Battery Charging

Thread Starter

EricSutton

Joined Oct 7, 2018
37
Hi, I would like to use 2 batteries for my project. They are Li-ion if it matters. I would like to use one to drive the circuit, until it is completely dead, or just about, then switch to the other. But while using that battery I would like the other to be charging.

I was told to use this as a good switching device: Link
But from what I understand is that this only lets you use the one that is at a higher voltage. But I want to have total control over which is being used at what time.

Any other component recommendations or if this one is sufficient maybe someone can help explain it better.

Thanks
 

mvas

Joined Jun 19, 2017
538
Yes, there is a Body Diode that will turn on at ~ 0.7 Volts differential.

Charged = 4.2 Volts
Discharged = 3.6 Volts
==================
Difference = 0.6 volts ?

Charging to only 4.1 Volts will significantly increase the life of the cell.
The "charged" battery will prevent the "discharged" battery from going below 3.5v - 3.4v.
Is that a bad thing at a 0.5 C or less Discharge Rate?
 
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MrChips

Joined Oct 2, 2009
19,269
That doen't make sense. If you are going to have a second battery on charge, why not have just one battery on charge all the time?
Normally, if you are going to use two battery packs for portable operation, you would exchange the battery packs.

For continuous mobile operation you would bridge the two batteries on separate connections so that there is zero down time. Use two Schottky diodes for that.
 

ronv

Joined Nov 12, 2008
3,768
Hi, I would like to use 2 batteries for my project. They are Li-ion if it matters. I would like to use one to drive the circuit, until it is completely dead, or just about, then switch to the other. But while using that battery I would like the other to be charging.

I was told to use this as a good switching device: Link
But from what I understand is that this only lets you use the one that is at a higher voltage. But I want to have total control over which is being used at what time.

Any other component recommendations or if this one is sufficient maybe someone can help explain it better.

Thanks
Do you have a mirco or something to oversee all of this?
 

crutschow

Joined Mar 14, 2008
23,292
You could use two P-MOSFETs in series, connected source-to-source.
That configuration will block in both directions when OFF (Vgs = 0V) and conduct in both directions when ON (Vg = gnd).
If the battery is less than 10V, the series MOSFETs need to be logic-level types (Vgs(threshold) is ≤2V max).

Below is the LTspice simulation of such a circuit.

upload_2019-5-16_9-42-5.png
 
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Thread Starter

EricSutton

Joined Oct 7, 2018
37
Yes, there is a Body Diode that will turn on at ~ 0.7 Volts differential.

Charged = 4.2 Volts
Discharged = 3.6 Volts
==================
Difference = 0.6 volts ?

Charging to only 4.1 Volts will significantly increase the life of the cell.
The "charged" battery will prevent the "discharged" battery from going below 3.5v - 3.4v.
Is that a bad thing at a 0.5 C or less Discharge Rate?
Where did you find this 0.7V turn on differential?

That doen't make sense. If you are going to have a second battery on charge, why not have just one battery on charge all the time?
Normally, if you are going to use two battery packs for portable operation, you would exchange the battery packs.

For continuous mobile operation you would bridge the two batteries on separate connections so that there is zero down time. Use two Schottky diodes for that.
The point is to hardly ever have to touch the system. Is there a schematic you could sketch showing the Schottky doides?

You could use two P-MOSFETs in series, connected source-to-source.
That configuration will block in both directions when OFF (Vgs = 0V) and conduct in both directions when ON (Vg = gnd).
If the battery is less than 10V, the series MOSFETs need to be logic-level types (Vgs(threshold) is ≤2V max).

Below is the LTspice simulation of such a circuit.

View attachment 177540
Aren't the top of R1 and R2 floating when the MOSFETs are off?
 
Last edited by a moderator:

ian field

Joined Oct 27, 2012
6,543
Hi, I would like to use 2 batteries for my project. They are Li-ion if it matters. I would like to use one to drive the circuit, until it is completely dead, or just about, then switch to the other. But while using that battery I would like the other to be charging.

I was told to use this as a good switching device: Link
But from what I understand is that this only lets you use the one that is at a higher voltage. But I want to have total control over which is being used at what time.

Any other component recommendations or if this one is sufficient maybe someone can help explain it better.

Thanks
All the literature I've seen says over discharging lithium cells ruins them (2.75V minimum AFAICR) - but all my E-cig batteries are salvage from the recycle bin and I've had very few problems.

