mart and get a deep-discharge 12V battery. It can be an RV battery, a trolling motor battery, or any other deep discharge battery.
It will be about the size of a car battery and will be 80 to 100 Amp-hours for about $80.
If you want something that will provide emergency lighting, do what I do . . . . . use a generator!!
:-)
You typically do not want to do that with batteries of different sizes. Will it blow anything up? Probably not, and it’s probably *less* of a deal since batt types are lead acid. But it’s not what’s recommended. Usually you want to isolate each battery via the use of diodes.
Do some research on “dual-battery systems”...typically for boats.
I personally would be far more careful. Two batteries aren't going to be exactly the same - not now and certainly not later, as they age. Two things result from this:
None of that is good. You can somewhat alleviate the problem #1 by using diodes. But you will be losing a lot of power on them, especially if the current is high (the forward voltage on a Shottky diode will be around 0.3V.) At 8A you will lose 2.4W per diode. This is unwelcome in itself.
The problem #2 may result in bad batteries, quickly. Batteries are generally delicate devices; more so as they get more and more sophisticated. A typical notebook battery has an integrated charging controller inside. It's because they will catch on fire if overcharged (and it happens now and then.)
To begin with, the whole idea of storing energy in a battery is not that great. Some people already commented on that. A battery that cycles so often won't last long. Note that car batteries and UPS batteries are used lightly, and kept charged all the time - that's how they get some reasonable lifetime out of them. Cell phones and notebooks have this problem - the number of full cycles of a LiIon cell is around 500, and then it's dead.
There are solutions that work. The most effective one is to use the electric grid as your battery. When you produce you send the energy into the grid, and the meter spins backward, giving you a credit. You can use that credit later on.
But, for purposes of the argument, if you want to use several batteries then the best way to connect them is in series. This is because batteries are charged by current, not by voltage. Voltage varies between cells, but it doesn't matter until the differences grow to be too large. Your 12V battery already contains several cells connected in series, and if you connect another box of them also in series then it won't change a thing, from the POV of one cell.
On top of that you need to build a small computer that manages the battery (regardless of how large it is.) This controller has to measure the charge (in coulombs.), this is usually done with an integrator. The controller then figures out how much energy can be transferred into or out of the battery, and allows that to happen. Since the voltage of the battery changes as it charges or discharges, you need a voltage inverter (DC-DC or DC-AC) to regulate the output. You will end up with a good part of Prius electronics.
If you want to install a solar (PV) system the best way to do it is by using panels and inverter(s) that are commercially produced. If you simply want to experiment with things, this particular setup is not the best. Talk to your local hams instead, they will throw a bunch of far better ideas at you, and on top of that they will advise you what to do (and what not to do :-)
You definitely need moe charging capacity, for AGM batteries like you have, figure on ~2% self discharge per day - you'll need to replace that even if you don't use that battery at all so the 18a/hr battery will need to be 'fed' about .36a/hr daily to maintain its charge. Also figure on only drawing down 50% of the battery capacity for decent battery life, the 18a/hr would be able to provide 9a/hr or 96w/hr before needing to be fully charged. Also, avoid "deficit" charging where you never fully charge the battery from the previous days' use. That will cause sulphation on the plates effectively ruining the battery, particularly AGM's since they can't be equalized like FLA.
One lesson I learned was never attempt to do car repairs unless I was prepared to pay the dealership to fix what I screwed up........
I repair battery operated commercial floor equipment. Just a quick post before I head to work...
1. IMO...It would be better in your instance to build a battery bank consisting of 4 6v batteries hooked in SERIES which constructs a 24v system...much more efficient than 12v...and much more Amp hour storage capacity per charge cycle. 24 and 36 volt systems are prevalant in my business...and there’s a reason for that.
2. A typical Lead Acid battery system...properly maintained and properly charge cycled...will last around 400 charge cycles.
The batteries are going to try to charge and discharge at different rates even though the voltages are tied together.
The end result is going to be much shorter battery life, probably for both of them, certainly for one of them.
All commercial/industrial battery back-up systems depend on having identical batteries to warranty the system. Replacing a battery with anything other than an identical battery will void the warranty on all the batteries.
I learned the hard way why you should not connect two lead acid batteries in parallel. I own two boats, each with two batteries. I used to leave the battery switch on “Both” (which connected the batteries in parallel) in order to simultaneously charge both batteries while the motor ran. I had to replace anywhere between two and all four batteries every year. I finally learned that I needed to leave the batteries connected separately, and keep them charged with individual shore chargers. I haven’t had to replace a battery since then.
They use a different technology. The batteries have the acid absorbed in a felt tupe material that is spiral wound in between the lead sheets.
It will not leak, even if a bullet is fired into the battery. That was one test OPTIMA did during development. We had one set of OPTIMA batteries last almost 15 years, and many lasted 10 or more years.
Here’s a guy who sells solar powered generators... he’ll send a free DVD if you request one...
www.MySolarBackup.com
Phone number: 877-327-0365