Skip to comments.Any FR electricians? (12v DC Lead Acid Battery, Basic Q)
Posted on 02/22/2011 8:18:28 PM PST by Cringing Negativism Network
Hi a question for FR electricians please.
Probably a basic, dumb question, but hey - I don't know the answer and it might just be useful for other readers...
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. :)
Check out homepower.org
I’m not an electrician. Yes you can connect batteries in parallel or in series and it doesn’t cause explosions.
You won’t hurt a thing.
Join these 2 groups. The first is extremely friendly. The second not as much but you learn a lot. You will really enjoy the camaraderie of the first group.
As long as the two batteries are the same type and voltage, no problem, just parallel ‘em. Lots of alternatives to consider such as having separate charge controllers to provide redundancy, keeping one battery in reserve, etc. Sounds like you’re on the right track - good luck.
You can run them in parallel. And that setup will approach the sum of the amp hour ratings of the two individual batteries (18+8=26 amp hours total).
You will need to make sure that both batteries are in a similar state of charge before connecting them in parallel. Use a digital volt meter and get them as close to the same voltage as possible.
If you have one fully charged and the other fully drained, you’ll have a huge current spike as the drained battery takes massive current from the charged battery to reach equillibrium. This will cause a lot of heat, and the battery that is charging could be damaged badly (a shorted cell is very likely to develop).
Before you parallel the batteries, fully charge each one separately.
You’ll be fine with them in parallel.
Battery banks consist of many batteries connected in series to provide the correct voltage for a system. Typical system voltages are 12, 24, 48, & 120 Volts DC. These can consist of many individual batteries, either 12, 6 or 2 volts connected in series. Occasionally batteries will be connected with 2 (or 3) parallel strings to increase the overall capacity of a battery bank.
Yea, you’re ok. The bigger concern is whether you’re deep-cycling batteries that are not intended to be deep-cycled. If you are, you’ll get about 3 to 5 cycles out of them and then plates will start to disintegrate. That’s why you get about 2 chances to leave your car lights on before the third time kills the battery for good.
Yes, you can connect it - in parallel that is + to + and - to -. You will still have the same voltage but the combined amperage of the two batteries.
Will just take your solar panel longer to charge it though.
You are going to find out the same thing I found out in college. Using wind and/or solar to charge batteries(especially lead acid batteries) is an utter waste of time. there is too much loss (ie efficiency is poor) in the charging and draining process in lead acid batteries. In short, they suck. They suck real bad. They also don’t last very long when you discharge them very deeply very frequently. They are designed to be used only a little bit and not very often.
lead acid is too inefficent and photovoltaics make too little electricity.
The concept will never go anywhere until batteries are a thousand times more efficient and photovoltaics put out a thousand times more juice. And no, I’m not exaggerating the numbers.
Who knew? What a lot of great advice, thanks!! FR rocks.
E x I/R
E is Voltage. That’s like pressure in a water hose, especially when talking about DC. Amperage is the AMOUNT of water that comes out the hose. You can hook them up in parallel, just make sure you use big enough wire. The thicker the better. The electrons won’t care.
I can’t imagine what you could do with a tiny battery like that. The basic minimum would be a group 24 deep cycle battery which is 96 amp hours (x 0.40 = 38 useable amp hours)
That might run a radio or a laptop computer during the summer, but to do it in the winter you need three times as much generating power, which means at least 360 watts of solar panels, which will cost about $750 - $850 on todays market.
Yes. The larger battery will drain faster as per it capacity but in the end they will reach their minimum at about the same time.
It is the total capacity that is your concern, not how much each individual contributes.
Right. That’s one lesson I’m learning already. Scaling up (big) is necessary to actually have a useful source of electricity.
Though it’s probably better for me (total novice) to start small, so when I epic fail numerous times, it’s at least cheaper.
Attempting this in SoCal, so low temps aren’t a factor much. Not trying to run a house with it, just learning how in a pinch - maybe a large quake perhaps - how to get by for a few days maybe a week or two worst case. I’ll perhaps scale it up later, right now just trying to build a subsistence sort of basic backup test system...
It’s fun anyway! Agreeing so far with all the advice (and reality checks) It’s ironic that conservatives are, I would say, more capable of the very sorts of energy advancements, that bubble-heads on the left dream about.
Thanks again all, for the great feedback.
The concept will never go anywhere until batteries are a thousand times more efficient
There are a couple of possible ways to do that (carbon nano tubes for one), but the the danger of a cascade type of failure and catastrophic instant energy release would be very bad to say the least.
Short answer >> YES
There are some considerations. If one of the batteries goes bad like a shorted cell, It can drag the other one down,If batteries are left in a discharged state too long they will go bad.
You can isolate the batteries from each other using diodes.
Anodes of diodes go to + of each battery,tie cathodes together and connect to load. - of both batteries connect together.You will drop some voltage across the diodes (0.7 to 1 volt or so depending on diodes chosen and current going through them.Make sure your diodes can handle the load cuurent.
