>The myth is that two batteries in parallel will discharge each other.
>They won't.
Providing both batteries exhibit identical electrical characteristics
you're right. However, the charge/discharge cycle results in cell-to-cell
differentials in plate thickness, plate roughness, and un-removed sulfates.
This changes the electrical characteristics so that over time even
batteries that start out identical end up being different. Some plates end
up with dendritic growth that causes high resistance shorts. Sometimes the
dendrites dissolve and disappear. Sometimes they become more significant.
Some SEM photos of sulfated plates can be seen at
http://www.thomasregister.com/olc/marinetics/t1.htm For more in depth
review of the sulfation and de-sulfation process see
http://vdcelectronics.com/desulphation.htm
The equalization phase is intended to minimize and/or reverse the effects
of sulfation. However, because the sulfation process involves mechanically
altering the composition of the plates through shedding of the sulfates,
there is an inevitable degradation of the plate structure over time. As a
consequence the cells exhibit differing voltage characteristics, with the
result that two batteries over time become electrically different. And they
will discharge when configured in parallel.
You can learn more about the most modern technique used for de-sulfation
in: Testing Pulsed Current Conditioning to BenefitVehicle Battery
Performance published by SAE 1999
For a more commercial view see http://www.webcasting.com/pulse/howit.htm
> Now if you think that means two different types of
>batteries can be paralled then you are wrong.
Well, they can be. They may suffer discharge faster.
> They need to be the same.
>However they still won't discharge each other if different, only one will
>be discharged.
This depends on many factors. What you say is absolutely true for NiCads
but not true for other chemistries. For example, Take the case where
battery A is charging battery B due to a difference in charge states.
Depending on their internal resistances, one battery will get hotter than
the other. Ordinary lead acid batteries produce more voltage when they get
hotter (to a point). We used to use this fact when I was a kid in Maine to
start cars in the winter when their batteries were too low. Momentarily
short the battery using jumper cables. Sometimes the battery would heat up
enough to start the car. (not recommended practice but it worked). Suppose
that the donor battery gets hotter. That means that its voltage will rise
somewhat during the charging process. Once it cools off, the voltage is
correspondingly lower. Then battery B tries to charge A. Whether or not
this process will continue depends on many factors.
So how much of a problem can this be? As a rule of thumb, most of the
usable energy in a lead-acid battery is in the top 0.5 volt out of 12.6. If
only one cell is low by 0.25V the combined battery bank will have a useful
capacity of about 70% of the full charge capacity. A problem for people who
need the full capacity but not proving mutual discharge.
Why can your diesel truck run paralleled batteries? Keep in mind that most
automotive type regulators never go above 13.9V This means that automotive
batteries are NEVER fully charged by the alternator. (I'm not certain about
the GMC but have no reason to suspect otherwise). The reason for the
paralleled batteries is to increase the amperage available for cranking.
Most of the time starters never get close to running a battery down -even
though it isn't fully charged. A fully charged battery bank could probably
run your starter for at least 10-20 minutes. But all of this is fine for a
starting battery since you start and run your truck frequently. Therefore
the compromises made by the engineers work OK for starting a diesel. And,
most consumers are conditioned to accept that they must buy a new battery
every 3-5 years depending on where they live. Bottom line is that you
really don't need much charge to start your truck.
I know from personal experience that two paralleled batteries can mutually
discharge. I had a bulldozer that used two 8Ds in parallel for starting.
Two new Sears Diehard batteries were put in it one weekend. The next
weekend they were both flat. No sneak paths, no draw, just the two
batteries duking it out. An extreme case for certain, but the principle is
there.
All of this is irrelevant to those who do not dry camp for extended periods
of time. I can go 3-4 days without recharging using my 2 T-105 batteries -
and plan to add two more. I don't want the potential loss of energy from a
parallel configuration to impact my dry camping.
> > If you put a new battery with a dying battery you will kill
> > both of the batteries.
