On Tue, 25 Jun 2019 13:14:00 +1300
nospam.Henri.Derksen@f1208.n280.z2.binkp.net (Henri Derksen) wrote:

>Hello Folderol,
>
>> Anything based on lithium batteries is going to be complex. SLAs are far
>> more easy going, especially when on constant trickle charge.
>
>Yes.
>
>> There is a relatively simple way this can be achieved, and get constant
>> genuinely uninterrupted power to the Pi.
>
>> You use a 12V SLA charged from about 15V via a series diode and small
>> 12V light bulb. The light bulb not only gives you current control,
>> it also gives you visible indication.
>
>That light bulb is placed in parallel I asume?
>What capacity in Watt's you need?

No! Series. 1 or 2 W is about right, but you need to find one that when run
like this gives about the right trickle charge current - for SLAs it's not
usually critical.

>> Both the battery, and the original supply have further diodes, both
>> cathodes going to a small DC-DC converter to give you the stable 5V out.
>
>Make that 5.1 or 5.2 Vdc=, as 5.0 V is to low for the Pi to operate safely
>in the long run.

>> Under normal conditions the original supply is higher than the battery
>> voltage so the battery's output diode is backed off.
>
>Yes, the highest Voltage will feed the load.
>Victron Energy in Almere.NL makes a battery combiner with two
>80 A Shottkey diodes for 12 or 24 Vdc= usage ;-)
>Used for dual power safety critical users, like VHF, AIS and ECDIS.
>I build an ECDIS with a Pi and OpenCPN chart viewer software.
>
>> If the mains fails, the voltage sags to the battery voltage minus
>> the diode drop, which will still be plenty.
>
>5.0 Vdc= minus 0.6 Vdc= diode Voltage drop is to low for the Pi.
>You need 5.1 Vdc= with 3 Adc= at the Pi's power input.

The diodes are *before* the 12V->5V regulator.

>> At the same time the light bulb will go out, and the series diode
>> prevents power being push in reverse to the power supply.
>
>Yes.
>
>> I've used variations of this on many, many occasions going back into the
>> 1970s and never had a problem.
>
>Only when the power outage is during too long ;-(.
>But then you can start a generator, or better use sunpanels and/or windmills.
>
>> If you want to be really fussy, add a relay to the output that drops out
>> if the battery is below a certain voltage.
>
>A relay makes it more complex and less faultproof.
>KISS = Keep It Stuped Simple ;-).
>
>Henri.
>


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W J G

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