You could make one of these
I suspect you are starting to get an idea of how ludicrius the whole solar/wind power idea of the Libs to replace our whole electrical grid is considering how much power the modern world uses.
But it is fun experimenting with it for selected uses.
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.
“The bigger concern is whether youre deep-cycling batteries that are not intended to be deep-cycled. If you are, youll get about 3 to 5 cycles out of them and then plates will start to disintegrate.”
Does immediate re-charge help mediate that?
Is there a percentage de-charged (for example on a 12-volt battery, down 25 percent percent to 9v remaining?) at which point the damage goes logarithmic?
"A DC Fester in an AC spaceship?"
The concept will never go anywhere until batteries are a thousand times more efficient and photovoltaics put out a thousand times more juice. And no, Im not exaggerating the numbers.
Yes, you are exaggerating the numbers. A flooded lead acid battery (one with "caps" that you can add water to) charge at about 90% efficiency, absorbed glass mat (AGM) charge at about 95% efficiency. I'd like to see you calculate how a ~90% efficient device can become 1000 times more efficient.
Consumer photovoltaic panels are in the neighborhood of 15% efficient in converting sunlight into electricity. The average solar insolation (total energy reaching the earth's surface) is about 1000w/m^2. My 10' x 10' array can produce ~1kw for ~5 hours per day. Not alot but it is something. If everyone had their roof covered in them, there might not be a reason to import oil from 6th century cavemen that would love nothing more than to slice your head from your neck or from turd world dictators that speak spanish.
Don't get me wrong, I'm not a proponent of "renewable" energy at all, I'm more of a slash-and-burn oil burner that enjoys going fast, cold air in the summer and hot air in the winter but spreading falsehoods about solar's capabilities is bull. Think about it this way, the more people that spend their money on those power sources, the more of the other there is for us.
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.
For pete sake. According to the Battery FAQ posted just before your post, that 25% SWAG I used as an example was right.
I pretty much totally pulled that out of thin air, after watching the voltage on my first attempt to boil water - I could see the power delivery to the hotpot drop significantly about that point.
At 75% of capacity, sulfation begins. Lots of information here...
Good suggestion. Thanks.
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 saw that discussed in a couple places, and another FReeper mentioned that upthread.
Gotta duck out now for some sleep, appreciate the feedback.
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.
Even lead acid batteries require a sophisticated solar charge controller. (~$120)
Even deep discharge rated batteries will only last around 400 charge-discharge cycles at an 80% discharge level.
Suggest you go to this site and do a lot of reading: http://www.windsun.com/Batteries/Battery_FAQ.htm
Got that right - electrical circuits will try to maintain power [watts] at a constant level. If voltage drops, amperage increases to maintain the wattage level.
I used to work for NEC in the Fiber Optic Transmission division. The designer of the power module for one of the fiber optic multiplexer models forgot to include an under-voltage cut-off circuit in the power module.
Note: The under-voltage cut-off is a mandatory BellCore spec.
Anyway, New York Telephone [now Verizon] performed the BellCore under-voltage protection test when it installed 100 of these multiplexers in a new Central Office installation.
They dialed down the voltage and ALL 100 power modules FRIED !!!
I can't seem to embed it..., perhaps because it's a pdf.
The negatives are common, eg; tied together.
The "bar" end of the diodes = cathodes go to the separate + terminals on the batts.
The "butt" ends of the diodes are common, eg; tied together
When we are charging the batts, this setup WITHOUT the selector switch *automatically* charges the weaker battery....REGARDLESS of whether we happen to have left the selector switch in "A" or "B" position. That sounds good and it probably is in most cases. (Ideally, we would really like to be fully aware & know how much current is being drawn by batt A and then switch to batt B so we are always in a position to compare them. But then again, sometimes "automatic" works very well.
I say again, notice how the charging current goes to BOTH batts, REGARDLESS of which position the switch happens to be left in. Hmmmm.
When we are DRAWING power from the batts, that same switch (The one whose center pole is within the dashed box labeled "Sure Power Isolator #31822 or #31922" and which connects to the anode or "butt" ends of BOTH diodes) >>ONLY<< lets us draw from one battery at a time.
You want it this way on a boat!! Because being able to start an engine can be very critical, when a current is washing you up on some rocks. So, if you have been using "batt A" all day long, drinking beers in the sun and are plowed when the current comes up and you suddenly you realize you had better freaking start your engine and scoot; your ability to start your engine off batt "B" is not a matter of whether you were a goofball and let the radio drain BOTH batteries: You didn't have the choice. You would have had to switch that big fat switch over to "B" and that big KACHUNK would have triggered those remaining functioning neurons to think something like "whoa, I better not run the radio off batt "B" because I'm gonna need to start this thing later.
