Can someone check my math on this? It feels… wrong:
After cycling a little over 8000km on our e-bike, our battery died. Rather inconvenient, but thankfully we had a variety of cycle shops nearby and one of them was able to find us a replacement.
Ours is a Bicicapace Just Long a fantastic cargo bike with a Shimano motor and battery. The battery that we originally purchased with the bike was no longer available, so the replacement battery is a newer model. It is however a legit Shimano battery rather than a cheap knockoff prone to exploding.
The total cost, just for the battery was £600. Interestingly, this is roughly what I paid for my entire road bike about six years ago.
The steep price tag got me thinking though: if all I can expect to get out of this battery is 8000km, what is the “mileage” of my e-bike? Math is not my strong suit, but the number I arrived at is not inspiring:
If we take the cost of travelling 8000km and ignore the marginal cost of electricity for the sake of my sanity, the cost per kilometre is:
£600 ÷ 8000km = £0.075/km
That feels… high. My kid’s school is almost 5 kilometres from our home, so every day we take her to school, we’re effectively paying £0.75 for the return trip and again to go pick her up, so £1.50. That’s £7.50/week (not including the weekends which are busier).
Given this, I wondered what it’d cost to do this with a car — only counting the fuel mind you — and the result wasn’t inspiring.
The average milage in the UK for a diesel car is 43MPG. I opted for diesel for this exercise 'cause that’s what it seems like everyone is driving here. Converting this to metric, you get:
1gal → 4.5461L
43mi → 69.2018km
69.2018km ÷ 4.5461L = 15.22 km/L
With this value, we can calculate how many litres of diesel one might use to travel 8000km:
8000km ÷ 15.22km/L = 525.62L
Finally, with the price of diesel currently here in Cambridge hovering at around £1.569/l, that means that the price to pay for the diesel alone for the same distance I travelled on that £600 battery was only a couple hundred bucks more:
1.569 * 525.62 = £824.70
That’s… not inspiring. It’s really hard to convince people that cycling is cheaper when the costs of regular use are so high compared to the ridiculously low cost of fossil fuels. Sure, the electricity cost is negligible, and there are many many other costs associated with cars, but having just bit the bullet on £600 battery after such a short time, let me tell you, that taste is bitter.
Bet you charge it back to full every trip. Li-ion should be good for at least 500 charges before noticeable range degradation and up to 1000 before being pretty useless, if the charge controller doesn’t die first. 8000/10 = 800 trips, so ballpark.
Instead, only charge when (nearly) empty. If you’ve got a practical range of 60, you can make 5 trips before charging with a 10 km safety margin, now you get 40000 km out of the battery, much nicer. Bonus points for keeping charge between 20% and 80% which will get you significantly more charges (they say it doesn’t matter, they lie).
Do this if you have LiFePO4 batteries, definitely. But for regular lithium ion, it’s best not to let it get below around 40% and to pull it off at around 85% or so. It won’t be a huge difference if you discharge it deeper because you need the range, but most damage happens when the battery is charging when mostly empty or almost full. As opposed to LiFeP04 batteries that do prefer being fully charged and nearly fully discharged, to the point that you should be careful to put them on the charger periodically (but don’t leave them plugged in) over winter if you live somewhere the weather prohibits bike use then. Sitting empty for long periods is significantly harmful to them.
Dont most batteries don’t his automatically though? What you see as fully charged and fully empty is 85-90% and 15-10%. Maybe gets a little lower than you’d want on a “full” discharge, but never to truly zero charge, and never to fully 100% on a charge.
They all protect against over discharge (but charge them to at least 50% as soon as possible if they cut off, due to potential damage from self discharge) and over charge, but full charge wears out almost all lithium batteries quicker, so it’s best to not charge over 80% unless you need the extra range. LiFePo batteries are not significantly damaged by charging to 100% so those can be fully charged; also LiFePo batteries have very flat charge curves so it’s worth fully charging them so the battery metre is more accurate
Wow, $600? I think you should expect a lot better range out of that battery!! Rechargeable batteries don’t like getting near ends of their range. 40 to 85% charge is about right.
If the charger is cheesy or broken, it could be charging to 100% (or worse yet, not stopping) That would be bad, so you might replace the charger … making sure that the replacement is doing the right thing for you.
I just watched a video where a Chinese taxi (car) battery was taken out of service after 800,000 km. (Probably in a couple of years!) That battery pack would have A LOT more cells in it, so the strain on each cell would be less. They’re expensive, so the charger is probably very high-quality.