Mixing Battery types in 1 Bank

[sailinglegend]

Originally Posted by colemj

No. While what you say is correct when speaking about AGM and FLA separately, when combined together in a single bank none of that matters at all with regard to charging.....

.....When the final voltage is reached (say 14.4V), the SOC of the two batteries will be so close together that there will be no problem with over-charging or under-charging either of them during the final stage of absorption.......

There are nearly 1000 watching this argument so we need to try and move this on without boring people.

You accept that FLAs and AGMs charge at different rates, that's positive. Let's just consider what has happened at the end of the Boost stage when the bank has reached 14.4v. You say their SOC will be so close - that must be impossible because the AGM accepts charge maybe 40% faster than the FLA. At 14.4v the AGM current will then decline and the FLA current will continue to rise, but it's SOC will always be lagging behind the AGM.

Talking about charging starter batteries and big service banks only confuses the argument. You don't seem to accept my point that it is voltage not current that overcharges batteries. I am happy to have that discussion on another thread but not here as it is not relevant here. IMHO!

We need some other input to this thread!!!

[colemj]

Originally Posted by sailinglegend

Let's just consider what has happened at the end of the Boost stage when the bank has reached 14.4v. You say their SOC will be so close - that must be impossible because the AGM accepts charge maybe 40% faster than the FLA. At 14.4v the AGM current will then decline and the FLA current will continue to rise, but it's SOC will always be lagging behind the AGM.

Talking about charging starter batteries and big service banks only confuses the argument. You don't seem to accept my point that it is voltage not current that overcharges batteries.

You don't seem happy to accept the fact that the AGM cannot reach a higher voltage than the FLA when they are combined into a single bank. It is not possible - I don't know how to get you to understand that single point that is holding you back.

It simply does not matter how fast the AGM can accept charging current because it will always be tied with the FLA in a bank. When the AGM reaches (say) 14.4V, it will only be when the FLA is also at that voltage. When the combined bank is held at that voltage while the charging current is tapered to the criteria that meets "Float", both batteries will be fully charged when "Float" is reached. Any difference between the two will be inconsequential and on the order of differences between cells within a single battery.

The example of a very small 99% SOC battery combined with a very large 50% SOC bank is exactly on argument and just the opposite of confusing. That small battery cannot accept current like the large bank and it would reach full charge way before the large bank if it was charged separately. Yet, you will not overcharge a mostly-full 20Ahr motorcycle start battery using a 120A charger when it is combined with a large, mostly empty house bank. This is an extreme example - much more so than combining a deep cycle AGM with a FLA at similar SOC.

When you understand that, you will understand how a combined AGM/FLA battery bank behaves during charging.

Perhaps someone else whose knowledge you do trust will chime in on this.

Mark

[colemj]

Originally Posted by sailinglegend

There are nearly 1000 watching this argument so we need to try and move this on without boring people.

BTW, if this was a criteria on CF, there would be very few thread posts indeed.

Mark

[FlyingCloud1937]

6.5.2 Wh-Efficiency
In addition to item “Ah-Efficiency”, average voltages during discharge and
re-charging have to be taken into account.
Discharged Ah • Average Voltage Discharge
Definition: Wh-Efficiency =
Re-charged Ah • Average Voltage Recharge
Example:
Discharge: Battery C10 = 100 Ah
10h discharge, rate: I10
discharged: C10 = 100 Ah
(100% DOD)
Average voltage during C10-discharge: 2.0 Vpc
(estimated)
__________________________________________________ __________
Classic-Handbook, Part 2 (Edition 4, January 2012) - 39 -
Industrial Power, Application Engineering Copyright – All Rights Reserved
Recharging: IU-Charging 2.23 Vpc, 1• I10
Expected re-charging time (incl. charge coefficient 1.20): 32 hours
Estimate for average voltage during re-charging: The voltage increases
from 2.1 Vpc to 2.23 Vpc during 9 hours
average 2.16 Vpc.
The voltage is constant at 2.23 Vpc for (32-9) hours = 23 hours.
Estimated average voltage during 32 hours: 2.21 Vpc
100 Ah • 2.0 Vpc
Wh-efficiency = = 0.754 = 75 %
120 Ah • 2.23 Vpc

[colemj]

Uh, OK.

What are you trying to say?????

Mark

[Rusty123]

Consider the connection in parallel of two random AGM and FLA batteries, without a charging source. Current will flow between the batteries until an equilibrium is reached, even though their post voltages are equal (by definition).

Does this not imply that batteries can be at a different state of charge (at least for a while), even when connected in parallel and the post voltages are equal?

