A friend has secondhand Smarta bike (about 10 years old) which needs a replacement battery, the type that slides in behind the seat tube. I think this type of battery is often referred to as a 'Silver Fish' battery and is used on a wide variety of older ebikes. There are many such batteries available on ebay and Amazon but we want to ensure that the replacement is safe to use, ie has good quality battery cells, a battery management system to prevent over-charging etc. Can anyone recommend a reputable source for a replacement? NB We know the ebike works as we've tried a battery from another bike (aJorvik trike) and all is fine.