Advice on power bank voltages/power

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williams01933
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Advice on power bank voltages/power

Post by williams01933 » Tue Jun 11, 2019 2:59 pm

I decided to test just how much power my external power bank could actually supply over a period of time so set up a simple 7.5 Ohm load resistor across its supposedly 5v 2.1A ouput. It is rated at 5600mah and I did not want to stress the battery too much.

The voltage immediately dropped to 3.77v and the current drawn was around 600ma.
I was not expecting that immediate drop in voltage.

Tried with a second power bank (different manufacturer but similar spec) and that did exactly the same thing.
Surprising thing was that it continued to supply 3.7v at 600ma for about 7 hours which was not far off its official rating of 5600mah

Am I missing something here or being naïve expecting the 2.1A output to hold steady at 5v for at least a few hours.
I would have thought a simple resistive load would have been no problem for either battery pack.

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dacflyer
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Re: Advice on power bank voltages/power

Post by dacflyer » Tue Jun 11, 2019 7:47 pm

many of the BMS boards have current limiter circuitry. you might be able to replace it with another BMS that can handle higher current.
Then again, you might have a weak cell in the pack ?
But i read that you tried another one as well..

williams01933
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Re: Advice on power bank voltages/power

Post by williams01933 » Wed Jun 12, 2019 2:04 am

I would think it almost a certainty that any pack would have a current limiter for safety.
However, the slot is rated at 2100ma (with a second one rated at 1000ma).
5v into 7.5 ohm would only draw 666ma so any current limiter would (should) not kick in.

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dacflyer
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Re: Advice on power bank voltages/power

Post by dacflyer » Wed Jun 12, 2019 5:32 am

Hmmm, only other thing i'd suspect is defective circuitry....or they lied on the specs..
can you try adding small loads, and see how high you can get before it drops off ? figure out what the max current is that you get before it cuts back the current.

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Externet
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Re: Advice on power bank voltages/power

Post by Externet » Wed Jun 12, 2019 8:10 am

There is also another factor while testing/evaluating the energy delivered by batteries, usually not published; or they come up with it when there is a complain : Test duty cycle.
Manufacturer may want 5 minutes drain, 10 minutes rest and repeat; or whatever they decide.
- Abolish the deciBel ! -

williams01933
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Re: Advice on power bank voltages/power

Post by williams01933 » Wed Jun 12, 2019 9:35 am

OK, so here is what I have learnt today
1 Do not use a breadboard to connect low voltage / high current components
2 'Jump' wires are really not good enough for this type of connections
3 Be careful where you measure your voltages

Because of the above my measurements were way out. The extra resistance in the contacts and the cables was totally confusing me.
With another voltmeter at the very front end of the test rig (close to power pack but 'distant' from the load) I can see that the input voltage is actually not dropping at all (just a few millivolts). It was the voltage directly across the load which was dropping because the high current through the poor connections was causing a voltage drop in the cabling/connections.

I have now dismantled the power pack and replaced the batteries (x2 18650) with known good quality ones.
Next time I try and measure the power output I will use solid, soldered connections and short 'heavy' cables.

Lessons learnt.

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haklesup
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Re: Advice on power bank voltages/power

Post by haklesup » Fri Jun 14, 2019 4:51 pm

Parasitic Series resistance, I've seen it toast and once melt a connector body down to a puddle. I recently cleared a fault on one piece of equipment where low voltage on logic 5V caused by connector series resistance dropped it so low (3.4V), the PC couldn't communicate with it.

Yes, anything higher than "Signal" levels usually needs more metal and good connections.

The true voltage drop across the battery terminals divided by the load current is the battery internal series resistance and is normally a good indicator of its health and would be pretty low for Li-Ion or LiPo

Some people mentioned current limiters. Be careful to monitor the temperature of the battery pack regardless of load current so you don't catch on fire. Current limiters try to prevent thermal runaway when presented by a short circuit

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dacflyer
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Re: Advice on power bank voltages/power

Post by dacflyer » Fri Jun 14, 2019 8:11 pm

williams01933 >>> yup, that'll do it too... glad you found it..
i had thought you was using a factory made BMS ( power bank )

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