Multiple SLA Batteries in Parallel

[Neil - Salem, MA USA]

I have never been to this newsgroup before, so I apologize if this question
has been posted before.

I am thinking of building a miniature backup power system for my living room
using eight 12-volt Sealed Lead Acid (SLA) batteries connected in parallel.
(There will be no series connections.) During a power outage, I'll use an
inverter to make my AC power from this battery assembly.

I have no need for a rapid charging process, and I have read that a
Float-Charge is safest for SLA batteries. I have not been able to determine
how much current a single 12-volt SLA battery would draw during a
Float-Charge. Whatever that current might be, I would need a Float Charger
that can supply 8 times that amount of current during a Float-Charge for
this parallel set of 8 batteries.

I am looking for any an all kinds of advice. Is my plan workable? What
Float Charger would you recommend that could Float-Charge eight 12-volt SLA
batteries in parallel? What SLA batteries would you recommend?

Thank you very much in advance,

Neil
Salem, MA USA

 

[SQLit]

I have never been to this newsgroup before, so I apologize if this question
has been posted before.

I am thinking of building a miniature backup power system for my living room
using eight 12-volt Sealed Lead Acid (SLA) batteries connected in parallel.
(There will be no series connections.) During a power outage, I'll use an
inverter to make my AC power from this battery assembly.

I have no need for a rapid charging process, and I have read that a
Float-Charge is safest for SLA batteries. I have not been able to determine
how much current a single 12-volt SLA battery would draw during a
Float-Charge. Whatever that current might be, I would need a Float Charger
that can supply 8 times that amount of current during a Float-Charge for
this parallel set of 8 batteries.

I am looking for any an all kinds of advice. Is my plan workable? What
Float Charger would you recommend that could Float-Charge eight 12-volt SLA
batteries in parallel? What SLA batteries would you recommend?

Thank you very much in advance,

Neil
Salem, MA USA

I will be the first to say I do not know what a SLA battery is.
8 batteries is 16 connections and places for a failure.
Bigger batteries less connections.
Are you planning on putting this contraption in the house??? ( humor
intended)
Have you thought about out gassing?
Ya out gassing, oh these batteries are sealed, right. Wrong, all batteries
out gas sooner or later. unless they are solid, no liquid.

Your charger would need to be large enough to not only float charge but full
charge the array.

Just how long do you plan on using this setup? Days, weeks?

 

[Neil - Salem, MA USA]

My recommendation would be: do not put 8 batteries in parallel! It's
asking for trouble. Seen it done, seen the hassles it can cause. Two in
parallel maybe, but preferably not even that. I'm sure everybody in the NG
will agree.....

An power engineering company I found on the Internet wrote the following:

"PARALLELING BATTERIES
"You should parallel batteries only to create a battery bank with a higher
capacity. We do not recommend paralleling batteries only for charging
purposes, the weaker batteries can hog all the charging current.

"When you parallel batteries, do so with batteries that are the same
voltage, same AH rating and with the same past history. We recommend using
new (identical) batteries. Don't connect new batteries in parallel with old
batteries. Don't connect 7 AH batteries in parallel with 100 AH batteries.
You should charge each battery separately, let them rest at least 12 hours,
and then connect in parallel to create a battery bank. No two batteries are
the same. Once connected, the weaker batteries will draw down the stronger
batteries and eventually all the batteries in the bank will equalize. Until
then, the weaker batteries will hog all the charging current."

Charles, is your recommendation to *not* put batteries in parallel because
of possible problems with charging the bank, as mentioned in the above
quote? If so, I could do as the quote suggests: I could charge them
separately and then connect them.

I thank you for your response.

Neil
Salem, MA USA

 

[Vaughn]

8 batteries is a lot to parallel, a REAL lot. I can think of no reason for
doing this unless you already have the batteries. I don't admit this often, but
I have had 4 in parallel for several years and it works just fine, but I agree
that it is not good practice. Why do I do it? Because I have the batteries and
I got them for free! (and because they are matched, identical batteries)

If you have not already obtained the batteries, I suggest that you buy
fewer, larger batteries, and/or go to a 24 volt or 48 volt system.

Vaughn

 

[Robert Morein]

I have never been to this newsgroup before, so I apologize if this question
has been posted before.

I am thinking of building a miniature backup power system for my living room
using eight 12-volt Sealed Lead Acid (SLA) batteries connected in parallel.
(There will be no series connections.) During a power outage, I'll use an
inverter to make my AC power from this battery assembly.

The other posters are correct.
Don't do this.
Your intuition might tell you that the load and wear will be shared equally
between the batteries, but this is not the case.
Batteries are close to constant-current devices. Minute differences between
the voltages of the batteries will cause a "pecking order", where the
strongest battery takes the beating.

Also, if these are gel cells, there is a real possibility that a short could
cause things to get very hot. Keep an axe handy to sever all the
connections!

The conventional wisdom is that no more that three batteries should be
paralleled, and that it's a second rate idea at that. A single string
completely eliminates the problem of battery imbalance.

 

[Neil - Salem, MA USA]

The other posters are correct.
Don't do this.
Your intuition might tell you that the load and wear will be shared
equally
between the batteries, but this is not the case.
Batteries are close to constant-current devices. Minute differences
between
the voltages of the batteries will cause a "pecking order", where the
strongest battery takes the beating.

Also, if these are gel cells, there is a real possibility that a short
could
cause things to get very hot. Keep an axe handy to sever all the
connections!

