I have a nitefighter bt21 which has developed 2 dead cells in the pack. I want to replace these as its far cheaper than buying a new pack but i am confused about the rating of the batteries. The sticker on the pack says 7.4v,5200mah,li-ion 18650 batteries. When i look at one of the batteries it says 2600mah, four of these dont equate to 5200mah so is this info referring to 2 cells on the assumption this is 2 x 2 cell packs put in series. What size of individual battery do i need to buy and any links to good sources would be a massive help? Thanks

I’m not an expert but I always understood you had to try and ‘balance’ cells to avoid issues – mixing old and new seems to go against that?

You’ll need four new cells as mix and match won’t work. It will be a series-parallel arrangement with two cells in series then in parallel with another two cells. Problem is they are likely to be welded so you need to buy cells with solder tabs which will limit your options for cell availability. Might be easier to buy a battery case (about £10 from ebay or dx) then you can fit whatever cells you like and they will be easy to swap in the future.

Nickjb – that sounds pretty much like what i am seeing. Any chance of a link to case so i know what i am looking for?

I never considered mix and match being a problem if i got the same rated batteries. I will look in to changing all four then i guess, makes sense i suppose as who is to say the other cells wont soon die on me as well.

This the case I have: http://www.gearbest.com/diy-parts-and-tools/pp_119294.html

You can also get them from ebay from a UK seller.

I’m sure others are available but this seems to work OK and has decent waterproofing. Only slight thing is that there is a small battery drain. Not an issue if you are using it regularly but if you want to store it for a while, say over summer, then you need to take the cells out.

Thanks nickjb – just ordered the same battery case in your link via ebay, its a few quid more but a UK seller.

Can anyone recommend the ultrafire batteries that are on ebay or are those liable to blow me up? I have seen torchys batteries which i assume are good but pricey.

Can you charge the batteries in that type of case or do you need a seperate charger?

You probably need a separate charger. The original pack would have had some circuitry to ensure the charge was balanced; li-ion batteries don’t behave the same way as nicads etc if charged together – effectively, if one reaches full charge before the other, it doesn’t stop charging and so overheats.

I have a individual cell charger for 18650 batteries that gives you a reading of internal resistance and capacity.

I then harvest dead laptop batteries for 18650s, usually only one or two cells is duff.

Then use one of the battery cases from deal extreme or eBay to assemble a pack of cells with the nearest matching capacities and internal resistance.

Be very careful especially harvesting cells, the cells contain a huge amount of energy and will give it up really quickly if short circuited and easily start fires.

Usually the cheap “new” 18650 cells on eBay or dx are recycled cells or really poor capacity. Better to harvest our own from quality branded laptop batteries.

Could i possibly use the circuit board from the original pack and put that in the new battery case?

Would a replacement battery pack from C & B Seen or Torchy be compatible?
Thats got to be easier.

I am now thinking by the time i buy all these bits its probably easier and cheaper to go to torchy or someone else.

The battery on my last set of cheap chinese lights died after 1 ride and I replaced it with one from C & B seen, I think it was approx £25 delivered but that was 12 months ago

C&b seen battery packs are ideal but sadly out of stock or something to that affect. It seems they cant sell them as a standalone product at present but do have them for warranty replacements or if you are buying the light bundle!

I’ve had several welded pack die due to the protection curcuits getting wet, just got 2 solarstorm boxes and swapped out the cells from one of the packs, and put some torchy ones in the other. I have also removed the USB lead and shortened the main lead. These zip tied to the bottle cage are great, and seem to be more waterproof!

Tom2000 -do you use a seperate charger then?

Yes, unless I am feeling lazy. I will let you know if my shed explodes.

Have you tried mtbbatteries for getting your battery pack fixed or a replacement with the correct connector supplied. Smudge is a good Lad to deal with.

It was one of their packs, I tried fixing it before I thought about sending it back.