Mixing Battery types in 1 Bank

[Maine Sail]

Originally Posted by sailorchic34

A 100 AH AGM can accept 500 amps charge load without overheating. It would be fully charged in less then 30 minutes. A FLA would barf, er overheat at a 500 amp charge rate.

A 100Ah AGM can not take 500A in bulk charge and continue to accept this.. The will nearly immediately hit absorption voltage then begin limiting current. A 100A AGM can take an in-rush of 500A but this is extremely short in duration. That short of a duration won't cause over heating because the battery will very quickly enter the acceptance phase when a charge source that big is applied. There is a big difference between charging at 500A and a 500A spike/in-rush... The Lifeline manual is quite misleading in this regard.

These batteries came up to voltage limiting nearly immediately at just 2C (200A for a 100Ah battery). This means no real bulk period just a quick path to current limiting/absorption voltage... At 5C (500A for a 100Ah battery) this would take seconds.....

At 0.5C (50A for a 100Ah battery) the batts took bulk current for a decent period of time, before hitting absorption, and a 0.25C even longer in bulk.....

At the 0.25C charge rate, this is about the same exact spot SOC wise, where FLA's hit absorption voltage.....

[sailorchic34]

Oh agree that the 500 amp load would drop rather quickly. The point is that AGM can take 500 amps without damage and that the internal resistance between AGM and FLA are very far apart. Sure a .015 to .017 ohm difference does not seem like much, but even .0005 ohms makes a big difference in charge rates between batteries.

At the 2C rate on the graph, I don't see a switch to absorption from bulk. I expect the 2C charger was slightly different then the .5C or .25C charger used for those (>25C &.5C) curves.

What the curves don't show is the actual time it took to reach absorption from 50% SOC. Only the SOC at that point.

[btrayfors]

Mainesail is basically correct, as usual, but there are three things to note:

1. The 5xCA absorption rate is for AGM batteries with a low state-of-charge, i.e., way below 50%, say ~ 20%. Charge acceptance rate would decrease rapidly. In just 4-5 minutes a SOC of 50% would be achieved, and at that SOC the battery would be accepting just over 100 amps.

2. At 50% SOC, the maximum acceptance rate is just over 1xCA or just over 100 amps for a 100AH AGM battery. The curves in the graph shown by Mainesail are all for AGM batteries which started at 50% SOC.

3. A 100% full charge would not be reached "in less than 30 minutes". No matter the size of the charger, reaching a full charge takes much longer than that -- something on the order of an hour and a half to two hours, as shown on the graph above.

It's that last 5-10% of charging before reaching full charge which kills you :-)

Here is a graph derived from the same test data (by Concorde Battery Company) showing charging current acceptance over time using four different size chargers.



And here's a graph which tries to show the total AH restored to the battery over time, by charger size. This is an approximation and calculated number. Those in the first graph are actuals.



Note that even with the big capacity chargers, it still takes 2-3 hours to reach anywhere near 100% charge.

Note also that many boats wishing to "take advantage of AGMs" don't have anywhere near even 10% CA charging capacity, much less 25% or 50% or 100%. These boats are way below the red line.

Bill

[colemj]

Originally Posted by btrayfors

Note that even with the big capacity chargers, it still takes 2-3 hours to reach anywhere near 100% charge.

Note also that many boats wishing to "take advantage of AGMs" don't have anywhere near even 10% CA charging capacity, much less 25% or 50% or 100%. These boats are way below the red line.

Bill

So Bill, do you think charging an AGM paralleled with a similar sized FLA, both having the same recommended absorption voltage, will reach 100% SOC and start boiling before the FLA reaches full charge? For a 1C, 0.5C and 0.25C charging rate? If so, how much sooner?

Will the AGM even reach absorption voltage before the FLA in this situation?

In your experience, how long would an AGM with an absorption voltage of 14.4V have to sit at 14.4V before damage (unnatural) to it started occurring?

Mark

[hellosailor]

Sailorchic-
"Gee Optima lists their AGM's at 2.8 milliohms, Century lists theirs at 4.8 milliohms. "
Apples and oranges again. Optima doesn't make "AGMs" any more than Lifeline does. Optima only makes spiral-wound AGMs, and that's going to be different from conventional flat-plate AGMs. In a number of ways. It isn't surprising that they would have different electrical properties as well.

[goboatingnow]

I see this is still going on.

Let's establish a couple of ground rules.

