The Power to Innovate

The perfect Smart Phone power bank?

17 January 2018

We get many questions about Power Banks, so thought it worth exploring a number of the key features and explain how these relate to Salom products.

1. Cell Chemistry

The principle types of cell chemistries used in portable power banks for consumer electronics are Lithium-ion (Li-ion) and Lithium-Polymer (Li-Po). At present, Li-Po is essentially the same construction as L-ion but with a different electrolyte, so it could more accurately be referred to as Polymer Lithium-ion! Nevertheless, they are different and there are pro’s and con’s with each that can be summarised as follows;

Li-ion offers 10-30% lower cost but is not as robust as Li-Po which can be made thinner with higher energy density and higher discharge rates. This means that whilst there are both types in the market, Li-Po based power banks are increasingly popular for the highest performance power banks.

2. Power Bank capacity.

The battery capacity is measured in milli-Amp hours (mAh), which is to say the discharge current that the battery can provide in 1Hr. This means that at twice the discharge rate, the battery would last 30minutes. So this implies that ‘bigger is better’ right? Well, not necessarily as you need to think about how you’re going to be using the power bank!

 Taking a typical power bank marked 10000mAh (=10.0Ah), then the power this can deliver when fully charged should be 10.0Ah x nominal output voltage (typ. 3.7V) = 37W. However, no Power Bank is 100% efficient as there are losses in the circuitry. This can vary from 60-90% depending on the quality of the product, where the inefficiencies convert to heat degrading component lifetime and slowing charge time. Assuming 80% efficiency in this case, we get 37W x 80% = 29.6W.

Now we need to consider the load. Smart phones have grown in complexity and performance which invariably leads to higher power consumption which then limits run-time. In a recent study, the Lenovo P2 led the way with an astonishing 28hrs 50mins battery life.

This long battery life is accomplished with the use of a 5100mAh battery. Using the same calculation as above, the load power is then 5.1x 3.7 = 18.9W, so a typical 10000mAh power bank would charge this ~1.6x. Considering the cell phone run time on a fully charged battery, this would equate to an extra 45hrs run time. Other mobile phone examples utilise smaller batteries, so the number of recharges would be more. Indeed, 3 x recharge of smaller smart phones is not uncommon resulting in several days potential run time. In short, the larger the power bank capacity then the greater potential to recharge a smart phone numerous times, but this is based on some critical criteria that is rarely published when focussed solely on the lowest cost solution.

 3. Cycle Count and Retention

The Cycle Count (sometimes called Cycle Time) is the number of times in its life that a power bank can be fully charged and discharged, so can be considered as a proxy for useful life time. Best in class power bank cycle count is 500, although many lower cost products use cheaper cells that have a significantly lower Cycle Count. 500 cycles may not sound much, but if you consider the use case where a power bank is used every 2 days, this results in 1000 days lifetime which is ~3years. This is longer with lower usage, but then we also need to consider the charge retention time which is the ability of the power bank cells to hold charge over time. For a low usage model, it would be pointless using a power bank to find that it’s self-discharged when you go to use it! Unfortunately, this is common as low cost cells with a low cycle count also often have a low retention rate, which means that they end up being used more which in-turn reduces their life due to the finite cycle count. However, both cycle count and retention also drop dramatically as the cells age due to stress. This includes environmental stress such as operating at extreme temperatures (which is also linked to efficiency outlined above), but also includes electrical stress induced when charging and discharging at high rates as explained next.

4. Charge Rate

The larger the power bank capacity, the LONGER it takes to charge. This reduces with increased charge rate which may have a negative impact on Cycle Count.

Charge and discharge rates of a battery are governed by C-rates. The capacity of a battery is commonly rated at 1C, so an empty battery rated at 10000mAh will be fully charged at 10.0A in one hour. Similarly, 2C rate would be twice the current in half the time. The same applies to discharge rates. Technology exists to charge and discharge at >1C , but great care needs to be taken as this can significantly reduce the cycle time of the battery as illustrated in the diagram above.



