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Old 09-05-2006
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Lightbulb Paralleling Batteries

There is a major disagreement on paralleling batteries on this forum. As an engineer who parallels batteries for a living and designs & builds chargers for them, I’ll give you one guess where I line up.

This conflict has hijacked more then one thread, so probably deserves a posting of its own. Here goes!

I have been considering two states of the battery bank while answering these posts;

1. Batteries on a charger and charging or,
2. Batteries off a charger and discharging into a load.

It was pointed out that there is a third state where the batteries are off the charger and are not discharging, i.e. no load.

Examining each state with a simplified battery bank of two parrelled batteries, both rated at 100 AH (amp-hours), and one “good” at rated capacity and one “bad” with only 50% capacity or 50 AH. We will have to assume good connections, since everyone does monthly preventive maintenance every month on there battery bank, right? We will also have to assume the “bad’ battery does not contain shorted plates. The loss of capacity will be due to plate corrosion, sulfation and/or electrolyte loss (drying-out).

Under charge, the batteries behave independently, each charging to there maximum capacitance. The “bad” battery will charge faster, given the same energy input, but with two in parallel, the amount of current may be limited by the “good” battery, depending on the charger. Once the float voltage level of the batteries has been reached, both batteries will be at approximately 80% state of charge and the two batteries will continue to charge the remaining 20% independently. Using the 80/20 rule of thumb, 80% of the charge takes 20% of the time, the final 20% taking 80% of the charge cycle time. Because of the limiting effect of the good battery, both batteries will reach full charge state at about the same time. (Assuming full discharge was the initial state of both batteries. You can start without full discharge and the charge time of both batteries may differ.)

At the end of the charge cycle, both batteries will be at full charge, one at 50 AH and one at 100 AH.

During discharge, the “bad” battery will discharge 1/3 of the current level of the “good” battery. For a 10 amp load, 3.33 amps will be coming from the “bad” battery and 6.66 amps from the good battery. Both batteries will be at the same voltage level and because of this discharge the same percentage of capacity. Both batteries will be at the same state of charge during the discharge.

If the batteries are left off the charger they will drift down to their open circuit voltage after several hours. If the specific gravity of the electrolyte has not undergone change, i.e. the loss of capacity of our bad battery is due to plate corrosion or drying-out, the open cell voltage of both batteries should be the same for given state of charge. (close enough anyway). Since there will not be a potential difference between the batteries there should be minimum to no current flow between batteries. Any difference will be equalized once the batteries stabilize any specific gravity differences. This will be done by discharging the battery with the higher specific gravity and charging the battery with the lower specific gravity. There is no guarantee which battery will be discharge as the one with the lower capacity may have the higher specific gravity electrolyte. In any case the overall system capacity will remain constant. The only loss will be a small resistive loss due to current flow (negligible compared to system capacity).

If the “bad” battery has a low specific gravity electrolyte due to sulfation (this is the only cause I can think of in a battery system) the parrelled batteries will once again try and equalize the open cell voltage and the “good” battery will continue to “charge” the “bad” battery until the specific gravity of the electrolytes are the same. However, any charge taken from the “good” battery will be stored in the “bad” battery minus resistive loss again. Again the battery system capacity should remain constant.

If any work is done by the system, i.e. hydrolysis of water within the electrolyte in to hydrogen and oxygen, that energy may be lost from the system. This should occur, especially in a system off the charger, only when there is physical damage such as shorting of a cell of one of the batteries. In this case the “good” battery will OVERCHARGE the “bad” battery causing electrolysis and gassing and possibly venting. This energy will be lost to the system.

Basically the conservation of energy prevails. Energy from one battery may move to another (current flow between batteries) but the overall system energy will remain the same (discounting the negligible loss from resistive heating).

Does this mean that you should not replace all of your batteries if you find one bad? Maybe, or maybe not. Only a load test of the batteries can answer that question and that is beyond the means of most sailors. Good practice for reliability is to change all of the batteries at one time. This helps insure that all batteries are in good condition and that you have the battery system capacity you think you do.

Comments please!
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Dave,

Very interesting post. Thanks for making this a separate thread.

I'm a bit dismayed and not a bit disturbed by some of the comments and "advice" I've seen on the subject of paralleling batteries. In particular, to state that this is "never a good idea" is pure poppycock.

For many cruising sailboats, particularly the larger ones, paralleling house batteries is the only practical solution to obtaining the needed AH capacity, where both cost and space considerations prevent the use of large industrial batteries. For many sailors, a large house bank constructed from, e.g., Trojan T-105 golf cart batteries (225AH 6V) in series/parallel is the most cost-effective solution.

