What battery pack?

[citoyen]

I have a Gemini Duo light which I've not used for ages because the battery pack is dead and won't take a charge. It seems to be easy enough to buy generic replacement battery packs with the same type of connectors, but I'm confused about what I need - and to be perfectly honest, electrickery is not my strong suit so I'd rather ask the advice of Those Who Know...

The label on the battery pack says 7.4V/5200mAh. Looks like it is made up of four 18650 cells bundled together with plastic shrink wrap. It comes with its own fabric case with a velcro strap to attach it to the bike.


However, while the battery pack says 7.4V, the label on the charger says 8.4V... and that's what is confusing me.


Does the apparent discrepancy matter? What does it even mean? Something to do with input vs output, I presume.

But most importantly, can anyone recommend me a suitable replacement battery pack? I've said to the boy he can have the light to use for winter commuting if he pays for a new battery pack, but I want to make sure he gets the right thing, and not some cheap Chinese shit that is going to catch fire...

(I'm assuming it is the battery that's the problem rather than the charger, but I don't really know how I would test that either.)

[grams]

Lithium batteries are nominally 3.7v per cell but when fully charged they're 4.2v. You can charge them (crudely) by just applying 4.2 volts across them until either current stops flowing or the overvoltage protection chip kicks in (if it has one).

Your pack will contain two strings of two cells in series, hence the voltages are double the above.

[citoyen]

Lithium batteries are nominally 3.7v per cell but when fully charged they're 4.2v.

Your pack will contain two strings of two cells in series, hence the voltages are double the above.

Aha! Thanks for the explanation. That makes a kind of sense, even to me.

You can charge them (crudely) by just applying 4.2 volts across them until either current stops flowing or the overvoltage protection chip kicks in (if it has one).

This has been suggested to me as a remedy before, but a) I don't have ready access to a means of charging in this way*, and b) I'm not sure I would trust myself to do it safely... a replacement battery pack seems by far the easiest option.


*Bench power supply was what was suggested, but I presume there are other ways.

[FifeingEejit]

the connector looks like the standard "Magicshine" one that pretty much every generic MTB light except newer magicshine  ones use

[citoyen]

the connector looks like the standard "Magicshine" one...

Aha! That does look very likely. Thanks for putting a name to it - I know of Magicshine but didn't know it was a de facto standard.

Would an actual Magicshine battery be a good choice? They seem quite pricey but are they worth it?

...that pretty much every generic MTB light except newer magicshine  ones use

Doh!

[grams]

This has been suggested to me as a remedy before, but a) I don't have ready access to a means of charging in this way*

I meant that's probably what the "charger" is doing. Although it does say "smart charger" on it, which slightly increases the chances it's not just an 8.4v DC power supply.

I think your setup has a few possible failure modes:
1. The charger is dead.
2. The pack has gone below some minimum voltage and the charger is refusing to charge it.
3. The cells are bad and won't take a charge.
4. One of the connections is bad.

(1) seems unlikely, but is easy to test if you have anything you can connect it to. Car light bulbs are good, or anything with a DC jack and similar voltage (check polarity!)
(2) This is what you need a bench PSU (or similar) to get around.
(3) Are there any signs of life when you connect the light? What if you charge for a while, and then connect immediately? Does it last longer.

[citoyen]

2. The pack has gone below some minimum voltage and the charger is refusing to charge it.

I had assumed this was the most likely scenario since the battery was left unused for quite a long time.

But it could be anything... Unfortunately there are no indicator lights or anything helpful like that on the charger. It may well be that the charger is dead. It could even be that the fuse in the plug has blown. Maybe I should try replacing it!

[sojournermike]

I’m using some generic 4 x 18650 battery boxes from the auction site

Something like https://www.ebay.co.uk/itm/NEW-Waterproof-4x18650-Battery-Storage-Case-Box-Holder-For-Bike-LED-Light-C8-SGH-/224046216398?_trksid=p2349624.m46890.l49292

With 4 decent branded 18650s - there are several suppliers but getting genuine is the harder part. You don’t need high current drain, so probably look at some 3200 or 3500mah cells. LG or Samsung or Panasonic. My last lot came from a vape store(!)

But a proper 18650 charger - mine is by nitecore (I’ve not used their batteries) - this will make sure charging is safer and better balanced. Protected cells are safer - you don’t want to accidentally short  unprotected 18650s by putting the into your (now fiery and spontaneously disassembled) battery box the wrong way.

Mike

[fuaran]

MTB Batteries should have something suitable, think it is pretty decent. https://www.mtbbatteries.co.uk/

[citoyen]

MTB Batteries should have something suitable, think it is pretty decent. https://www.mtbbatteries.co.uk/

Thanks, that looks just the ticket. I was looking at some sub-£10 options on ebay, but I'd be worried about the house burning down... those are a bit more expensive but they look like a reputable company!

[FifeingEejit]

YEs I'm majorly out of date on MTB lighting.

