Master Switches

On Friday, April 26, 2013 9:09:48 AM UTC-5, Tom Gardner wrote:
rk wrote:



Let me repeat: there is no good reason for parallel connection, do not use it.



Precisely.



I haven't yet seen anybody articulate any significant advantages of

direct parallel connection, let alone why they overwhelm the

disadvantages.

Tom, let me be the first to 'articulate significant advantages of parallel':
http://www.amazon.com/dp/B005KLOYO2/..._M3T1_ST1_dp_1
http://www.amazon.com/BALLISTIC-PERF...=pd_sim_auto_1
These are both Lifepo4 batteries that would be well suited for glider usage.. Note that the higher capacity battery has 8 cells but still 13.2V nominal voltage. Obviously, inside are two of the first kind batteries connected in parallel. This has to do with the standard cell (3.3V, 7,000mah) they are using, makes sense to me.

Herb

On Friday, April 26, 2013 11:28:31 AM UTC-4, wrote:
On Friday, April 26, 2013 9:09:48 AM UTC-5, Tom Gardner wrote:

rk wrote:







Let me repeat: there is no good reason for parallel connection, do not use it.







Precisely.







I haven't yet seen anybody articulate any significant advantages of



direct parallel connection, let alone why they overwhelm the



disadvantages.



Tom, let me be the first to 'articulate significant advantages of parallel':

http://www.amazon.com/dp/B005KLOYO2/..._M3T1_ST1_dp_1

http://www.amazon.com/BALLISTIC-PERF...=pd_sim_auto_1

These are both Lifepo4 batteries that would be well suited for glider usage. Note that the higher capacity battery has 8 cells but still 13.2V nominal voltage. Obviously, inside are two of the first kind batteries connected in parallel. This has to do with the standard cell (3.3V, 7,000mah) they are using, makes sense to me.



Herb

Herb: No! Much better off with a battery pack that includes current limiting protection. These are starting batteries. There are several good looking 9 - 12 AH LFP battery packs available now that include in-the-pack battery management. Over charge protection, under charge protection, cell balancing and over current protection (all somewhere between "really good ideas" and "essential" for safe LFP use). It remains to be seen how well these systems work over the long haul. Technology is still a little young to say for certain imo. Vendors include K2, Bioenno, Starck. If you choose to go any of these routes, get the spec sheets, ask questions, do some diligence..

best,

Evan Ludeman / T8

wrote:
On Friday, April 26, 2013 9:09:48 AM UTC-5, Tom Gardner wrote:
rk wrote:



Let me repeat: there is no good reason for parallel connection, do not use it.



Precisely.



I haven't yet seen anybody articulate any significant advantages of

direct parallel connection, let alone why they overwhelm the

disadvantages.

Tom, let me be the first to 'articulate significant advantages of parallel':
http://www.amazon.com/dp/B005KLOYO2/..._M3T1_ST1_dp_1
http://www.amazon.com/BALLISTIC-PERF...=pd_sim_auto_1
These are both Lifepo4 batteries that would be well suited for glider usage. Note that the higher capacity battery has 8 cells but still 13.2V nominal voltage. Obviously, inside are two of the first kind batteries connected in parallel. This has to do with the standard cell (3.3V, 7,000mah) they are using, makes sense to me.

It appears that, with that specific chemistry, design, and
manufacturer, parallel operation is possible. Presumably the
manufacturer did RTFM, so that's fine!

But, what is the *significant* *advantage* of such direct parallel
connection over indirect parallel connection?

Whether or not they are suitable for glider operation I don't
know since I haven't seen any assessment of the technology.

On Apr 25, 5:36*pm, Tom Gardner wrote:
Analogies are always dangerous! You are analysing the analogy
in more depth than can be justified.

I have to agree that the chain analogy is not particularly applicable
to the subject at hand. I would also like to point out that it is not
my analogy.

