Single cell Li-Ion battery protection

[Northy]

Hi,

I'm new here, but I hope someone can help.

I have a single cell Li-Ion battery that has a protection PCB built into it, but I would like to build a little more protection into the circuit to cut it out of circuit before the built in protection cuts in, or I'll have no way of charging it again.

So I was thinking of using a PFet high side switch and a micro power voltage monitor chip to drive it. If I used one with a reset pin then the on board micro would be able to reconnect the battery to charge it again.

Would that seem a sensible solution? Or is there a better way?

The device will be powered from the battery, but will be plugged into USB to charge it.

My thinking was this:

Device slowly discharges the battery while it is not in use - sleep mode.
When the battery gets to the trigger voltage the Fet disconnects the battery - meaning only self discharge now drains the battery.
The device is plugged into USB to charge - powering up the circuity and re-connecting the battery to charge it.

If the extra protection wasn't there, the circuit in sleep mode would just drain the battery down to the built in threshold and then it would never be able to be re-charged?

Thanks,

G

 

[Divedeep]

Can i have some more info please.

What current will your device use in sleep mode?,
How big is the cell in mAH?,
How long do you need the device to function for between charges?

I have just finished designing a device that has built in Li-Ion charging and monitoring. I used a microcontroller to monitor the voltage of the battery when not charging. This allows me to do 2 things.

Monitor the battery voltage,
Shut the device into standby when the voltage gets to 3.4v.

I dont actually disconnect the battery from circuit in any way i just limit the circuits current draw down to a few mA.

Providing the device isnt left discharged for a lengthy period of time then the cell should last for quite a while.

 

[Northy]

1) I'm not sure on the sleep current, as little as possible, but that partg isn't finalised yet.
2) Again not sure on the size of the cell yet. Big enough for the device to run and measure for 24 hours once we know how much current it takes when measuring. Maybe around 1000mAh.
3) Between charges it might sit in a cupboard for 6 months or a year - that's why I'm scared of the built in protection kicking in on the pack. If I don't disconnect the battery and the sleep current triggers that protection then the battery won't charge again will it?

The micro will moitor the voltage, but I still think I need a standalone way to remove the battery from circuit that can then be reset with the micro when we get usb power again?

G

 

[Divedeep]

Li ion battery chemistry is quite complex.

I am by no means the expert, but i know they dont like being over charged. They dont like being run down below a certain voltage. I believe that voltage is 2.7v or something in that region. This is the point at which the inbuilt protection will cut all outgoing power from the cell.

If the cell were to enter this state and be charged in a short space of time, then the cell will be functional but may loose small amounts of charge capacity.
If left for an extended period of time, then the cell will most likely be permanently damaged.

I think what you need to consider here is that li ion may not be the most ideal choice of cell.
NiMH for example is much more forgiving. Downside is the space that is occupied.

In terms of your cutoff for the battery, i am assuming that once the uC disconnects the battery it will completely shut down the device. The only thing that can bring it back to life is to put the device on charge.

Im thinking that this may be just as bad as if you dont charge the battery before putting it away for 6 month - 1 year then the cell will most likely be damaged anyway.

Final thought i had, is it possible to implement a kill switch into your design or simply unplug the battery??

 

[(*steve*)]

I believe that voltage is 2.7v or something in that region. This is the point at which the inbuilt protection will cut all outgoing power from the cell.

That assumes the cell is "protected"

 

[Northy]

Yes the battery is protected, here is the spec:

3.1 Absolute Maximum Ratings (for Ricoh R5402N204KD)
3.2 Supply Voltage: -0.3V to 12 V
3.3 Operating Temperature: -40℃ to 85℃
3.4 Storage Temperature: -55℃ to 125℃

4. Electrical Characteristics (for Ricoh R5402N204KD)(T=25℃ )
The followings is referring to the specs of R5402N204KD of Ricoh (for details,
see R5402N204KD ). These specs are guaranteed by design not by production tests.
4-A.1 Input Voltage: 1.5V (min) 5.0V(max)

4-A.2 Overcharge Detection : 4.175V (min) 4.200V(Typ) 4.225V(max)

4-A.3 Output Delay of Overcharge: 0.7s (min) 1.0s(Typ.) 1.3s (max)

4-A.4 Overcharge Release : 3.85V(min) 3.90V(Typ) 3.95V(max)

4-A.4 Over-discharge Detection : 2.438V (min) 2.500V(Typ) 2.562V(max)

4-A.5 Output Delay of 14ms (min) 20ms(Typ.) 26ms(max)
Over-discharge:

4-A.6 Over-discharge Release : 2.925V (min) 3.000V(Typ) 3. 075V(max)

