Electronics Repair (sci.electronics.repair) Discussion of repairing electronic equipment. Topics include requests for assistance, where to obtain servicing information and parts, techniques for diagnosis and repair, and annecdotes about success, failures and problems.

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Old October 10th 16, 04:25 PM posted to sci.electronics.repair
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Default Batteries

The first question : which fuse blows ?

Stephen Wolstenholme a écrit :
A friend has a disabled buggy for short distance trips. It is supposed
to do 25 to 30 miles on fully charged batteries. Last week it stopped
dead after about 10 miles. It was retrieved by the insurance company
and taken to the workshop for repair. A fuse had blown and so both
batteries needed replacing. Can anyone explain how a fuse blowing
results in the batteries needing to be replaced. They are sealed lead
acid batteries.

Any ideas?

Steve



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Old October 10th 16, 08:39 PM posted to sci.electronics.repair
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In article ,
Chris Jones wrote:

Would the workshop receive more money if they decide that the batteries
need replacing, even if the batteries are ok?


This.

It's also why some dealers advocate that, for best life on the Lead Acid
batteries, you should always run them fully down, before recharging
them.

Is it ignorance, or just a way to ensure you hammer the batteries
into the ground?

At which point they can sell you new ones at ridiculous markup,
as "specialist items" which can't be bought elsewhere ...
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Mike Brown: mjb[-at-]signal11.org.uk | http://www.signal11.org.uk

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Old October 10th 16, 09:15 PM posted to sci.electronics.repair
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On Monday, October 10, 2016 at 1:29:43 PM UTC-4, Phil Hobbs wrote:


Sure, but if it ever had enough torque to start up against the load,
that amount of current wouldn't blow the fuse. Of course it might be a
super slow blow variety that eventually went, but saying that the
current went up because the voltage went down makes no sense unless it's
using a SMPS.


Phil:

Here is the scenario:

a) Go-Buggy has what passes for a full charge, and off the rider goes.
b) At some point, the battery starts to "run down". Voltage drops.
c) There is a magic moment when the battery *cannot* put out enough current (combination of voltage *and* amps) to start the motor under load. Say, a small hill.
d) As this very cheap device has a brush-type DC motor, the battery is now dead-shorted across the fuse. As you suggest, that fuse is likely a very slow blow device, also rated well over the actual 'normal' operating current. It is designed to protect real-estate vs. the device.
e) *POOF* goes the fuse, along with, effectively, the battery.

It is not so much that the current went up. The battery DEAD SHORTED.

Try it yourself. Get an inexpensive 12V DC brush-type motor. Feed it with a sealed lead-acid battery and measure the current. Get a fuse at 3X that rating. Clamp the motor shaft so that it cannot turn. Turn on the power. *POOF*. Even if the battery is on its last legs, making say.... 9V, the amount of current a lead-acid battery can put out is pretty massive even then...

Again, this is a BRUSH-TYPE motor.

http://www.monsterscooterparts.com/m...-motor-brushes

Nor are the controllers much of anything.

http://www.monsterscooterparts.com/2...MG7RoC42bw_wcB

I pay more for my R/C submarine control devices - which do have current limiters and also high-temp shutoff capacity. And a fail-safe that blows ballasts tanks and 'floats the boat'.

Peter Wieck
Melrose Park, PA
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Old October 11th 16, 04:17 AM posted to sci.electronics.repair
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Phil Hobbs wrote:



Sure, but if it ever had enough torque to start up against the load,
that amount of current wouldn't blow the fuse. Of course it might be a
super slow blow variety that eventually went, but saying that the
current went up because the voltage went down makes no sense unless it's
using a SMPS.


** Battery current goes up with diminishing voltage due the actions of the driver increasing the throttle setting and so the duty cycle of the PWM drive.

The fuse copes with normal start up since it has a long thermal time constant. But if the motor is unable to get the scooter moving for any reason and the driver increases the throttle to full, the time constant is soon exceeded.

Also, most SLA batteries incorporate an internal fuse as a last line of defence against a short or near short. This is also slow acting and just maybe was the culprit in the OP's example.

