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A question on Lead Acid Batteries

M

martin griffith

I have a question regarding lead acid batteries, specifically regarding
the concept of holding them at a float voltage.

I am wondering what are the advantages and disadvantages of holding the
batteries at a float voltage as opposed to letting them self discharge
for a short while, say 24 hours, and then performing a charge top off.

The batteries are in a system that requires that they be maintained in a
readiness state, which is why I suggest only letting them self discharge
for a few days at most before performing a top off.

As I said, I am curious about the relative advantages and disadvantages
of the two methods. Does anyone have an insight or experience that may
help me decide which method to use?

http://www.batteryuniversity.com/partone-13.htm may help
quote
"
After full charge, remove the battery from the charger. If float
charge is needed for operational readiness, lower the charge voltage
to about 13.50V (2.25V/cell). Most chargers perform this function
automatically. The float charge can be applied for an unlimited time.
"


martin
 
G

GregS

I have a question regarding lead acid batteries, specifically regarding
the concept of holding them at a float voltage.

I am wondering what are the advantages and disadvantages of holding the
batteries at a float voltage as opposed to letting them self discharge
for a short while, say 24 hours, and then performing a charge top off.

The batteries are in a system that requires that they be maintained in a
readiness state, which is why I suggest only letting them self discharge
for a few days at most before performing a top off.

As I said, I am curious about the relative advantages and disadvantages
of the two methods. Does anyone have an insight or experience that may
help me decide which method to use?

When bringing up batteries to full charge, overcharging is standard
practice. While on constant float, it does not get that overcharging
which helps get rid of sulfation, and increases storage.

greg
 
N

Noway2

I have a question regarding lead acid batteries, specifically regarding
the concept of holding them at a float voltage.

I am wondering what are the advantages and disadvantages of holding the
batteries at a float voltage as opposed to letting them self discharge
for a short while, say 24 hours, and then performing a charge top off.

The batteries are in a system that requires that they be maintained in a
readiness state, which is why I suggest only letting them self discharge
for a few days at most before performing a top off.

As I said, I am curious about the relative advantages and disadvantages
of the two methods. Does anyone have an insight or experience that may
help me decide which method to use?
 
M

Martin

Noway2 said:
I have a question regarding lead acid batteries, specifically regarding
the concept of holding them at a float voltage.

I am wondering what are the advantages and disadvantages of holding the
batteries at a float voltage as opposed to letting them self discharge
for a short while, say 24 hours, and then performing a charge top off.

The batteries are in a system that requires that they be maintained in a
readiness state, which is why I suggest only letting them self discharge
for a few days at most before performing a top off.

As I said, I am curious about the relative advantages and disadvantages
of the two methods. Does anyone have an insight or experience that may
help me decide which method to use?

My understanding is that for battery health, either float or isolation
with periodic recharge are both approximately equally acceptable.

In either case, you will need to initially fully charge the battery,
so the equipment for that will be equivalent.

After that, float is MUCH easier to do automatically ...

to float, you hook up to a constant voltage source and you're done.

with periodic recharge, you have to have some kind of timer or voltage
sensor, a way to connect to the battery,. a way to tell when the
battery is charged and disconnect again.

If you have parasitic loads connected to the battery, it doesn't
affect the float scheme at all (assuming the float source can handle
the extra current)
The periodic recharge scheme would perhaps need the float interval
length adjusted.
 
G

Gibbo

Noway2 said:
I have a question regarding lead acid batteries, specifically regarding
the concept of holding them at a float voltage.

I am wondering what are the advantages and disadvantages of holding the
batteries at a float voltage as opposed to letting them self discharge
for a short while, say 24 hours, and then performing a charge top off.

The batteries are in a system that requires that they be maintained in a
readiness state, which is why I suggest only letting them self discharge
for a few days at most before performing a top off.

As I said, I am curious about the relative advantages and disadvantages
of the two methods. Does anyone have an insight or experience that may
help me decide which method to use?

With both flooded lead acid and Gel or AGM lead acid batteries there is
a slight advantage with constant float charge as it prevents the build
up of sulphation. Each time the battery is discharged, no matter how
little, and no matter whether it's by way of a load or self discharge,
some of the sulphate remains as sulphate even after a full recharge.
This is just a part of the natural battery ageing process.

Maintaning a constant float charge reduces the extent of this effect
dramtically over the extent when the battery is allowed to stand then
recharged.

With wet cell (ie flooded) lead acid batteries there is a slight problem
in that during float, the electrolyte stratifies and the first discharge
cycle after an extended float does not produce the full battery
capacity. This effect doesn't happen with periodic recharge following a
few days with no charge. Well it does happen during the standing period,
but the following full charge solves it.

For this reason many advanced chargers run float with a periodic full
charge at some interval between about 48 hours and 28 days. Different
manufacturers have different ideas what works best.

