Mixing Battery types in 1 Bank

[TeddyDiver]

Originally Posted by Andina Marie

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?)

She was assured it was the only way to get rid of stupid questions ad nauseum.

[skipperr100]

I have 2x105ah agm, two years old, I want to add two more of the same, will the age difference be a problem?

[TeddyDiver]

No if your girl friend is at least 21 and 22 years younger than you :-)

[skipmac]

Originally Posted by skipperr100

I have 2x105ah agm, two years old, I want to add two more of the same, will the age difference be a problem?

Based on the comments in this thread so far one of two things will happen.

1. The second you connect the new batteries they will exploded and destroy the entire marina.

2. It will work perfectly and your combined batteries will last forever.

All kidding aside, it will probably be fine. If you add two identical batteries as long as the old ones aren't defective then any impact will be minimal.

[skipmac]

Originally Posted by TeddyDiver

No if your girl friend is at least 21 and 22 years younger than you :-)

Also depends on what color your boat is painted.

[sailinglegend]

Originally Posted by TeddyDiver

She was assured it was the only way to get rid of stupid questions ad nauseum.

+1

Why should we believe anything Andina says that just promotes her battery combiners?

EDIT:

You can ask questions in such a way that you will get the answer YOU want!

[sailorchic34]

Lovely discussion. While mixing batteries (AGM and FLA) will work in the short term, but will effect battery life. The differences in internal resistance will affect charge rates and SOC. That is the AGM's will charge and discharge faster then the FLA. Depending on how the battery charger is set up, either the FLA will be undercharged or the AGM will be overcharged.

When the batteries are discharging the AGM will be discharging at a higher rate, based on V/R=I. AGM has 2.5 to 3.4 ish milli ohm resistance. FLA is somewhere around 20 milli-ohm or roughly 6 times more then AGM. This greatly effects both charge acceptance and discharge rates.

So while it will work short term, long term use of mixed battery types will generally shorten the life of all batteries in the mixed battery bank.

[skipmac]

Originally Posted by sailinglegend

+1

Why should we believe anything Andina says that just promotes her battery combiners?

EDIT:

You can ask questions in such a way that you will get the answer YOU want!

I can't see where you get this. It seems very clear to me that the posts from Andina were based on their knowledge of batteries and charging. No different than the numerous posts from members arguing discussing the various aspects of battery chemistries and charging.

Never saw one word in any post from Andina that was in any way a sales pitch for their products, directly or indirectly.

[skipmac]

Originally Posted by sailorchic34

So while it will work short term, long term use of mixed battery types will generally shorten the life of all batteries in the mixed battery bank.

Thanks you.

Cut through all the nitpicking and BS and in my opinion this is the bottom line.

As I mentioned in jest in a previous post, no your batteries won't explode if you mix chemistries in one bank but there will certainly be differences in how they charge and discharge. Not dramatic but not the ideal situation for the best long term health of the batteries.

[sailorchic34]

Originally Posted by skipmac

T Not dramatic but not the ideal situation for the best long term health of the batteries.

Oddly it will be quite dramatic, at a 20 amp discharge the AGM will be providing 16.6 amps of the load, while the FLA will only provide 3.3 amps (assuming 50/50 split). That's at a constant voltage. As the voltage drops things get a little wonky, but the AGM will be dropping current faster and once the load disappears the AGM's will be balanced from the FLA. That is current will flow from the FLA to the AGM's till the rest voltage is balanced.

Same in charging, assuming a 40 amp charge current, the AGM's will accept 33 amps while the FLA will accept only 6.6 amps at a given voltage. Very small amounts of resistance make huge differences in amps.

It's not quite that straight forward as you have to take into account resistance in the wires. But its close.

Myself I would not reccommend mixing battery types.

[skipmac]

Originally Posted by sailorchic34

Oddly it will be quite dramatic, at a 20 amp discharge the AGM will be providing 16.6 amps of the load, while the FLA will only provide 3.3 amps (assuming 50/50 split). That's at a constant voltage. As the voltage drops things get a little wonky, but the AGM will be dropping current faster and once the load disappears the AGM's will be balanced from the FLA. That is current will flow from the FLA to the AGM's till the rest voltage is balanced.

Same in charging, assuming a 40 amp charge current, the AGM's will accept 33 amps while the FLA will accept only 6.6 amps at a given voltage. Very small amounts of resistance make huge differences in amps.

It's not quite that straight forward as you have to take into account resistance in the wires. But its close.

Myself I would not reccommend mixing battery types.

Actually I meant not dramatic in the context of one type of battery seriously overheating or dying in a very short term. Don't know enough about the details of battery chemistry, charging, internal resistance, etc to form even a semi-educated opinion so on that matter I will just take your word for it.

If I had no choice, like having to replace a defective battery while trapped in Tierra del Fuego or Mogadishu or the like then I would not hesitate to mix batteries to keep me going. But given the option then yes I will stick with all one or the other, in my case FLA for now.

[colemj]

Originally Posted by sailorchic34

Same in charging, assuming a 40 amp charge current, the AGM's will accept 33 amps while the FLA will accept only 6.6 amps at a given voltage. Very small amounts of resistance make huge differences in amps.

The rate at which individual batteries accept charge current during charging is meaningless because at the end of the charge cycle, both batteries will be charged. Neither battery will get charged first or overcharged because the combined voltage will not allow it.

Again, this is the same principal behind why one can combine small, mostly charged start batteries with large under charged house banks for a full charging regime and not have any problems.

Likewise, for discharge from a combined bank, the terminal voltage is the same for both batteries, so one battery will not shut down the other and provide the bulk of the current for the discharge of the bank over time. In an instantaneous sense, if one battery draws down faster, its voltage will drop and the other battery will supply current until its voltage is drawn down similarly. In other words, when the bank is at a resting voltage of, say, 12.2V, both batteries are at a 50% SOC - not one at 75% and the other at 25%.