I have some experience with SLA and wet cell batteries used with a 12V starter motors for RC boats using high performance, glow plug engines. This involves putting a severe load on the battery for a few seconds at a time, sometimes pulling the terminal voltage down to near half if the engine is flooded with fuel.

SLA battery life was fairly short used in this way, so I eventually upgraded to a 40AH maintenance free car battery with a 20A in-line blade fuse for safety. The fuse kept popping so got replaced with a domestic AC supply circuit breaker, also rated at 20 amps. This was *perfect* as it protected the battery from a dead short ( 2kA breaking capacity ), rarely opened in normal use and when it did could be reset in 2 seconds.



..... Phil



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Old October 13th 16, 07:54 PM posted to sci.electronics.repair
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Stephen Wolstenholme wrote:
On Mon, 10 Oct 2016 14:18:27 +0100, Stephen Wolstenholme
wrote:

On Mon, 10 Oct 2016 08:55:33 -0400, Pat wrote:

On Mon, 10 Oct 2016 13:25:23 +0100, Stephen Wolstenholme
wrote:

A friend has a disabled buggy for short distance trips. It is supposed
to do 25 to 30 miles on fully charged batteries. Last week it stopped
dead after about 10 miles. It was retrieved by the insurance company
and taken to the workshop for repair. A fuse had blown and so both
batteries needed replacing. Can anyone explain how a fuse blowing
results in the batteries needing to be replaced. They are sealed lead
acid batteries.

Any ideas?

Steve

My guess is that it's the other way around. The bad batteries
(voltage lower than spec due to bad cells) caused the electronic
controller to draw too much current in an attempt to maintain speed.
That's just a guess, though. It is hard disagnose something with no
information. It is also possible they replace the batteries whenever
they do any other repair to avoid another trip to the shop as the
batetries reach end-of-life.


The batteries were brand new about two months old!

Steve


I should have typed "two months ago!"


two months is a good run for chinese batteries.
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Old October 13th 16, 09:01 PM posted to sci.electronics.repair
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On Monday, October 10, 2016 at 3:50:05 PM UTC-4, Mike wrote:

It's also why some dealers advocate that, for best life on the Lead Acid
batteries, you should always run them fully down, before recharging
them.



That applies pretty much only to NiCd (nickle/cadmium) batteries that will get a 'memory' if not fully discharged with each use. That memory can be dispersed with care, but it can be a real PITA when the submarine in question is about 5 feet down and 40 feet away in a very fast stream. And why I stopped using them.

Peter Wieck
Melrose Park, PA
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Old October 14th 16, 02:45 AM posted to sci.electronics.repair
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wrote:




That applies pretty much only to NiCd (nickle/cadmium) batteries that
will get a 'memory' if not fully discharged with each use. That memory
can be dispersed with care, but it can be a real PITA...



** The so called "memory effect" in NiCd batteries is basically a myth.

https://en.wikipedia.org/wiki/Memory_effect


However, loss of capacity of a NiCd pack is very common and caused by one or more of the following.

1. Over-charging so the cells get hot.

2. Over-discharging so some of the cells go to zero or reverse voltage.

3. To fast discharging so the cells get hot.


Each of the above damages cells so some or all of them have reduced capacity or develop a fault condition where the cells discharge themselves in a much shorter time than the others - ie days instead of months.

Since all the cells in a pack are charged in series, the cells that contain more charge than others soon become over-charged and heat up while others remain less that fully charged. Such a pack will show low capacity as one or more cells goes flat early.

Such a pack can usually be fixed by discharging each cell individually to 1V, but if any cells have developed high self discharge then even this does not work.

The term "memory effect" is regularly trotted out when one of the above scenarios needs explaining.


..... Phil





















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Old October 14th 16, 12:12 PM posted to sci.electronics.repair
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On Thursday, October 13, 2016 at 9:45:17 PM UTC-4, Phil Allison wrote:


The term "memory effect" is regularly trotted out when one of the above scenarios needs explaining.


.... Phil


And why I put '' around the word. It is a convenient term for an inconvenient problem.

Peter Wieck
Melrose Park, PA
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