Qualcomm have developed such quick charge technology with the Quickcharge ‘QC’ standard. For instance, a QC3.0 enabled charging process applied to a 2750mAhr cell would charge from 0-80% in only 35mins which equates to ~1.5C charge rate. This optimises charge time without compromising battery cycle life due to the authentication protocol which accurately identifies the specific (load) battery requirements. However, this is only possible if the charging source AND the load have implemented complementary quick charge technology. In the case of a Power bank where the requirements are both to quick charge the power bank AND use the power bank to quick charge the load (mobile phone), this would require QC support on BOTH the input and output of the power bank, as well as in the supporting Power bank charger and mobile phone. A typical example of a compatible mobile phone charger supporting QC3.0 technology is the Motorola Turbo-Power-15;

In the absence of such advanced charging technology, it is also possible to implement ‘pass-through’ charging technology that enables the power bank battery to be charged with a large source in parallel with the smart phone load when plugged into the power bank, although this is not as effective as an intelligent charging protocol to reduce charge time and enhance safety through additional cell monitoring features.

5. Safety Features

The key benefit of a power bank is being able to safely charge an end device whilst travelling away from a fixed charging infrastructure. This may invariably involve passenger flight where the rules are governed by IATA. At present, power-banks are classed the same as spare rechargeable batteries and are assessed on their Watt-hour (Wh) rating.

Power banks with a rating of <100Wh are NOT ALLOWED in checked-in baggage, but ARE ALLOWED AS CARRY ON BAGGAGE without operator approval. Taking a 10000mAh power bank with nominal 3.7V cell voltage would result in ~37Wh, so comfortably within IATA limits, even when carry other spare batteries. Additionally, IATA dictates that the terminals must all be protected from short circuit. In the case of a battery, this can be accomplished by ensuring the battery is in its original packaging or taping over its terminals. In the case of a power bank where the battery cells are already shielded in a housing, this is accomplished through these other means.

As introduced initially, a Li-Po cell is very similar in structure to a Li-ion cell and whilst more robust due to the lower rigidity of the cell structure, they still require careful handling. This is why there is no change in classification of a Li-Po based power bank by IATA over a standard Li-ion power bank.

A high quality product should therefore have a range of internal safety features including thermal protection, short-circuit protection, over-voltage / current and power protection all in a robust housing considering the mobile nature of the typical use-case.


In summary, the key elements to consider in the purchase of a Power Bank for Smart phone support are:

Cell Chemistry                    Li-ion for cost, Li-Po for size

Battery Capacity (mAh)   Larger isn’t necessarily better. You need to consider how you’re going to use the product to determine the correct size for you.

Efficiency (%)                     Higher is better. Typically ranges from 60-85%. Higher efficiencies extend product lifetime and minimise charge time. No product info detailed this should be of concern!

Cycle Count and Retention   Higher is better. Again, this depends on usage, but 500 cycles is generally best in class. No product info detailing this should be of concern!

Charge Rate and Time          Lower charge time (higher charge rate) is better, as long as this uses a smart process to ensure Cycle Count isn’t compromised.

Safety features                        More is better. Over-voltage / current, Short circuit and Thermal protection in a robust housing is an absolute minimum requirement.

Salom has reviewed a range of popular smart phones and determined that ~10000mAh capacity is the optimum to support a minimum of 2 extra days typical smart phone run time. This easily falls within IATA limits for hand carriage on aircraft.

To realise the lightest and thinnest profile for ease of carriage, we have utilised high quality Lithium Polymer cells with a minimum 500 cycle performance resulting in typically 1000 days (~3years) continual usage when fully discharging every 2 days, although lifetime may be considerably longer with a reduced use model.

The designs have been optimised to minimise losses and ensure maximum power transfer both in to the power bank and out to the smart phone through the use of smart charging technology. This enables the power bank to charge at 84% efficiency, increasing to 90% efficiency as the load charges. With smart phones utilising Qualcomm SnapDragon QC enabled processors, charge time of the smart phone is minimised through the use of QC3.0 technology in the UPOW106Q. This also significantly reduces charge time of the power bank by 28% (100mins) over a standard 5V/2A USB charger or a massive 48% (233mins) over a standard 5V/1A USB charger when using a compatible QC charger.

Weighing at only 155g, the end result is a family of market leading light weight, thin profile power banks with cutting edge performance and a huge range of internal safety features to complement industry leading smart phones.

These are not, nor are they intended to be, the lowest cost power banks on the market. However, these are intended to be the highest performance products in their class to complement high performance smart phones. We’re very pleased with the results and have no doubt that you will be too!

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