The conventional wisdom is that no more that three batteries should be
paralleled, and that it's a second rate idea at that. A single string
completely eliminates the problem of battery imbalance.

Well ...I think you folks have convinced me! OK ...perhaps I can achieve my
goal of a backup power system using a series of 12V SLA batteries ..or maybe
I'll just stick with one big 12V battery.

My initial ambitions were probably excessive. I really only need a backup
power system that can handle 5 amps at 110 volts for 5 to 10 hours, without
overly discharging the batteries. (SLA batteries don't like to be
discharged too much.) I probably could achieve that with one 100 or 140
Amp-Hour 12V SLA battery, couldn't I?

Sincerely and gratefully,

Neil
Salem, MA USA

 

[jmar]

Well ...I think you folks have convinced me! OK ...perhaps I can achieve my
goal of a backup power system using a series of 12V SLA batteries ..or maybe
I'll just stick with one big 12V battery.

My initial ambitions were probably excessive. I really only need a backup
power system that can handle 5 amps at 110 volts for 5 to 10 hours, without
overly discharging the batteries. (SLA batteries don't like to be
discharged too much.) I probably could achieve that with one 100 or 140
Amp-Hour 12V SLA battery, couldn't I?

Sincerely and gratefully,

Neil
Salem, MA USA

120v at 5a is about 48a DC 12.6v. A 100 or 140ah battery would not last
more than maybe two hours at most and you would be below the 20% level.
I have 9, 130ah wet, 12v batterys hooked parallel with a shutoff switch
hooked to each bank of three. I can turn on 3, 6 or 9 or shut off all if
I want. The shutoff switches are hooked to a 3 foot long piece of angle
iron that allows me to run off either of the three banks. Most of the
time I run of all three banks. I do this because I want a lot of backup
power. So far the power has went off a couple of times and I was the
only house with the lights still on and watching tv like nothing
happened.. I only have 21 amps of solar panels and that is no where
enough to pull all 9 batteries up to bulk charge while under load
(barely can with no load)or EQ the bank. So I had to split them up.
Works great. The water bubbles and I am able to float all 4 in my gage.
I will buy two panels at a time till I have 60 amps and then I plan to
never break the bank apart again. But I know nothing about GEL. So I'm
just guessing and willing to bet Neil is correct.

Jmar

 

[[email protected]]

Very easy answer to this part of the question. Hawker Genesis EP
Series batteries - pure lead (not alloyed) plates - EXTREMELY low
internal resistance, and accept very high charging currents,plus very
long life. Not cheap, but quality seldom is.

 

[[email protected]]

The basic problem is that the entire battery bank will only be as good
as the weakest battery. As one starts to 'fade' it will pull all the
others down with it, and you will have to disconnect the entire array to
find the weak one. Take some advice based on many years of living on an
island where there is no grid supply.... just use one large battery, or
a number of lower voltage ones in series. (Especially in the case of SLA
batteries, which don't have the longevity of flooded cells).

Charge multiple batteries separately, and discharge separately. When
one battery reaches a given depth of charge switch to the second
battery with a "make before break" switch.

And don't paint all SLAs with the same brush. some can, and regularly
do, outlast FLA batteries.

 

[Vaughn]

On Sat, 18 Jun 2005 21:53:23 -0400, "Neil - Salem, MA USA"

Your output will be limited by the weakest battery,

While I agree that paralleling many batteries is a strategy to be avoided,
the above, often repeated, statment is simply not true for paralleled batteries
and should not go unchallenged. Multiple batteries in series can be redrawn as
a series-parallel circuit with ideal batteries in series with resistors that
represent the internal resistance of the batteries. As the batteies age, their
internal resistance increases at a rate that varies among even identical
batteries. The "weaker" batteries will thus have a higher internal resistance
and will contribute less current to the load, but the total currrent available
is limited only by the equivilant parallel resistance of all of the internal
resistances, not by the "weakest batery".

In the case of a series string of batteries, the output is limited by the
weakest battery because all of the internal resistances are in series.

and the lifetime of the bank will be shortened.

Possibly true: In the case of series-parallel strings, one bad battery
effectively removes an entire string which can dramatically increase the DOD for
the remaining string(s) and drastically reduce battery life.
The real danger of paralleling batteries is if one of the cells in one of
the batteries should develop a short. Depending on how "bad" the short is, you
could develop very high currents which could cause a fire and/or cause batteries
to spew acid fumes. If you are lucky you might just see what looks like a large
leakage current and a battery bank that never seems to be at full charge.

This is due to the inevitable differences in internal
resistance that will develop over time.

True only in the case series or series-parallal connections, because of
what I have written above.

Two strings in parallel is
acceptable; three strings is borderline. You're better off with a single
string of the required current.

I have been struggling with the above for years and I have not yet made up
my mind. In my work, I deal with large UPS's that use strings of many
batteries. One bad battery in a system that consists of a single string means
that you are finished, but parallel strings have their own problems. Which is
best? Get back with me in 10 years.

I agree with all that Ron has written below:

Vaughn

 

[Vaughn]

Vaughn, ..

Do you believe that cross-tying helps with series-parallel setups? It is
mentioned in the Trace SW manual as desireable if there are multiple,
paralleled strings.

I have never seen it done. I could list multiple theoretical advantages
and disadvantages, but have no personal experience to offer.

Vaughn