Mixing Battery types in 1 Bank

[colemj]

Originally Posted by goboatingnow

Ahem, a little Kirchoffs here would go along way.

Yes, I mentioned Kirchoff in post #72 and talked about needing to treat this as an equivalent circuit in several other posts. I'm glad you reinforced this point.

It is important to note that we have been discussing combining two equal sized batteries whose manufacturers recommend an absorption voltage setting of 14.4V. Not a small battery with a big battery, nor charging at higher or lower voltages.

Originally Posted by goboatingnow

In discharge Two batteries with differing series resistance ( i.e. equivalent output impedance) may actually have currents flowing into OR out of the combining node. Ie at any time, one battery could be charging , while the other is discharging and vice versa or they could be both discharging, the exact current distribution can only be computed knowing the series resistance at the given SOC.

Hence in discharge both batteries will contribute current, but as Sailorchic says the one with the lower resistance will "work " harder , i.e. it will see deeper discharges BUT only if the current demand is such that the smaller battery cannot actually deliver its proportion of the current. In normal use with fractional C discharges, the batteries will lower their SOC percentage evenly.

Yes, this is as I described several times earlier. The dynamic situation during discharge has the two at any given instantaneous point in time not at equilibrium or in the same state. But at the removal of any reasonably normal load used for a reasonably normal time, the SOC of both batteries are the same.

Again, I pointed out several times that one could design an experiment with a rapid, very high load - after which the batteries could be disconnected from each other and have different SOC's. I also pointed out that that experiment would not have any real-life relevance.

So you agree with my earlier attempts at describing the situation during discharge.

Originally Posted by goboatingnow

On charging both batteries will ultimately charge up according to their acceptance rates ,thats true ( up to a point) , (sorry edit here bad english) both will reach the absorption point at the same time ( i.e. the same voltage) but the time in absorption mode can mean that the FLA or the AGM depending on relative capacities will be too long possibly resulting in gassing.

Depending on the settings for the absorption mode, this extended period could ( i stress could ) cause gassing and venting in sealed batteries.

While it is correct to say that the FLA and the AGM terminal voltage will rise together during charging, that does not means the % SOC is rising together, Again using Kirchoffs , you can see that in charging there can be current flowing from one battery to another, Thats not an issue. ultimately the system will reach full charge

Yes, I also spent a lot of words describing the dynamics during charging, where at any instantaneous point in time, the SOC between the two batteries are not the same. But by the end of a charge cycle, both will be at full charge.

Your description is opposite of the view that the AGM will suck huge amounts of current (600% more) during charging and be boiling by the time the bank reaches 14.4V while the FLA is still sipping current in a discharged state. My description was also opposite of that view.

As for gassing, that would depend on voltage and time. Note that all along we have been talking about similar sized batteries with 14.4V absorption voltages. I don't believe that when the combined bank reaches 14.4V, the SOC between the two batteries will be so out of balance that the AGM will gas and vent before the FLA reaches full charge. Unless one has a dumb charger set to hold an absorption voltage for a very long time - at which point the FLA will also be boiling away.

So you agree with my description of what happens during charging. Do you agree with my above thoughts on gassing and venting?

Originally Posted by goboatingnow

Where the problems arise is in typical real life we do not recharge to full, hence you will find that one battery , typically the FLA, will be charged to a lower SOC then the AGM, resulting in it having poor discharge recharge cycle, resulting in premature failure.

No argument there. However, in real life charging where batteries never hit full SOC, it appears that those boats with AGM's are suffering the most compared to those with FLA's. Enough so that the AGM companies are now warning that AGM lifetimes will be severely curtailed unless these batteries are brought to full charge almost daily. Lifeline even goes as far as recommending periodic equalization charging of their AGM's.

Do you think typical charging in real-life is to the point where a combined AGM would reach full SOC before the FLA and then charging terminated? In other words, do you think that typical charging is to >95%?

So I will disagree with you on which battery will suffer the premature death in a combined bank. Do you disagree with the manufacturers that AGM's need to have regular full charges to meet their expected lifetimes?

Originally Posted by goboatingnow

As an aside remember that batteries given a big enough charger, do not in fact self regulate their charge. LA is greedy , it will allow its terminal voltage to be held up and current delivered enough to destroy itself. It only self regulates within an agreed voltage range

I also mentioned that very large charging sources were not part of this discussion. However, even with large sources, there is an "agreed voltage range", isn't there? I don't see how at, say, 14.4V, a 1000A charger can dump its potential.

