Power bank with replaceable 18650 batteries

[darkandmuddy]

Just bought one of these online to power my bike light using my caving light 18650s. It takes two or four and has a USB output and an 8.4v output. Claims to have protection included as well. Seems well made and the unit looks quite well protected against water and is quite tough. Power ports would probably need protecting more for underground. I can report back once tested next week but looks ok so far. I'll be charging my cells outside of this so won't be testing the charging capabilities it claims since I don't trust it for that!

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[Ian Ball]

One of the best names for a product that can catch fire if incorrectly charged ;-)

I think that is spot on but I think the cables are a weak point, you can get them for about ?10 on ebay. Looking forward to your review.

 

[royfellows]

These have been around for quite a few years and it was a few years back that I obtained and tested one as a possible lamp battery case. I found that the waterproof integrity was very weak. Recommend testing in this way without the batteries.

 

[Ed]

Without going into the question of why anyone would want to take, and be able to recharge, a PDA or a phone underground...

To answer the question you didn't ask, for surveying. 

Of course!
All you need now then is a waterproof phone to go with the waterproof power bank

Got one - goes with me caving instead of a camera these day

 

[darkandmuddy]

Done a bit of testing and further poking and in terms of waterproofing, it's got a reasonably convincing seal between the lid and box but this is very dependant on tolerances so might not be reliable. I think the weakest point is the blue knurled knob and possibly the buttons and lights. However for cycling it's fine in a bag. For caving it would be fine as long as it's in something else as well.
In terms of using, the polarity is very important as it uses two batteries in series and then the other two in parallel with these. There are tiny symbols to show this but I've added white paint to make it easier and also to the lid to get the right position. This is otherwise only lined up with a small hard to see plastic bit.
I also tested the USB charging using only two cells fitted. The result was that the two 3500mAh cells (7.4v in series) managed to add about 3500mAh to a single 3.7v mobile phone cell. So some losses here between the various DC to DC conversions and charging. However this gives an idea about the capabilities. Some more efficient things might be possibly with external circuitry using the other (bike light) output which is labelled 8.4v which is top end for two cells.
All in all, for what it cost I'm moderately impressed and should work well for me to let me use my caving cells for cycling to work!

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[ZombieCake]

I noticed that Nitecore have brought out a series of waterproof power banks in various capacities between 5 and 20 Ah using 21700 batteries (as opposed to the more common 18650s), e.g.

https://www.nitecore.co.uk/Shop/Batteries/Power-Banks/14024-Nitecore-NPB2-Power-Bank.html

 

[Ian Ball]

If I win the lottery ;-)

 

[AlexR]

Those standard Chinese 4 cell receptacles are probably ok if anyone is looking for a rough and ready solution, but a couple of things should be kept in mind:

The aforementioned poor water resistance.

Piss poor efficiency. The 50% darkandmuddy mentions is probably what you can expect. Resistance is the enemy of efficiency, and anything that has cells connected by some flimsy spring (or even a not so flimsy one) will lead to high internal resistance. If I did buy that 4 cell holder my first act would be to check resistance on the springs, as they could be made out of all sort of wonderful metals. [If anyone wanted to build this thing themselves and have replaceable cells with the components BenJ and Ari posted I'd suggest a 2p arrangement and 5V booster (for 2 cells). At least that way your internal resistance is lower rather than running them in parallel and bucking the voltage.]

Cells that you slot in should have at least roughly the same charge and ideally the same capacity, plus be individually protected. If one of the cells in a parallel pair is nearly empty and the other full, you might be in for a fiery surprise without over-current protection as they'll equalise rather rapidly. The lowest charged cell pair/ lowest capacity cell pair in the 2s2p arrangement will determine how much you can get out of it.

USB is a terrible design for caves. I'd take XT connectors over JST RCYs, but the RCYs are still miles better than USB in terms of resistance and tolerance to grit and mud.

 

[royfellows]

yes, and also if the cells are in series a sharp bang will temporarily break the contact, this could be an issue if you are running something directly off it.

 

[ChrisJC]

Piss poor efficiency. The 50% darkandmuddy mentions is probably what you can expect. Resistance is the enemy of efficiency, and anything that has cells connected by some flimsy spring (or even a not so flimsy one) will lead to high internal resistance.

The 50% efficiency will be the cumulative effect of various switched mode power converters (i.e. step up and step down again).
It will not be a function of contact resistance of the battery contacts.

Chris.