[TriEmbed] Battery charging while under load

[Shane Trent]

Charlie,

As Pete mentioned, isolating the two power sources using Shottky diodes
inline with each power source is a good way to go if you can spare the
diode drop (and lost useable voltage range) on the battery pack. The
article below goes into more detail on configurations of OR-ing diode
configurations and using active diodes (controlled MOSFETs) to reduce the
voltage drop when running from a battery pack.
https://www.maximintegrated.com/content/dam/files/design/technical-documents/design-solutions/ds50-cut-your-losses-with-an-ideal-diode.pdf

Several vendors offer active diodes or smart diodes for use in power OR-ing
or solar panel applications. The TI SM74611 looks like it might fit the
bill. One of those for each power source and your sources are isolated with
a forward voltage drop of 26mV at 8A.
https://www.digikey.com/product-detail/en/texas-instruments/SM74611KTTR/296-35688-1-ND/3911155


Good luck!
Shane

On Tue, Feb 18, 2020 at 1:17 PM Pete Soper via TriEmbed <
triembed at triembed.org> wrote:

> You typically use a schottky diode that can handle the current with a low
> voltage drop to keep current from flowing in an unintended direction. It
> isn't immediately obvious how your batteries could go flat with the charger
> connected and the system running, but an ammeter can help you determine how
> much current is going where in the various scenarios. Maybe the charger is
> being tricked into turning off? Hard to guess.
>
> -Pete
> On 2/18/20 1:10 PM, Charles West via TriEmbed wrote:
>
> Hello!
>
> I'm still working on my robot project and I'm starting to make tentative
> steps toward something other people could actually use.  There are a few
> open problems remaining, but there is one in particular I was wondering if
> you guys might have any ideas for.
>
> How do you make a LIPO battery system that acts like your laptop?  In
> particular, how can you make it so you can run your system off of adaptor
> power when it is plugged in/charge the battery and then automatically
> switch to battery power when the adaptor power is removed?
>
> I've tried in the past just leaving my system plugged in while the battery
> was charging with an external charger.  The result was dead LIPOs.
>
> The typical LIPO pack has 2 thick wires coming out with all of the cells
> (3 or 4 in my case) in series and a smaller JST connector which exposes
> each of the cells in parallel.  My first thought is that you could have
> your adaptor voltage higher than the battery series voltage and use one or
> more reverse current protection circuits to make the battery stop
> discharging in that case and apply power to the charger.  Does that make
> sense as an approach?
>
> They also have battery management ICs (such as the Maxim line) which
> include charging, but I'm not sure if they can handle the level of current
> that the motors could draw.  What do you guys think?
>
> Thanks,
> Charlie
>
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*Shane D Trent*
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