2-cell LiFePo USB charger with battery UVLO

[Calvin]

Hi,

I´m looking for a solution to charge 2 series (or more) connected LiFePo batteries on a mobile device.
Besides the 5V USB input capability, I´m looking for battery management and Battery undervoltage protection.
Also I need standalone functionality ... hence no fancy programming of register settings ... just plain ´ol hardware 
Now I found the MP2672A from Monolithics Power which is almost perfect for this task apart from the fact, that a attached load will drain the batteries till exitus. 
So far I found no charger that will prevent battery drainage with a fixed attached load.   
What I need is a circuit or device that ´disconnects´ either the load or the batteries to prevent the drainage below ~3.0V per battery cell.
Almost all load switches with a comparator EN-input are resticted to 5.5V of supply line voltage or draw too much idle current.

Does anybody habe an idea or knows a suitable device or circuit?

regards
Calvin

[thm_w]

You mean a BMS. Whatever IC they use on cheap bms module would likely work:
https://www.aliexpress.com/item/1005001733302623.html
https://www.aliexpress.com/item/32370020541.html

https://www.eevblog.com/forum/projects/looking-for-a-lifepo4-overundervoltage-overcurrent-protection-chip-or-circuit/

but you'll want to verify the voltages.

[Calvin]

Hi,

thanks for the links.
Actually I´m not primarily looking for a BMS circuit, that is already integral to the MP2672A.
This little gem has it all, but a UV protection for the batteries.
Its a 5V-USB Boost charger, with BMS and a number of protection functionality.
But it discharges the batteries into the attached load.
It clearly misses a UV-protection/battery switch ... as seem to do all charger circuits I could find so far (those that protect the battery are only single-cell systems restricted to 5.5V supply line voltage).
Ideally I´d like to find a load/battery switch that automatically disconnects the battery or the load and switches on again when the charger supplies for sufficient voltage (precise comparator enable input).
All in all I´m looking for a device like the TP4056 charger, but for 2 or more battery cells.

regards
Calvin



 

[tooki]

You are looking for a battery protection IC (which the TP4056 isn’t, incidentally). TI for example makes tons of them, with internal or external MOSFETs, with various cutoff voltage and current combinations available.

Bear in mind that these are intended as your last line of defense against undervoltage; you should still design your load to turn off below your design “empty”. For example, with a regular 3.7V Li-ion, nearly all the energy is used up by 3.5V. Pulling out the last few percent of energy down to the absolute cutoff of 3V stresses the battery a lot for very little yield. So while the protection IC is for 3V, you still want to design your load to shut itself off at 3.5V. (You can use the last 0.5V as a “reserve” to display a dead battery screen or flash an LED or something when someone tries to turn it on.) The exact values are given in the datasheet.

In a project of mine, I use a fuel gauge IC to monitor state of charge, and when the battery is empty (= 3.5V), the system shuts down and the MCU sends a command to the charger IC to disconnect the battery. But if you don’t have an MCU, use a comparator and a load MOSFET or something.

[Calvin]

Hi,

TI for example makes tons of them

.
Yes, I understand, but if You search for Battery Protectors, filter for 2 cells, only 5 devices remain ... none of which features UV-protection, but just OV-protection.
If You search for Load switches with low Iq instead, most are for 5.5V only, 8V beeing the highest.
Load switches for >9V supply only the TPS22810 remains ... but with an unacceptable high quiescent current (~50µA), which it enters into when the EN-input falls below 1.13V.
It enters its low Iq mode with only 500nA when EN falls below 0.75V ..... so this device is also not practical at all.
So, seemingly TI has nothing to offer for this application. 

regards
Calvin

[tooki]

The much smaller selection of products for series packs is why I went with a 1S3P configuration in my last project, followed by boost converters to get the voltages needed.

You don’t have to use a load switch IC. You can just use logic-level MOSFETs.

Again, don’t forget that the load switch isn’t the protection, it’s the main switch. You still need protection behind it.

Does your device not have an MCU? Monitor the battery voltage, and when the voltage drops below the “knee” in the discharge curve (somewhere between 3.3V and 2.5V per cell, depending on the discharge current), turn off the load switch or MOSFET. If you don’t want to do it via MCU, then use a comparator and some little voltage reference.

(No idea why 1.13V vs 0.75V should matter: use a pulldown resistor to tie it to ground. Then apply your logic “high” to turn it on.)

[Calvin]

Hi,

a MCU would make things probabely easier, but Im totally lost with this new stuff .... I´m the hardware type guy. 

regards
Calvin

[tooki]

So use a comparator with hysteresis. It’ll swing to 0V, more than enough to pull your load switch low. Remember that this is not your protection circuit, just your load switch. You want two levels of switching: the load switch at the nominal “empty”, and the protection cutout at (or above) the minimum cell voltage per the cell datasheet.

[Calvin]

Hi,

in the best case, yes, but a single treshold switch would suffice.

regards
Calvin

[tooki]

Hysteresis is optional but desirable (and given that it only takes 2 resistors to implement on a comparator, there’s no reason to not use it).

Having different voltage thresholds for your primary load switch and your undervoltage protection circuit, however, are really not optional. You need that buffer to prevent the battery from routinely going below the undervoltage threshold (since once triggered, there will still be further voltage drop over time, due to leakage current through the MOSFETs and self-discharge in the cell).

Again, to be crystal clear since I am not certain you’ve truly understood this point: you must have two distinct layers of switching, each with a different voltage threshold.