[TriEmbed] Battery charging while under load

Wed Feb 19 21:10:26 CST 2020

Thank you guys for excellent answers.  That charger/wall adaptor diagram is
exactly the sort of situation I am talking about.  I'd heard of using
MOSFETs to act like a diode, but it's cool to see that they actually have
IC packages that are specifically implementing that trick to act like a
more efficient Shottky diode.  I will have to do some more research, but
this is definitely looking like a promising direction.

For anyone else on the list that might be interested in this sort of thing,
I thought it might be good to share some of the stuff I've found so far.
One of the things that threw me for a while is that all of the chargers for
LIPOs I've seen hook up to each cell individually, while most of the
charger ICs I've looked at seemed to hook up to all of the cells in
series.  What I'm starting to realize is that the ICs I'm looking at seem
to have split functionality into 2 parts.  You have one IC which is
responsible for under/over voltage protection of the cells and cell voltage
balancing (a balancer which typically stays with the battery) and a
seperate one which feeds power to the balancer combination (constant
current, then constant voltage).  Essentially, my manual charger is
combining the two functionalities into one module.  However, if I get a
battery which has a built in balancer then I only have to worry about the
charger side of things.

Another useful thing:  There are several different lithium battery
chemistries floating around and some are better for certain things.  I
would really like my robots not to catch on fire and the battery not to die
for several years.  Consequently, lithium iron phosphate batteries are
looking much better than LIPOs if there current is high enough (which I
think it will be).

1. A good guide to LIPOs
https://rogershobbycenter.com/lipoguide

2. Decent summary of pros/cons of different battery chemistries
https://owlcation.com/stem/Comparing-6-Lithium-ion-Battery-Types

3.  Battery I'm considering using for the robot.
https://www.amazon.com/Miady-Phosphate-Rechargeable-Self-discharge-Replacement/dp/B07Q7FY8CC?th=1

Thanks,
Charlie

On Tue, Feb 18, 2020 at 7:35 PM Shane Trent via TriEmbed <
triembed at triembed.org> wrote:

> 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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>
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