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Li ion Battery charge and use circuit

Royrdan Ross

Feb 3, 2017
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Hi guys! I'm new to electronics (self taught) and still trying to wrap my head around a few things. Ive got a Li ion battery pack (4 x 18650 batteries) rated at 14.8v. Ive built a robot with 2 x 12v motors for motion, a raspberry pi and some ultrasonic sensors. What I want to do is the robot run on battery power but to be able to charge from a wall plug without interrupting the power supply. Ive drawn up a circuit that explains how I think I can set this up but not sure if this will work? (Sorry if it's dodgy) I haven't been able to find any difinitive information on this.
The wall provides 24v which is regulated to 16.8v through a charge controller to charge the battery. And regulated to 12v for the motors. Which is then regulated to 5v for the pi and powering the sensors etc. I believe the battery won't discharge and will charge if there is a higher voltage against it. So when the power is disconnected. The battery takes over with 14.8v(average) and regulated to 12v to 5v etc as before. I believe I'll need a diode from battery to 12v regulator so 24v isn't sent straight to the battery.

Can anyone tell me if this will work?
If not, how can this be achieved or is there an easier way of doing this?
Thanks for any help!
 

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Bluejets

Oct 5, 2014
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24v to 12v regulator is going to put a lot of unnecessary strain on your 12v regulator.
Better to come from the battery.
Only problem then is when the battery starts to get down in voltage, the regulator may not do it's job quite as well.
Perhaps use a low drop out version.
LiFePo4 require dedicated chargers to monitor min and max charge voltage of the cells, not just any old off the shelf charger.
Cells may need protection pcb boards fitted as well just to be on the safe side.
 

Royrdan Ross

Feb 3, 2017
3
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Feb 3, 2017
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Thanks for your reply. What do you mean by 'come from the battery'? Like drain the battery as it charges? Ive read that is bad practice. I was going to use a buck boost regulator with auto cut off when voltage goes low (battery too empty) as the charger regulator and the battery pack already has a PCB for over and under voltage protection. I'm just using a generic transformer wall plug at the moment but also have 12v ones but this would not be enough to power the 16.8v for the battery.
 

(*steve*)

¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd
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Jan 21, 2010
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Like drain the battery as it charges? Ive read that is bad practice

It is perfectly OK. Depending on the load current vs the charge current, you will either discharge the battery slowly or charge it slowly.

Your charge current must be limited and the voltage be no higher than the fully charged battery voltage
 

Royrdan Ross

Feb 3, 2017
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Feb 3, 2017
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Depending on the load current vs the charge current, you will either discharge the battery slowly or charge it slowly.

So if the battery needs to charge at 2amps do I leave the regulator at only 2amps and only charge the battery slowly with whatever the load doesn't use? Then from the battery to the motor driver and 5v regulator? Does the battery take whatever current is available? Or does it only take what it needs? I have a pcb overvoltage protector on the battery pack.
Can I run power backward through the battery charger(regulator) for the load? or will i need to connect straight from battery after the charger regulator, but then the charge regulator will not be able to regulate the battery properly with a varying load. I have been searching online for days but cannot find defining information.
 

(*steve*)

¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd
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Jan 21, 2010
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You can't put power through a regulator backwards.

Set the maximum voltage to the charge cutoff voltage, and the current to the appropriate value for the battery.

If there is no load, the current will tail off as the battery charges.

This approach is simple and reasonably safe, however it is not the fastest, and won't work well if your load current is very close to (or exceeds) the charge current.

If you have more than one cell, balance charging is highly recommended
 
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