And in turn, does this not imply the same can occur during a charging evolution?

[colemj]

Originally Posted by Rusty123

Consider the connection in parallel of two random AGM and FLA batteries, without a charging source. Current will flow between the batteries until an equilibrium is reached, even though their post voltages are equal (by definition).

Does this not imply that batteries can be at a different state of charge (at least for a while), even when connected in parallel and the post voltages are equal?

And in turn, does this not imply the same can occur during a charging evolution?

For the first and second questions: current cannot flow between two batteries when voltages are equal. Without an external current source, if batteries are at the same voltage, they have the same state of charge and no current will flow between them. If batteries are at different states of charge, their voltages will be different and current will flow until the voltage is equalized.

Ask yourself what happens with the individual cells in a battery being hooked together. Is current flowing between them?

For the third question: sure, all batteries (and all individual cells of a single battery) are at dynamically different states of charge during a charge routine. I tried to make that point earlier. One battery (or individual cell of a battery) may get to a higher voltage than the others, but then its internal resistance rises so that it cannot accept more current at that voltage level, so it "waits" for the others to catch up and then starts off again.

At the end, when all the batteries (and individual cells of those batteries) are at the same voltage and no longer taking current at that voltage, then all batteries are charged.

I made a point above to mention individual cells of a battery. A combined bank is nothing but a series/parallel string of individual battery cells. It does not matter how those cells are packaged, how many of those cells are in a package or how many of those packages are in a bank. The charger sees a single "battery" consisting of all of those cells together with single equivalent circuit resistance and voltage.

Mark

[Rusty123]

Originally Posted by colemj

If batteries are at different states of charge, their voltages will be different and current will flow until the voltage is equalized.

Completely concur. But I think this is where the apparent quandary comes from – i.e., how can their voltages be different when they are connected together? The answer, I believe, lies in the recognition that for a parallel bank, the battery terminal voltage has no particular significance in terms of overall bank SOC (unless the bank is quiescent, and voltage has equalized across the bank).

Originally Posted by colemj

Ask yourself what happens with the individual cells in a battery being hooked together. Is current flowing between them?

I don’t think that is directly applicable, since the individual cells are in series, and so current will not flow since there is not a complete circuit.

Originally Posted by colemj

At the end, when all the batteries (and individual cells of those batteries) are at the same voltage and no longer taking current at that voltage, then all batteries are charged.

I think this is a key distinction – only when the bank is quiescent (no current flow in, out, or between) does the voltage at the terminals indicate the overall charge of the bank. Since for practical purposes, it’s all but impossible to get the bank to this point (assuming the charger even had a way to recognize when it occurs), the charger will shift to float mode at some point sooner, at which point there will, in fact, be a difference in the SOC of the batteries.

The magnitude of the SOC difference, and whether it is significant enough to matter in the real world is a different question, and would depend on many variables (most of which are eliminated, or nearly so, by using similar batteries)

[colemj]

Originally Posted by Rusty123

Completely concur. But I think this is where the apparent quandary comes from – i.e., how can their voltages be different when they are connected together? The answer, I believe, lies in the recognition that for a parallel bank, the battery terminal voltage has no particular significance in terms of overall bank SOC (unless the bank is quiescent, and voltage has equalized across the bank).

Yes, but we are talking about a bank, not quickly connecting two batteries together to see what happens. While voltage is a poor way to measure SOC, in a bank of two parallel batteries, a resting voltage is proportional to SOC as per your disclaimer. If you mean that voltage is not significant during the charge cycle where the current is cut suddenly and the voltage measured quickly, then that is the same for any two batteries of any type.


I don’t think that is directly applicable, since the individual cells are in series, and so current will not flow since there is not a complete circuit.

There is a complete circuit while charging and discharging. There is also a complete circuit between all but the last two cells in a series string. Each cell is a battery in and of itself. They are strung together only to get higher voltages.


I think this is a key distinction – only when the bank is quiescent (no current flow in, out, or between) does the voltage at the terminals indicate the overall charge of the bank. Since for practical purposes, it’s all but impossible to get the bank to this point (assuming the charger even had a way to recognize when it occurs), the charger will shift to float mode at some point sooner, at which point there will, in fact, be a difference in the SOC of the batteries.

No, I don't agree this is correct. First, it is very easy to get a bank of 12V batteries to, say, 14.4V. And keep them there forever.

