To the collective relief of the group this will be my last comment on
this OT subject.
On Fri, 7 May 2004 21:49:58 -0500, "Bernard Randall"
|This thread is discussing the situation as it applies to a sump system which
|appears from the supplied information to be unattended. You claim to be an
|EE, all the experienced engineers I have worked with don't look at
|situations through rose colored glasses as you appear to advocate. Anyone
|with an engineering background will have to take into account what happens
|in the fault condition of a critical piece of equipment. In this case the
|battery is one of those items. As I said in my initial post, and now
|expand, if it is a critical application I would use 2 batteries with
|separate chargers and a differential relay to auto change the batteries when
|the first reached its rated dischage voltage.
No, this thread began with:
|"I know this is off topic but I have gotton so much information here I hope
|to get an answer. I have an in line emergency battery powered sump pump that
|will run about 5-6 hrs on a full charge. I wish to connect a second battery
|to double the pumping time but I am not sure of the proper wiring."
The OP simply wanted to know whether to connect a second battery in
series or parallel. Many respondents before I gave him the correct
Note that the OP simply wanted to increase the Ah capacity of his
battery. He did not ask about redundancy, automatic changeover, or
anything about unattended operation. Perhaps he should have, but he
didn't seem to be concerned about it, so neither am I.
One respondent (probably repeating something he had read) gave out
totally erroneous information thus:
Because the correct answer was given before I read the OP, I did not
reply until the foregoing was posted. I offered a simple analogy in
layman's terms of why this statement was incorrect and explained the
operation of battery isolators in the same gentle terms.
Apparently itching to complicate things, you started off with:
|"It is not good practice to directly connect batteries in parallel...."
and then offered as a reference a document that says:
"Up to this point, we have attempted to define the scope of the issue,
and examine the failure mechanisms and their consequences to parallel
configurations. We find nothing alarming about such practice in either
liquid or sealed battery systems....."
Your reference and I seem to agree.
I'm gonna go cut some wood now.
Actually I have used this configuration.... (Storage Batteries in parallel)
Back when cars were still 6 volt. Living in a cold winter climate I decided
to use two dissimilar 6 v. batteries in series to start my truck on cold
winter mornings using some relays and solenoids, paralleling them when not
starting, charging them together. Worked fine the first day. The next
morning they were both almost completely drained. After checking and
rechecking my circuits this continued until I added a switch to disconnect
one battery from the charging system when idle and connecting it only while
the truck was running. Walla ! solved the problem and worked great except
when I forgot to flip the switch when parking overnight. I devoted some time
and effort to find out why this had occurred and passed what information I
gathered along. If you have real experience with this situation, I would be
interested in your results. In theory, both theory and practice work. In
practice theory doesn't always cooperate.
|Actually I have used this configuration.... (Storage Batteries in parallel)
|Back when cars were still 6 volt. Living in a cold winter climate I decided
|to use two dissimilar 6 v. batteries in series to start my truck on cold
|winter mornings using some relays and solenoids, paralleling them when not
|starting, charging them together. Worked fine the first day. The next
|morning they were both almost completely drained. After checking and
|rechecking my circuits this continued until I added a switch to disconnect
|one battery from the charging system when idle and connecting it only while
|the truck was running. Walla ! solved the problem and worked great except
|when I forgot to flip the switch when parking overnight. I devoted some time
|and effort to find out why this had occurred and passed what information I
|gathered along. If you have real experience with this situation, I would be
|interested in your results. In theory, both theory and practice work. In
|practice theory doesn't always cooperate.
I understand what you were doing, but without seeing a schematic of
the way you did it I cannot understand totally what the problem was.
Are you sure that one of your solenoids/relays wasn't energized in the
off (not running) condition.
Solenoids that handle the currents you were dealing with (particularly
on a 6V system) draw considerable coil current and could discharge the
batteries if left energized for long periods. If your added switch
(the "cure") was also disconnecting this parasitic load, then that is
the answer to your problem.
Otherwise, there is no reason why having two (or more) functional
batteries in parallel will increase the self-discharge rate. My
travel trailer used to sit for months at a time with two deep-cycle
12V batteries in parallel without charging. The trailer has a
built-in charger of very poor design that didn't properly maintain the
charge and also generated huge amounts of radio-frequency
interference, so it was usually turned off. I have since designed and
built a "smart" three step charger that is electrically quiet and can
be connected indefinately.
