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I just bought a Priority Current ebike that has a battery integrated in the frame. The main disadvantage of this is that there is not third-party version of this battery, a replacement can only be bought on their website. Thinking ahead, I am afraid that once Priority stops making the Current, it will also stop selling a replacement battery and I won't be able to find one when I need it. Is it worth buying a spare now even though I probably won't need it for years? Don't unused spare battery also degrade if not used, even if well taken care by keeping them in cool area and moderately charged.


Thanks everyone for the helpful answers.

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  • 3
    To clarify - the battery is clip in-out or is it really integrated in a way that makes a replacement swap difficult? That is, if you owned two, could you swap batteries while out on a ride with minimal tools ?
    – Criggie
    Jul 16 at 11:18
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    @Criggie it looks like the battery goes in a modified downtube, rather than the very common custom case moulded to fit the downtube. It may not even be intended to be (easily) user-swappable, let alone field-swappable.
    – Chris H
    Jul 16 at 12:01
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    Chance is that the life expectancy of the stored spare battery is about the same as the on-bike battery.
    – Carel
    Jul 16 at 18:38
  • As the answers say, you are focusing on just one thing. You should be focusing on everything that is a model-specific part for that bike. That probably includes all the control electronics for example, and even the frame if it is a "nonstandard" design to fit the battery inside. Don't just think about things "wearing out" - think what might get broken if you crash for example. The bottom line is probably that the whole bike will be pretty much worthless in 5 years - so start saving up to buy replacement now!
    – alephzero
    Jul 18 at 1:53
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Probably better asked on Electrical Engineering SE.

From my RV experience, batteries have a limited shelf life no matter how well they are stored. Shelf life reports vary from 2-6 years (I thought longer). Regular recharging (6months to a year) will extend the storage life. Lithium have a typical useful cycle life of 1000-2000 cycles. If you are riding every day, that is 3 - 6 years, the bike would be well on the way to worn out by 6 years.

Battery packs are almost always made up of standard cells (e.g. the 18650). In this case, its possible to get the battery pack rebuilt with new cells, and with current advances in the tech, making better than new.

Should you buy another battery - its unlikely its going to extend the life of the bike much beyond having one battery. I would find out from the manufacturer what cells they use, if a common one like the 18650, then don't worry.

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    "Probably better asked on Electrical Engineering SE." No, it's not. Questions about consumer electronics that are not about the design or modification of these products are outlawed.
    – Mast
    Jul 17 at 6:10
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    I think you mean "off-topic".
    – chepner
    Jul 17 at 15:20
  • “its possible to get the battery pack rebuilt with new cells, and with current advances in the tech, making better than new.” — Both of this is speculation. Yes, they are usually made up of standard cells, but often these seem to be integrated so tightly (possibly with protective electronics) that it would be impossible to disassemble without destroying the pack. And I haven't seen any evidence for the often-made claim that lithium batteries are really getting much better anymore. On the contrary, the remarkable improvements in the 2000s seem to have stalled around 2012~14. Prove me wrong. Jul 17 at 15:45
  • (What is certainly true though is that batteries keep getting cheaper, as mentioned in juhist's answer.) Jul 17 at 15:53
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Li-ion batteries lose some capacity in prolonged storage no matter the state of charge, so I certainly wouldn't buy one now.

Apart from the option in mattnz's answer (battery rebuild services, which can be slow and cost more than a replacement), you have an alternative: Keep an eye on the condition of the rest of the bike, the battery capacity (as measured by the indicated remaining charge just before you plug it in, and the company's condition. Then in 3 or 4 years, think about buying a new battery while this one is still useful, and if the company seems to be going through a bad patch, getting taken over, etc. grab a spare while you can.

Other parts on e-bikes can also be tricky to replace, being slightly unusual, but I don't suggest keeping stock of those either. Common examples are brake levers with integrated motor cutoff switches, common between brands but hard to find, the motors themselves, and the short spokes used with hub motors (though custom cut spokes are easier to fins than they used to be)

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Stocking up on replacement parts that you might need someday (uncertain) but don't need right now makes sense only in some cases:

  1. You suspect the part shortage continues, or you suspect the part will be discontinued
  2. The part is likely to fail or wear
  3. The part is not very expensive and not likely to become much much cheaper soon, instead maintaining its value and usability very well even if technology changes
  4. The part doesn't degrade over time unused or degrades so slowly that it doesn't matter -- for example even though tires degrade, if not exposed to sunlight the calendar life is so good that I always recommend keeping one spare tire in stock despite possible rubber degradation.
  5. You have enough money to stock on replacement parts
  6. You are able to cover a very large fraction of the probable replacement needs

For example, if you have a mid-drive e-fatbike that uses non-standard cranks you are unable to find, it is very likely the reason for the bicycle becoming unusable is not battery failure but rather crank failure. Plenty of these fail. So if you are left with a bike with broken cranks, no replacement cranks to be found, and a spare battery, and you still need to buy a complete new bike due to lack of replacement cranks, it wasn't a very good idea to stock on the spare battery, was it?

