jondallimore
Posts: 309
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Run a pi off a car batt and buck converter

Sun Jun 13, 2021 5:45 pm

I'm looking to run a Pi off a car battery with a buck converter.
I've added a 3amp fuse and everything is working so far (not plugged a pi in yet but getting 5v from the buck)

I'm thinking adding an on/off switch between the buck converter and the battery.
Would it be best to add it on the positive terminal? Is there any reason not to?
Will the buck converter actually drain any power if theres nothing plugged into the 5v end?

Thanks

drgeoff
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Re: Run a pi off a car batt and buck converter

Sun Jun 13, 2021 6:25 pm

The buck converter will still draw a small current from the battery even if there is nothing taking power from its 5 volt output.
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W. H. Heydt
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Re: Run a pi off a car batt and buck converter

Sun Jun 13, 2021 6:32 pm

jondallimore wrote:
Sun Jun 13, 2021 5:45 pm
I'm thinking adding an on/off switch between the buck converter and the battery.
Would it be best to add it on the positive terminal? Is there any reason not to?
Positive terminal is the correct place to put the power switch.

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CaxtonPi
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Re: Run a pi off a car batt and buck converter

Sun Jun 13, 2021 6:34 pm

What is a buck converter?

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neilgl
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Re: Run a pi off a car batt and buck converter

Sun Jun 13, 2021 6:35 pm

How is it going to shutdown safely i.e. not just turning off the 12v (12V to 5V via DC to DC adapter aka “buck converter”)

jondallimore
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Re: Run a pi off a car batt and buck converter

Sun Jun 13, 2021 7:39 pm

by shutting it down first...
the switch is just to prevent any power draw.
I'm also intending to run motors from the same car battery, so want a master switch

drgeoff
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Re: Run a pi off a car batt and buck converter

Sun Jun 13, 2021 7:51 pm

CaxtonPi wrote:
Sun Jun 13, 2021 6:34 pm
What is a buck converter?
All search engines down again?
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jondallimore
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Re: Run a pi off a car batt and buck converter

Sun Jun 13, 2021 7:55 pm

https://letmegooglethat.com/?q=what+is+ ... nverter%3F

Good to know it will draw a bit of current - master switch it is

LTolledo
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Re: Run a pi off a car batt and buck converter

Sun Jun 13, 2021 9:57 pm

jondallimore wrote:
Sun Jun 13, 2021 7:55 pm
https://letmegooglethat.com/?q=what+is+ ... nverter%3F

Good to know it will draw a bit of current - master switch it is
as a "master switch", better cut off both sides (positive and Gnd) as well...
DPDT switches are not that expensive....
(unless 2-pole breakers are used as master switch)
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jondallimore
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Re: Run a pi off a car batt and buck converter

Sun Jun 13, 2021 10:52 pm

Why?
What advantage is there to full isolation?
I would think better to keep gnd connected at all times.

ganzgustav22
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Re: Run a pi off a car batt and buck converter

Mon Jun 14, 2021 7:49 am

jondallimore wrote:
Sun Jun 13, 2021 5:45 pm
Will the buck converter actually drain any power if theres nothing plugged into the 5v end?
This one: https://www.pololu.com/product/3782 uses around 2mA quiescent current. Not very much, doesn't really matter if you leave it connected all the time given that typical car batteries have capacities above 50.000mAh.

jondallimore
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Re: Run a pi off a car batt and buck converter

Mon Jun 14, 2021 11:49 am

aye, its not going to be much. However I also think a kill switch would be useful for the other stuff I'm connecting to it - so switch it is.

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davidcoton
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Re: Run a pi off a car batt and buck converter

Mon Jun 14, 2021 3:57 pm

jondallimore wrote:
Sun Jun 13, 2021 10:52 pm
What advantage is there to full isolation?
The equipment still stops if one side of the switch fails.
Since we are only talking about 12V, it is a comparatively minor consideration that a part functional two-pole switch could break the 0V line leaving 12V on the equipment -- 12V isn't going to do much damage when there is no direct return path.

Aside: With mains equipment, a two-pole main switch is strongly recommended, and sockets with a single pole switch, while not illegal, are deprecated.

Remember that a ground connection and an 0V connection are not the same thing. Low voltage equipment is often isolated rather than grounded, especially where switch-mode power supplies are involved.
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emma1997
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Re: Run a pi off a car batt and buck converter

Mon Jun 14, 2021 6:55 pm

ganzgustav22 wrote:
Mon Jun 14, 2021 7:49 am
jondallimore wrote:
Sun Jun 13, 2021 5:45 pm
Will the buck converter actually drain any power if theres nothing plugged into the 5v end?
This one: https://www.pololu.com/product/3782 uses around 2mA quiescent current. Not very much, doesn't really matter if you leave it connected all the time given that typical car batteries have capacities above 50.000mAh.
What worked best for me with many battery setups is to use the enable pin on the converter. Popular LM2596 only draws couple microamps when shut down. Virtually same as fully disconnected, won't drain even a coin cell. Not to mention orders of magnitude longer switch life.

Using 2 pole switch for DC is rare. I've never seen this and, as mentioned, don't make a lot of sense.

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