I need to build another one to maintain the battery in my '99 Camaro,
which goes dead while it is parked and we are traveling in the RV.
There is apparently a substantial parasitic load from the ECM and
anti-theft system that discharges the battery. After the last 3-month
trip, I had to replace the battery.
He's not looking for a solution for "a critical application", he's looking
for a longer run time on a system that is working satisfactorily with a
single battery. He's not dealing with a discharged or damaged battery,
he's dealing with two batteries both of which will unless something
malfunctions be fully charged and in equilibrium at the start of operation.
Thus none of your concerns apply and I don't really understand why you're
on about this.
Reply to jclarke at ae tee tee global dot net
The only person who knows if he is dealing with a critical application is
the OP. The OP as in many of the posts on this and other NGs by the
nature of his question is asking a basic question which shows either
directly or indirectly that he does not know the full ramifications of his
actions. Hopefully, through discussion, he will take the appropriate action
depending on how critial he considers the application.
When dealing with batteries you are using an element which will ultimately
fail, fact of life, depending on how it is used will affect how soon it will
fail. However, battery powered equipment has many underlying potential
problems, only a few of which were even touched on it the original thread.
The bigest issue which was not discussed was what type of battery charger
was being used. From the OP it appeared that the battery was being
permanently float charged. Here I quote "A cheap, unregulated 'trickle'
charger or manual two stage charger can overcharge a battery and destroy
it". The full text of this can be seen at this link
Maybe mistakenly I try and look beyond the basic question posed, and if I
see potential problems I try and bring them to the OPs notice, if he doesn't
consider my viewpoint applies he just ignores it, isn't that one of the
benefits of NGs?
Option 2 will likely burn out your pump as it will deliver 24V to it.
Option 1, parallel, is the way to do it. However it could shorten the
battery life due to circulating currents between the batteries.
The best solution is to get a battery isolator from an RV or boating
supply house and hook the batteries up thru it.
Wire them together using Option one, but instead of hooking the pump
directly to one battery, put the positive from the pump on one battery and
the negative from the pump on the other battery. That way, the bank will
appear to the charger and pump as one huge battery and you will eliminate
the problem of discharging one battery instead of both.
Who are you writing to? And why? If you don't like the thread, pass it by. If
you think you're some kind of net cop, you're due for a big surprise.
"Don't let yesterday use up too much of today." Will Rogers
ROFLMAO at all the 'experts'...
...parallel batteries will not self discharge below the level of the lowest
voltage battery... period. Unless of course you are applying a load to
it/them and allow the discharge to continue until the battery capacity is
depleted. We ar talking relatively good batteries here.
Typical non-cellular CO TELCO batteries are float charged at
typically -54Vdc and 'equalize' charged at approximately -55.5Vdc -56Vdc.
All the small 1.2 - 1.3Vdc cells add up to the appropriate voltage. They
are monitored on a regular basis with an electrolyte check and are replaced
if the electrolyte or the cell voltage tests below a certain point (varies
per manufacturer). If you leave them without maintenance to
sulphate/short/self discharge, whatever you wanna call it , they will self
destruct and bring the overall voltage down to whatever the sum of the cell
voltages are, minus the lost cell until the internal resistance of the
defective battery causes more extensive problems in voltage level.
We are not talking TELCO batteries here...
Charging car batteries in parallel will work, with all things being
approximately equal, with the main concern being that the current capacity
of the charging equipment is adequate to charge the batteries at a trickle
rate. You are doubling the current capacity of the batteries and if the
battery charger design is good then no problem. It will take longer for the
batteries to fully charge though. If the dead batteries try to charge at a
rate higher than the charger is capable of providing on a steady basis, the
voltage will drop the batteries will not charge and the charger will likely
blow a fuse/pop a circuit breaker or whatever it does to protect itself when
excessive current is drawn for any length of time.
In a battery series circuit, a defective cell (battery) will cause problems
for the rest of the cells in the series. Unless you want the voltage to
increase, do not put them in series. If you want higher current capacity,
put them in parallel.
...just add me to the list of 'experts'
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