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The battery's voltage normally is 13.8V when it fully charged. Say I have 5.7Ah motor bike's battery.

The laptop's charger will be connected to a battery. The positive port of the charger to positive of the battery, and so on.

Will it damage the battery?

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  • \$\begingroup\$ Just a sidenote, modern laptop chargers will only charge if they're connected to the correct battery. \$\endgroup\$ – po.pe Feb 20 '19 at 13:04
  • \$\begingroup\$ Indeed, a "charger" is not necessarily a simple power supply. That's part of why this question about sticking together unspecified consumer products for which no engineering specifications are available is an off-topic "usage" question rather than an on-topic engineering design one. But yes, it's also, a bad, unsafe idea which must not be attempted. \$\endgroup\$ – Chris Stratton Feb 20 '19 at 15:53
  • \$\begingroup\$ you could trickle charge it through a buck or voltage divider, but i wouldn't want to charge charge it... \$\endgroup\$ – dandavis Feb 20 '19 at 20:42
  • \$\begingroup\$ That is what I am thinking, using serial resistor with the battery to devide the voltage so it ecactly drop the required voltage on the battery. As serial, the current still remain the same as.the charger output, but drop voltage is different. \$\endgroup\$ – AirCraft Lover Feb 20 '19 at 21:15
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    \$\begingroup\$ using serial resistor with the battery to devide the voltage so it ecactly drop the required voltage on the battery. That will not work. Resistors don't work like that. The battery would still overcharge only more slowly. What would work is use a voltage regulator module like: ebay.com/itm/… set its output to 13.8 V and connect that to the 12 V battery. As this is an LM317 chip based regulator module it will also limit the current to a safe value. \$\endgroup\$ – Bimpelrekkie Feb 20 '19 at 21:23
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Will it damage the battery?

If the power adapter (that's the proper name for a "charger") survives and keeps delivering current while the battery voltage increases": Then yes, the battery voltage will increase too much, electrolysis will take place, hydrogen will be formed which can explode when mixed with air. Your battery will not be the same as it was. So yeah, it will be damaged.

The power adapter might also be overloaded, best case, it will blow an (internal) fuse so then you cannot use it anymore either. I think this is best case because this will also stop charging the battery so win-win.

Worst case, it might overheat and catch fire, then you also cannot use it anymore.

Summary: just don't. Buy a proper Lead-Acid battery charger. They're not that expensive.

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  • \$\begingroup\$ But hold on. My laptop battery's voltage is 10.8V. The power adapter (charger) is 19.2V. How could it didn't damage the battery? Mine is Toshiba. \$\endgroup\$ – AirCraft Lover Feb 20 '19 at 20:48
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    \$\begingroup\$ That's why I wrote that the "charger" isn't actually a charger, it is a power supply. It generates 19.2 V for the charging circuit. That charging circuit is inside the laptop. That charging circuit controls the charging. It needs to be inside the laptop so that it can closely monitor the battery's temperature and voltage. That circuit uses 19.2 V and converts it to a voltage between 10.8 V (battery almost empty) and 12.6 V (full battery) while controlling the charging current. \$\endgroup\$ – Bimpelrekkie Feb 20 '19 at 21:08
  • \$\begingroup\$ That is needed because charging a Lithium based battery like used in your laptop needs to be done with care. If that is not done with care the battery will overheat / smoke / catch fire. \$\endgroup\$ – Bimpelrekkie Feb 20 '19 at 21:08
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The battery will most likely be damaged. It will get hot and another consequence would be the emission of gases.

As Solar Mike said, don't do it.

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  • \$\begingroup\$ My laptop battery's voltage is 10.8V (consist of 3 serial @3.6V, paralled with another 3 serial @3.6V). But the adaptor is 19.2V. How could it didn't damage the battery? \$\endgroup\$ – AirCraft Lover Feb 20 '19 at 20:50
  • \$\begingroup\$ @AirCraftLover: Because there is a charging circuit between 19.2V power supply and 10.8V battery. The 19.2V power supply is not directly connected to battery. Lithium batteries need to be charged with very intelligent circuitry, they are not charged like lead-acid car batteries, where 13.7V can be directly applied to battery forever to charge it. \$\endgroup\$ – Justme Feb 20 '19 at 22:14
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Note that some cars have charging systems that go up to 15.3V (my car is one of them - that's how I know and I have the workshop manual as well)...

But 19V is too much - regulate it.

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  • \$\begingroup\$ My friend. My laptop battery's voltage is 10.8V (consist of 3 serial @3.6V, paralled with another 3 serial @3.6V). But the adaptor is 19.2V. How could it didn't damage the battery? The battery should has been exploded. \$\endgroup\$ – AirCraft Lover Feb 20 '19 at 20:51
  • \$\begingroup\$ Because the laptop has a charge control circuit inside that does the regulating to the battery. \$\endgroup\$ – Solar Mike Feb 20 '19 at 20:53
  • \$\begingroup\$ Interesting. In my understanding, we need to "drop" 10.8V voltage to the battery. Is it possible to just put serial resistor with the battery? As in serial the current will be remain the same. \$\endgroup\$ – AirCraft Lover Feb 20 '19 at 21:06
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    \$\begingroup\$ Is it possible to just put serial resistor with the battery? Yes in principle that is possible but since that would require a very large resistor, generate a lot of heat and cannot be controlled (control the charging current) it is not done like that. Instead a switching converter is used. Go google "DCDC converter" and "buck converter" to learn how those work. They're not that simple btw. \$\endgroup\$ – Bimpelrekkie Feb 20 '19 at 21:12
  • \$\begingroup\$ After I tried many times, my car battery and the charger are just fine. The battery was charged well, better than the common charger sold in the market. The most important thing is, during charging, we need to monitor the voltage of the battery by taking off connection from the charger to the battery and measure the voltage. Once it reach 13.8V, than it is done. If still below, then continue charge. Trust me, it worked well. And it is the idea behind the fast charging. \$\endgroup\$ – AirCraft Lover Nov 28 '19 at 2:39
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What must be understood here that the device you believe is a laptop charger is not a charger. It is a power supply. The actual circuitry that is actually responsible for battery charging is inside the laptop or battery pack. So no, connecting power supplies directly to batteries without a proper charging circuitry that matches the battery chemistry must not be attempted.

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  • \$\begingroup\$ That is interesting, as my laptop battery's voltage is 10.8V (consist of 3 serial @3.6V, paralled with another 3 serial @3.6V). But the adaptor is 19.2V, and it didn't damage the battery. \$\endgroup\$ – AirCraft Lover Feb 20 '19 at 20:58
  • \$\begingroup\$ Once I have opened the battery that's why I know how to replace the cell in case it got wear. Rather to buy new batter, just replace all the cells. Cell itself is just a common battery like a bit bigger than AA size. But one thing, there is not many circuits there. \$\endgroup\$ – AirCraft Lover Feb 20 '19 at 21:01
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    \$\begingroup\$ But one thing, there is not many circuits there They're 18650 cells. Mistreat these and you can be in trouble (fire etc.) so perhaps you should not mess with them. The small circuit you saw in the battery pack is a simple battery protection circuit it protects the battery from overcharging/overdischarging and overheating. The charging circuit is in the laptop, not in the battery pack. \$\endgroup\$ – Bimpelrekkie Feb 20 '19 at 21:16
  • \$\begingroup\$ I replaced the cell and it work fine. I forgot the cell number, but seems that you are correct. \$\endgroup\$ – AirCraft Lover Feb 20 '19 at 22:19

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