Measuring capacity of power banks

[joe384]

I got a USB power tester some time ago.

Charging my Anker powerbank from empty came to around 96Wh.
However I allowed it to discharge connected to a dashcam and when the cam switched off it showed around 56Wh!

What am I doing wrong here to get such different numbers? When the power bank was charging it was showing around 2A @ 5v and when it was discharging it was a lot less, probably around 0.3A. Surely the Wh reading should be consistent?

Should I be using a resistor load instead of the dashcam when discharging, do I need to charge at a slower rate?

 

[Nigel]

There should be some inefficiency, most voltage converters are only around 90% efficient and the 5 volt USB voltage has to be converted to battery voltage (average 3.7 volts) when charging and back to 5 volts when discharging, so two losses of around 10%.

That is a big loss though, and more than I would expect to see, but I've never tested an Anker. Most likely your meter isn't very accurate due to the fact that the camera is a digital device and tends to use power in short bursts, for example the memory card is written to maybe once per second for maybe 1/10th of a second, if the meter only checks the power consumption once per second then it could miss all the power consumed by the memory card!

 

[joe384]

That is a big loss though, and more than I would expect to see, but I've never tested an Anker. Most likely your meter isn't very accurate due to the fact that the camera is a digital device and tends to use power in short bursts, for example the memory card is written to maybe once per second for maybe 1/10th of a second, if the meter only checks the power consumption once per second then it could miss all the power consumed by the memory card!

Yes, I think it's probably not measuring the current often enough or it's just a rubbish meter
I think I will get a resistive load to use instead for discharge testing.

 

[SawMaster]

Charge rate shouldn't matter in regards to capacity as long as it's within specified limits; it's main effect shows up mostly in the usable product life. Measured capacity is going to vary with differing load types (intermittent, variable, steady rate, and also the rate amount), so unless you know how the manufacturer measured things you're not likely to get the same results. A resistive load will offer the best repeatable method for comparison testing, and with some empirical testing of the cam you will find a rough but calculatable ratio you can transfer into expected runtime for the cam with different powerbanks.

The way I look at things, I'm only interested in real-world results since that's where I live. If I begin living in a laboratory then lab results will be what I go for

Phil

 

[Nigel]

The way I look at things, I'm only interested in real-world results since that's where I live. If I begin living in a laboratory then lab results will be what I go for

Yes, the only really interesting figure is how long it will run your chosen dashcam for, and even that will reduce over time as the powerbank wears out.

The actual watt hours is slightly interesting, but a powerbank may produce more watt hours discharging into one dashcam than another, or into a resistive load, since efficiency will depend on rate and smoothness of power draw and can vary quite significantly with increasing loads.

Also note that a brand new powerbank can require several charge/discharge cycles to reach peak capacity, some more than others.

Getting a measurement that is anywhere near accurate and also useful is very difficult, a better general guide is to weigh the powerbank, the heavier it is the longer it will run your dashcam, if you buy a 20,000 mAh powerbank that weighs the same as your 10,000 mAh powerbank then it is almost certainly a 10,000 mAh powerbank with false advertising, and there are a lot of those about.

 

[EsQueue]

Wow, the 900s is power hungry, I was able to get close to 20 hours on my battery bank before updating it.

 

[SawMaster]

a better general guide is to weigh the powerbank, the heavier it is the longer it will run your dashcam, if you buy a 20,000 mAh powerbank that weighs the same as your 10,000 mAh powerbank then it is almost certainly a 10,000 mAh powerbank with false advertising, and there are a lot of those about.

As long as the case construction is similar, weight makes a good comparative. Most powerbanks are bought by the unenlightened masses who don't know they're getting ripped off by false capacity claims. As long as that continues so will the lies and exaggerations Buy something bigger than you think you need and try it with your cam- it's the only way to be sure what you'll get and that it will be enough

I just bought a 12V 18A AGM battery to resurrect an old inverter/jump pack I found. It's the same size as the old SLA it replaced which was supposed to be a 20A battery but according to what I found never was, even though it was marked as such It's going to eventually be my dashcam powerbase but I'll first have to re-engineer the storage spaces for it, as everything is now taken up with tools which will need to move elsewhere. Going to be some time getting that done as there's a lot of cabling to be moved with all the cams I've got in there; an all day job probably!

Phil