Originally Posted by goboatingnow

The analogy of combining start batteries and house batteries is misleading, typically the starter battery is virtually at full charge and hence receives little current from the combining process and most LA can withstand absorption level voltage unto about 14.8 virtually indefinitely. However run a higher long absorption phase and that battery will indeed suffer.

In practice, combining batteries of different configurations with fixed charging regimes can and does lead to lazy battery syndrome, poor cycling and premature failure.

Again we are talking 14.4V, not 14.8V. In the context of other posters here convinced that the AGM will suck current and be fully charged (and still sucking current) well before the FLA battery, I think a combined small start battery with a large house bank is a good analogy. How do you see that as different in this context?

As for the general sense of a combined start battery suffering from long absorption phases - we recently replaced our small 14 year old starting batteries, which have spent their entire lives combined to a 675Ahr house bank during charging to a 14.8V absorption setpoint and full absorption times. They are even combined during equalization charging to 15.5V for up to 5hrs.

Those start batteries outlived 3 house banks, and I think it was the lightning strike that actually took them out since they stopped working the day after the strike (which was the first day we tried to use them).

Maybe this is just an anomaly? I know I'm not the only one with this experience, so maybe I only know anomalous people?

Again, I agree that mixing is not a good general practice, but disagree on which battery will suffer. I also disagree that the failure problem will be significant (if even a problem) if the charging regime is good. I do agree that an improper charging regime is bad for batteries of all types, and probably worse for mixed types.

So, other than those couple of small, aside points, where exactly do you disagree with my reasoning and description?

Mark

[colemj]

Originally Posted by sailinglegend

Many thanks for introducing the Water Tank analogy - it is often the only way to explain volts amps and resistance to people who just can't understand it.

BUT, it still appears that it doesn't work for some people. but I would persist in using this analogy, but you need to make the tank analogy more realistic.

May I suggest that each tank really has a series of many tanks inside it and each internal tank discharges into the next and when that is full it discharges into the next, and so on. The pipe connecting these internal tanks together is 2" in the AGM tanks and 0.5" in the FLA tanks.

Now your two tanks can be joined together by a very large pipe fed by a constant flow pump. This will fill up the first tank within both the FLA and AGM tanks - the water pressures will be the same so this is the end of the absorption phase. A water level detector senses this and switches the pump to a constant pressure pump and the AGM tank with its bigger internal pipe will be filling up more slowly because it has to force water through a 2" pipe to the next tank, but it will still be filling much faster than the FLA tank. At this point the "adaptive charging algorithm" in the water pump sets a timer for say three hours, by the end of which the last of the internal tanks in AGM tank will be nearly full and the pump will switch to its lowest pressure which is just their to maintains leaks and evaporation. If the pump was switched off then some of the FLA tanks would still be empty and the AGM would start to allow its water to slowly help to fill the FLA tanks. In doing so some of the AGM's water would be lost to the FLA tanks. If the pump was left on the FLA tank would eventually fill also.

So in this scenario it is not the AGM battery that overcharges, but the FLA that under charges.

Now apply this to tank discharging. The AGM will allow more water to flow out more quickly than the FLA tank, and when the flow stops the FLA will have to help top up the AGM until the water levels equalize. So an already partly empty FLA tank has now lost more of it's water to the AGM tank.

I don't know if this helps??????

Again, the water tank analogy only works if the pipe connecting the two tanks is much larger than the pipes connected to the "loads".

Your newer description of it is not accurate in any way with regards to a battery. For example, do you really think that individual battery cells are charging/discharging sequentially? And that the resistance between them is the same as the total resistance of the battery?

It has nothing to do whether it "works for some people" - particularly those who do understand volts, amps and resistance, especially as they are applied to batteries. You are letting your predetermined conclusion drive the construction of your analogy into a very complicated situation to support that conclusion.

But if it "works" for you, that's OK with me. But doesn't help in correctly describing a battery bank.

Mark

[colemj]

Originally Posted by sailorchic34

Ah, Kirchoffs. I could not remember the fellows name. Thanks Dave. I pretty much agree with what you said.