Furthermore, the idea of separating your house batteries into, e.g., bank 1 and bank 2 and using them independently is not good practice. Paralleling all your batteries into a single large bank has numerous advantages, including faster charging and longer life. A large capacity bank may be charged more rapidly (more AH replenished per unit of time) than a smaller capacity one. This means shorter run times for your engine or generator, with attendant savings.

The percentage of discharge on a large capacity battery bank per unit of time as a result of the typical house load (lights, instruments, refrigeration, etc.) will be less than the percentage discharge caused by the same load on a smaller capacity battery bank. Typically, this will mean that the house batteries are drawn down less when banks are combined than they are when banks are used separately, resulting in longer life (more discharge/charge cycles) for all the batteries.

Finally, all house batteries are treated identically in the combined pattern. If they are treated well, their life will be greatly extended and overall costs will be reduced.

Starting batteries, of course, need to be treated separately. I favor an approach wherein a large capacity alternator/smart regulator charges the combined house battery bank, and a small EchoCharger is used to maintain the starting battery which is completely separate. All battery charging devices (shore power, onboard generator, wind powered generator, solar panels, etc.) also charge the combined house battery bank.

Batteries on sailboats don't die. They are murdered by bad installations, improper maintenance, and ignorant practices.

BTW, there's no such thing as a battery with no discharge. They all are self discharging, even with no load attached. Just a matter of rate!

Equally, they are all sulphating. From the moment after manufacture when sulphuric acid is added, they begin sulphating. How far and how fast this chronic process progresses depends on how the battery is treated. Proper 3-stage charging will help minimize sulphation. Failure to fully charge a battery and/or letting it sit without charging for any appreciable time contributes mightily to sulphation.

Just my $.02 :-)

Bill
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Thanks Bill.

Bill,

I agree with all of your comments. There are a lot of things going on in batteries and battery systems and I was trying to keep it simple to start.

The number one killer of batteries is the lack of proper charging. A lot of posts I have read indicate that the engine is run until the batteries reach float voltage and then turned off. That is only giving the batteries an 80% charge at best. Other posts describe symptoms that indicate bad connections in the system which will also prevent proper charging.

Hopefully others will put in there two cents. Maybe we can start a fight club for electricial engineers.
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Very good suggestions, thanks!

Quote:
Paralleling all your batteries into a single large bank has numerous advantages, including .......
This point is also made by Nigel Calder:

Boatowner's Mechanical and Electrical Manual

AMAZON: ($39 new // $25 used)
http://www.amazon.com/Boatowners-Mec...e=UTF8&s=books
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Well, Dave, I guess I'm the Devil's Advocate here.

GE, in their old industrial battery book, says right out that parallel batteries can work perfectly well. But then goes on to say they will work best when each set of CELLS is paralleled, and then the parallel banks of cells are connected in series. (i.e., three cells in parallel, six banks of them, to make 18 cells combined in the final battery. Not, two six-cell batteries put in parallel.)

I would guess that they suggest this because it "plays the odds" better, averaging the capacity of each group of cells rather than playing one group of six against another, where the variations in SIX cells accumulate in each battery.

But, as you note, there are real life considerations to be taken view of and I suspect it is the real life variations versus the pure ideal where the problems can arise. Or, be eliminated by simply not paralleling.

So, I'd like to suggest some investigation along these lines:

Consider your case of two 100AH batteries running in parallel from one charger, which for systems of that capacity usually means one 60-90Ah automotive alternator with integral automotive regulator. I'm not sure where to begin quantifying anything that specific. We know that the regulator will throttle back the charging amperage when it sees about a 90% charge, and we know that there will only be one charge sense lead, which is reading the average of the two batteries. So we can begin by noting that when the AVERAGE state of the two batteries has reached 90% capacity (80, 90, whatever, that's another example of the differences in specific equipment) capacity, the system will go into trickle charge mode. Which is a problem regardless of battery configuration, but even more so with parallel batteries I'd think one of them will be cooking (boiling out electrolyte and overcharging) while the other is undercharging more than it would alone.

Or don't you think this "charge abuse" will be greater for two batteries in parallel than it would be for separate banks?

Have you actually measured or metered parallel batteries to see the charge states they reach, versus the same batteries each separately charged?

And, given the variation in batteries these days, have you compared mutiple batteries "off the shelf" to see how closely they do or don't compare in voltage and capacity, even when they are twins off the same line? (Assuming twins stay together all the way into the boat?)

"...but with two in parallel, the amount of current may be limited by the “good” battery, depending on the charger." Current limited by the good battery? Not when there's just the one charge sense lead, reading the average of them. Of course, as soon as that average (pulled up by the good battery) reaches a set point, the *voltage* produced by the alternator will be dropped and the current reduced. But AFAIK in the common automotive alternators, they may be limiting voltage or current, there are a number of different schemes and float ranges.