Anyway on
https://www.mtbbatteries.co.uk/mountain-bike-batteries/7-5v-li-ion-batteries/

"Most commonly used connector" is the "standard" one it looks like you have.
Magicshine went through a phase of doing weird ones for specific lights but looks like they do 2 sets of plugs now; the MJ models with the standard round plug and the ones that go in the units.

I also note there's a lot more "wanky piss" in their set ups too, what was wrong with the button on the light unit and a set of cells and electronics taped up with duct tape...

[citoyen]

Anyway on
https://www.mtbbatteries.co.uk/mountain-bike-batteries/7-5v-li-ion-batteries/

"Most commonly used connector" is the "standard" one it looks like you have.

Yep, I'd come to the conclusion that's exactly what I need, but I'll probably drop them a line just to make sure - sounds like they're pretty clued up so will know for sure. 

(They may be more expensive than the ebay Chinese specials, but I note the official Gemini replacement battery, listed on Wiggle but not currently available, is £120, which is just silly.)

[Kim]

This is probably a good time to determine whether the light works properly if you feed it 5V.  Because if it does, the world of USB powerbanks is your mollusc^[1].


_{[1] Even if it's not, careful abuse of QC3 or PD could gain you something in the 9V range, with a suitable pack.}

[citoyen]

This is probably a good time to determine whether the light works properly if you feed it 5V.  Because if it does, the world of USB powerbanks is your mollusc^[1].

As in powering the light directly from a USB powerbank? Interesting thought, hadn't considered that. Would there not be waterproofing issues?

Also, do you know where I would be able to obtain a suitable lead?

[Goldcrank]

I bought a cheap LED light which had USB fittings. Dumped the battery and use a ANKER power bank. It's great. I can carry a smaller battery if I only need a couple of hours or swap it out to something bigger for audax. I recently bought a new 10,000 mAh mini bank and it lasted for 6 hours on full beam (I normally only use 1/4 power on the road and it is more than bright enough)

[Goldcrank]

This is probably a good time to determine whether the light works properly if you feed it 5V.  Because if it does, the world of USB powerbanks is your mollusc^[1].

As in powering the light directly from a USB powerbank? Interesting thought, hadn't considered that. Would there not be waterproofing issues?

Also, do you know where I would be able to obtain a suitable lead?

It might be a case for a bodge...

https://www.youtube.com/watch?v=uOG-v3e-Or8

[citoyen]

It might be a case for a bodge...

I was worried someone would say that. 

[Kim]

This is probably a good time to determine whether the light works properly if you feed it 5V.  Because if it does, the world of USB powerbanks is your mollusc^[1].

As in powering the light directly from a USB powerbank? Interesting thought, hadn't considered that. Would there not be waterproofing issues?

Certainly.  My go-to solution for waterproofing USB things on a bicycle is to keep them in waterproof luggage.  I'm then more concerned about stress on the PCB-mount USB socket in the battery pack, which is designed for the back panel of a computer, not rattling around on a bicycle.

I'm not convinced this is a particularly good solution compared to the dedicated batteries, particularly if you're doing actual off-roading.  But it's a useful option.

Also, do you know where I would be able to obtain a suitable lead?

Splice the plug from your dead battery pack with a suitable USB cable?  That was my approach, but I've got a soldering iron and heatshrink to hand...

[citoyen]

Splice the plug from your dead battery pack with a suitable USB cable?  That was my approach, but I've got a soldering iron and heatshrink to hand...

I have a soldering iron and heatshrink. It's the competence to use them that I'm not sure I possess.

[Frank9755]

I've ridden thousands of miles with lights (front and rear) powered by USB power banks. 

Power banks don't seem to come to any harm in bike luggage for a week or two.  My favourite are Anker as they come with a little bag that protects them.  I slip them into a plastic bag if there is rain around.  I've never had any issues with USB sockets degrading.

They are great for ultra races as you can carry a few plus a 4-socket charger and recharge them all overnight when you find a hotel.

If you don't fancy the soldering iron and heat shrink, at a pinch, stripping the wires, twisting the appropriate ones together and taping would also work.  There is a risk of the connection working loose at some point if you don't do it carefully, but it is a small one, perhaps smaller than that of solder coming unstuck if you are not confident with it.

[citoyen]

I’m using some generic 4 x 18650 battery boxes from the auction site

Something like https://www.ebay.co.uk/itm/NEW-Waterproof-4x18650-Battery-Storage-Case-Box-Holder-For-Bike-LED-Light-C8-SGH-/224046216398?_trksid=p2349624.m46890.l49292

Sorry, missed this post earlier but thanks - was looking that these and considering them as an option so it's good to have a recommendation.

[citoyen]

If you don't fancy the soldering iron and heat shrink, at a pinch, stripping the wires, twisting the appropriate ones together and taping would also work.

Yep, my dynamo lights are connected in this way. The vid Goldcrank linked to earlier also recommends splice connectors. I'm sure I could manage to bodge something one way or another.

[Frank9755]

Someone on lfgss is selling a Magicshine battery pack

  https://www.lfgss.com/conversations/356315/#comment15635043