So I am either confused or have a large comprehension problem (would not surprise me if both are true).

You say you agree with this, but it goes against most of what you were saying in your earlier posts.

Mark

[TeddyDiver]

Originally Posted by colemj

So I am either confused or have a large comprehension problem (would not surprise me if both are true).

You said it
No fence but you are too fixated to voltage which is pretty much irrelevant becouse it's the same in both batteries.
Thou you are right about water analogy not fitting to combined 2v cells in series.

[goboatingnow]

So, other than those couple of small, aside points, where exactly do you disagree with my reasoning and description?

Never said I did, merely wrote a piece based on my professional understanding

and yes in a less then 100% recharge AGM because of their arrangement suffer more then FLA;s, but that not a function of the paralleling

dave

[sailorchic34]

Originally Posted by colemj

So I am either confused or have a large comprehension problem (would not surprise me if both are true).

You say you agree with this, but it goes against most of what you were saying in your earlier posts.

Mark

Actually Mark, it doesn't. Dave mentions that each battery could have a different SOC when discharging even though the voltage is the same.

I'm sorry I can't explain this clearly enough for you to understand. Your perception that the same discharge voltage across the bank with different battery chemistry's in parallel will indicates the same SOC is in error. That is the case when all the batteries in a group are of the same chemistry. But mix different battery chemistry's with way different internal resistance curves and all bets are off. Voltage is just part of the equation.

The Rate of discharge current (or charge acceptance) is based on V/R=I in all cases . Just because you tie the two batterys in parallel does not at all mean both batteries behave the same way. YES. Voltage will be the same, but the Rate of discharge and recharge between the AGM and FLA ARE NOT THE SAME. The Battery chemistry is completely different and the internal resistance is not at all the same. So the discharge rates will not be the same.

Put it this way, A FLA has a peak charge rate of about C/3. AGM has a peak charge rate of C*5. So a 100 amp FLA can take 33 amps without damage. A 100 amp AGM can take a 500 amp charge current with no overheating or out gassing. Part of that is due to chemistry and part is due to internal resistance. When you join the two different battery types together in parallel, the internal resistance curve in each battery type stays the same. So the AGM resistance will always be way less then FLA, even when charging.

Under low load the discharge rates will be close to the same but as more amps are drawn the AGM will draw down a bit faster,and faster as amp load is increased. The FLA will be partly back feeding the AGM under lower loads. So it will look like they are putting out the same current. But measure the current flowing at each battery type and the rates will not/can not be the same.

The end result is that the FLA will not reach 100% SOC under charging and the AGM's will be worked harder. This will reduce the life of both batteries rather quickly. Its Physic's and a bit o chemistry.

OK I'm done trying to explain this.

[colemj]

Originally Posted by TeddyDiver

No fence but you are too fixated to voltage which is pretty much irrelevant becouse it's the same in both batteries.

Yes, that was my whole point and the key to understanding this. I wasn't fixated on it - I was trying to get people to understand this point (which you seem to imply I had the opposite view, which is incorrect).

Mark

[colemj]

Originally Posted by goboatingnow

Never said I did, merely wrote a piece based on my professional understanding

Well, thank you. It is interesting to see people here agree with you and disagree with me! Including trying "one more time" to get me to understand the points that you are agreeing with me on.

Mark

[hellosailor]

Sounds like we need to groupthink a "fair" test scenario and then ask Mainesail to bolt up some batteries and see how they fare after a year of it.

A wet lead SLI, a wet lead deep cycle, an AGM, a lithium phosphate...You know, might as well abuse a whole bench full and get some definitive answers.

[goboatingnow]

Originally Posted by sailorchic34

Actually Mark, it doesn't. Dave mentions that each battery could have a different SOC when discharging even though the voltage is the same.

I'm sorry I can't explain this clearly enough for you to understand. Your perception that the same discharge voltage across the bank with different battery chemistry's in parallel will indicates the same SOC is in error. That is the case when all the batteries in a group are of the same chemistry. But mix different battery chemistry's with way different internal resistance curves and all bets are off. Voltage is just part of the equation.

The Rate of discharge current (or charge acceptance) is based on V/R=I in all cases . Just because you tie the two batterys in parallel does not at all mean both batteries behave the same way. YES. Voltage will be the same, but the Rate of discharge and recharge between the AGM and FLA ARE NOT THE SAME. The Battery chemistry is completely different and the internal resistance is not at all the same. So the discharge rates will not be the same.