A charger understands when to go into float because one has told it when the proper point is. Our charger, for example, knows how many amps it needed to supply through bulk and absorption and calculates when to go to float based on time, number of amps being accepted, and rate of change of that acceptance. When our charger cuts to float, the bank has either been in absorption for 10hrs (and the charger is probably saving our boat and any remaining good batteries here), or the amperage acceptance is 2% of the bank capacity or the rate of change of charge acceptance after 2hrs is very low (these are all user-settings, so they are being used as a personal example of what I consider "charged").

At this point, the charger shifts to float mode and there is no difference between the SOC of the batteries - or any difference is indistinguishable from that of individual cells in a battery.

However, even less stringent float requirements will bring one to this point. Bringing the mixed AGM/FLA bank to 14.4V, holding it there for a set 2 hours, and going into float - the individual batteries will be at the same nominal SOC.


The magnitude of the SOC difference, and whether it is significant enough to matter in the real world is a different question, and would depend on many variables (most of which are eliminated, or nearly so, by using similar batteries)

OK, here is everyone's out on this. The absolute magnitude of SOC at the end of a charge cycle could be different between the batteries. It will not be significant or any different than that between individual cells of each battery, and it will be the same magnitudes as if the battery types were the same - but it could be different. If anyone wants out of this - here is your chance.

However, it is a very weak way out with no supporting argument.

There really isn't any other "variables" to eliminate. But this is an academic discussion for the OP - he would be silly to pay for AGM's to add to his existing FLA bank. However, there would be no problem if he did so.

There also would be no problem if one found themselves with an AGM bank and could only source FLA batteries.

Mark

[FlyingCloud1937]

Sorry about that Mark,

I typed additional comments, below and posted, then left the computer. They vaporized.

So will try again.

A fully charged VRSLA resting voltage is 12.9 to 13 volts, depending on manufacture.

A fully charged FLA resting voltage is 12.7 to 12.6, again depending on manufacture.

It takes +/-3 tenths a volt, for current to flow from charge source to battery, this could be a charger or a battery at a higher voltage. This is due to its internal resistance, VRSLA's being less then FLA's.


Now battery banks can be used in many different ways, ie: Back-up, un-interupted power supply, load sharing, cycle discharge/charge, and PSOC cycle discharge/charge.

On boats we typically uses battery banks in two ways

1. Cycle discharge/charge, for the boats that are usually tied to the dock, and go out for the day, weekend, or even maybe a week or two. They have in inverter charger, or a stand alone charger, and an alternator. Maybe even a generator. These boats will recieve a full charge to their batteries when tied to the dock and plugged in for more then 48 hrs. Then they will float until the next excursion.

Using a mixed bank in this situation, will eventually damage the VRSLA due to being held to a higher then required voltage for complete re-charge. They will charge faster then the FLA's, and the excess voltage will generate heat in the VRSLA's which cause undue venting and eventual dry-out. Or it will lead to hard sulphation on the plates of the FLA's because of constant under charging, and the inability to equalize charge them.


2. PSOC cycle discharge/charge, for these boats, they are cruisers. They seldom tie to the dock for extended periods. These boats typically operate in the 50% to 80% SOC. They may have a generator, usually an alternator, solar, and maybe even wind. They seldom reach a full state of charge, as this takes 25- 35 hours depending on charge source, and mostly never are fully charged.

Problems with using a mixed bank in this situation are. VRSLAs need to be fully charged as often as possible, else they start to build dendrites and cause micro shorting. Mostly they can't be equalized except for Lifelines. They like to charge at temp compensated to 14.4 volts for boost stage. They are a poor choice for bat banks in this operation.

On the other hand FLA's like to charge to a temp compensated 14.8 volts. Like the VRSLA's they also would prefer to be fully recharged at the end of a cycle, else they start to hard sulphate. This can be overcome by an equalize cycle, which you can't due in a mixed bank(caveat Lifelines). They can tolerate over charging ie: held to extended high voltages, but they lose water at a faster rate, and also suffer grid corrosion on the positive plates, which will dramaticly shorten their life. This is caused by being held to an over charge voltage, not an over charge current. Charging an FLA to less then 14.4 volts absorption will result in stratification of the electrolyte. This ends up causing corrosion on the bottom of the plates, and electrolyte starvation on the top of the plates, this area of the plates will hard sulphate, and reduce the battery capability.

Last but not least, VRSLA's are exothermic, and FLA's are endothermic. This means that in an thermo run away in the VRSLA's, it can be fueled by the FLA's.

In the end it is just silly to mix two different types of batteries.

Why do it. Batteries don't dye of old age they are murdered.

Lloyd

Originally Posted by colemj

Uh, OK.

What are you trying to say?????