Modelling a battery as a perfect voltage source with a series resistor is a first order equivalent circuit, crude and in fact a very poor model of what goes on. Equivalent input resistance during charging and equivalent output resistance during discharge are non linear dynamic values.

The relationship between terminal voltage and SOC is non linear, very non linear in certain parts of the charge cycle.

The same is true for the discharge cycle and equally the charge and discharge curves are not mirror images of each other.

Two batteries in parallel. It's irrelevant as to their chemistries. Will both happily charge up to 100% assuming that the charge cycle lasts as long as necessary to charge both.

During bulk charge the current will divide into each batteries as a proportion of the equivalent dynamic input resistance at that point in tine..

The terminal voltages of parallel batteries will obviously always rise together.

However at any point in the charge cycle, the % SOC is not the same , it may be close as would be expected in a LA chemistry. But depending on various parameters it may not be that close

Hence when the charger decides to enter absorption mode at a set point, and enter constant voltage mode, the two parallel batteries may in fact have a % SOC discrepancy. This is one do the reasons why similar capacity and chemistry is recommended.

AGMS will typically have regained more percentage SOC then FLAs at that point typically 90% for AGMs and 80% for Flooded LA.

Absorption mode will end when the charger determines either the current falls below a set point or a safety timer runs out. The batteries may or may not be at 100% at that point. Based on the charge acceptance during absorption, typically the AGM will complete absorption before FLAs. At that point onwards it is being subjected to an absorption cycle voltage that it's doesn't need , even though little current is flowing into the AGM. This is a common occurrence with systems that have loads active during chsrging. unless the absorption voltage is excessive nothing will happen. Ultimately the timer will expire and the charger will drop out of that mode.

So in partial recharges, both batteries will likely have unequal % SOC. This is bad for FLAs , bit it's much worse for AGMs

Now in discharge. Where the load currents are large , ie the equivalent load resistance is low. Both batteries will contribute current roughly in proportion to their capacities but modified by their state of charge at any point.

Hence , the work each battery does will not necessarily be proportional to their respective capacities.

This is even more pronounced where currents are small as a proportion of capacity. You will find typcally the bigger capacity will do more work. Also you will find in certain cases the smaller battery trying to charge the larger and vice versa. Because of the non linear inverse relationship between charging and discharging , this parasitic charging results in efficiency losses ( ie capacity )

Again halting the discharge at any point will reveal that there CAN be a divergence of % SOC. this shows up as a weak/lazy battery syndrome.

Ultimately if the discharge continues, both batteries will balance their SOC at 0 . This of course is not a good idea.

In general, once neither battery absolute specifications are exceeded and 100% charging is carried out and the usual provisions of not discharging too deeply are followed, premature failure of the batteries WILL NOT occur .

What will occur is that perceived system capacity is not the sum of the two batteries , ie unequal batteries and or chemistries often result in less then expected real capacity.

Premature failure IS potentially exacerbated in this scenario where typical partial charge cycles occur, especially where % SOC is significantly out of balance. Which one fails first ( or is degraded first) is a complex relationship. But partial charge kills AGMs anyway , irrespective as to what they are paralleled with.

Hence the reason similar capacities, cell age and chemistry is reccomend for paralleling is too ensure that as much as possible %SOC remains the same, both batteries share loads, charge up together and delivered capacity is closer to stated capacity.

So no , the system will not burst into flames or die tomorrow. It simply will not perform optimally


As an aside AGMs have no place on a typical boat. Their good characteristics cannot be exploited and their bad ones cannot typically be mitigated. Don't waste money on them
.
Dave .




Sent from my iPad using Tapatalk

[Maine Sail]

Originally Posted by btrayfors

Mainesail is basically correct, as usual, but there are three things to note:

1. The 5xCA absorption rate is for AGM batteries with a low state-of-charge, i.e., way below 50%, say ~ 20%. Charge acceptance rate would decrease rapidly. In just 4-5 minutes a SOC of 50% would be achieved, and at that SOC the battery would be accepting just over 100 amps.


Bill

I only used that chart because it applies more realistically to the real world and how we use our batteries. In the real would as you noted AGM's can not be charged in 30 minutes.... No LA battery I know of can, though I am soon to be testing a new LA technology that seems quite interesting, if the "claims" are anywhere near true... You can always discharge lower than 50% but you will eat into cycle life.....

[sailorchic34]

I pretty much agree with just about everything Dave says, except I think that mixed type batteries "MAY" fail faster depending on a whole slew of variables. I know that Dave knows more about this stuff then me, so I would not be at all surprised, if he is correct on that point too.