>
>Well thats true, did you think paralleling a good battery with a bad one
>would save the bad one ?
It really boils down to replace batteries in sets if you insist on
paralleling them permanently.
> > It is recommended that when a high capacity
> > battery bank is needed 2 6 volt high capacity batteries in series work
> > better and have a much longer life than a parallel battery layout.
>
>Nothing above really supports this.
The best 12V battery that I've found that matches up more or less favorably
to 2 T-105s is an industrial 8D deep cycle. But at three times the cost of
the golf cart batteries I'd call the GC batts the best deal. And, for
certain they are less sensitive to cell degradation than paralleled 12V
batts. (for paralleled batteries, the lower voltage cells will control how
much of a charge the paralleled batteries can accept)
>If the connections are bad you will have problems with both setups.
>if they are dirty and leak charge at different rates than one will
>be overcharged if they are in series. That does not happen if they
>are parallel. If the parallel system has bad connections one battery
>may do all the work, this does not happen with serial connections.
Your paralleled batteries are made up of series cells. Same potential
problems. Same results.
>And once again, of the dozen or so vehicles I have owned that came
>with two batteries they have all been 12 volt and all in parellel.
>Why do you suppose John Deere, Internation Harvester, Case, GMC,
>and Mack(4) all did it that way?
>
>Perhaps deep cycle applications like house batteries fare better with
>2x6 volts but I doubt that the difference is significant (opinion !!)
>so I use 12's, purchased and replaced as an identical set.
They fare better with deep cycle batteries. Golf Cart batteries are a
reasonable approximation to true deep cycle batts at a reasonable cost.
Gelled or AGM batteries theoretically can do better than GC batts but the
lifecycle cost is much higher. FWIW, it's the plate chemistry that makes
the difference between the batts. And, the best rule of thumb indicator for
capacity and longevity is battery weight - absent exotic technologies.
Solid plates versus porous plates. More lead versus less.
If parallel batteries work well for you in your GMC, go for it. They don't
meet my requirements.
Henry
>They won't.
Providing both batteries exhibit identical electrical characteristics
you're right. However, the charge/discharge cycle results in cell-to-cell
differentials in plate thickness, plate roughness, and un-removed sulfates.
This changes the electrical characteristics so that over time even
batteries that start out identical end up being different. Some plates end
up with dendritic growth that causes high resistance shorts. Sometimes the
dendrites dissolve and disappear. Sometimes they become more significant.
Some SEM photos of sulfated plates can be seen at
http://www.thomasregister.com/olc/marinetics/t1.htm For more in depth
review of the sulfation and de-sulfation process see
http://vdcelectronics.com/desulphation.htm
The equalization phase is intended to minimize and/or reverse the effects
of sulfation. However, because the sulfation process involves mechanically
altering the composition of the plates through shedding of the sulfates,
there is an inevitable degradation of the plate structure over time. As a
consequence the cells exhibit differing voltage characteristics, with the
result that two batteries over time become electrically different. And they
will discharge when configured in parallel.
You can learn more about the most modern technique used for de-sulfation
in: Testing Pulsed Current Conditioning to BenefitVehicle Battery
Performance published by SAE 1999
For a more commercial view see http://www.webcasting.com/pulse/howit.htm
> Now if you think that means two different types of
>batteries can be paralled then you are wrong.
Well, they can be. They may suffer discharge faster.
> They need to be the same.
>However they still won't discharge each other if different, only one will
>be discharged.
This depends on many factors. What you say is absolutely true for NiCads
but not true for other chemistries. For example, Take the case where
battery A is charging battery B due to a difference in charge states.
Depending on their internal resistances, one battery will get hotter than
the other. Ordinary lead acid batteries produce more voltage when they get
hotter (to a point). We used to use this fact when I was a kid in Maine to
start cars in the winter when their batteries were too low. Momentarily
short the battery using jumper cables. Sometimes the battery would heat up
enough to start the car. (not recommended practice but it worked). Suppose
that the donor battery gets hotter. That means that its voltage will rise
somewhat during the charging process. Once it cools off, the voltage is
correspondingly lower. Then battery B tries to charge A. Whether or not
this process will continue depends on many factors.