Put it this way, A FLA has a peak charge rate of about C/3. AGM has a peak charge rate of C*5. So a 100 amp FLA can take 33 amps without damage. A 100 amp AGM can take a 500 amp charge current with no overheating or out gassing. Part of that is due to chemistry and part is due to internal resistance. When you join the two different battery types together in parallel, the internal resistance curve in each battery type stays the same. So the AGM resistance will always be way less then FLA, even when charging.

Under low load the discharge rates will be close to the same but as more amps are drawn the AGM will draw down a bit faster,and faster as amp load is increased. The FLA will be partly back feeding the AGM under lower loads. So it will look like they are putting out the same current. But measure the current flowing at each battery type and the rates will not/can not be the same.

The end result is that the FLA will not reach 100% SOC under charging and the AGM's will be worked harder. This will reduce the life of both batteries rather quickly. Its Physic's and a bit o chemistry.

OK I'm done trying to explain this.

Most of what you state is bang on the button , even if a little long winded

The last paragraph is only wrong, if for some reason the absorption cycle was stopped for no reason.

two batteries together will both reach 100% , i.e. both will reach the absorption voltage together and then the absorption phase will complete that. What is does mean is the battery that finishes first will be subject to a longer absorption voltage then is necessary, this can or may cause gassing depending on exactly what voltages are used,

But it is wrong to say that the FLA will not reach 100% SOC merely because its parallel with an AGM.

dave

[sailinglegend]

I'm sorry if some didn't understand my expanded water tank analogy, but this is a standard Nigel Calder explanation of battery charge acceptance, but he uses membranes within the tank not additional tanks with a restrictive pipe in between. So if you disagree with my analogy then argue with Mr Calder. I have taken his analogy with two tanks with different internal resistances, or different membranes, and I have even joined them together with a very large pipe. Some people here just do not read posts properly before jumping in with both feet. That is plain rudeness. Nowhere did I suggest these internal tanks were cell analogies, they are representing the difference between the surface areas of the plates and the internal areas.

[colemj]

Originally Posted by sailorchic34

Actually Mark, it doesn't. Dave mentions that each battery could have a different SOC when discharging even though the voltage is the same.

I'm sorry I can't explain this clearly enough for you to understand. Your perception that the same discharge voltage across the bank with different battery chemistry's in parallel will indicates the same SOC is in error. That is the case when all the batteries in a group are of the same chemistry. But mix different battery chemistry's with way different internal resistance curves and all bets are off. Voltage is just part of the equation.

The Rate of discharge current (or charge acceptance) is based on V/R=I in all cases . Just because you tie the two batterys in parallel does not at all mean both batteries behave the same way. YES. Voltage will be the same, but the Rate of discharge and recharge between the AGM and FLA ARE NOT THE SAME. The Battery chemistry is completely different and the internal resistance is not at all the same. So the discharge rates will not be the same.

Put it this way, A FLA has a peak charge rate of about C/3. AGM has a peak charge rate of C*5. So a 100 amp FLA can take 33 amps without damage. A 100 amp AGM can take a 500 amp charge current with no overheating or out gassing. Part of that is due to chemistry and part is due to internal resistance. When you join the two different battery types together in parallel, the internal resistance curve in each battery type stays the same. So the AGM resistance will always be way less then FLA, even when charging.

Under low load the discharge rates will be close to the same but as more amps are drawn the AGM will draw down a bit faster,and faster as amp load is increased. The FLA will be partly back feeding the AGM under lower loads. So it will look like they are putting out the same current. But measure the current flowing at each battery type and the rates will not/can not be the same.

The end result is that the FLA will not reach 100% SOC under charging and the AGM's will be worked harder. This will reduce the life of both batteries rather quickly. Its Physic's and a bit o chemistry.

OK I'm done trying to explain this.

Since you won't read my explanations, please go back and read Dave's.

You will see that he doesn't agree with this description in practical use. I have always said that an experiment can be done with transient currents and separation of batteries to prove your point. However, in real-life, those experiments have no relevance.

