CTW is for a bike not an ancillary part even a battery critical for it's working.
As Tony has kindly linked to:
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Rule 3 - what can I get?
The cycle to work scheme is awesome as it enables you to get all manner of cycling gear at a significant discount (whilst also spreading the cost).
You can get any accessory that keeps your bike wheels turnings, for instance:
- Safety accessories - that help keep you and your bike safe; helmets, bike lights, reflectors, mirrors, bells, locks...
- Essential accessories - necessary to cycle commute; clothing and shoes, mudguards, panniers, backpacks, child safety seats…
- Components and parts - that keep your bike in great shape; pumps, puncture repair kits, cycle tool kits, tyre sealant, groupsets, wheels, cranksets, cassettes...
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However stuff on websites is often a bit generic, and its not super specific on whether an ebike battery is certainly allowed, and also, while I know the last time I did do the CTW scheme, you could get accessories AS WELL as the bike, I can't recall if you could get ONLY the accessories ?