High amperage USB charging - is there acurrent limit on external devices?

Thread Starter


Joined Oct 10, 2014
I have a number of devices that are basically battery packs (Li-ion) with various accessories. These have a USB charging port which takes forever (well, relatively) when charging a 50-80 watt hour battery. The one device is a really nice work light and the other is a jump starter and device charger. The manual doesn't say not tu charge faster but it does limit to a USB device.

I know it's possible to use a 5v charger for these devices and I can make it work with USB but my charger does 4, 5 or 6.5 amps for a small wall wort - then there is the case of using a PSU 5v rail.

does anyone know if devices can be damaged by this? I would think that my car jump starter can handle higher amperage (hell it wasn't fully charged after 90+ hours on a 2.1A charging port!) as this thing discharges at 350 - 450 amps at 14.8v.

So what is the consensus?


Joined Sep 9, 2010
The problem here is lack of information. It's highly likely that the battery packs charge themselves properly when fed 5V from any source, regardless of the capacity of that source.

But "likely" doesn't cut it. If the battery charger design relies on the current limits specified by the USB standards, the battery may be damaged by charging at an "unlimited" rate.

It would be relatively easy to verify how much current it could draw. Just see what current a discharged battery pack briefly draws from your 5V PSU. Unfortunately I don't see how you'd know what current level is safe, without more data about the battery pack and how it charges itself.


Joined Nov 4, 2013
The two main issues is the source amperage limit and the battery pack voltage conversion.

If the source is only good for a few maps that all you can pull from it. Then on top of that is whatever method of voltage boosting and charge contorl the actual packs have built into themselves which again were most likely designed around the 1 - 3 amp current limits of USB ports so even if you managed to make a USB port aht could support more current drain than that the battery packs own internal power conversion system would still only draw the limit it was designed to work at and no more.

Given that, if you want to charge the actual battery cells faster you have to bypass the built in 5 volt booster and charge control circuit and direct feed the cell sets set from a higher power charging source.
At that point you are only limited by what ever current limit your charging source can supply or the batteries own peak current limits which for most decent batteries recharging them at their rated Ah value for a hour or two is not a problem. Especially so for packs designed for high C rate packs like booster units or RC toy power packs that easily handle 10 - 20+ C discharge rates.

As far as what the limit for charging a cell or set of cells at is largely what you want to do it at.
I for one have no issue or hesitation in hooking up batteries to constant voltage sources that have comparatively huge current capabilities in regards to the batteries own Ah ratings and just letting them take all the current they want for the first few minutes . Reason being the peak inrush currents for a near stone dead battery are very brief and rapidly taper off down into the normal recharging values within a short time well before they can overheat or be damaged.

Realistically I would say that if you can get power to the pack cell sets directly with a higher power capable source doing recharging cycles of ~ 1 - 2 hours at the packs rated Amp Hour value should be possible without causing concern and doing 15 - 30 minute charging at 3 - 4X the packs cells Ah ratings on high C rated packs like a booster unit uses would not be a problem either.


Joined Jun 13, 2008
You got to be careful when charging lithium ion or lithium polymer batteries as they can explode if left for more time that they require to fully charge, also you should monitor the temperature of the batteries while charging. The best way to charge a battery is with a power supply with constant current and where you can adjust the voltage.