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Old 15-02-2013, 23:17   #1
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Homemade backup battery for small electronics

I have a piece of electronics I want to keep on all the time - it is currently wired directly to my master house battery switch (with an inline fuse).

However if someone turns off that switch, when the power comes up again the unit goes in standby. It draws little current when in idle - ready to self turn on - but in standby after off, it does not turn on without a mechanical button press. I want it on.

So, I want to make a small backup battery. Say a motor cycle battery - and need a capacitor inline so that the backup does not float at the house battery charge level, but rather at full. How do I calculate the capacitor size? And am I doing this the right way?

Thanks.
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Old 16-02-2013, 00:34   #2
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Re: Homemade backup battery for small electronics

I am not sure how you plan to use a capacitor inline to achieve your goal. Capacitors block dc. A relay which would drop out and not keep your auxiallary battery connected to your main battery, when the main is switched off the load might be a better solution. Keep in mind that you don't want that battery connected when you try to start your engine. A good system to do this is to control the disconnect relay with an oil pressure switch. That way the relay only connects the auxilliary battery to the main battery after the engine starts and the oil pressure comes up and automatically drops out when the engine and charging stops.
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Old 16-02-2013, 01:06   #3
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Re: Homemade backup battery for small electronics

Quote:
Originally Posted by alessandra View Post
I have a piece of electronics I want to keep on all the time - it is currently wired directly to my master house battery switch (with an inline fuse).

However if someone turns off that switch, when the power comes up again the unit goes in standby. It draws little current when in idle - ready to self turn on - but in standby after off, it does not turn on without a mechanical button press. I want it on.
.
Just connect it to the other pole of your house battery switch (the one that leads to battery). Then it will stay on irrespective of the position of the boat battery switch.
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Old 16-02-2013, 20:45   #4
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Re: Homemade backup battery for small electronics

Well as others have said a capacitor will not work. A Schottky Diode will work. Put one on the feed from the house battery and your good to go. Voltage drop is only about .5V so probaby ok for your use

1N5822: Diode Schottky Barrier Rectifier 3A 40V DO-201AD

Need more then 3 amps, then put two in parallel. Easy..
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Old 16-02-2013, 20:58   #5
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Re: Homemade backup battery for small electronics

Sorry, but diodes don't play well together in parallel.
Minute differences in forward conduction make a big difference in which one does the heavy lifting.
In this case, it doesn't sound like that would be a problem as his load is supposedly small.
A battery combiner would be a better choice. Open circuit until the batteries are being charged, then virtually zero voltage drop between house and backup battery.
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Old 16-02-2013, 23:05   #6
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Re: Homemade backup battery for small electronics

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Originally Posted by senormechanico View Post
Sorry, but diodes don't play well together in parallel.
Minute differences in forward conduction make a big difference in which one does the heavy lifting.
In this case, it doesn't sound like that would be a problem as his load is supposedly small.
A battery combiner would be a better choice. Open circuit until the batteries are being charged, then virtually zero voltage drop between house and backup battery.
Well true on the parallel issue but for a small load and using schottky diodes I still think it would work as long as the total amps are oh 60% of peak for two. One might run more amps but the other would still take part of the load too. Putting both on a common heat sink would help some too

Oh you could get a pair that was grossly missmatched, and one could go into thermal runaway and die, but his load is probably less then 3 amps anyway. There are also 30 and 100 amp diodes also.

The problem I see with the battery combiner is that the house bank feeds the electronics and so would the backup. So they would be in the same circuit not on opposite sides of a combiner. In my mind the house is charging the backup, where the .45v forward drop of the schottky would make it a fair match to keep the little battery at a good state of charge. When the house dies or is switched off the backup takes over with no loss of power.
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Old 17-02-2013, 00:09   #7
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Quote:
Originally Posted by noelex 77 View Post
Just connect it to the other pole of your house battery switch (the one that leads to battery). Then it will stay on irrespective of the position of the boat battery switch.
Noelex's solution is so much simpler than diodes, second batteries, or combiners. This is typically how other uninterruptable devices are wired up like bilge pumps and CO detectors.
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Old 17-02-2013, 10:48   #8
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Re: Homemade backup battery for small electronics

The cheapest isolation for a small load instrument battery if it is being charged from the starting battery is to wire an automobile headlight in series with the charging line. Incandescent lamps have the delightful characteristic of having a cold resistance that is 10% or less than operating resistance. So a 60 watt headlight has a running resistance of about 2 ohms but when cold it is only about 0.2 ohms.

With charging currents significantly below 1 amp to run the instruments on a small tractor battery there will be minimal voltage drop. But if the starting battery drops to a low voltage when starting the engine, the headlight will have more voltage across it so some current will flow from the instrument battery to the starting battery, the lamp will heat up and limit reverse current flow to a maximum of 2 amps. This will limit any voltage drop on the instrument battery to safe levels.
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Old 17-02-2013, 11:10   #9
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Re: Homemade backup battery for small electronics

I used a sealed motorcyle (small) battery for my HF Radio. I isolated it from the 12v system by using a "wall wart" float charger wired into the AC circuit. I had a on/off switch to that AC outlet. Worked great. I still have that charger around here somewhere.
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