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This Luxos U dynamo powered bike light has a cache battery that provides backup power for the light when the dynamo isn't providing power, and it also provides power to smooth the USB charging that it also supports. The nominal power output of the dynamo hub that feeds it is 6v, 3 watts.

I have not been able to locate a replacement battery given the identification on the existing, 2-cell battery. I'm hoping to figure out how to remedy this since I have some nearing end of life, and I'm considering buying more of them -- but I don't want to do that if I can't refresh them with new batteries when they need them.

The internal battery pack label reads:

802025p LiFePO4 6.4V 150 mAh 0.96Wh 1710

It looks like 2 (3.2 volt) cells wired in serial with another yellow wire coming off -- a thermistor?

enter image description here

The cells measure 8mm thick x 20mm wide x 25mm long, so it's clear that ID number refers to the dimension of an individual cell.

The only cells I can easily dig up with the same dimensions are similar to these (https://www.aliexpress.com/item/33052412361.html) that is, 3.7 volt polymer lithium batteries, which, if I understand correctly, are more energy dense, but at the expense of operating temperature range and service life.

The device itself is very easy to open up, so if a suitable, less long-lived replacement could be created by wiring together a couple of those 3.7 volt, 300 mah cells, that would be a reasonable replacement.

Would I need to worry about the 7.4 volts that would make, versus the original 6.2? I haven't tried to pull apart the original packaging, so I don't know what that yellow wire is attached to. If it's a thermistor, I assume I can just recycle it by attaching to the new penultimate cell and maintain the same performance. If I understand correctly, these little cells often have overcharge protection built into the electronics between the cell and the leads anyway.

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  • \$\begingroup\$ LiFePO4 batteries have lower voltages then regular lithium-ion or lithium-polymer batteries (around 3.2V nominal, 3.7V max. charge VS. 3.7V nominal, 4.2V max. charge). A concern for replacing them with lithium-ion would thus be that the existing charging circuit would only partially charge them. \$\endgroup\$ Oct 31, 2020 at 8:07
  • \$\begingroup\$ The original choice of battery technology may have been affected by the raw output voltages for efficient charging. As mentioned by @Unimportant the charge voltage may need to be adjusted and that may require more advanced diagnostic and soldering skills. Keep looking, yo may bee able to find cells with closes match. Also if you can fit larger cells you will have larger storage capacity and probably longer cell life. \$\endgroup\$
    – KalleMP
    Oct 31, 2020 at 10:03
  • \$\begingroup\$ @Unimportant I was concerned with that as well. The hub can definitely put out higher voltage, but the charge rate is presumably conditioned by the electronics in the light, and is probably somewhere about 3.7, meaning it may not charge them up. I'll probably need to find LiFePo4s. There's not a lot more room inside the case for bigger cells. A bit thicker would work, and extra width maybe, but not on length. \$\endgroup\$ Oct 31, 2020 at 11:59

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Since I can't find a suitable replacement cell, I think the best answer is to find an easily obtainable lithium phosphate cell pair and wire them up, build an external piggyback for the light, and wire it in.

Something like a AA size (14500) LiFeO4 cell seems to be pretty common as they're used in solar lights, and they're inexpensive. In the AA size, it would be simple to get a 2xAA battery container, wire it up, waterproof it, and call it done.

At a cell price of 4 for $16, and a holder price of 4 for $12, that would be a simple and cheap solution in perpetuity, and wouldn't require any re-jiggering of the electronics.

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  • \$\begingroup\$ Hi, thank your for your feedback ! Did you manage to make the change? And also I was wondering, what did you do about the Yellow (safety ?) wire? \$\endgroup\$
    – Vincent
    May 2, 2021 at 9:28
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    \$\begingroup\$ I have not made a change to these yet, but I can confirm that if you unplug the battery from the main board, you can lengthen the control cable by cutting and splicing it (another thread). If I go the route described above, I will put the thermistor on one of the cells. The US distributor for these lights states he will not sell a replacement battery, and the OEM in Germany states they cannot send a battery because of flight regulations. That is clearly a lie, since they ship them to their distributor, and EU retailers ship the lights, battery included, to the US. \$\endgroup\$ May 7, 2021 at 12:25
  • \$\begingroup\$ Another option would be to buy this ready-made, lead-wired pack, open it up, place the thermistor, and piggy-back it on the light. batteryspace.com/… \$\endgroup\$ May 7, 2021 at 12:29
  • \$\begingroup\$ i have same issue replace battery but i not found it , i saw you post ask this page i want to know can you replace successfully? if done please share me information ? thank you \$\endgroup\$
    – auntip
    May 12, 2021 at 3:33
  • \$\begingroup\$ It does not appear possible to find the original batteries. Unless you can get an authorized dealer to send you one, you will be left trying something else. Peter White Cycles is the authorized US distributor, but his page suggests he will neither send a replacement battery out (in-house service only) nor will he work on lights not purchased in the US. The problem there is that Peter White charges $300 for a Luxos U (peterwhitecycles.com/b&m-hl.php) and they are available for $100 direct from EU (bike24.com/p238271.html). \$\endgroup\$ Jul 21, 2021 at 14:15
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There is a full how too on putting in a couple of replacement batteries that will fit inside the shell at this link: https://www.ifixit.com/Guide/BUMM+Luxos+U+battery+replacement+and+upgrade/139939

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