Does/does not hub dynamo make sense on an E-bike?

Trying to get my head around this. I have hub dynamos on my two non-ebikes. One is my Randonneur bike, a Salsa Casseroll. The other is my commuter, a folding Bike Friday Tikit.

I really like the convenience of knowing I always have lights on those bikes and am wondering whether I should consider that for my Xtracycle with Stokemonkey. But does that make sense with an EBike since there is always a battery and power? One argument for the hub dynamo is that even if the battery runs out, there are still lights. Counter argument would be to say "Yes, but how likely is that?" I also looked at the draw on these lights. With a hub dynamo, front is 2.6 watts and rear is .4 watts for total of three watts. If I've got this right, that means three watts per hour, so if I have a 540 wh battery, that means It could run the lights for 180 hours. That seems like pretty minimal drag.

Or looking another way, for a long trip I don't use my stokemonkey much at all, so average around 10-12 watt hours per mile. I rarely push it and try to budget for under 500 watt hours for any longish distance.

For short trips to the store, I might use 25 watt hours per mile. So using that example, if go total of eight miles at top speed, I might use 192 watt hours and take around 20 minutes give or take. In 20 minutes, I use maybe one watt hour of light. So it seems, very little power is used for the lights.

So it seems to me unless I am really pushing the limits of my battery, might as well use it to power the lights. Getting a hub dynamo would cost a little more and only save me a few watt hours.

Am I going in the right direction here, or am I missing something?

Thanks in advance,

Ty

I think any sensible person can decide whether it's a good idea to use a battery to provide electricity to turn a motor that turns a dynamo to produce electricity to power a light, when you can use the electricity to power a light directly.

A lot of people don't understand that a hub dynamo produces drag in proportion to the amount of power that you take out of it. Why would you want that?

I also have hub dynamos on all my regular bikes. As you note, for most cases it probably doesn't make sense to have one on an e-bike given you have a big battery already there and lights draw a minimal amount. I ended up with one on my cargo bike because (1) I retrofitted a hub motor a year after getting the bike and (2) I plan to do longer tours without e-assist down the road (can easily swap the rear hub motor with a regular wheel) so having the hub dynamo is useful for those situations.

With a stokemonkey it's not so easy to remove your e-assist so I doubt you'd have any benefit of a hub dynamo. BTW if you're used to dynamo lights, I recommend checking out the e-bike versions of B&M and Schmidt lights ( we have the B&M Cyo 80 lux one on my wife's cargo bike and like it a lot)

I just question the hub dynamo on the regular bike. Now that the lights are leds, your tiny battery for them lasts days, instead of an hour or so.

Dynamos made some sense back in the incandescent bulb days, but not anymore.

What low voltage protection do you have? If you can have the controller cut off before the bms, you still have lights after the motor stops working.
An unheard of version is to use the main pack to charge an auxiliary lighting pack. Then you have the auxiliary packs capacity after the main pack fails. Many bike lights come with batteries in a separate little lump so you just have the charger to think about. A 110v one if you have high pack voltage (70+ iirc) or a tiny dc-dc setup to supply just a trickle. Maybe not lithium for an emergency lighting pack like this. Such a design would be lighter and more efficient. You could also be pioneering something many would want to think about. Or you could just use 18650 battery powered lights, and carry a spare battery because we all know the battery does go flat, in every battery operated thing we own. Perhaps power the torch like light from the pack using a dummy battery, and keep the 18650 in your bag somewhere. Just in case.

There are a number of ways to avoid needing the dynamo with it's electrical inefficiencies and parasitic needs. Not forgetting their attractiveness to thieves

I just added a SP DP-8 dynomo hub on one of my bionX bikes that I sometimes use without the bionx System (just replace rear wheel, battery and console in 2 minutes). I payed 135 Euro for that wheel.

Headlight is a Philips saferide 60 that runs either on the DC Output of the BionX System (at 7,5V) or on the SP Dynamo hub.

Drag of the SP-8 Dynamo hub is 2W at 30km/h (it isn't 0W in a Standard hub), weight is around 400g (so 250g more than Standard front hub).

http://fahrradzukunft.de/14/neue-nabendynamos-im-test/

Efficiency at 3W is around 60%. I do not know the Efficiency of the DC-DC converter in the BionX battery, maybe 85-90% (???)

