Mixing Battery types in 1 Bank

[SimonV]

It really comes down to your charger. AGMs like and need a higher charge rate so a flooded battery will work with an Agm charge setting, gel will not. Flooded will work with Gel if the charge setting is for gel. You will slowly kill an Agm on a gel setting and will kill a gel on an Agm setting.

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[sailinglegend]

Originally Posted by SimonV

It really comes down to your charger. AGMs like and need a higher charge rate so a flooded battery will work with an Agm charge setting, gel will not......

This is confusing and misleading.

You are confusing current charge rates with voltage. Chargers only set the charge voltage, and they can only deliver a maximum current set by the size of the charger. AGMs like a lower voltage than FLAs but can accept a much higher charge current. At all times it is the battery that accepts how much current it will accept. Also an AGM has a much higher charge acceptance rate, or charge efficiency, so it will charge quicker than the FLA so the charger will switch to Float before the FLA is charged. You will also kill an FLA and an AGM on a Gel setting. Trojan FLAs need 14.8v, a Gel stating could be as low as 14.1v! Mixing battery types is never a good idea. It's all to do with reaching the gassing voltage which gently stirs up the electrolyte. If they don't gas slightly batteries not only sulfate but it will also lead to stratification, or concentration of the acid at the bottom of the battery which can damage the plates.

[colemj]

Originally Posted by sailinglegend

Also an AGM has a much higher charge acceptance rate, or charge efficiency, so it will charge quicker than the FLA so the charger will switch to Float before the FLA is charged.

The rest of your post is true, but the second part of this statement is not.

Mark

[sailinglegend]

Originally Posted by colemj

The rest of your post is true, but the second part of this statement is not.

Mark

So I could have added a lot more detail, like this from the firststart AGM batteries site:

1. When charging, the AGM will reach full charge before the wet cell. This means that the charge system will do one of two things. It will keep charging until the wet cell is fully charged and may over-charge the AGM or it will switch off when the AGM is fully charged which means the wet cell is only partially charged.

2. The wet cell will try to draw power from the AGM and this will reduce the life of both batteries because of under-charging then draining them will naturally damage both batteries but will kill the wet cell quickly.

This general statement I agree with, but what actually happens depends on a lot of factors. If there is a potential problem why put one or other of the batteries at risk and shorten their life.

If any of this is not true then please correct me. That is what I try and do on my posts - add more information, not just say black isn't white!

[colemj]

I think they are making some assumptions in their statements, and they don't fully understand what they are talking about. First, they recommend their AGM's to be charged at 14.1V. If they are assuming that one would try to charge to higher voltages to suit the FLA, then yes, this will overcharge an AGM where 14.1V is critical.

Most AGM's are recommended to be charged at much higher voltages - close to those for FLA's. At 14.4V, for example, this is fine for FLA, so let's use that in analysis.

A bank consisting of an AGM and FLA battery in parallel will not reach 14.4V until both batteries are at 14.4V. Even though the AGM may be taking more current than the FLA during the bulk phase of charging, there is no way the AGM can reach final voltage before the FLA.

If both batteries are at 14.4V during charge, there is no way the FLA can "try to draw power" from the AGM.

When this mixed bank is brought to 14.4V and held there, each battery's individual resistance will control its current acceptance - in the case of a mostly charged AGM, that current acceptance will be close to nothing and the majority of the current will go to the FLA. There is no way the AGM can be overcharged while the FLA is still accepting current. When both are charged, the charger will cut the voltage.

During discharge, each battery will provide current proportional to its internal resistance and both batteries will stay at the same voltage doing so. Neither will be harmed or "used up" faster during their lifetimes.

Mark

[SimonV]

Originally Posted by sailinglegend

This is confusing and misleading.

You are confusing current charge rates with voltage. Chargers only set the charge voltage, and they can only deliver a maximum current set by the size of the charger. AGMs like a lower voltage than FLAs but can accept a much higher charge current. At all times it is the battery that accepts how much current it will accept. Also an AGM has a much higher charge acceptance rate, or charge efficiency, so it will charge quicker than the FLA so the charger will switch to Float before the FLA is charged. You will also kill an FLA and an AGM on a Gel setting. Trojan FLAs need 14.8v, a Gel stating could be as low as 14.1v! Mixing battery types is never a good idea. It's all to do with reaching the gassing voltage which gently stirs up the electrolyte. If they don't gas slightly batteries not only sulfate but it will also lead to stratification, or concentration of the acid at the bottom of the battery which can damage the plates.

I don't see what is confusing about it at all. If you must mix type, flooded will cope. Say your house bank is OK and your starter is shot, then if only flooded are available don't sweat it. Flooded batteries will cope with a charging raigeme that is set for Agm or gel, but not the other way around.

Flooded: (not sealed)

Typical absorption voltage range 14.4 to 14.9 volts, typical float voltage range 13.1 to 13.4 volts.

Flooded: (sealed)

Typical absorption voltage range 14.2 to 14.7 volts, typical float voltage range 13.1 to 13.4 volts.


