How can I tell if a lithium battery has over discharge protection?

Thread Starter

AlbertHall

Joined Jun 4, 2014
10,558
I have a HW903475 7.4V (2S) lithium battery. It has a three pin connector for the charger so I guess that the charge balancing is done by the charger but do I need to provide overdischarge protection or is it built in to the battery pack.

I could connect a resistor and monitor the voltage but if it doesn't switch itself off, how low can I allow the voltage to get before any damage is done, but low enough to have triggered the battery protection circuit if it has one.

Any better way of doing this?
 

Dodgydave

Joined Jun 22, 2012
9,666
If there is no PCB on the battery pack, then I would say the charger will have some form of intelligence built in. Have you looked inside the charger?
 

Thread Starter

AlbertHall

Joined Jun 4, 2014
10,558
If there is no PCB on the battery pack, then I would say the charger will have some form of intelligence built in. Have you looked inside the charger?
The battery pack is shrink wrapped and I can't feel anything that might be a PCB (but that doesn't mean there isn't one). It has the three wire connector for the charger so I expect the charger is smart. However the battery output comes from two separate much thicker wires. While the battery is discharging the charger would not be connected.

These are commonly sold as drone batteries but I could find no datasheet for them.
https://www.amazon.co.uk/Swiftswan-2000mAh-Battery-Replacement-Quadcopter/dp/B07FQSK99Q
 

Dodgydave

Joined Jun 22, 2012
9,666
If it monitors for Over voltage then it will cut off to prevent discharge of the battery through the charger, the cells alone will discharge naturally, how do they perform in the quadcopter or whatever you use??

I don't think manufacturers would make a lithium pack without any protection..
 
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Thread Starter

AlbertHall

Joined Jun 4, 2014
10,558
It will not be going in a drone, it is just a convenient size, shape and capacity to fit the tracked chassis I am working on.
 
This pack is *highly unlikely* to have any over OR under discharge protection. The 3 wires would be the appropriate number for a 2S pack. You can determine if this is so by checking the voltage between the 3 wires - black to white should be 3.7 (nominal), and white to red should be the same -- assuming a "balanced" battery pack.
 

Thread Starter

AlbertHall

Joined Jun 4, 2014
10,558
Voltages as described.
The charger it comes with, I presume, has overcharge protection as it does stop charging after a time depending on how discharged it was,
I just wanted to know if I should monitor the voltage while it is discharging and switch off at some voltage.
 

Irving

Joined Jan 30, 2016
1,069
If its anything like the drone packs I have then no, it has no internal 'protection'. The last thing you need in a drone is for an errant BMS to switch the power off in mid-flight due to a power spike/surge, that does wonders for 'protection'.

Its very hard to tell from the outside if a BMS is fitted, other than to dismantle it, or, less destructively, discharge it and watch the voltage. If it drops below 2.9v/cell (5.8v/pack) then definitely unprotected. Doing this once at low current (<0.5A) won't harm the cells.

Overcharge management and balancing is handled by the charger.
 

Irving

Joined Jan 30, 2016
1,069
Overkill... if all you need is low voltage cutoff then a tl431, a large n-channel mosfet, a pnp bjt and some resistors.

What would a 'tracked chassis' be doing while you were asleep? :rolleyes:
 

Audioguru again

Joined Oct 21, 2019
2,061
Of course a charger has nothing to do with discharge protection.
My Eflite airplanes pulse the main motor to warn me that the battery voltage is getting low then if I ignore the warning and keep the airplane flying then the low voltage protection circuit in the airplane turns off the main motor but allows servo power for me to control a glided landing. The receiver is still powered and the battery will be destroyed by a discharge too low if I do not disconnect it. The battery is destroyed if the airplane spends some time in a tree.

Modern Eflite airplanes and transmitters have telemetry where the receiver radios to the transmitter then the transmitter gives me a warning or tells me the voltage.
 

alan01346

Joined Aug 13, 2020
14
With 18650 lithiium cells there are at least 2 or 3 types of protection circuits. Some you can reset by sticking some sharp point in beside the button on the end. Some you can reset by connecting the tripped cell in parallel with a working one for an hour or so.

Most chargers don't really "taper off", what they do is output a fixed voltage like 4.2 or 13.8 volts. As the battery charges it comes closer and closer to that voltage so it's drawing less and less current. When the current gets low enough like 50 MA a circuit detects that and turns on the charged light and stops charging.

The battery pack with 3 wires is probably 2 lithium cells in series. They have to be charged separately to be safe so the charger is really 2 chargers in series, each has it's own voltage regulator to 4.2 volts. There's probably a transformer with 2 windings so they float relative to each other, then they're tied together once they're regulated. Overcharge a lithium battery and it can catch fire.
 

Irving

Joined Jan 30, 2016
1,069
Most chargers don't really "taper off", what they do is output a fixed voltage like 4.2 or 13.8 volts. As the battery charges it comes closer and closer to that voltage so it's drawing less and less current. When the current gets low enough like 50 MA a circuit detects that and turns on the charged light and stops charging.
That what a bad charger will do, which means the cell is never fully charged, maybe only to 90 - 95%. A good charger starts with a constant current (CC) at the prescribed %C rate (where C is the battery capacity), which could be anything from 30% to >>100% depending on which lithium chemistry it is. Li-ion and LiPo generally 0.5 - 1C, LiFePO4 can go to 3 or 4C. It will maintain that CC until the cell volts reaches the prescribed Max charge voltage (4.1-4.2v for most Li-ion or LiPo, 3.65 for LiFePO4 - the charger therefore has a higher voltage than the final cell, or n * cell, voltage). The charger then switches to constant voltage (CV) mode at that voltage until the charge current drops to typically 1%, this will take anything from a few minutes to a few hours after which the cell is truely 100% charged (my 8S 24v 200Ah LiFe packs take typically 3.5h from 40% to 100% at 40A CC for 3h and 30min at CV absorption charge. I'm currently developing a 100A CC charger.)

The battery pack with 3 wires is probably 2 lithium cells in series. They have to be charged separately to be safe so the charger is really 2 chargers in series, each has it's own voltage regulator to 4.2 volts. There's probably a transformer with 2 windings so they float relative to each other, then they're tied together once they're regulated. Overcharge a lithium battery and it can catch fire.
Possibly true on cheap low current 2S chargers, but most higher cell-count chargers will charge the whole pack at CC and then balance charge during the CV phase to ensure all cells have the same voltage by discharging higher voltage cells into the lower voltage ones.
 
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Thread Starter

AlbertHall

Joined Jun 4, 2014
10,558
I set up the gadget linked in post #15 to discharge down to 5.8V, into a 220Ω resistor - about 33mA.
This morning the gadget had disconnected the load resistor and the battery voltage had bounced back to 6.4V. It is now on charge.

So the battery does not have protection built in and this was a worthwhile exercise.
 
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