Agree completely that the charge current and internal resistance is non-linear, and that SOC will vary in mixed type battery bank's while voltage will be the same.

Oddly I even agree that AGM's make poor cruising batteries, as its hard to get the AGM's (really any battery) back to 100% SOC daily on the hook.

BTW the 500 amp/ 30 minute charge info was on lifeline's website. But agree that marketing might have written that statement. :-)

[colemj]

Originally Posted by sailorchic34

I pretty much agree with just about everything Dave says

Look, I know pointing this out seems snarky, and I do respect your opinions in many other things, and have enjoyed debating with you and your willingness to engage, but frankly, pretty much everything Dave just posted was OPPOSITE of most of your earlier comments. Particularly in regards to charge acceptances, the practical implications of the relative differences in internal resistances of combined banks, reaching absorption voltages at largely different times, the damage that would be done by the end of an absorption cycle, whether both batteries would be fully charged at the end of a charge cycle, how soon the failures would occur, and much more.

Also note that Dave's point about partial charge/recharge cycles being bad for the AGM is true whether that AGM is paralleled with a FLA or not. There is no practical difference between a pure AGM bank and a chimera bank for that situation.

I pointed this out after you agreed with his earlier post.

On the other hand, Dave's comments agreed with the ones I have been making, so if you agree with me also, then we have nothing to debate!

Mark

[Stu Jackson]

THANK YOU ALL FOR A DELIGHTFUL CONVERSATION.

I used to think that only ybw.com attracted the "How many engineers can dance on the head of a pin?" and "Ask what time it is and they'll tell you how to build a watch" types.

Glad to see you're all here together.

Really, really good discussion.

Wandered all over the place, but certainly entertaining and illuminating.

Thanks, I learned a lot. Really.

Keep it up.

Please, please, please, though: don't wander into LiOpPo4 or whatever it's called. I have enough issues with my 390 ah FLA house bank and our 60ah FLA reserve bank. On a combiner, no less. But, but, but, you've already covered that...

Anyone for a 1-2-B switch discussion" Oh, well, the guys in the UK at ybw.com don't understand that yet, even though Maine Sail has tried to explain it to them for years. Like, do you charge the reserve bank first or the "leisure" bank? What, indeed, IS a "leisure" bank? Do you lean back at the dinner table?

Inquiring minds wanna...

Seriously, though, thanks for a great thread.

[sailorchic34]

Originally Posted by colemj

Look, I know pointing this out seems snarky, and I do respect your opinions in many other things, and have enjoyed debating with you and your willingness to engage, but frankly, pretty much everything Dave just posted was OPPOSITE of most of your earlier comments. Particularly in regards to charge acceptances, the practical implications of the relative differences in internal resistances of combined banks, reaching absorption voltages at largely different times, the damage that would be done by the end of an absorption cycle, whether both batteries would be fully charged at the end of a charge cycle, how soon the failures would occur, and much more.

Also note that Dave's point about partial charge/recharge cycles being bad for the AGM is true whether that AGM is paralleled with a FLA or not. There is no practical difference between a pure AGM bank and a chimera bank for that situation.

I pointed this out after you agreed with his earlier post.

On the other hand, Dave's comments agreed with the ones I have been making, so if you agree with me also, then we have nothing to debate!

Mark

We must be reading Dave's comments completely differently. Dave was saying that the SOC would vary between AGM and FLA, which is what I have been saying.

Forgive me, but I thought you were saying that SOC would be the same for AGM and FLA during the charge and discharge cycle based on voltage being the same.

Dave Said: During bulk charge the current will divide into each batteries as a proportion of the equivalent dynamic input resistance at that point in tine..

To me I read that as, the AGM will accept more current as its dynamic internal resistance is lower in the AGM and will be lower then LA at any point in the charge/discharge cycle. Dave if I'm misreading this, please let me know.

Dave Said: Hence when the charger decides to enter absorption mode at a set point, and enter constant voltage mode, the two parallel batteries may in fact have a % SOC discrepancy.

My point exactly. Yes if left to charge long enough, both will reach 100%. But on the hook. day in and day out, that rarely happens.

Dave said:So in partial recharges, both batteries will likely have unequal % SOC. This is bad for FLAs , bit it's much worse for AGMs.

Pretty much what I've been saying too.

