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Connecting Two Li-Ion Battery Charger? Answered

Hi, can I connect this(please see image) two Li-Ion battery charger in parallel, to increase the amp. Thank you. Regards.

Sorry image not uploading. Here is the link.




Best Answer 9 months ago

Lithium battery chargers are essentially constant-current power supplies. They have a high output impedance, which simply means that the output current does not change when the voltage across the battery changes. However, once the cell is at 4.20v, the charger goes into the Constant Voltage charging mode to prevent overcharging the cell. Now the device tries to maintain a constant voltage and allows the current to steadily drop off towards zero to top off the battery.


The device you have pictured appears to be a very basic linear regulator charger circuit, and because connecting constant current outputs in parallel should not be a big deal. a particular charger is going to "see" the same load resistance regardless if there are other chargers connected to the same device.


Near the end of charging, however, when it goes into Constant voltage regulating, then you might run into issues, as each supply tries to regulate it's own slightly different voltage. The result is that one may end up doing all the work, or some very strange behavior. The solution would be to have some low value load-sharing resistors in series with each output so that the 2 chargers are not directly in parallel.

While I do think this will work fine, I do NOT suggest that you actually do this!! It would require that you know what you are doing, and closely monitor the behavior of the system design over the entire charging interval and that you do so in a safe manner in case some strange behavior arises which leads to overcharging the cell, possibly causing a catastrophic failure.


It's much safer and easier to just order a lithium battery charger that can be configured to charge at a greater rate.


9 months ago

Thank you so very much, my good brothers. Your time and help, helps an
Imbecile:) person like me a lot. I will give it a try and if it smoked, I
will post here:) thanks much. Regards.


9 months ago

NO, not a good idea that regulator IC will interpret the sister regulator as a weird Lion battery...

+1 !
These chargers check the battery and regulate their output depending on the charging state.
Using them in parallel will in the best scenario force both chargers to shut themself off as they detect a problem.
In the worst case one or both charger will be fried or the battery damaged - up to the point of fire...

I don't know if anything bad would necessarily happen, since these are linear devices and have high output impedance. I am fairly certain they don't do much at all beyond a basic CC-CV charging characteristic with maybe a few basic safety shutoffs.

When connected in parallel to a battery, the main impedance either charger is going see would be just the battery. The CV mode would be a bit more interesting. It would be essentially equivalent to connecting 2 low impedance sources to each other. If the voltage difference between them is high enough, I would suspect one charger would go into some safety overvoltage shut off while the other continues to CV charge the battery as it was designed to. Load balancing resistors are typically used when paralleling the output of voltage regulators, so the same technique should work for this case, that that it's worth the effort and possible safety hazards.