Yes... A few decades ago I designed rechargeable
batteries

Out of curiosity. Were they NiCd/NiMH batteries, by any chance?

into equipment, and the manufacturer and
general engineering lore said it was A Bad Idea.

If they were lead-acid, I would really appreciate more data. Both
about the manufacturer and the source of the "engineering lore." Even
if it is "guy in the next cubicle said so." I like learning new
things.

So, to clarify things: are you or are you not claiming that it is
"generally recognised good practice" [....]
I have not made any claims in that area for SLA batteries.
All I'm saying is "It is generally recognised that Here There
Be Dragons",

"...generally recognised good practice..."
"It is generally recognised..."
"...general engineering lore..."

OTOH, *you* are the one making specific claims which it
would be prudent to be able to justify, e.g. your post starting
* *Two (or more) batteries wired in parallel will basically act as a
* *single battery. Basically, the voltages will track, during both
* *charging and discharging. Moreover, types and capacities do NOT have
* *to be identical.

This was an answer to a specific question. Also, I happen to stand by
*this* claim. If someone happens to read from it more than I actually
wrote - well, I can't help it.

In particular, I did not state that the current rating of this
combination will be a sum of individual ratings, either during
charging or discharging. I also did not recommend such a connection or
stated that it offers any advantages compared to alternatives. As a
matter of fact, I can think of reasons *not* to connect batteries in
parallel, but they have nothing to do with the question I was
answering.

But absence of recommendation is not a prohibition.
Actually often the *effect* is exactly that of a prohibition!
Oh, and try finding a battery manufacturer that says it is OK to use
their products in a glider.
That's a red herring, of course.

Reductio ad absurdum. Quotes above rearranged to make it obvious.

Bart

In article writes:
rk wrote:

Let me repeat: there is no good reason for parallel connection, do not use it.

Precisely.

I haven't yet seen anybody articulate any significant advantages of
direct parallel connection, let alone why they overwhelm the
disadvantages.


Perhaps you didn't read my posting , where
I articulated the 25% greater capacity obtained by the slower discharge rate.

I recall that folks wanted longer operating time from their batteries for
cross-country or contest flights. 25 percent more seems worthwhile.

Alan

On Thursday, April 25, 2013 12:43:03 AM UTC-4, Eric Greenwell wrote:
On 4/23/2013 5:44 AM, kirk.stant wrote:

Each bus can be powered by either battery 1 or battery 2 (or turned

off). With 8AH batteries I rarely run low on either battery (I

recharge both each night, rotating 3 batteries through the system.



Why not just connect them in parallel and leave them that way? It would

be simpler.



--

Eric Greenwell - Washington State, USA (change ".netto" to ".us" to

email me)

Me? I would just buy good automotive ones. These are gliders not space ships. Real glider pilots don't need no stinkin electronics anyway!

-PC

From the web page:

These batteries are manufactured for Motorcycles, ATVs and Jet-Skis, any
other manner of use voids the warranty.

Some people care about warranties...


wrote in message
...
On Friday, April 26, 2013 9:09:48 AM UTC-5, Tom Gardner wrote:
rk wrote:



Let me repeat: there is no good reason for parallel connection, do not
use it.



Precisely.



I haven't yet seen anybody articulate any significant advantages of

direct parallel connection, let alone why they overwhelm the

disadvantages.

Tom, let me be the first to 'articulate significant advantages of parallel':
http://www.amazon.com/dp/B005KLOYO2/..._M3T1_ST1_dp_1
http://www.amazon.com/BALLISTIC-PERF...=pd_sim_auto_1
These are both Lifepo4 batteries that would be well suited for glider usage.
Note that the higher capacity battery has 8 cells but still 13.2V nominal
voltage. Obviously, inside are two of the first kind batteries connected in
parallel. This has to do with the standard cell (3.3V, 7,000mah) they are
using, makes sense to me.

Herb

Bart wrote:
On Apr 25, 5:36 pm, Tom Gardner wrote:
Analogies are always dangerous! You are analysing the analogy
in more depth than can be justified.