Mark

Originally Posted by FlyingCloud1937

6.5.2 Wh-Efficiency
In addition to item “Ah-Efficiency”, average voltages during discharge and
re-charging have to be taken into account.
Discharged Ah • Average Voltage Discharge
Definition: Wh-Efficiency =
Re-charged Ah • Average Voltage Recharge
Example:
Discharge: Battery C10 = 100 Ah
10h discharge, rate: I10
discharged: C10 = 100 Ah
(100% DOD)
Average voltage during C10-discharge: 2.0 Vpc
(estimated)
__________________________________________________ __________
Classic-Handbook, Part 2 (Edition 4, January 2012) - 39 -
Industrial Power, Application Engineering Copyright – All Rights Reserved
Recharging: IU-Charging 2.23 Vpc, 1• I10
Expected re-charging time (incl. charge coefficient 1.20): 32 hours
Estimate for average voltage during re-charging: The voltage increases
from 2.1 Vpc to 2.23 Vpc during 9 hours
average 2.16 Vpc.
The voltage is constant at 2.23 Vpc for (32-9) hours = 23 hours.
Estimated average voltage during 32 hours: 2.21 Vpc
100 Ah • 2.0 Vpc
Wh-efficiency = = 0.754 = 75 %
120 Ah • 2.23 Vpc

[sailinglegend]

Originally Posted by colemj

You don't seem happy to accept the fact that the AGM cannot reach a higher voltage than the FLA when they are combined into a single bank. It is not possible - I don't know how to get you to understand that single point that is holding you back....

I am now going to act as a moderator and stop any further discussion between us as your posts are now becoming insulting. I have reported you to the Forum moderator.

Nowhere in any if my posts do I say or suggest that the AGM reaches a higher voltage than the FLA when connected together by a piece of wire in a bank. I say their SOC can be different because the open circuit voltages of different batteries at ant given state of charge can be very different. This changes with age which is why old and new batteries shouldn't be mixed in a bank. Disconnect your fictious bank of AGM and FLAs at any stage in their charging cycle and let them stabilise and their open circuit voltage will be different.

That is my last input to any conversation between us, and thanks to the others who have contributed to this thread.

I challenge your, or any of your supporters out there, to find a battery or a charger manufacturer (not a battery combiner manufacturer) to say that combining AGM and FLAs permanently in a service bank is a good idea. It will work, but it will shorten battery life.

[SimonV]

Originally Posted by sailinglegend

(note I have removed some of you quote that I do not wish to reply to. SimonV)

I challenge your, or any of your supporters out there, to find a battery or a charger manufacturer (not a battery combiner manufacturer) to say that combining AGM and FLAs permanently in a service bank is a good idea. It will work, but it will shorten battery life.

After reading your reply I re read the OP. My mistake, I did not read it correctly and do not recommend mixing battery types in a bank. I was thinking on the lines of a large gel bank for the house and an FLA as a start battery. Using the charge controllers set on gel.

Sent from my GT-N7105T using Cruisers Sailing Forum mobile app

[TeddyDiver]

Originally Posted by colemj

OK, here is everyone's out on this. The absolute magnitude of SOC at the end of a charge cycle could be different between the batteries. It will not be significant or any different than that between individual cells of each battery, and it will be the same magnitudes as if the battery types were the same - but it could be different. If anyone wants out of this - here is your chance.

Can't decide weather to cry or laugh.

[donradcliffe]

I was trying to stay out of this, as I don't think anyone disagrees that it is not a good idea to mix battery types--but I can't let some of the misinformation in this post stand unchallenged.

Originally Posted by FlyingCloud1937

Sorry about that Mark,

I typed additional comments, below and posted, then left the computer. They vaporized.

So will try again.

A fully charged VRSLA resting voltage is 12.9 to 13 volts, depending on manufacture. Perhaps a bit high, but you are right

A fully charged FLA resting voltage is 12.7 to 12.6, again depending on manufacture.Right again

It takes +/-3 tenths a volt, for current to flow from charge source to battery, this could be a charger or a battery at a higher voltage. This is due to its internal resistance, VRSLA's being less then FLA's.The voltage difference depends on the internal resistance AND the current--for small currents, the voltage difference can be much less than 0.3 volts.

Because the batteries in a mixed bank are in parallel, there is essentially no difference between the voltage of each battery type, AND THERE WILL BE NO FLOW BETWEEN ONE BATTERY TYPE AND THE OTHER. However, due to the differences in battery chemistry, one type will take more of the load current, and there will be differences between the SOC of the battery types. However, in a typical operating cycle, both types will share the load.



Now battery banks can be used in many different ways, ie: Back-up, un-interupted power supply, load sharing, cycle discharge/charge, and PSOC cycle discharge/charge.