The diodes have to be current and voltage rated to handle your maximum charging current, which, if you are using solar cells, is miniscule. Very, very few diodes will have a problem with 12 volts. OTOH if you're using an automotive alternator that can pump out 85 amps, you will need some FAR more serious diodes and heatsinks. You could need that (eg; high-current diodes) if you have hundreds and hundreds of sq feet of solar panels, as well. These pictured are prolly 35-50 AMP diodes, and, would require mounting on a heatsink to truly be able to handle that amount of power.
That's so true, the problem. A month ago I was designing a switching (DC-DC) power supply, and the low end of the input voltage range is the most troublesome. Not all circuits "will draw down more" but some do, if they are too smart for their own good. The regulator that I picked does have an internal cutoff at something like +3.6V, but that is to be tested once the board is back from assembly. Then I will know if it works or not :-)
For what it is worth I generally agree with what you said (Greysard) with one exception.
It is a very bad plan to wire batteries with different capacities in series. The smaller capacity battery will charge first and then be overcharged to reach full charge on the larger capacity battery. In addition when they discharge the smaller capacity battery will discharge first and can be reverse biased by the larger battery. Very bad idea.
So, do not wire different capacity batteries in series.
It is not normally a good idea to wire batteries in parallel for the reasons you stated. And if you do so, they should absolutely be fused between batteries for safety. In addition the batteries should be physically close together so they are both the same temperature. Temperature affects the output voltage and output voltage differences between paralleled batteries means large currents flowing between batteries.
Honestly, I would not wire batteries in parallel unless there’s no other practical solution. And if you are going to wire them in series they need to be the same capacity and about the same age and then it is okay.
“Is there a percentage de-charged (for example on a 12-volt battery, down 25 percent percent to 9v remaining?) at which point the damage goes logarithmic?”
I’m not sure what a good number is for lead acid battery that’s not meant to be deep-cycled is, but I don’t see 25% as a problem. So if your battery is 10 Amp-hrs, you pull out 2.5 Amp-hrs, so you have 7.5 Amp-hrs left.
A bigger problem (from what you wrote) is that you’re trying to use voltage rater than Amp-hrs to determine battery state of charge. Don’t do that. Voltage generally will not drop much until the battery is almost depleted. In other words, you’re probably 90% drained by the time you see 9 Volts. You pretty much have to measure how much energy you’ve pulled out of the battery, rather than look at voltage.
“For pete sake. According to the Battery FAQ posted just before your post, that 25% SWAG I used as an example was right.”
The way your post started, I thought I pissed you off. Anyway, I follow now - you made a guess and nailed it - good job.
You can see my point about voltage on that same FAQ page. One thing to keep in is that lead acid batteries for cars, or other engine start applications, are optimized for putting out a lot of power, very fast - which leads to a lot of plate area, which leads to thin plates, which leads to plates that fall apart with deep cycling.
If you don’t need high power levels, then you’re looking at deep-cycle batteries, which have much stronger (thicker) plates and can be run down lots of times (maybe hundreds, not sure), but they are meant to be run down completely. They’re typically used to power electric motors in boats, where the motor is small and the power levels are pretty constant (i.e., nothing like trying to start a V8 engine when it’s 10 below zero). The only deep-cycle batteries that I’m familiar with are about the size of car batteries, which is a lot larger than you’re talking about. However, if you want to get 18 Amp-hrs out of a conventional lead-acid, you’ll need a 72 Amp-hr battery to start with (i.e., use 25% of it). A 72 Amp-hr battery is about the size a car battery (I believe), so at that point, you may want to shift over to a deep-cycle and not have to worry about destroying it (and who knows, there may be smaller ones out there).
“Its fun anyway! Agreeing so far with all the advice (and reality checks) Its ironic that conservatives are, I would say, more capable of the very sorts of energy advancements, that bubble-heads on the left dream about.”
LOL. I like that comment. Yea, I’ve been dealing with batteries for a while. I know a bit about solar too. Once in a while I go to a display of ‘green energy’, and it just kills me to see the greenies trying to talk about stuff that’s COMPLETELY out of their league. They cross up units all over the place and draw conclusions that are totally out to lunch (a 48 volt solar panel puts out twice as much power as a 24 volt panel of the same size because the voltage is higher...LOL...they put out the power...different current, but same power). And then, as weak as they are, they even have to try to talk down to the public, who really (understandably) has no clue how this stuff works - although they’re learning in California that it is not cheap...as it starts getting worked into their electric bills.
“Im not an electrician. Yes you can connect batteries in parallel or in series and it doesnt cause explosions.”
Only connect similar batteries in series.
After reading some other stuff here...even with a deep cycle, you probably shouldn’t pull it down more than about 50% (rather than all the way down)...but it will take a lot of cycles at 50%. Anyway, you can research it, I don’t remember the sweet spot on those, and you may not be cycling much, so you may be able to go lower.
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