I have to agree that the chain analogy is not particularly applicable
to the subject at hand. I would also like to point out that it is not
my analogy.

Yes... A few decades ago I designed rechargeable
batteries

Out of curiosity. Were they NiCd/NiMH batteries, by any chance?

NiCd. Hence my caveats about the chemistry.

OTOH, *you* are the one making specific claims which it
would be prudent to be able to justify, e.g. your post starting
Two (or more) batteries wired in parallel will basically act as a
single battery. Basically, the voltages will track, during both
charging and discharging. Moreover, types and capacities do NOT have
to be identical.

This was an answer to a specific question. Also, I happen to stand by
*this* claim. If someone happens to read from it more than I actually
wrote - well, I can't help it.

In particular, I did not state that the current rating of this
combination will be a sum of individual ratings, either during
charging or discharging. I also did not recommend such a connection or
stated that it offers any advantages compared to alternatives. As a
matter of fact, I can think of reasons *not* to connect batteries in
parallel, but they have nothing to do with the question I was
answering.

Fair enough. It sounds like we probably agree with each other.

On 4/25/2013 3:13 PM, Tom Gardner wrote:
Alan wrote:


Power Sonic seems ok with it:

http://www.power-sonic.com/images/po...r_Parallel.pdf


Fair enough.

However, it is worth keeping this class of experiences in mind:

Subject: Master Switches
From: rk
Injection-Date: Thu, 25 Apr 2013 14:16:43 +0000
I had two parallel batteries years ago. On one flight, other
battery suddenly died. It took less than 15 minutes to drain
the other one completely flat. All the time I could just watch
the voltage indication and plan my way back home using uncontrolled
airspace. I use now simple selector switch to choose between
2 or 3 batteries. I run one empty and then switch to other one.
Please never connect batteries parallel, it's just asking for trouble.

I wonder how much energy was dissipated, and where, and consequently
whether any temperature rises were noticed.

I don't see what we learn from that experience, without any indication
of what happened. You can always protect against any particular
situation, but the solution might add other ways for things to go bad.
Maybe you forget to charge both batteries, the extra wiring isn't done
well and shorts out, and so on.

If you read the SLA datasheets and technical manuals, you'll see
paralleling isn't a problem. As one poster pointed out, many hundreds of
DG 400 and 800 motorgliders were manufactured (and still are, I believe)
with four 6 volt SLA batteries in series-parallel configuration, so it's
a common glider technique.

--
Eric Greenwell - Washington State, USA (change ".netto" to ".us" to
email me)

On 4/25/2013 11:56 AM, Don Johnstone wrote:
Please never connect batteries parallel, it's just asking for trouble.

Absolutely, if you are not convinced ask yourself why aircraft
manufacturers go to the added expense of designing and fitting expensive
and complicated fuel crossfeeds. The simple solution would be to connect
all tanks so they all feed, the problem with this approach is that if one
tank has a leak it will drain the fuel from all the other tanks. Same thing
applies to batteries.

The two tank solution solves one problem and introduces at least one
other: many pilots have mismanaged the switching between tanks, causing
big problems they would not have had with a single tank.

Losing instrument battery power isn't nearly as dangerous as running out
of fuel. My preference is to install a single battery that can easily
operate the glider for at least two very long flights; second choice is
paralleling two identical batteries to obtain that duration.

I have considered my options for a dead battery in flight. Because it is
such a rare occurrence for me - never in 35 years of soaring - I'm only
concerned with how it would affect my safety, and not convenience or
running a flight recorder (eg, during a contest or record attempt):

* Where I normally fly (Pacific NW), I'll land at an airport.
* At more challenging areas I fly at infrequently, like out of Ely, I
carry a handheld GPS, or soon, use something like the ClearNav vario
with a primary battery good for an hour or two, so I can actually find
that airport to land on.

--
Eric Greenwell - Washington State, USA (change ".netto" to ".us" to
email me)