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How do you correctly charge a 12V lead acid battery using a 5V phone charger and step-up converter? Answered

I'm making a bluetooth capable, battery powered portable speaker - I have some 12V lead acid batteries lying round and I want to use those since the amp accepts 12V power. Its important to me that I can use a 5V micro-usb cable as a power supply because they're so common/universal. Everything in my setup works fine except for charging the battery. I've tried two different step up converters. One overheats massively when power in is connected to the 5V supply, and the other is a slightly fancier buck-boost converter which comes with a small LCD which displays voltage and current out - however this one, once connected to the battery, shows a tiny jump in current and then goes down to zero. I'm fairly certain the converters aren't inherently faulty since I've tried running the converter output straight to the amp rather than the battery and then everything seems to work fine.

How am I meant to charge this battery? Is there some component I should be adding in somewhere?

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Josehf Murchison
Josehf Murchison

11 months ago

How many amp hours are the batteries?
You might need a current limiter.
The step up converter that gets hot screams over current.
The one with the LCD flash of current then drops to 0 screams over current shutoff.
You might want to add a circuit like this.

LM317 Current Regulating.png
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KentsOkay
KentsOkay

11 months ago

Does your battery test good? I cant recommend trying to go the route you are, the USB charge would not even be a trickle by the time you up it to 12v. I'd get a cheap 12v wall charger, and then maybe add a 5v charging circuit output off of the the battery to charge your devices, or step the whole system down to 5v, but that would require a lot of new hardware.

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Orngrimm
Orngrimm

11 months ago

https://batteryuniversity.com/learn/article/chargi... has a nice and simple diagram for that.
Basically Constant current, Then Constant voltage, then float.

Depending on your DC/DC-Convrter, you may already have all thats needed: If your board has 2 potentiometer you very well may be all set.
One pot regulates max current (For the Constant current-phase) the other the max voltage (for Constant Voltage-phase).
The float-pashe... Well... What you could do is simply switch in a bunch of diodes or a LDO into the lead via 2 Relais to drop the voltage. Or have your DC/DC-convrter put out a lower voltage by means of actually changing the constant voltage potentiometer (Like with an electronic potentiometer and a Arduino) or by manipulating the feedback-resistors and adding a parallel R ro it to trick the DC/DC into believing he sees a higher voltage then it really is so it lowers the voltage a bit without changing the potentiometer but the feedback...

To your problems:
Depending on the Battery, the current may very well be MUCH too high to handle either by the DC/DC or also by the USB.
USB is not built for much over 5W @ 5V. If you or your battery and therefore the DC/DC tries to pull more, the Voltage of the USB may drop and the DC/DC will stop working.
Limit the current on your 12V-Output-side to maybe 400mA, better 300mA. If this doesnt work, please post more information on your DC/DC-Modules...