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Hey guys, I'm in the middle of my 48V Lithium electric motorcycle build and I'm pretty sure that road rules in Australia require a separate battery for the 12V system so you can still run hazard lights etc if the main pack quits. I'm wondering whether you guys think I will need a DC-DC converter or whether I should just go with a beefier battery for the 12V system. What Ah battery would I need to run a motorcycle for an hour without the headlights dimming? The converter I was looking at is this one, but its about AU$130 and I would rather spend the money adding to the energy storage not finding a way to subtract from it.

I can't see a bike using more that like 100W even running the controller, contactors etc and headlights. Running time should be about an hour so I'm thinking about 15Ah deep cycle lead acid battery should do it without DC-DC. That's about 7Ah usable to 80% DoD after peukerts. Does that sound about right?
 

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You don't *need* a dc-dc converter and a 15Ah battery is probably about right for the job, but you won't escape the headlights dimming if you go with just a battery. Even at 12V most headlights look pretty dim and that won't get any better over the course of an hour.

As a sometime motorcycle owner, I personally don't care about the brightness of the headlight during daytime riding, but I sure do when riding at night.

So if you do a lot of night riding you might want to consider using a 5A/13.8V DC-DC converter along with a smaller battery (even a gel cell will be fine on most bikes - the electrical load is relatively puny). I have no idea who makes such a product. If it doesn't need to be isolated it could be a simple buck converter. Could probably knock one together with one of National's Simple Switcher series for about $15 with off-the-shelf parts from Mouser/DigiKey (US distributors - sorry- I think JayCar is popular in Australia?)
 
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If you forget to charge your extra battery you will be in trouble. Get the DC/DC so you can keep that battery charged and so you can power the lights which do take a bit from a single 12 v battery. Make sure the DC/DC has an output of at least 13.5 v.

Pete :
 
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