Dave Said: Now in discharge. Where the load currents are large , ie the equivalent load resistance is low. Both batteries will contribute current roughly in proportion to their capacities but modified by their state of charge at any point.

Hence , the work each battery does will not necessarily be proportional to their respective capacities.

Here I'm unclear. To me I see each batteries contribution as a combination of each SOC and of each batteries internal resistance plus Pukert rating. In parallel, I don't think that the internal resistance of each battery type normalizes to a mixed average resistance. This would be the case in series batteries, but not in Parallel.

[darylat8750]

Originally Posted by goboatingnow

The issue being debated seems to be on recharge

both batteries will essentially recharge together its only in absorption mode that one battery make take longer to finish leaving the other to ( potentially ) fizz.

what happens is during bulk , the voltage will only rise as both batteries recover SOC. ( this assumes there is isn't a massive capacity differential ) This is exactly the same as when you recharge your battery with a load on the system, the system loads don't suddenly stop just because the battery is low and sucks all the current from the charger.


in practice during recharge , based on dynamic inout resistance of the battery , current can flow into or out of both batteries at various stages, both batteries will arrive at the absorption voltage together ( they have to ) essentially the batteries will load balance as they charge up. however at absorption mode things are slightly different and the battery that completes absorption mode first , will be left exposed to longer absorption cycles then necessary.

dave

I'm not an EE nor did I sleep in a Hol. Inn last night. However, Dave's post makes sense to me. The issue that I haven't seen addressed is that living on anchor (without solar in my case) the batteries would be close to equally charged at the end of bulk. Most of us charge a ways into absorb then quit because it takes a lot of time, fuel and noise to finish the charge to 100%. At that point the FLA would be lagging behind to some extent. So in my use I would expect the FLA to not last as long as the other one.

The OP was not talking about a FLA and an AGM. Wasn't he talking about two FLA batteries of different plate construction?

[sailorchic34]

Originally Posted by darylat8750

I'm not an EE nor did I sleep in a Hol. Inn last night. However, Dave's post makes sense to me. The issue that I haven't seen addressed is that living on anchor (without solar in my case) the batteries would be close to equally charged at the end of bulk. Most of us charge a ways into absorb then quit because it takes a lot of time, fuel and noise to finish the charge to 100%. At that point the FLA would be lagging behind to some extent. So in my use I would expect the FLA to not last as long as the other one.

The OP was not talking about a FLA and an AGM. Wasn't he talking about two FLA batteries of different plate construction?

As Emily Latilla would say: Oh that's different..... Never mind.

Actually the main issue of the OP is the age of the existing 4 batteries. Mixing two new with old batteries, old being over 3-4 months is generally a bad idea. Mainly as the internal resistance of the old batteries would be higher then the new batteries. So you would have mismatched load between the old and new batteries, as the older LA would have a higher internal resistance then the new batteries would.

[monte]

well we are yet to find additional batteries, however after noticing a continuing gassing smell in the battery area we checked the system and found that the factory had installed open flooded cell batteries but set the battery chargers for sealed/Gel.
The charger settings are
Boost ON
They were set for Gel at 13.8 with boost off and 14.4 with boost on for max 4 hrs
They should have been 13.4 with boost off and 14.1 with boost on for max 4 hrs
So I am wondering if any damage would have occurred to the batteries. They have been operating like this since new 6 weeks ago. If there is a risk they have been damaged or battery life shortened I will be replacing them.
Also is there any test advisable to check the health of the batteries?
Thanks again
Monte

[colemj]

Monte, if your current batteries are flooded, then the proper absorption charging voltage for them should be 14.4-14.8V. Gel batteries are charged at much lower voltages than flooded.

I don't understand the "boost" thing, but think it may be an equalization charge? If so, that should never be done to Gel batteries, and the values you list are too low for any other battery.

Anyhow, if you have been charging FLA's at 13.8V, then the only damage will be a bit of sulfating from the undercharging. This should not have effected them much, if at all, in this short time. Particularly since they have probably been sitting on a float charge after being fully charged.

As for the smell, you really should not be smelling anything. If they are gassing during charging, it will be hydrogen and oxygen, which do not have a smell. If you are smelling rotten eggs, then you have a problem and there is potential damage. You should not charge them if you smell rotten eggs. A very light rotten egg smell could be due to sulfuric acid vapors on the nasal membranes, which would be OK, except that they should not be gassing that vigorously.

However, I fail to see how they are gassing at all if you are charging them below 14.2V.

Mark