Powerbank Question

[peter yetman]

Do powerbanks have some sort of boost circuit in them so that they can bring something to a state of charge higher than themselves?
Imagine two similar containers of water, one full the other empty. If you connect them together the water will flow until the containers are equaly full. If you had a pump between them the water could be comepletely transferred for one to the other.

I only ask because I've just got a second Xtar PB2 and with a pair of 2500mah cells in it only charges my iPhone to 70%.
I'm now trying with a pair of 3000mah cells to see if there's difference. My other PB2 has a pair of 3500mah cells in and charges to 90%.

Thanks,
P

 

[Lynx_Arc]

Yes, most power banks have parallel lithium ion cells in them and a boost circuit to bump the voltage to 5v (USB out). The efficiency of the power bank boost circuitry combined with the wiring and cells and your phones charging system can all add to losses in power when boosting/charging your phone. My rule of thumb when deciding on a power bank is take your phone battery size and double it for about 1 charge even though better power banks can get you better efficiency.
One other thing is the higher the current drawn from the power bank the more loss in boosting can occur plus some cells can handle higher current easier than others so the cells internal resistance figures less in power losses.

 

[WalkIntoTheLight]

Also, keep in mind that power banks will tell you their capacity in terms of the internal battery voltage, not the 5v that is used as output. So, a power bank that claims 10,000 mAh, means that it has 10,000 mAh at 3.7v. But that's not the voltage that it provides. At the 5v of USB output, it will only have about 7,000 mAh, or probably less due to inefficiencies of the boost circuit.

A device being charged might just have a circuit that "throws away" excess voltage that it doesn't use during charging. So it probably sees the 5v capacity, not the capacity that the power bank advertises.

 

[Lynx_Arc]

Also, keep in mind that power banks will tell you their capacity in terms of the internal battery voltage, not the 5v that is used as output. So, a power bank that claims 10,000 mAh, means that it has 10,000 mAh at 3.7v. But that's not the voltage that it provides. At the 5v of USB output, it will only have about 7,000 mAh, or probably less due to inefficiencies of the boost circuit.

A device being charged might just have a circuit that "throws away" excess voltage that it doesn't use during charging. So it probably sees the 5v capacity, not the capacity that the power bank advertises.

Not to mention the heat loss generated in discharging/charging regardless of efficiency of the circuitry involved. IMO with todays smart phones I recommend a minimum of a 10,000mah power bank if you want to make sure you get a full charge this would be 3-4 18650 batteries in the power bank. The single cell varieties would be fine to tide you over for quick phone calls till you get to a power source while the 2 cell banks probably get you a half charge. I read a few reviews that equate generic power banks in the 50% range of efficiency while some top end ones get efficiency in the 90s but if you take a 10,000mah power bank at 3.7v that is 37Wh and at 5v that is 7400mah take that at ~50 efficiency gives you about 3700mah to your device while at 90% gives you about 6700mah.
The problem with the higher efficiency power banks is the cost is a lot higher probably 2-3 times (or more) as much as a generic brand.
I have several generic power banks and a few Anker brand power banks and rarely use them to charge my phone most of the time they are used for LED lighting. My phone supports Qualcomm fast charging and I don't heavily use it for gaming or long phone calls so sometimes I only have to charge it every 3-4 days and 30 minutes of charging on a fast charger can give me 2 days of power for it.
If you find yourself using a power bank a lot may be a good idea to get 2 smaller power banks than one that is twice as big as the ones that don't support them being fast charged (QC3.0) take a half a day or so to recharge.

 

[peter yetman]

Thanks chaps, that's all very helpful.
P