battery protection using diodes

[DiDBoGDaN]

Hi, I need a little help with my project. I am trying to create an uninterruptible power supply for a router (9V 500mA) and a fiber optic modem (12V 500mA). I have already soldered the circuit using the following components 18650 lithium ion batteries 3 pcs
MT3608 DC-DC step up modules - 2 pcs
ip2312 charger for lithium ion batteries 1s with fast charging function and current up to 3A
 bms 1s - battery protection module
Below is a diagram of the components.

I also read on the Internet that such a scheme is bad, because the batteries are constantly connected to the load, regardless of whether there is power from the power supply unit or not. Accordingly, batteries are simultaneously charged and discharged, which is harmful to the batteries themselves
. And I also read that with the help of two diodes connected by locks to each other, it is possible to make it so that when there is power from charging, the batteries only charge and do not give off energy, and when there is no charging, the batteries start feeding the load. I have several diodes with a maximum current of 10A.
 So the question is, where exactly should I connect such diodes, or is it possible that there are better (and necessarily simple and cheap) methods of battery protection?

[PGPG]

In such task using MEAN WELL DRC-40A seems for me rationale.

[DiDBoGDaN]

you're right, moreover, this module costs about as much as the whole assembly cost (all elements are purchased), but I decided to do something with my own hands and learn something new

[ledtester]

Take a look at Microchip Application Note AN1149:

Designing A Li-Ion Battery Charger and Load Sharing System With
Microchip’s Stand-Alone Li-Ion Battery Charge Management Controller

https://ww1.microchip.com/downloads/aemDocuments/documents/OTH/ApplicationNotes/ApplicationNotes/01149c.pdf

It shows how to disconnect the battery from the load with a PMOSFET and diode when it is charging.

You can use a TP4056 with this scheme as described here:

https://www.best-microcontroller-projects.com/tp4056-page2.html

[fourfathom]

Accordingly, batteries are simultaneously charged and discharged, which is harmful to the batteries themselves

Let's discuss this.  I submit that as far as the battery is concerned, there is no such thing as "simultaneously charged and discharged".  They are either charging, or discharging, or no current is flowing through the battery.  There may be reasons to isolate the charging and load currents, but battery harm is not one of them.

[J-R]

As mentioned in the linked article, the issue is that the load keeps the charger going which means the cells will be float charged at 4.2V all the time.  This is bad for their health, although how bad can be a longer discussion and definitely revolves around the specific cell used.

So float charging or constantly topping off Li-Ion should be avoided, but you can get away with it a bit longer if you reduce the voltage slightly.  So I would suggest simply replacing the IP2312 charger with a power supply set for ~4V (and perhaps 2A). This will keep the cells full enough without causing much damage and no extra circuitry is needed, although you should have a blocking diode to prevent the cells from feeding into the supply.


An alternate solution is to change up the design and go with something based on a lead acid chemistry, which enjoys float charging immensely.

[SteveThackery]

An alternate solution is to change up the design and go with something based on a lead acid chemistry, which enjoys float charging immensely.

Or LiFePO4 batteries, which are used as substitutes for lead acid and are also suitable for float charging.

[Peabody]

I think the circuit below would be the app note version.  But I don't have any information about the BMS, so I'm not sure about that.  But otherwise, if the charge current is present, the charger will charge the battery, but the mosfet will be turned off, so the battery will be isolated from the load.  If the charging current is not present, the mosfet will be turned on, allowing the battery to power the load, but the schottky diode will block current from flowing back into the charger.  The mosfet is P-channel.