As regards battery life, the difference between the two methods is
actually quite minimal. But the effect of the reduced first discharge
following extended float may sometimes cause problems.

With AGM and Gel cells there is no doubt that maintaining a constant
float is the better option.
 
P

Pete D

Noway2 said:
I have a question regarding lead acid batteries, specifically regarding
the concept of holding them at a float voltage.

I am wondering what are the advantages and disadvantages of holding the
batteries at a float voltage as opposed to letting them self discharge
for a short while, say 24 hours, and then performing a charge top off.

The batteries are in a system that requires that they be maintained in a
readiness state, which is why I suggest only letting them self discharge
for a few days at most before performing a top off.

As I said, I am curious about the relative advantages and disadvantages
of the two methods. Does anyone have an insight or experience that may
help me decide which method to use?
We use the method of float charging with a constant regulated 27.6V (2
series 12V gel type) We have encountered no problems, some battery
manufacurers recommend a salightlu lower voltage for float charge but as
I said we've had no problems with approx 2000 units out in constant use.
We do recommend replacing the batteries sooner than battery
manufacturers suggest, but the application is a fire safety system and
needs to be reliable.
 
N

Noway2

We use the method of float charging with a constant regulated 27.6V (2
series 12V gel type) We have encountered no problems, some battery
manufacurers recommend a salightlu lower voltage for float charge but as
I said we've had no problems with approx 2000 units out in constant use.
We do recommend replacing the batteries sooner than battery
manufacturers suggest, but the application is a fire safety system and
needs to be reliable.

First off, I would like to thank everyone for taking the time to reply
to my inquiry. The responses have given my some ideas to consider on
both sides of the fence.

This application is also part of a fire safety system and reliability
is my foremost concern, which is why I can make arguments for both
ways. One of my biggest areas of concern is float voltage versus the
temperature, which can vary wildly with different installations.

As it stands, I am thinking about defaulting to a float condition, but
leaving the option for a periodic top off charge instead as a
programmable / selectable feature.
 
J

James Beck

We use the method of float charging with a constant regulated 27.6V (2
series 12V gel type) We have encountered no problems, some battery
manufacurers recommend a salightlu lower voltage for float charge but as
I said we've had no problems with approx 2000 units out in constant use.
We do recommend replacing the batteries sooner than battery
manufacturers suggest, but the application is a fire safety system and
needs to be reliable.

We do the same sort of thing.
A current limited bulk charge with a float at the end and you can leave
it floating if/all you want.
Lots of units in the field, no problems.
I went with the classic 13.7V for the float, but with resistor
tolerances it can go +/- .1V anyway.

Jim
 
R

Rich Grise

First off, I would like to thank everyone for taking the time to reply
to my inquiry. The responses have given my some ideas to consider on
both sides of the fence.

This application is also part of a fire safety system and reliability
is my foremost concern, which is why I can make arguments for both
ways. One of my biggest areas of concern is float voltage versus the
temperature, which can vary wildly with different installations.

As it stands, I am thinking about defaulting to a float condition, but
leaving the option for a periodic top off charge instead as a
programmable / selectable feature.

It wouldn't do any harm to monitor the batteries' temperature, albeit it
would add complexity to your circuit. But a battery with a float voltage
that's adjusted for temperature is a very happy battery. :)

Good Luck!
Rich
 
G

g

It wouldn't do any harm to monitor thebatteries' temperature, albeit it
would add complexity to your circuit. But a battery with a float voltage
that's adjusted for temperature is a very happy battery. :)

Good Luck!
Rich

The TI chip I used several years ago, took all that into
consideration. It was a Unitrode chip and just need a pass transistor.
greg
 
G

g

It wouldn't do any harm to monitor thebatteries' temperature, albeit it
would add complexity to your circuit. But a battery with a float voltage
that's adjusted for temperature is a very happy battery. :)

Good Luck!
Rich

The TI chip I used several years ago, took all that into
consideration. It was a Unitrode chip and just need a pass transistor.
greg
 
J

James Beck

The TI chip I used several years ago, took all that into
consideration. It was a Unitrode chip and just need a pass transistor.
greg
The TI's I've seen need a bit more than that.
Resistors to set the bulk charge rate, peak voltage, float voltage, and
so on.
The UC3906 is a prime example.
Nice IC's use them in our upper end stuff, but they do need a bit more
than a pass transistor.

Jim
 
R

rebel

The TI's I've seen need a bit more than that.
Resistors to set the bulk charge rate, peak voltage, float voltage, and
so on.
The UC3906 is a prime example.
Nice IC's use them in our upper end stuff, but they do need a bit more
than a pass transistor.

Indeed they do, but IMOE they are one of the best SLA/gel charge controller
chips around. I highly recommend the O/P read the TI/Unitrode appnote U-104
for them, to be found at: focus.ti.com/docs/prod/folders/print/uc3906.html
 
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