AGM:

Typical absorption voltage range 14.4 to 15.0 volts, typical float voltage range 13.2 to 13.8 volts.

GEL:

Typical absorption voltage range 14.0 to 14.2 volts, typical float voltage range 13.1 to 13.3 volts.

Flooded can take it both ways.

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[hellosailor]

"AGM:
Typical absorption voltage range 14.4 to 15.0 volts, typical float voltage range 13.2 to 13.8 volts."

And let's compare that to Deka's recommendation for one of their G31 AGMs:
At 75F, a nominal temperature:
Bulk charge, 14.3 Optimum, 14.6 Maximum
Float charge, 13.4 Optimum, 13.7 Maximum

Consider that "optimum" is what you want for eight year battery life instead of three or four. Incidentally, they've also said (verbally) that 1/10th of a volt here or there is not important.

But what happens when your FLA charger is putting out 14.4 volts because the FLA battery is still getting a bulk charge, while the AGM is already charged up and wants to be seeing a 13.4 volt float charge?

It may or may not happen that way, but these are the things that *can* easily happen with mixed charging setups, that most owners won't really become aware of until they find the batteries have prematurely died.

[SimonV]

Hello sailor, it's all well and good to give an exact voltage but what do you do when your charge controllers have switches for battery type?

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[FlyingCloud1937]

Everything you want to know about Exide Batteries

Lloyd

[colemj]

Originally Posted by hellosailor

But what happens when your FLA charger is putting out 14.4 volts because the FLA battery is still getting a bulk charge, while the AGM is already charged up and wants to be seeing a 13.4 volt float charge?

No, it doesn't work that way. See my post above. Batteries connected together in a bank cannot reach independent voltages from a charge source. They will all reach the same voltages together. There is no possibility of overcharging one and not the other unless the charging voltage is set too high for one.

Mark

[sailinglegend]

Originally Posted by colemj

I think they are making some assumptions in their statements, and they don't fully understand what they are talking about.....

Reading more of Firststart's website and they do appear to be a bit lacking in knowledge. their chargers are a maximum of 20 amps, so they are missing the main point of AGMs in that they will charge faster. They do say charge at 14.2 to 14.4 for AGMs. I was just using them as an example of what battery manufacturers are saying.

Originally Posted by colemj

...If both batteries are at 14.4V during charge, there is no way the FLA can "try to draw power" from the AGM.

This is a contentious issue and not worth debating in detail here, but similar 'power drain' happens when mixing old and new batteries where the batteries really each want to be at different voltages. Maybe this is where they are coming from?

Originally Posted by colemj

... There is no way the AGM can be overcharged while the FLA is still accepting current. When both are charged, the charger will cut the voltage.

This I believe is where your reasoning is flawed. If the AGM charges much quicker than the FLA it will sit their taking no current as you say, but it is not current that overcharges at battery, but voltage. Sitting at its gassing voltage for a long time whilst the FLA continues to charge causes the AGM to gas excessively. This is bad for any sealed battery.

The other scenario where the FLA could be undercharged is because when both batteries have reached 14.4v the charger will enter the Absorption phase for a pre-set time. The AGM will be 80% charged but the FLA will not be. The charger switches down to float after a time of say 3 hours, not when it thinks the combined batteries are fully charged. At this point the AGM will be nearly fully charged but the FLA will not be.

It's worth noting that battery manufacturers recommend NOT to mix battery types in a bank.

[colemj]

Originally Posted by sailinglegend

This I believe is where your reasoning is flawed. If the AGM charges much quicker than the FLA

This is only (possibly) true if you have a charger capable of delivering greater than 0.5C current for long periods of time. For a typical 450-650Ahr bank, that will be 200-400 amps before the AGM stretches its legs appreciably above the FLA. Most people do not have this charging capacity. For almost every charger in common use with even modestly small banks, my reasoning stands.

However, I don't even think the above will happen because the AGM cannot charge "quicker" when connected in parallel with a FLA. The total bank voltage will rise as a whole because the charger only sees the combined bank - not each individual cell in the bank. Locally, the AGM may get to, say, 13.8V, but then its internal resistance will limit the current it can accept and its voltage will sit at 13.8V until the FLA battery's internal resistance rises enough to allow for the bank voltage to rise further.

This is also why small starting batteries combined into large house banks and charged with (relatively) high current sources do not get over-charged.

Originally Posted by sailinglegend

The other scenario where the FLA could be undercharged is because when both batteries have reached 14.4v the charger will enter the Absorption phase for a pre-set time.

This speaks to the quality of the charger, not the issue here. A good charger uses an adaptive algorithm to determine the time spent in absorption.

The admonishment against mixing battery types is more about proper charging voltages and equalization than how the battery's electrolytes are contained and distributed (note that they are all the same chemistries - lead/acid).

Mark

[hellosailor]

Simon-
All charge controllers are not created equal. Some may allow you to set specific voltages while others dumb it down and just set "types". Others will charge you extra to custom-program controllers to match whatever your battery maker claims is ideal. At a certain point...I think most owners would rather just set the "type", ignore the technical points and replace their batteries more often because it is less painful than doing the math and trying to match things up.
So as they say at the horse track, "you pays your money and you takes your choices."

