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I have an electronic bicycle; 350 motor that runs off a 17.5 Ah battery at 36 volts. I've just ordered three 12 volt, 150 watt solar panels, that I'll wire in series to total 450 watts; giving 12.5 amps at full rating. I've got an MPPT controller to boost the charge voltage.

However, the battery is limited to 2.5 amp charge current, meaning that the 10.5 will be wasted I suppose?

Would it be better to wire the panels direct to the motor, where any excess can top up the battery, which can be used when solar power is less then what's demanded by the user.

Is this possible? Or will the MPPT controller sort this out?

Thanks for any help

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  • \$\begingroup\$ Most of the time it will probably be not running anywhere near maximum so you have to run it through the MPPT anyways. \$\endgroup\$ – DKNguyen Jun 21 '19 at 21:20
  • \$\begingroup\$ I think you mean "350 W" in your first sentence. Your post is not clear: are you installing these panels on the bike? (It sounds like it if you are thinking of wiring them to the motor.) What size are they? \$\endgroup\$ – Transistor Jun 21 '19 at 21:23
  • \$\begingroup\$ "... wire the panels direct to the motor, where any excess can top up the battery ..." What makes you think you can backfeed from the motor, through the speed controller to charge the battery? Does it have regenerative braking? \$\endgroup\$ – Transistor Jun 21 '19 at 21:24
  • \$\begingroup\$ Was this your idea? Both battery charge and motor and be supplied if both are regulated \$\endgroup\$ – Tony Stewart Sunnyskyguy EE75 Jun 22 '19 at 2:17
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Is this possible? Or will the MPPT controller sort this out?

It is physically possible, but you would need a combined controller for the MPPT, battery charging, and speed control. It would be a fun project for an experienced circuit designer or two, but it's not something that you could likely make from off the shelf boxes.

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This really difficult to answer without a schematic of your e-bike system.

But you almost certainly cannot connect your panels to the motor and expect the battery to charge. I've never seen an off the shelf e-bike that has any sort of regenerative system, I assume because the amount of recovered energy simply wouldn't be worth the bother.

You could rig it up to both charge the battery and supply the motor. This would be a more complicated system. Does your MPPT have more than one output? Or does it just have the LiPo charging output?

There are some simple ways to do it. They aren't great, but for example you could use a manual switch. In one direction it would connect your battery to the charging port and disconnect it from the motor while connecting the other output of the MPPT to the speed controller.

Not exactly elegant, but allows you to charge the battery and power the motor from your panels. Then switch it back when you need battery power.

To do it properly you would need to do as TimWescott said, with a combined system. Possible, but you wouldn't be able to just go out and buy the part you need.

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  • \$\begingroup\$ "I've never seen an off the shelf e-bike that has any sort of regenerative system ..." Usually the free-wheel mechanism prevents regen. There was a freewheel in the hub motor on my first e-bike and there's one on the Panasonic chain-drive motor on my current e-bike. The free-wheel mechanism allows operation as a regular bike without the additional load of turning a dead motor and gearbox. \$\endgroup\$ – Transistor Jun 22 '19 at 8:46
  • \$\begingroup\$ @Transistor that makes sense, though I imagine you could have some sort of hub breaking mechanism that would allow you to feed back to the battery. \$\endgroup\$ – hekete Jun 24 '19 at 6:59
  • \$\begingroup\$ Hub braking would be better. d:^) \$\endgroup\$ – Transistor Jun 24 '19 at 7:46
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Thanks for the answers. Apologies for lack of clarification in the question. I will look at the MPPT charger and see if it has two outputs. Otherwise it sounds like a custom job for an electrical expert. The switch idea did occur to me Hekeke. It's probably best to rig it up, and see if there are any performance issues and take it from there.

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  • \$\begingroup\$ This doesn't appear to be an answer to the question. StackExchange sites don't work like forums, answers have to be answers, and they float up and down by votes received or user sorting preferences. As a result any attempt at using the Your Answer box for discussion turns into a mess. \$\endgroup\$ – Transistor Jun 28 '19 at 16:00

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