Power banks for tramping

Re slow-charging: Yes. The power bank i'm using has capacity of 1A output which it happily delivers to other usb equipment. I'm testing on a miboxer power bank as that has a built in ammeter. But I note that samsung phones drop to a trickle charge when using non samsung chargers. Likewise on my 2.5A usb wall socket the samsung slow-charges. === Regarding 85% efficiency - where does that figure come from? I can see that 5v usb dropped to 4.2v charging voltage across an ideal voltage regulator will give you 84% of the supplied power delivered to the battery. But there is also loss before that stage: at what's delivered to the USB wire by the powerpack. The measured usb output for the full discarge cycle of my 2600mAh battery is approx 6.3Wh-7.3Wh at the usb compared to 9.36Wh nominal for the battery (The range is because I have only a range for the time it ran for of 3-3.5hr). So thats 67%-78% efficiency across the voltage step-up stage. If this is achieved with a voltage doubler follower by regulator - I'd expect a max theoretical efficiency of that setup to be 69% (5/(3.6*2)) due to the power/voltage dropped across the regulator. So my theoretical max efficiency across the supply and charging circuits is .69*.84=57.9% So what wrong assumptions have I made @bernard?

Good topic, I've come to the conclusion I'm going to go with a power bank because my phone provides more entertainment and has more function than any other single thing: - camera - watch - Video - notepad - recording my voice, notes - apps for the trail - podcasts - e books I've more or less landed on the Belkin 10,000, which I'm assuming will do 3 charges, so that's 4 charges from the time I leave the car https://www.dicksmith.co.nz/dn/buy/belkin-pocket-power-10000mah-power-bank-silver-f7u039btslv-belkin/?utm_source=google&utm_medium=product_listing_ads&gclid=CjwKCAjw9dboBRBUEiwA7VrrzVTOnp2Zb3eMNg7kfjkUQEzqCWwPL2r8UoqocVJ9GZZY3PHSBMnMhxoCbxAQAvD_BwE

@madpom, the formula is this: Labeled capacity of the power bank x 3.7 / output voltage of power bank) x 0.85 / battery capacity of device = Total number of recharges

Well ... were pretty close to agreeing. By your formula: (3.7/5) x .85 = 0.74 x .85 = 62.9% By my calculations based on the components I believe the circuit uses: (5/(3.7*2)) * (4.2/5) = .67 * .85 = 57.9% Either way the max theoretical charge you will get out of a usb powerbank is around 60% of its labelled capacity, and experience shows you might not even get that. Buyer beware.

Charging any battery is subject to lots of variables not least of which is the type, e.g. sealed lead acid, NiMh, NiCd, lithium and so on. Not withstanding the stretched claims that some of these power pack manufacturers may be making, lithium batteries need to be constant CURRENT charged. So,depending on how the charging circuit applies voltage potential to the cells, as long as the constant current parameters are met then the lithium battery should recharge at a proper curve with minimal heat. Battery quality and internal resistance will vary from manufacturers so these claims aren't even necessarily misleading, just highly subjective.

there is enough electronics in most bat packs and I would hope most other devices to take care of the constant current charging

@madpom the formula doesn't give you the efficiency, but [the number of charges](https://techzillo.com/power-bank-recharges/). If we assume a 6000 mAh battery, you can charge your phone 1.5 times. That's an efficieny of 75%. I.e. the naive calculation is 6000 mAh / 3000 mAh (size of power bank divided by size of phone battery). So you would expect 2 charges. But we get "only" 1.5.

@bernard. Your formula does not give the result 1.5 times (75% of expected charge). For a 6000mAh pack charging a 3000mAh battery it gives: 6000x(3.7/5)*.85/3000 =1.258 charges Which is 62% of nominal charge

@madpom, sorry, should have added I used 4.2V for charging. That's the value you mentioned as you seem to have measured that. But yes I agree with your value if you take 5V.

I bought a mini-power bank that charges stuff via USB from (replaceable) 18650 batteries as a result of this discussion: 'DIY Portable USB External Power Bank Case Pack Box 18650 Battery Charger' - (DIY because the batteries are not included) https://www.trademe.co.nz/mobile-phones/accessories/chargers/other/listing-2294498359.htm There are dozens of people selling these on trademe / ebay for $5-$10 I promised to report back on how it performed - so here it is. Description ========== http://www.360customs.de/wp-content/uploads/2017/07/1xblack.jpg The charger is a simple aluminium tube with a USB A and USB-C connector at one end and a threaded aluminium cap at the other. The USB-A is to plug devices in to to charge them from the power pack. The USB-C is to charge the pack itself (e.g. from a solar panel). Unscrewing the end-cap the charger slides out of the tube allowing you to insert any 18650 battery into it to use as the power source for charging your devices. The unit is slightly larger than an 18650 battery: 94mm long x 21mm diameter (batteries are 65mmx16mm) and weighs ~10g (below the minimum reading of my scales, sorry) (without battery). Samsung 3000mAh batteries are 42g each. The unit has a 2-colour LED. Blue to indicate battery pack discharging, red for charging. http://www.360customs.de/wp-content/uploads/2017/07/1xsilver_inner-600x255.jpg The case is robust and I would have no issues with the unit being bashed around inside a tramping pack (in a dry bag, etc). However, once opened the plastic compartment that the battery slots into was flimsy plastic and it would be easily broken or the battery contacts easily broken off the circuit board by rough handling. So the unit is only rugged once closed and care is required when changing the battery. Electronics & efficiency ========== The charger uses a TP4333E chipset which is capable of delivering 1A at 5V USB, with 15-20% power loss in the process (source: http://www.360customs.de/en/2017/07/vergleich-powerbank-fuer-iot-esp8266-esp32-arduino-gps-bluetooth/). When combined with the 25% loss experienced in charging a 3.7v battery from a 5v USB source this gives the expected efficiency as 60-55% depending on the current being drawn (i.e. dependent on charging speed). As expected, a single full 18650 3000mAh battery charged my Samsung S5 (also 3000mAh) to about 60% from flat with the phone turned off. I didn't monitor how long the charge took exactly - but somewhere around 2.5-3 hours indicating that the system was running at near the max output of 1A for the chipset. I use 3000mAh Samsung 18650 INR batteries for my torch, so this device allows me to use spare torch batteries to charge other devices. These are readily available from vaping supply shops. === Advantages over traditional 'power packs' ========== Weight. The charger plus 3 batteries weighs in at 130g (10g for charger, 42g each 3000mAh battery) compared to 200-400g for a commercial 10000mAh charger. The saving is because the case only needs to be large enough to house 1 battery. Volume. charger plus 2 spare batteries (total 9000mAh) is: 32.9 + 13 + 13 = 59cc compared to 140cc for Samusung commercial 10000mAh power bank Control. You know that if you leave a device charging it can only flatten a single battery - not the entire pack Multi-use. If you use a torch that takes 18650 batteries then the spare batteries you carry for it can be used to charge other devices too Cost - The unit is under $10 delivered - if you already have the batteries. And top-of-the-line 3000mAh Samsung batteries are about $9 each if you need to buy them. Disadvantages ============= Ruggedness - whilst the case was rugged it would be possible to break when changing the battery. These devices are not waterproof, whilst some commercial chargers are. Charging speed - many commercial chargers have faster charging speeds Partial charge - you'd need to change batteries mid charge to full charge any high-capacity devices Conclusions =========== A handy back-up power source for anyone who already carries 18650 batteries. A cheap option for someone who may want the occasional topup charge without the bulk/weight/cost of branded chargers. Probably not the way to go if you're a heavy electronics user requiring frequent charges in the bush due to slow charge time and lack of robustness when making frequent battery changes.