You need a UVLO circuit which probably involves a comparator and power management transistor.
 

Thread Starter

EricSutton

Joined Oct 7, 2018
37
What is charging the battery on standby?
Why not use a single battery?
I want to have 2 batteries, so power is always on for the system. One to power, and one to charge while the other is running. Then once one gets sufficiently low the system will automatically and safely switch to the other battery. Then the dead one will begin charging.

I want to have 2 batteries, so power is always on for the system. One to power, and one to charge while the other is running. Then once one gets sufficiently low the system will automatically and safely switch to the other battery. Then the dead one will begin charging.
Solar panels will be charging the batteries
 

MrChips

Joined Oct 2, 2009
19,269
I want to have 2 batteries, so power is always on for the system. One to power, and one to charge while the other is running. Then once one gets sufficiently low the system will automatically and safely switch to the other battery. Then the dead one will begin charging.


Solar panels will be charging the batteries
Still doesn't make sense to me. Kept one battery on charge when the sun is out. At night, the battery powers the system.
Why do you need two batteries?
 

BobTPH

Joined Jun 5, 2013
2,016
Do you understand that a battery can be charged without disconnecting it? Then whenever charging current is available it will be partially powering the circuit and partially charging the battery. There is no need to have two batteries and disconnect one to charge it. Does your phone have two batteries? Have you ever used your phone when connected to a charger? Why do you think can your device will not work the same way?

Bob
 

crutschow

Joined Mar 14, 2008
23,292
Aren't the top of R1 and R2 floating when the MOSFETs are off?
With respect to ground, yes.
But they keep both MOSFETs' Vgs=0, so they stay off, independent of the input or output voltages.
Also, if you have 2 PMOS and you connect them source to source. Current can't flow because then it would go drain to source, which for PMOS you cant do
Au contraire.
Where did you get that misinformation?
MOSFETs conduct equally well in either direction when biased ON.
They just can't block in both directions due to the substrate diode.
 

Thread Starter

EricSutton

Joined Oct 7, 2018
37
Do you understand that a battery can be charged without disconnecting it? Then whenever charging current is available it will be partially powering the circuit and partially charging the battery. There is no need to have two batteries and disconnect one to charge it. Does your phone have two batteries? Have you ever used your phone when connected to a charger? Why do you think can your device will not work the same way?

Bob
Yep I understand. But when the battery dies eventually. I want the system to keep running, hence a backup battery.

I do not think my device will work the same way because of the amount of current draw I will be having. I have not finalized the design so I do not have numbers. I am assuming a worst case scenario where I draw so much power that it will not last the entire time the sun is up, including solar panels powering it partially.
 

Thread Starter

EricSutton

Joined Oct 7, 2018
37
With respect to ground, yes.
But they keep both MOSFETs' Vgs=0, so they stay off, independent of the input or output voltages.
Au contraire.
Where did you get that misinformation?
MOSFETs conduct equally well in either direction when biased ON.
They just can't block in both directions due to the substrate diode.
I guess I always just assumed that they would only go one direction for some reason, but now that I think about it, it sounds silly to say... Knowing how it physically works I guess I should have known that. I just never thought about that.
 

MrChips

Joined Oct 2, 2009
19,269
Yep I understand. But when the battery dies eventually. I want the system to keep running, hence a backup battery.

I do not think my device will work the same way because of the amount of current draw I will be having. I have not finalized the design so I do not have numbers. I am assuming a worst case scenario where I draw so much power that it will not last the entire time the sun is up, including solar panels powering it partially.
Have you completed an energy budget?

The total energy consumption has to be provided by the sun.
If you are going to consume more energy that what the sun can provide on a typical day then your battery will die, regardless of whether you use one or two batteries.

If one battery can be charged during sunlight hours and provide the energy when there is no sun then one battery will do.
 

Thread Starter

EricSutton

Joined Oct 7, 2018
37
Have you completed an energy budget?

The total energy consumption has to be provided by the sun.
If you are going to consume more energy that what the sun can provide on a typical day then your battery will die, regardless of whether you use one or two batteries.

If one battery can be charged during sunlight hours and provide the energy when there is no sun then one battery will do.
I see what you're saying, crap. You are definitely right. I guess I was thinking that it makes more sense to use 2 because then one is being charged and one is providing power. But I am wrong. Thank you, that makes perfect sense.
 
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