So how much of a problem can this be? As a rule of thumb, most of the
usable energy in a lead-acid battery is in the top 0.5 volt out of 12.6. If
only one cell is low by 0.25V the combined battery bank will have a useful
capacity of about 70% of the full charge capacity. A problem for people who
need the full capacity but not proving mutual discharge.
Why can your diesel truck run paralleled batteries? Keep in mind that most
automotive type regulators never go above 13.9V This means that automotive
batteries are NEVER fully charged by the alternator. (I'm not certain about
the GMC but have no reason to suspect otherwise). The reason for the
paralleled batteries is to increase the amperage available for cranking.
Most of the time starters never get close to running a battery down -even
though it isn't fully charged. A fully charged battery bank could probably
run your starter for at least 10-20 minutes. But all of this is fine for a
starting battery since you start and run your truck frequently. Therefore
the compromises made by the engineers work OK for starting a diesel. And,
most consumers are conditioned to accept that they must buy a new battery
every 3-5 years depending on where they live. Bottom line is that you
really don't need much charge to start your truck.
I know from personal experience that two paralleled batteries can mutually
discharge. I had a bulldozer that used two 8Ds in parallel for starting.
Two new Sears Diehard batteries were put in it one weekend. The next
weekend they were both flat. No sneak paths, no draw, just the two
batteries duking it out. An extreme case for certain, but the principle is
there.
All of this is irrelevant to those who do not dry camp for extended periods
of time. I can go 3-4 days without recharging using my 2 T-105 batteries -
and plan to add two more. I don't want the potential loss of energy from a
parallel configuration to impact my dry camping.
> > If you put a new battery with a dying battery you will kill
> > both of the batteries.
>
>Well thats true, did you think paralleling a good battery with a bad one
>would save the bad one ?
It really boils down to replace batteries in sets if you insist on
paralleling them permanently.
> > It is recommended that when a high capacity
> > battery bank is needed 2 6 volt high capacity batteries in series work
> > better and have a much longer life than a parallel battery layout.
>
>Nothing above really supports this.
The best 12V battery that I've found that matches up more or less favorably
to 2 T-105s is an industrial 8D deep cycle. But at three times the cost of
the golf cart batteries I'd call the GC batts the best deal. And, for
certain they are less sensitive to cell degradation than paralleled 12V
batts. (for paralleled batteries, the lower voltage cells will control how
much of a charge the paralleled batteries can accept)
>If the connections are bad you will have problems with both setups.
>if they are dirty and leak charge at different rates than one will
>be overcharged if they are in series. That does not happen if they
>are parallel. If the parallel system has bad connections one battery
>may do all the work, this does not happen with serial connections.
Your paralleled batteries are made up of series cells. Same potential
problems. Same results.
>And once again, of the dozen or so vehicles I have owned that came
>with two batteries they have all been 12 volt and all in parellel.
>Why do you suppose John Deere, Internation Harvester, Case, GMC,
>and Mack(4) all did it that way?
>
>Perhaps deep cycle applications like house batteries fare better with
>2x6 volts but I doubt that the difference is significant (opinion !!)
>so I use 12's, purchased and replaced as an identical set.
They fare better with deep cycle batteries. Golf Cart batteries are a
reasonable approximation to true deep cycle batts at a reasonable cost.
Gelled or AGM batteries theoretically can do better than GC batts but the
lifecycle cost is much higher. FWIW, it's the plate chemistry that makes
the difference between the batts. And, the best rule of thumb indicator for
capacity and longevity is battery weight - absent exotic technologies.
Solid plates versus porous plates. More lead versus less.
If parallel batteries work well for you in your GMC, go for it. They don't
meet my requirements.
Henry