Mark-
I don't know, I haven't field-tested enough setups to know who to believe, since just about every source differs on their advice. Do bear in mind that there are all sorts of charge sources out there, some voltage controlled, some amp controlled, some the gods only know how controlled. Or internally limited. And at least one MPPT controller charges based on leading the battery voltage by "this much more" volts, subject to an amperage limit that is actually based on the capacity of the battery bank, using a microprocessor and look-up table. All trade secrets, if you ask them.

In practical terms if you have to mix, you mix. If you don't have to mix...maybe you don't mix so there's one less question to ask four years from now. "Have to" being an economic, technical, or whatever criteria.

Since some folks get three or four years when others get eight or more, there's obviously more to batteries than meets the eye. I think we can agree on that much anyway.

[sailinglegend]

Originally Posted by colemj

This is only (possibly) true if you have a charger capable of delivering greater than 0.5C current for long periods of time. For a typical 450-650Ahr bank, that will be 200-400 amps before the AGM stretches its legs appreciably above the FLA...... my reasoning stands......

Now I see where your reasoning is faulty.

AGMs main advantage is they can charge much faster AND accept higher charging currents, but not all brands make these claims, so potential new buyers are often confused.

Even without higher charging currents AGMs will charge faster than FLAs, even from modest solar arrays, because their “"coulombic or charge efficiency” is much better. Wet cells may be only 70% efficient which means about 140Ah must be put in to actually raise the capacity of a battery by 100Ah.(70% of 140Ah is 100Ah) Lifeline AGMs may be up to 98% efficient so only 102 Ah needs to be put in. That’s nearly 40% faster charging and it doesn’t take a large charger to do it.

NET- charge efficiency is a non-linear function of battery state-of-charge. It's the value you put into your good Battery Monitor, the default setting is about 85%. The problem is that from 50-80% State of Charge (SOC) the overall battery charging efficiency is about 95%, and above 85% SOC it can be less than 50%. These figures depend on battery age and type, so having two batteries in the same bank with different charge efficiencies doesn't make much sense.

So charge efficiency is why the AGM can accept more charge as the bank reaches 14.4v and the "adaptive algorithm" starts the absorption phase, based on the AGM having reached 80% SOC. If the charger drops to Float before the FLA has taken in all the charge it can it will be less charged than the AGM.

[colemj]

Originally Posted by sailinglegend

Now I see where your reasoning is faulty.

AGMs main advantage is they can charge much faster AND accept higher charging currents, but not all brands make these claims, so potential new buyers are often confused.

Even without higher charging currents AGMs will charge faster than FLAs, even from modest solar arrays, because their “"coulombic or charge efficiency” is much better. Wet cells may be only 70% efficient which means about 140Ah must be put in to actually raise the capacity of a battery by 100Ah.(70% of 140Ah is 100Ah) Lifeline AGMs may be up to 98% efficient so only 102 Ah needs to be put in. That’s nearly 40% faster charging and it doesn’t take a large charger to do it.

NET- charge efficiency is a non-linear function of battery state-of-charge. It's the value you put into your good Battery Monitor, the default setting is about 85%. The problem is that from 50-80% State of Charge (SOC) the overall battery charging efficiency is about 95%, and above 85% SOC it can be less than 50%. These figures depend on battery age and type, so having two batteries in the same bank with different charge efficiencies doesn't make much sense.

So charge efficiency is why the AGM can accept more charge as the bank reaches 14.4v and the "adaptive algorithm" starts the absorption phase, based on the AGM having reached 80% SOC. If the charger drops to Float before the FLA has taken in all the charge it can it will be less charged than the AGM.

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.

The bank presents a single resistance and voltage to the charger. The charger works using that resistance and voltage. The AGM cannot reach a higher voltage than the FLA or charge independently of it when viewed as a total bank.

The AGM cannot take a proportionally higher charge current than the FLA from the charger unless the charger is a whopping huge one with current to spare.

While the AGM may in a transitory sense reach a state of intermediate charge before the FLA, its internal resistance at that instantaneous state will slow its current acceptance until the FLA catches up and the total bank moves into a higher voltage.

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.

I understand this is counter-intuitive when just reading AGM brochures, but you need to think of a combined bank - not how individual cells react to charging. For example, it is very common for boats to have large deep-cycle house banks and small starting batteries that are combined into a single bank for charging with higher-current charge sources. In other words, small, mostly charged lightweight batteries are being hit by a high-current charge source simultaneous with the large heavy house bank. Ask yourself why that small start battery never gets overcharged even though it sits fully charged on that high current source for several hours every day?

Here is another puzzler for you - we have a tiny 12V 20Ahr motorcycle starting battery in our dinghy that is used for starting the outboard and operating a bilge pump. When the dinghy is not being used for long periods, I often combine this battery with the 660Ahr deep cycle house bank and put it on the 120A charger. This tiny motorcycle battery never over-charges and we get many years of use out of it. Why?

The answer you come up with will solve your confusion about the AGM/FLA.

Mark