Biox DC-DC converter is limited to around 12W Maximum power.

A dual light Setup would be possible using a Dynamo light + an extra (offroad) battery powered light.

Cephalotus said:

Drag of the SP-8 Dynamo hub is 2W at 30km/h (it isn't 0W in a Standard hub), weight is around 400g (so 250g more than Standard front hub).

Efficiency at 3W is around 60%.

Click to expand...

I guess you mean that the drag of 2w is with the lights switched off. As soon as you switch the lights on, the drag has to go higher than the power of the lights.

The few logical reasons i can think of to have a dynohub on an ebike, is if you have an ebike that isnt always going powered.
Maybe you have a very light setup and can leave the battery at home for a close to reular bike experience?
Other than that the dyno could cover lights when youre completey out of juice in the battery.
(not very likely as the draw is very small compared to the motor, and minimal voltagesag will let the lights run for a while even if running the motor means low voltgae cutoff).
A dynolight might make the electrical installation more simple, with fewer cables to the handlebars.

So in my head, a dyno on the standard ebike makes sense only in specific cases.

Prima facie, its an inefficient way to power a light if you are carrying a battery, to power a motor, to power a dyno, to power a light.

Unless, the power you are using to drive the light is not parasitic of the power used to drive a motor. I.e. use it from braking instead. This leads to considering a drive drive motor and using it for driving and regen.

Alternatively, I thought about using a dyno with a switched clutch design. Switch on, engage clutch and regen away, Switch off, disengage. However, the hub motor can produce more powerful regen and therefore more power doing the same thing and has the benefit of driving the bike as well.

OR you aren't using a battery to make power in the first place - which is what it is originally designed for on an ordinary bike - its converted your human power into electricity.

Gave up on the dynamo idea at this point, couldn't see it being really worth it on an ebike. I would consider small DD motors instead if anything - conhis has some interesting tiny motors which I am considering buying just to experiment with....

d8veh said:

I guess you mean that the drag of 2w is with the lights switched off. As soon as you switch the lights on, the drag has to go higher than the power of the lights.

Click to expand...

Obviously.

It's 2W drag at an electric of 0 W at 30km/h (vs. 0,5W drag on a standard front hub)
It's around 5-6W drag at an electric output of 3W at 30km/h (vs. 4W(?) from battery + 0,5W from Standard hub).

So at the end you loose around 2W of mechanical power with a high quality dynamo hub at 30km/h vs. battery powered lights no matter if the light is on or off.

This is around 1% of the power needed to drive 30km/h with an efficient (electric) bicycle.

Weight penalty with a SP or SON hub is around 250g, this is also roughly 1% of the weight of my electric bicycles.

So if you think that you have a good reason to use a dynamo hub (as I do because I sometimes ride this bike without the electric drive train) than the loss seems to be acceptable to me.
It would be possible to use the BionX hub as a high Dynamo hub (+DCDC converter + DC light, NOT Dynamo light), too. BioX motors output DC voltage at the BAAS connectors, roughly 1V per 1km/h, when not connected to other bionX components like battery and console, but you have that heavy motor in the rear wheel. I do not like that. I ride my Cute 85 pedelec without battery sometimes, because that motor is lightweight and free wheeling, but not my BionX pedelecs.

So I have am not that fond of hub generators. Expense, Weight, Drag and wiring are my issues.

I am not a total weight freak but every pound counts.

I would much rather use an LED removable light with it's own batteries than a dynamo. I often have a pair of LED lights on the handlebars. Failure rate is very low but if one fails I have the other.

If I get a flat tire or need a light when I am off the bike my lights work great---your dynamo light will not. In fact riding at night I feel I must have a flashlight and the removable one on my bars qualifies.

I am also into as few things on my bike as possible. Day rides in nice weather, the fenders and lights come off.

As a bicycle mechanic I have spent enough time tracing down a broken wire on dynamos internally routed, I have no desire to do it on my own bike.

So in my opinion---for me--- no way, however if you like them, enjoy.