The ohm equation is good for separate batteries in a static state, but you must also use Kirchoff and apply appropriate dynamics to ohm during a charge/discharge of a combined bank. The combined bank is a single equivalent circuit to the charge source and the ohm equation only describes an instantaneous point in time.

And for the record, I have already stated it would be silly for the OP to add AGM to his existing FLA (for cost reasons).

In the OP's case, the manufacturer's recommended charge voltage for both his existing FLA batteries and AGM's is 14.4V. It is difficult to believe that this would leave either battery undercharged or overcharged when a charging regime has completed. This debate has been on electrical characteristics of LA batteries and whether there are problems in store for someone who did mix AGM and FLA.

Mark

[TeddyDiver]

Mark, read carefully what SailorChic wrote about resistance and then if you find a flaw in that explanation tell us what you think is wrong. The flaw in your reasoning is the assumption all batteries being fully charged, or becoming fully charged at the same time. Internal resistance of the batteries is different so is the current what ever the voltage..

[sailorchic34]

Originally Posted by colemj

The rate at which individual batteries accept charge current during charging is meaningless because at the end of the charge cycle, both batteries will be charged. Neither battery will get charged first or overcharged because the combined voltage will not allow it.

Again, this is the same principal behind why one can combine small, mostly charged start batteries with large under charged house banks for a full charging regime and not have any problems.

Likewise, for discharge from a combined bank, the terminal voltage is the same for both batteries, so one battery will not shut down the other and provide the bulk of the current for the discharge of the bank over time. In an instantaneous sense, if one battery draws down faster, its voltage will drop and the other battery will supply current until its voltage is drawn down similarly. In other words, when the bank is at a resting voltage of, say, 12.2V, both batteries are at a 50% SOC - not one at 75% and the other at 25%.

The ohm equation is good for separate batteries in a static state, but you must also use Kirchoff and apply appropriate dynamics to ohm during a charge/discharge of a combined bank. The combined bank is a single equivalent circuit to the charge source and the ohm equation only describes an instantaneous point in time.

Oh agreed there are some pretty neat chemical dynamics evolved more then just V/R=I. If the batteries were in series circuit I would agree that the batteries would be in better balance. Your assumption that amp flow rate is only governed by Volt drop and is balanced across the different batteries is not quite correct.

With parallel batteries, one FLA, one AGM, At a given voltage drop, the AGM will flow more amps then the FLA, due to the lower internal resistance. There is a 600% difference between in the internal resistance between AGM and FLA. AGM will flow more amps per unit of voltage drop. Once the load stops, current flows from the FLA to the AGM till the voltage is balanced again. OK That's over simplified, but the loads are not at all balanced between the batteries.

Gee just having unbalanced conductors, that is wires of longer circuit length, where the ohms are a bit more on one set battery(s) from another set will cause some batteries to work harder/charge faster. With AGM and FLA, that is taking it to an extreme. To the Boat owner, they will appear to work in unison, but that's not exactly what's happening and it will cause premature battery failure.

[FlyingCloud1937]

I'll point you to an extreme case, so you understand, batteries are dynamic in nature. Charge and discharge cures are not liner, they follow a bell curve based on IR, SOC, DOD as well as amount current moving.

Extreme example
Firstly, if you take one discharged cell and one fully charged cell and put them in parallel, the voltages will be equal immediately. It won't take any time (well, a microsecond maybe...). This is because the point where the the positive (or negative) ends are connected is a single point. A single point has a single potential, by definition.

The way this is explained is that current will start flowing from the charged cell to the discharged cell, and this current will produce a potential difference through the internal resistances of the cells. The charged cell will experience a voltage decrease due to the current it is delivering, and the discharged cell will experience a voltage increase due to the charging current it is receiving. The current will adjust itself until these exactly balance each other.

Now if you attach an external load, some of the current will start flowing through the load instead of the discharged battery. This means the empty battery will be discharging slower and the full battery will be discharging faster.

Depending on the size of the load (i.e. its resistance) the proportion of current that flows each way will vary.

One thing to avoid is the idea of "average voltages". There is no theory of voltage averaging, only Ohm's law. If you know the potentials and the resistances you can work out the currents, but involves solving simultaneous equations...

Lloyd

Originally Posted by colemj

The rate at which individual batteries accept charge current during charging is meaningless because at the end of the charge cycle, both batteries will be charged. Neither battery will get charged first or overcharged because the combined voltage will not allow it.

Again, this is the same principal behind why one can combine small, mostly charged start batteries with large under charged house banks for a full charging regime and not have any problems.

Likewise, for discharge from a combined bank, the terminal voltage is the same for both batteries, so one battery will not shut down the other and provide the bulk of the current for the discharge of the bank over time. In an instantaneous sense, if one battery draws down faster, its voltage will drop and the other battery will supply current until its voltage is drawn down similarly. In other words, when the bank is at a resting voltage of, say, 12.2V, both batteries are at a 50% SOC - not one at 75% and the other at 25%.

The ohm equation is good for separate batteries in a static state, but you must also use Kirchoff and apply appropriate dynamics to ohm during a charge/discharge of a combined bank. The combined bank is a single equivalent circuit to the charge source and the ohm equation only describes an instantaneous point in time.

And for the record, I have already stated it would be silly for the OP to add AGM to his existing FLA (for cost reasons).

In the OP's case, the manufacturer's recommended charge voltage for both his existing FLA batteries and AGM's is 14.4V. It is difficult to believe that this would leave either battery undercharged or overcharged when a charging regime has completed. This debate has been on electrical characteristics of LA batteries and whether there are problems in store for someone who did mix AGM and FLA.

Mark