4-A.7 Over Discharge-Current Detection : 0.185V (min) 0.20V(Typ) 0.225V(max)
4-A.8Overcharge-Current Detection : 0.17V (min) 0.20V(Typ) 0.23V(max)
4-A.9 Over Discharge-Current Value: 3.0A(min) 4.0A(Typ) 7.0A (max)
4-A.10 Over charge-Current Value: 3.0A(min) 4.0A(Typ) 7.0A (max)
4-A.11 Output Delay of 8ms (min) 12ms(Typ.) 16ms (max)
Over-Discharge-Current
4-A.12 Output Delay of 5ms (min) 8ms(Typ.) 11ms (max)
Over-charge-Current:
4-A.13 Short Protection Voltage: 0.55V (min) 0.8 V (Typ) 1.0V (max)
4-A.14 Output Delay of 230(Min) 300 µs(Typ ) 500µs(max)
Short Protection:
4-A.15 Supply Current (active status): 4.0µA (Typ) 8.0µA (max)
4-A.16 Supply Current (Standby): 1.2µA (Typ) 2.00µA (max)
4-A.17 PCM Resistance : 35mΩ(min) 50mΩ(Typ) 60mΩ(max)


I know Li-Ion batteries quite well from racing RC cars, and that is why I want to protect it so well.

The charging will be taken care of via a Ti bq24250.

My fear is that if the very low sleep current of the circuit discharges the battery down until it's built in protection kicks in and disconnects the battery cell then it will never be able to be charged again.

What I want to do is essentially mimic the built in low voltage cut off but at a higher voltage (just under 3V?) which would then mean that the only thing discharging the battery was it's own internal self discharge - and possibly the micropower voltage monitor I was planning on using.

It won't be possible to have a switch or unplug the battery.

Thanks,

G

 

[Northy]

Just a little update.

I built the circuit with a TPS3837 and a P channel fet and it works, but as the voltage drops and the fet disconnects, the voltage comes back up with no load and turns the fet on again, so I need to change it to latch.

However the theory works.

G

 

[Northy]

So a little update as I'm looking at this again. (it's only a simple block diagram):

The system can be powered either from the USB port (also used for charging up the lipo) or from the single lipo cell.

The device could be sat unused for a long length of time, and it's possible that the micro in sleep mode eventually discharges the lipo down until it's internal low voltage protection trips. Now that's fine from a safety point of view, but now the battery can not ever be re-charged as the built in protection PCB has disconnected it from the wires connecting it to the circuit.

To try and prevent that I'd like to have an extra layer of protection and disconnect the battery from the circuit at 3.0/3.1V ish. Then there is just the voltage monitor circuit discharging the battery, which should mean the battery doesn't get discharged down to the actually batteries low voltage cut off protection too quickly.

Once in this state, and the battery is disconnected, I'd like it to latch in that state so if the voltage creeps back up with no load on it it doesn't re-connect.

Once the device is plugged into USB again, the circuit will power from USB and the uP will know the battery has been disconnected (battery voltage is monitored by an A2D at the input to the board), so it will reset the latch in the monitor circuit and re-connect the battery so the charger IC can re-charge the battery.

All this is to hopefully stop batteries being damaged when units are not used for a long time.

What is a good way to implement the latching? The monitoring circuit needs to be as low power as possible (as it' will always be connected to the battery), and it needs to stay latched off when the micro is powered down when the battery is disconnected.

Thanks,

G

 

[(*steve*)]

There are single chip solutions to the lipo protection issue. I'll try to look one up for you tomorrow if nobody beats me to it

 

[Northy]

Did you get chance to look up the part?

 

[(*steve*)]

Here's one. There are many other chipsets. If you look at digikey (for example) and search for "battery controllers" you'll find they come in a large variety.

Oops, I forgot to press "Post"

 

[Northy]

Hmm, well the battery pack itself already has something very similar built into it, it's just that I want to add another layer of protection.
I want to disconnect the battery at a higher voltage and latch it off until the uP reconnects it.

G

 

[Northy]

Also, the data sheet recommends not discharging lower than 2.75V, but the built in protection only disconnects the battery 2.4V

G

 

[Northy]

I've looked extensively at the dedicated chips available but I haven't found one that latches, and I'd really like this circuit to latch off and disconnect the battery. I don't really want the no load voltage creeping back up and reconnecting the battery.

I know there is protection built into the battery pack, but I'd like to use that as a last resort. I'd really like to disconnect the battery at a higher voltage to give the device more shelf time before the battery gets deeply discharged and potentially damaged.

There is a slight complication as to where I connect the charger to the battery. I can see how it would be possible to connect as shown, but the charger I am using is the bq24073 that you connect the battery and USB power to and it sorts out the power supplied to the rest of the power supply.

I've been prototyping a little circuit (I've attached a picture of it and added system blocks around it to explain it better) and it seems to work, but it now raises some other questions.

The MAX835 monitors the voltage, disconnects the battery and latches off. I've now connected the clear line to the +5V from the USB, because it won't be possible to charge the battery again without connecting the device to a USB 5V source, so that might as well clear the latch and re-connect the battery.

However I've realised that because of the charger used the load switch needs to sit between the charger and the battery. So..... to charge the battery current needs to flow backwards through the PFET. I've tried it and it seems to work, but I'm not sure it's the best solution.

So is reverse current through the PFET a bad idea? Will it damage it?
Would a dedicated load switch be better?

I'd also like to minimise the quiescent current that the monitor circuit uses as ultimately once the battery is disconnected that is what affects the 'life' of the battery.

I'm really enjoying working on this but I find myself just spending endless time going round and round in circles looking at data sheets!

Thanks,

G