And then, at what point, at what size of battery banks, do we assume boat owners have switched to real marine outboard regulators and real marine high power alternators?? Because at that point, charging can or will change, won't it?

"Because of the limiting effect of the good battery, both batteries will reach full charge state at about the same time. (Assuming full discharge was the initial state of both batteries. ..."

Ouch! Assuming full discharge?! That's outright abuse. Who ever recommends taking batteries below the 50% discharge point? Makers may claim they get the most "lifetime power" out of a battery by cycling it to 40% or 70%, but they all seem to center on 50% and they all say a full discharge can kill a deep cycle battery at least 10x faster than any lesser charging state.
And reaching a state of full charge? Well, again, unless you've been motoring all day, or using a marine regulator, the more common automotive type won't get you above a 90% charge for hours after that point. If you're just running the engine as much as you need for recharging, and shutting it down after that...odds are you'll never see more than a 90+% charge.

"During discharge, ... Both batteries will be at the same state of charge during the discharge." Is that from theory, or observation? At what discharge rate?

And I think it really should be pointed out, the case of cruisers who are regularly cycling their batteries with daily cycling, is going to be very different from the typical weekend sailor--where the batteries may be sitting (one hopes OFF) for five days, then sitting in parallel but feeding only a small load, like the instruments, for six or eight hours during the day.

"If the batteries are left off the charger they will drift down to their open circuit voltage after several hours." That's too generous. I've seen battery makers who suggest that it will take 10-24 hours for the electrolyte to equalize out the charge in the battery, not "several" hours. With an older 12v battery, I've seen that initial charge burn off overnight but then continue to settle, visibly, for longer.

"If ...the open cell voltage of both batteries should be the same for given state of charge. (close enough anyway). Since there will not be a potential difference between the batteries there should be minimum to no current flow between batteries. " But that's a gross assumption. Unless the batteries come from the same maker and the same production batch, the alloys used in them can differ. Different makers and lines intentionally use different alloys making that the greatest problem, but battery making is not watchmaking, mechanical differences in assembly are normal.

"Any difference will be equalized once the batteries stabilize any specific gravity differences. " I'd been told that's not the case, and that is the problem. They WILL equalize, yes. And then as each battery is different, and has a different rate of self-discharge (among other things caused by the physical changes and sulphation happening differently in each battery), as soon as they have equalized they begin to drift apart and the current loop between them starts to flow again. This process does not stop, it continues and it drags them down.

I suppose the simplest way to test this would be to take two "good" batteries, put them on a very smart charger to make sure they are properly charged, and then record their discharge rates over the next month. Then repeat the process, alternating between batteries that were in series and in parallel. Perhaps using two sets to get better data. (What, a month if too long? OK, two weeks? One week? How about run the month, and let's see where the data cross?)

I'd volunteer to do that, but don't have the resources to do it.

"However, any charge taken from the “good” battery will be stored in the “bad” battery minus resistive loss again. Again the battery system capacity should remain constant." So you're familiar with the endless looping...but isn't that the catch? Just resistive loss? Can we put a number, a percent, on that? Again, measured, and with varying installations?

"Does this mean that you should not replace all of your batteries if you find one bad? Maybe, or maybe not." I'll agree with that!

But given all the variables, I still think that paralleling batteries gives the system opportunies for failure that simply do not exist when using batteries in series, or separate banks. Given optimization of the charging system (capacities and rates matched, etc.) I don't see anything to be GAINED by paralleling, except the chance to buy cheap 6V or 12V batteries from convenient sources.

Maybe we can get Practical Sailor to give us a stipend to do the lab testing, because without that...all we can say is "if, maybe, it depends, it might". Personally, I'd rather use a system that doesn't rely on those terms.

Last edited by hellosailor; 09-05-2006 at 03:00 PM.
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Advantage of Parallel

Without going back to read Nigel Calder....

The life of a cell is maximized by operating as nearly as possible to a fully charged state. When one increases (doubles) the ampacity of the bank by means of paralleling, for a given draw on the bank, one draws down the ampacity/energy pool LESS (as a % of total) and so operates closer to fully charged. I seem to recall that Nigel Calder in the book referanced in my post above outlines the case with specific examples and generates expected life extension of the battery bank which is much much more than negligable.

I will go back and read it ASAP, but it will certainly be sorted out in this thread before then.
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Oh good, a challenge

Hellosailor, I knew I could count on you!

