Any FR electricians? (12v DC Lead Acid Battery, Basic Q)

(vanity)

[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...

[dickmc]

General rule of thumb is that the overall cycle efficiency (charge-discharge) will be around 70%.

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

[Lmo56]

Also, as battery voltage drops during use, amperage will go up.

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 !!!

[Attention Surplus Disorder]

I tried to draw a schematic but my diode symbols are too damn small and it's late & I'm lazy. Study this diagram some...page 2...this shows how it is (properly) done. In this case, the two diodes are enclosed in a heatsinkish box that insulates and provides a convenient means of connection. Electrically speaking though, that box is just 2 diodes.

http://www.surepower.com/pdf/180031c.pdf

I can't seem to embed it..., perhaps because it's a pdf.

Note how:

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.

[Greysard]

They dialed down the voltage and ALL 100 power modules FRIED !!!

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 :-)

[DB]

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.

My 2cents...

[BobL]

“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.

[BobL]

“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).

[BobL]

“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.”

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.

[RFEngineer]

“I’m not an electrician. Yes you can connect batteries in parallel or in series and it doesn’t cause explosions.”

Only connect similar batteries in series.

[BobL]

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.

[Fresh Wind]

Series is OK as long as all the batteries in the series string are identical.

[Hot Tabasco]

Figured a couple basic lessons already:

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........

[Vigilantcitizen]

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.

[thackney]

I would suggest:

Optima Battery Model: D31M
http://www.optimabatteries.com/optima_products/bluetop/specs.php

Capacity: 75 Ah

http://www.amazon.com/Optima-Batteries-8052-161-BlueTop-Purpose/dp/B00099DW6G
$230.49

[thackney]

The main problem you are going to have is mismatched impedance (resistance).

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.

[mad_as_he$$]

What they said AND do not drain lead acid batteries below 10.8 volts. That will severely diminish their lifetime. INMHO solar is only cost feasible if your have absolutely no other choices.

[Cvengr]

Just hook it up, in your garage, in parallel with one of these batteries and it will work great.

[thackney]

I was always partial to this UPS system.

http://search.abb.com/library/Download.aspx?DocumentID=9AKK101130D0196&LanguageCode=en&DocumentPartID=&Action=Launch&IncludeExternalPublicLimited=True

[norwaypinesavage]

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.

[mamelukesabre]

Theres a danger of catastrophic energy release with gasoline and we all seem to manage.