About the points for batteries:

  1. Battery production is on the increase and battery manufacturing is not bike specific, so I don't believe the battery shortage is as severe as bike part shortage. It generally doesn't requires lots of factory investment to convert individual cells into e-bike battery packs, whereas for example chain or cassette manufacturing requires special tooling. For name brands you probably will be able to buy replacement batteries 10 years from now, but for off-brands this might not be the case (but for off-brands it might be possible to dismantle a pack and replace the cells).
  2. Batteries do fail and wear, so both of these are true.
  3. Unfortunately, e-bike battery cost as a percentage of the total bike cost is rather high. So it might not make as much sense to stock on batteries as it makes to stock on spare chains or brake pads. Battery cost, by the way, is likely to go down and especially e-bike battery cost even more because today car batteries are selling for $50 per half a kilowatt hour (if there was a car that only needs half a kilowatt hour... well perhaps modular batteries might have half kilowatt hour modules, many of these) whereas for bikes you may need to pay $500 for the same. Five to ten years from now, a half kilowatt hour car battery will cost $25. Will a bike battery cost less than $250? I suspect it will because unexplainable profit margins are likely to go away. We might not see $25 cost for the pack but something like $100 could be very probable. There is no standard for e-bike batteries so a battery bought today won't fit a bike bought 10 years from now.
  4. Batteries do degrade over time whether or not used, and if unused there's the additional danger of over-self-discharging. Some Honda NiMH hybrid batteries failed due to being only used in city driving where the controller algorithm never fully charged the battery, leaving it always at partial charge (and Honda prevented further failures by improving the algorithm) -- I'm not sure if li-ion batteries need occasional full charge but it might be very possible that an actually used battery sees better calendar life than a unused battery despite seeing cycling.
  5. This is entirely dependent on your financial condition so I can't answer.
  6. It might not be the battery that fails, it might be something entirely different. Does your bike have some special parts that can't be bought anymore? Crank sensor? Wheel sensor? Motor? Head unit? Control computer? If so, you might be left with a broken bike, a spare battery you don't need, no possibility to replace the broken part, and the need to buy a complete new e-bike with all its parts that won't be compatible with your already existing spare battery. Although it is true that battery is the most likely to fail of these special parts, but it can fail too if unused, sitting on the shelf.

Personally, I'd say don't buy a spare battery unless it can be swapped without tools and you are planning to ride long distances, so long that you need to carry the spare battery. And even then, it might be possible to carry a fast charger with you and charge your existing battery at a stop. Good e-bikes have 100-150 km ranges, and that amount of riding usually requires some time to rest before continuing, which might be an opportunity for a fast charge.

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Partly in response to @ChrisH, I thought I should check how much capacity a lithium ion battery potentially loses in prolonged storage. The answer is complicated. I think that potentially, lithium ion batteries can last several years in storage with only some loss of capacity. However, the answer is a bit more complicated than that.

Battery University has an unsourced table showing the (estimated?) capacity remaining in a lithium ion (cobalt chemistry) battery when stored at 40% capacity for one year. At 25 Celsius (77 F, around room temperature), it's 96%, and 98% if stored at 0 C (32 F). I don't know if the remaining capacity function is linear with respect to time or if it's bathtub shaped. Nevertheless, you'd probably want to arrange to store the battery at low temperature - above whatever the manufacturer specified minimum is, at any rate, but that minimum will probably be below freezing anyway. Storing the battery at full charge is harder on its life.

The article didn't explicitly talk about periodic recharging of a stored battery. However, all lithium ion batteries self-discharge, i.e. if not plugged in they still lose some charge over time. I think that if the battery completely discharges, that will kill it - the Battery University page recommends discarding the battery if the cells are under 2.00V for more than a week. I'd assume you need to periodically charge the battery, and to maintain optimal capacity that does mean partially charging it and then unplugging it. This site cites a recommendation from Panasonic to do exactly that.

If you are not able and willing to do that, then it may be better to just try to source a replacement when you need it or when you perceive that the company may be going under or may be discontinuing that model and its logistical support for the model. That's easier said than done, but so is periodically charging a stored battery. I can keep up with the batteries I have in service, but I don't think I would be able to maintain a stored battery like that. I suspect that the issue of future battery replacements is an industry-wide issue that hasn't been solved. You might be better off going with an established e-bike brand, since you could assume that they're more likely to support older products - but I don't know that this is necessarily true either.

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  • 18650 cells once under 2.3V won’t be brought back to life, nominal cut off for an 18650 is around 3.2 which is when most would be charged back up to 4.2V. Some branded 18650 cells can be brought back but not with standard chargers so it’s normally new cell time anyway
    – Dan K
    Jul 17 at 12:00
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I bought a second battery two years ago soley to increase my range. Cost about $1,000. Today same battery sells for about $1,400.

A second battery might be very useful for you since you intend to keep your battery only 60% charged. Less than 2/3 charge is also less than 2/3 range.

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