All battery manufactures of lead-acid batteries recommend a constant voltage, or float voltage, method for maintaining the charge level of their lead-acid batteries. This is what the voltage regulator of any engine alternator does; maintain the output level at a constant voltage. Most battery manufactures also recommend limiting the charge current into the batteries to about 20% of the rated 8 hour amp hour capacity, for a 200 AH bank this equates to 40 amps. Charging the batteries with much more then this current would promote self heating, electrolysis and constitutes charging abuse. Generally this current is limited by reducing the voltage output once the charging has reached this current limit and the output is regulated by current until the float voltage level is reached. Some class of chargers will do a secondary charge level, termed boost, and allow the current to be maintained until a higher voltage is reached, on the order of 14.4 to 14.8 V, and then the float or maintenance level is resumed, somewhere between 13.3to 13.8V. Equalize charging should be done only when recommended by the battery manufacturer and using the manufactures specific procedure. This helps overcome plate sulfation and can reestablish correct specific gravity levels in all of the cells. Other special charge cycles are possible but not recommended unless you can monitor temperature, outgassing, current draw, etc.

Practically one limits (or increases) the size of the alternator to limit the current flow to the battery. In the case stated above, a 60A alternator would be about the maximum size suggested.

As the batteries charge, the electrochemical state within the battery changes and the equilibrium cell voltage rises. For a smaller capacity battery, the “bad” battery in our example, this equilibrium voltage will increase faster then the larger capacity battery for a given current. If the two are connected in parallel, the smaller capacity battery will require less current to charge to the same voltage level as the larger. This is the mechanism for the current being “shared” by the two batteries. The larger one will use twice the current as the smaller one to get to the same equilibrium voltage level.

Using our example of one “bad” battery and one “good” with a 40 amp charge current, the “bad’ battery will charge at 13.3 amps, or 26% of the rated 8 hour capacity, and the good battery will charge at 26.6 amps, also 26% of the rated amp hour capacity. This is a little high but not catastrophic. We can also see in this mechanism that the paralleled battery system is “self healing”, one battery will not be overcharged and one will not undercharge.

This is from both theoretical and empirical data. I have measured currents under similar conditions and the only time I did not get these results some other factor was present.

The discharge sharing is similar to the charging. The same reasoning applies.
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sailandoar,

Exactly right! Cycling a deep-cycle battery to only 30% discharge rather than 50% discharge on a regular basis can GREATLY extend the number of useful life cycles....as much as 3 times!

That's one of the principal reasons it makes good sense to parallel your batteries into a large battery bank. This larger bank will be drawn down less (smaller percentage) from full charge than would a smaller bank, given the same load.

Bill
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Sailandroar-
"The life of a cell is maximized by operating as nearly as possible to a fully charged state. When one...paralleling...and so operates closer to fully charged."
A perfect example of fallacious logic! You are confusing the two issues, of discharge depth and parallelling. By using the correct cells, you can accomplish the same low-discharge-cycling WITHOUT paralleling. If you want to make a proper comparison, the comparison would be using a series of cells (a simple battery) versus a parallel array, of the same total capacity. In which case you'd find the cycling effects from the parallel setup to have no advantage at all.

Remember, if you have two batteries of "x" amp hours each...You can still cycle each one separately to 20-30-50% depth and still get the same total number of amp hours in and out of them--regardless of whether they are paralleled, or used sequentially. The depth-of-discharge argument holds no electrolyte here, if I may coin a pun.

Dave-
Regarding boost charges and such..."Other special charge cycles are possible but not recommended unless you can monitor temperature, outgassing, current draw, etc." All of which can't be accomplished properly with parallel batteries, since there is normally one charge voltage sensor, one battery temperature sensor, one &cetera and now, the question is which battery or bank do you hook it up to? And of course, that's all stuff that the smaller systems using automotive alternators just aren't going to have.

"This is from both theoretical and empirical data. and the only time I did not get these results some other factor was present" And what other factors did you get? Factors related to the parallel construction, or factors that might, again, affect it differently?

Further:
Let's look at another issue that has been ignored. Cell failure. Whatever the odds of cell failure are (1:1000? 1:100,000?) for any individual cell, once you start using parallel batteries, you multiply those odds. If you have six cells, each with a 1:10,000 chance of failing, you now have a 6:10,000 chance of having a battery failure from a bad cell. But, if you parallel two such batteries...you've doubled the odds of having a failure, to 12:10,000. And one cell failure will still take down both batteries as it starts those current loops and failure modes that you say can be ignored--except in the case of a bad cell.
Keep the batteries separate, and use them sequentially, and your risk of failure is now effectively halved, since you've got redundant and separate banks. Use them in parallel...and all your eggs were in the one basket. I can't call that a good idea. Or do you dismiss that?

Last edited by hellosailor; 09-05-2006 at 04:52 PM.
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I doubt that hellosailor has really had much experience cruising in the typical power-hungry sailboat with minimal battery capacity. If he had, he'd know that there aren't many sailors who are willing to keep their eyes glued on the voltmeter and, worse, get up at 3 or 4AM to switch from battery bank A to battery bank B so as to avoid a deeper than desired discharge.
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