You are still convinced that somehow AGM's are storing and discharging 6x more energy than a similar sized FLA. This is incorrect. While at any instantaneous point at one voltage you can say that the AGM resistance is lower and it will be accepting more current, at the next instantaneous point at that voltage the resistance will be higher and it will be taking less current. You cannot hold V and R constant while increasing I.

Whether in that equivalent circuit the FLA is supply current to, or taking from, the AGM vs. the load/source is inconsequential. Both batteries will be at the same SOC when it is all done.

Are you still willing to say that during discharge, an AGM will tap out hours earlier than a FLA and just sit there while the FLA powers the load? Conversely, are you still sticking to your point that during charging, the AGM races up to full charge and then sits there boiling away while the FLA pokes along at a much lower SOC?

These are the exact points you have made with all of your explanations, yet they are profoundly wrong.

Let me ask one more question: After 100 full cycles of this combined bank, how many cycles do you think each battery has experienced?

You say I'm wrong and that you agree with Dave's explanation. Yet here are some quotes from his post:

"In normal use with fractional C discharges, the batteries will lower their SOC percentage evenly."

"On charging both batteries will ultimately charge up according to their acceptance rates ,thats true ( up to a point) , (sorry edit here bad english) both will reach the absorption point at the same time ( i.e. the same voltage)"

"Again using Kirchoffs , you can see that in charging there can be current flowing from one battery to another, Thats not an issue. ultimately the system will reach full charge"

I have been saying these exact things (as Dave mentioned later), only being told I was wrong.

It is OK that you are done trying to explain this to me - I agree that it is a waste of your time.

Mark

[colemj]

Originally Posted by sailinglegend

I'm sorry if some didn't understand my expanded water tank analogy, but this is a standard Nigel Calder explanation of battery charge acceptance, but he uses membranes within the tank not additional tanks with a restrictive pipe in between. So if you disagree with my analogy then argue with Mr Calder. I have taken his analogy with two tanks with different internal resistances, or different membranes, and I have even joined them together with a very large pipe. Some people here just do not read posts properly before jumping in with both feet. That is plain rudeness. Nowhere did I suggest these internal tanks were cell analogies, they are representing the difference between the surface areas of the plates and the internal areas.

Your explanation is still complicated and confusing. Each "tank" from the original analogy is a battery. You then divided each battery into smaller "tanks". If those smaller tanks aren't individual cells, then what are they? If they are the difference between the surface areas of a single battery's plates and their internal areas (I am assuming you actually mean volume here), then that is an internal cell.

I don't know how you can surmise I did not read your post "properly". Sorry my inability to understand your writing correctly seems rude. Perhaps you should tell the moderators.

Mark

[TeddyDiver]

Mark, if the voltage is the same and resistance differs, which route the current mostly goes?

[colemj]

Originally Posted by goboatingnow

Most of what you state is bang on the button , even if a little long winded

The last paragraph is only wrong, if for some reason the absorption cycle was stopped for no reason.

two batteries together will both reach 100% , i.e. both will reach the absorption voltage together and then the absorption phase will complete that. What is does mean is the battery that finishes first will be subject to a longer absorption voltage then is necessary, this can or may cause gassing depending on exactly what voltages are used,

But it is wrong to say that the FLA will not reach 100% SOC merely because its parallel with an AGM.

dave

If that last paragraph is wrong about charging, then the discharging part is wrong also.

Again, I have always stated the instantaneous case of charge/discharge differences between the two batteries. I have agreed that at any given point in time, they are behaving this way, but stressed that they need to be understood in terms of an equivalent circuit. I have also described experiments that would result in this case, but in those experiments they would no longer be in an equivalent circuit.

What I don't agree on is that at the end of the load/charge period, the batteries are sitting with different SOC's. This is real-life usage - fractional C loads are applied over time and fractional C charging occurs at least up to absorption voltage.

I don't know anyone with unity C or greater charging systems in real-life cruising (we can generate 330A of charging, and that is only 0.5C of our modest bank). I also do not know anyone who generates unity C or greater loads.

In SC's example above, there is no way a charger could/would supply 500A to a combined bank. It may momentarily supply a large current to the AGM, but that battery's internal resistance would build to the point that the current acceptance would be low until the internal resistance of the FLA built up enough to allow the combined bank to increase in voltage.

Is this last paragraph wrong? Is the one above it wrong?

If they are, then my understanding is wrong. If they are not, then I am correct.

Mark