On boats we typically uses battery banks in two ways

1. Cycle discharge/charge, for the boats that are usually tied to the dock, and go out for the day, weekend, or even maybe a week or two. They have in inverter charger, or a stand alone charger, and an alternator. Maybe even a generator. These boats will recieve a full charge to their batteries when tied to the dock and plugged in for more then 48 hrs. Then they will float until the next excursion.

Using a mixed bank in this situation, will eventually damage the VRSLA due to being held to a higher then required voltage for complete re-charge. They will charge faster then the FLA's, and the excess voltage will generate heat in the VRSLA's which cause undue venting and eventual dry-out. What excess voltage?? The only conceivable argument for excess voltage I can see is that IF you have a charger which drops its voltage from the absorption voltage from the float phase based on current flow, it will stay at absorption for a little while longer--but I really doubt that is going to murder the VRSLA batteries if you do it once a weekOr it will lead to hard sulphation on the plates of the FLA's because of constant under charging, and the inability to equalize charge them.This is not true for boats plugged into the dock--FLA batteries will not sulphate at float voltages


2. PSOC cycle discharge/charge, for these boats, they are cruisers. They seldom tie to the dock for extended periods. These boats typically operate in the 50% to 80% SOC. They may have a generator, usually an alternator, solar, and maybe even wind. They seldom reach a full state of charge, as this takes 25- 35 hours depending on charge source, and mostly never are fully charged.

Problems with using a mixed bank in this situation are. VRSLAs need to be fully charged as often as possible, else they start to build dendrites and cause micro shorting. Mostly they can't be equalized except for Lifelines. They like to charge at temp compensated to 14.4 volts for boost stage. They are a poor choice for bat banks in this operation.Not all VRSLA batteries are AGMs---gel batteries are much more tolerant of sitting at lower charge states than AGMs or FLAs, and they are a good choice for this operation.

On the other hand FLA's like to charge to a temp compensated 14.8 volts (not mine--too much water loss). Like the VRSLA's they also would prefer to be fully recharged at the end of a cycle, else they start to hard sulphate. This can be overcome by an equalize cycle, which you can't due in a mixed bank(caveat Lifelines). They can tolerate over charging ie: held to extended high voltages, but they lose water at a faster rate, and also suffer grid corrosion on the positive plates, which will dramaticly shorten their life. This is caused by being held to an over charge voltage, not an over charge current. Charging an FLA to less then 14.4 volts absorption will result in stratification of the electrolyte. This ends up causing corrosion on the bottom of the plates, and electrolyte starvation on the top of the plates, this area of the plates will hard sulphate, and reduce the battery capability. Fortunately this tale of gloom doesn't play out in real life--as all of you who have FLA batteries can attest, chronic undercharging may reduce battery life by a year or two, but is not to be confused with taking the battery down to less than 50% SOC and leaving it there for a week or two,which WILL cause rapid failure.

Last but not least, VRSLA's are exothermic, and FLA's are endothermic This means that in an thermo run away in the VRSLA's, it can be fueled by the FLA's.Maybe you can give a reference for this claim??.

In the end it is just silly to mix two different types of batteries.

Why do it. Batteries don't dye of old age they are murdered.

Lloyd

The reasons to mix battery types/ages/sizes may be cost driven and/or schedule driven. The important thing to understand is that mixed bank batteries may not be perfect, but they will work for months or even years in both operating scenarios. You don't have to ruin your cruise waiting around for the 'optimum' batteries to arrive.

[Andina Marie]

About 5 weeks ago one of my customers said that the supplier of their batteries had advised against using our Combiner100 to parallel batteries of different chemistry.

So I sent an enquiry to Interstate Batteries. "I am adding a new deep cycle AGM battery. Can I put it in parallel with another battery that is lead acid or do I have to install 2 new batteries?

They replied that I will need two new batteries.

I wrote back and said that the lead acid battery was only 1 year old and was still working fine and I hated to just dump it, what was wrong with putting it in parallel with a new one?

They sent back a nice detailed explanation that the new battery would "Not pull the old one up but in fact the old battery would DRAG THE NEW ONE DOWN." (An argument you often hear.)

Again I wrote back and said that if the old battery and the new battery were both fully charged and I put them in parallel then they would both be at the same voltage so there would be no current flow from battery to battery. How is the old one going to pull the new one down?

She wrote back to say she took my question to their battery engineer and he said there was no problem putting the batteries in parallel.

Kids working for a large company answering emails don't necessarily know the answers and perpetuate the myths they have heard. (Or are they instructed to sell batteries even if the customer doesn't need them?)