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Modifying my bike...

Posted: Wed Sep 26, 2007 10:40 pm
by badnewswade
I have a Sakura electric moped, and while I'm fine with the low speed, I could really do with some improvements in acceleration and hill climbing. Range would be neat too, of course.

So here's what I think... :idea:

Would it be completely suicidal of me to stick a lower wattage 36v battery (or series of 12v units) in the back and wire it to the motor alongside the output from the controller, or should I buy another controller? What about if I had the battery charge a capacitor (I have a 3300 MicroFarad unit that will go up to 50v) which I then discharged into the motor? My main concerns are safety (don't want battery and / or capacitor to explode obviously) and that I don't damage the motor.

I'm also thinking I could use the second battery unit to extend the life of the primary battery, say if I use cheap SLA's from Maplins sort of thing.

Anyone got any ideas?

Posted: Mon Oct 01, 2007 9:07 pm
by ChrisB
I'd be concened about damaging the controller by over volting it :(

You might be better off just increasing the actual Ah capacity of the battery pack and increase the range rather than top speed.


Posted: Tue Oct 02, 2007 12:41 am
by badnewswade
Yeah, I was wondering how to do that...

So let's see - if I put extra amps into the controller I will get more range and won't damage anything, correct? Should I only use 36v batts to do this or can I use other ones? (I'm guessing only to put 36v into a 36v system)

And shouldn't I put blocking diodes into the batteries though to stop them charging each other up, or am I liable to interfer with any "regenerative braking" (such as it is)?

Also I still want to discharge a capacitor directly into the motor on a pushbutton basis for better acceleration (3,300 microfarads - it's about the size of a Red Bull can) - what do you think about that? I'm mainly concerned about trashing the bike, but there is also always the danger of damaging myself as well. If the battery's a 50-watter (36v at 1.5Ah) and the capacitor is a 50v-capable, 0.3 Farad, does that mean that the capacitor can supply 50 watts at 36v to the motor, and what will this do to it?

(Considering that the motor is 200 watts, I'm hoping for a nice mild jolt of power, about the same as turning the throttle a quarter of the way)

Whaddya think?

Posted: Tue Oct 02, 2007 11:36 am
by EVguru
First thing you need to do is find a good basic electrical theory primer, you're mixing up terms and units.

Current is measured in Amperes (A).

Electromotive force is measured in Volts (V or E)

The ability of a battery to supply a given current for a given time is measured in Amp/hours (Ah).

The power being supplied by a battery is the current multiplied by the voltage and is measured in Watts.

The capacity of a battery is the time it can supply a given power for and is measured in Watt/hours (Wh). You can also determine the capacity by multiplying the Ah value by the voltage.

For a fixed voltage, increasing the Ah rating increases the capacity.

Posted: Tue Oct 02, 2007 11:45 am
by badnewswade
Thanks - will get back to you on that.

So if I want more acceleration, I should wire up my extra battery serially, and (I should imagine) not use the controller (I don't want to blow up two important bits of bike when I can only blow up one :P )

Hmmmm...... (strokes beard)

PS love your website!

Posted: Tue Oct 02, 2007 2:20 pm
by aminorjourney
... and don't forget, if you Join the BVS you'll get the Plugged In Magazine, which actually will have a series of articles on converting/tweaking your own electric bike. PLUS our very own Meeeeester John (John Honniball) is going to do an article or two about basic electronic circuitry :)

Back to the Sakura:

I do have a spare controller here somewhere which I'm sure we can come to an arrangement on. Secondly, I would suspect that the quickest and dirtiest way to getting it to go with a bit more poke would be to upgrade the controller. The stock one is limited to 15mph top speed before the controller stops producing any more acceleration. That's to stay legal of course.

If you want more power a more capable (and non-restricted) controller should be sourced. The ones in the Sakura aren't designed to handle much power. You could probably get away with over-powering the bike by adding a 6 V battery in series with the main pack (make sure the chemistries and AH match though) , but charging it would be a problem with the stock charger.

As someone who ran her EV on 42 V for a while even though the controller is was at 36V I can say that it IS possible to over-voltage a controller by a small amount. However, you're putting strain on the system and eventually you should expect it to go bang. I resolved the problem on mine by using a controller rated to take 36-48V rather than the original 24-36V controller; which blew on me after I installed 43V of lithium and then passed high 'hill climbing' currents through it.

So, in short:

Tinker, by all means :) It's a great way of finding out what works and what doesn't.


The basic Sakura controller isn't all that well built, or that power-able. Upping the voltage by no more than 6V should be okay on that bike - but it will wear out the controller eventually, things will run hotter, and your range will drop slightly but with an increased top speed (if you can work out how the bike knows that it's doing 15 mph and switch it off!)

I'll also doubly-state that you do know making the bike go faster than that is technically illegal unless you MOT Tax and Insure it :) Of course, you're a big boy now so I hope you don't mind me stating the legal position. Just don't blame us if someone tells you off ;)

Good luck and let us know how it goes!


Posted: Tue Oct 02, 2007 5:46 pm
by badnewswade
Thanks for that, might PM you at some point re spares... So over-amping the controller won't cause any problems? I ask because I need more range almost as much as I need more speed!

And what about my idea of cutting out the controller "middleman" entirely and using a capacitor to give the motor a bit of kick?

Posted: Tue Oct 02, 2007 8:33 pm
by aminorjourney
Trying to put more power than the controller is rated at is an eventual recipe for disaster :(

Adding an additional 6V to a 36 V controller may let it operate at a higher voltage,tit may just fry the circuits too!

I'd invest in a new controller. A 36 Volt controller rated at a higher current would work pretty well :) But I certainly wouldn't push more current through the standard one than you ought to!

Posted: Wed Oct 03, 2007 12:02 am
by badnewswade
Yes, of course, but I'm talking about isolating the controller from the extra voltage with blocking diodes, so it never gets fried in the first place. Is this feasable?

If I do over-volt the controller rather than zapping the motor directly (which I probably I will have to do after all, I'm just trying to think out of the box here) I'll probably want to use sets of two 10 Amp NiMH D-Cells. They're small enough that they can go in that glove compartmenty thing below the on-off key switch and then that way I wouldn't have to put a whole 6v through- 3.6v would give a 10% improvement which would be fine. That said, I'd only want to overvolt for a few seconds at a time - which is partly why I think I can get away with using D-Cells.

I still don't understand why I can't just use a capacitor and blocking diodes to by-pass the bikes native controller in the first place. My guess is that just won't work and may cause really serious damage due to potential over-volting and ampage mismatches. Is my guess correct?

BTW, also I'd compensate for the reduced range by overamping with a 36v power tool battery at 1.5Ah. That sounds more feasable by the minute...

Posted: Wed Oct 03, 2007 7:16 am
by ChrisB
I recon the first thing to do is go back to Pauls post (EV guru) and take a few measurements of whats going on while your driving it.

Find out a few things like Current consumed at say max speed and voltage at the motor at max speed and voltage on the controller at max speed etc etc

Once you know the current that the motors using and the voltage you can work out some sort of Ah discharge then you can match the Ah discharge to a set of batteries.

Without any measurements you could spend a lot of time fitting x,y or z batteries only to improve the range by a mile or less :cry: