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I want to connect my router with its adapter and a battery for backup. Simply without BMS or any regular or charger, adapter power my router and charge barriers when needed. The router will have two power sources.

Is it safe to keep the batteries connected to the adapter all the time?

Details:

  • The adapter output 12.4 V and 1.5 A
  • The battery is three 18650 in series output 9 V to 12 V and 2.2 A
  • The router can only work from 9 V to 12 V so the batteries can't go lower than 9 V.

From what I understand. Is this scenario there is no risk of over charging or over discharging the batteries?

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    \$\begingroup\$ You need protection circuitry to use a lithium ion battery. \$\endgroup\$
    – Hearth
    Commented Dec 13, 2023 at 14:49
  • \$\begingroup\$ From what i understand. Is this scenario there is no risk of over charging or over discharging the batteries \$\endgroup\$
    – user359329
    Commented Dec 13, 2023 at 15:06
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    \$\begingroup\$ Just because the router doesn't work below 9V doesn't necessarily mean it won't draw current below 9V. Unless you have extra information in that regard, overdischarge seems quite possible. \$\endgroup\$
    – Mat
    Commented Dec 13, 2023 at 15:24
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    \$\begingroup\$ There is absolutely risk of overdischarging at the very least, and overcharging seems likely too. \$\endgroup\$
    – Hearth
    Commented Dec 13, 2023 at 15:32
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    \$\begingroup\$ Without balancing or per cell measurement, it will become unbalanced and over and undercharged on each cycle. Don’t. \$\endgroup\$
    – winny
    Commented Dec 13, 2023 at 17:43

4 Answers 4

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You clearly don't have the experience to do this by yourself if you don't see the issue of over discharge or overcharge in this application or why a charger is required.

No cells are equal, so what happens over cycles is that cells will start to drift from each other, and eventually one will be either overcharged or over discharged. You need a balancer.

In case your cells are almost empty when power comes back they will draw much more current than your router power supply can handle and probably damage the power supply. You need a charger.

Lastly, even though your router is only specified to work in a range of 9 to 12 V, if you apply less voltage it will still draw a current. An unspecified current, which might even be larger than the normal working current. It will drain your batteries empty, probably below 1 V where finally an input protection diode will no longer conduct any current. Very few devices are designed in a way that they will draw close to no current when they are not inside their operating window. You need an over discharge protection.

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    \$\begingroup\$ Apart from possible damage to the AC/DC power supply, the high current when power is restored may cause production of gas within the batteries and the subsequent buildup of pressure leading to rapid unscheduled disassembly. \$\endgroup\$
    – Ben Voigt
    Commented Dec 13, 2023 at 16:27
  • \$\begingroup\$ @BenVoigt "unscheduled disassembly" :-) \$\endgroup\$ Commented Dec 14, 2023 at 1:06
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Lithium 18650 battery:

I would buy a finished product that has a Battery Management System inside.

I would never assemble my self such a product because Lithium batteries may explode if shorted or overcharged.

Amazon is full of Chinese back-up battery systems for routers and modems.


Lead Acid 12 V batteries:

You may safely use "Lead Acid 12 V batteries" or "Lead Acid 6 V batteries". You can connect them in series or parallel without any risk. They are rechargeable. They can't stay in charge forever. All Alarm systems in the world use this kind of batteries. They don't need a Battery Management System. They are cheap. They don't explode. Just don't flip them up side down or sideways.

Ciao

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  • \$\begingroup\$ We don't have those in Egypt. What we have expensive ups. And local made 12v backup batteries and I am trying to avoid them \$\endgroup\$
    – user359329
    Commented Dec 13, 2023 at 15:16
  • \$\begingroup\$ From what i understand. Is this scenario there is no risk of short or over charging or over discharging the batteries.. \$\endgroup\$
    – user359329
    Commented Dec 13, 2023 at 15:18
  • \$\begingroup\$ @Osama There is. You can't connect batteries directly to a power supply. If that was possible then all lithium chargers would be unnecessary. But lithium chargers are needed. \$\endgroup\$
    – Justme
    Commented Dec 13, 2023 at 15:28
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No, that is not safe.

You cannot charge batteries by connecting them to a power supply.

The fact that 12V is less than fully charged batteries is least of your problems. On the other hand, lithium batteries should not be kept at float charge anyway.

You have a 12V power supply. If you connect that to say slightly empty 11V battery pack, the power supply tries to provide as much current needed to raise the voltage back to 12V. In which case, the power supply either hits overcurrent limit or undervoltage protection and shuts down because batteries prevent the voltage from rising and they can sink a lot of current.

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Using a BMS helps the cells to charge and discharge properly and also monitors if undervoltage condition has occured or not. With every charge and discharge cycle some of the cells will be more discharged than the rest.If these cells will be charged without a BMS they will never fully charge as the other cells and the rest of the cells will always charge before this cell .To prevent this a BMS uses cell balancing networks which ensures all of of the cells remain at the same voltage throughout it's life thus ensuring equal load on each cell. If a BMS will not be used any 1 cell in the pack will have more load thus reducing it's life ,thus reducing the entire battery packs life. It is also very dangerous if the battery enters thermal runway there wont be any way to disconnect the battery from the utility it is connected to thus damaging the utility as well.

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