W. H. Heydt
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Re: Portable Monitor for raspberry pi?

Tue Oct 08, 2019 10:16 pm

RossDv8 wrote:
Tue Oct 08, 2019 9:27 pm
Thanks davidcotton. I know with most traditional battery chemistries using batteries in parallel is common, with the caveat that one should not mix types and sizes. and try to avoid the batteries being at different states of charge,
The most common batteries run the cells in series: Autmotive (and the marine versions) generally use 6 2v Lead Acid cells in series. Flashlights use anything from alkaline (used to use zinc-carbon) AAA to D cells in series. This is why your typical battery holder alternates to orientation of the individual cells. I have an old UPS that uses 2 sealed lead acid 12v batteries...in series, giving 24v.

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Gavinmc42
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Re: Portable Monitor for raspberry pi?

Wed Oct 09, 2019 12:22 am

LiPO's and Li-Ion can be paralleled HobbyKing and Tesla do it.
But circuit efficiency mean series connected cells and then a step down converter is better.

Power banks step up 3.7-4.2volt cell to 5V, there is a higher % loss in conversion stepping up.
The currents are higher so parts are more expensive.....
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RossDv8
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Re: Portable Monitor for raspberry pi?

Wed Oct 09, 2019 7:43 am

Putting Li-ion cells in parallel is subtly different from paralleling the outputs of DC-DC converters.
[/quote
And because a Li-Ion power bank is a DC-DC converter, I would be connecting the output of the converters in parallel - NOT the cells in parallel.

I suppose the simplest way to find out is to make sure two power banks are fully charged, then plug each male USB plug of the Y-cable into a 2A outlet on each of the power banks, and plug the Pi into the female USB socket on the Y cable, grab a fire extinguisher - and boot up the Pi :D :D :D

Having said that, talking about ways to power the Pi and the portable monitor, has sort of drifted this topic a little sideways... Sorry :oops:
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davidcoton
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Re: Portable Monitor for raspberry pi?

Wed Oct 09, 2019 10:22 am

RossDv8 wrote:
Wed Oct 09, 2019 7:43 am

And because a Li-Ion power bank is a DC-DC converter, I would be connecting the output of the converters in parallel - NOT the cells in parallel.
Exactly the wrong conclusion. Many (not all) cell types can be paralleled, given identical cells of identical age. They are "designed" for it, but consult the manufacturer's datasheet.

Mains-derived supplies and voltage converters are often built as cheaply as possible, and working with another supply in parallel is not a design feature. Supplies should only be paralleled through reverse blocking diodes, which introduce their own problems of voltage drop and power dissipation. You need a good understanding of what you are doing before trying it. Don't assume the designers made it safe -- even if they did, "marketing" may have removed the safety elements.
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RossDv8
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Re: Portable Monitor for raspberry pi?

Thu Oct 10, 2019 12:11 am

The power banks I have lying around are Chinese cheapies off eBay, and the Y-cables are also Chinese from eBay.
I'm pretty sure everything is engineered to the most exacting tolerances and safety standards...
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