How to work out what battery capacity I need?

[stinkwheel]

Looking into this for my project. I intend to fit a slightly oddball charging system because reasons. It'll have a maximum output of 50W because of the available space for those reasons to fit in.

The ignition is seperate and self-excites. No electric start, kick only.

So. I've worked out that by using LEDs, I can make it have a maximum constant draw of around 30W with the lights on. Maximum intermittant draw (with brake light and flashers on) of around 40W. Horn breaks the bank but is only very intermittant.

Those wattages equate to a maximum 2.5A constant draw or 3.2 intermittant.

I'm not sure which variable (or combination of them) determines the capacity of battery I need.

I'd best work it out now, it'll determine the size of the battery which will in turn determine where it goes. Diddier the better. I'm thinking maybe 3Ah? Or can I push it smaller still? There are some really diddy 2.3Ah AGM batteries out there which I could probably hide somewhere cool like in the old tea caddy air filter box with the rest of the electronics.
____________________
“Rule one: Always stick around for one more drink. That's when things happen. That's when you find out everything you want to know.”
I did the 2010 Round Britain Rally on my 350 Bullet. 89 landmarks, 3 months, 9,500 miles.

[cb1rocket]

I would thought only enough for brake light, rear light, headlight and horn and indicators is all you need. The smallest you can find with a capacity of 4Ah should do? The charging system should be ample if the system is mostly led and the headlight is a 35 watt HID setup or similar.

Sorry in advace if you were looking for a mathematical answer.
____________________
Honda H100S2 Rebuild - https://www.bikechatforums.com/viewtopic.php?t=253852

[Luckyfish13]

I'd think u need to look at the minimum power your charging system outputs (at idle) and then use the battery to fill the power difference between minimum output and max peak power usage.

Assuming 12V system....a 1Ah battery will provide 12 Watts for an hour, or 24W for 30 mins...etc....

Then u can work out how long roughly you'll be at idle or at max charging output and go from there....

As the ignitions separate and there's no electric start, the batterys just there as a backup to run the electrics when (if) the charging system isn't producing enough power...
____________________
Daytona 650
XT 125R

[The Shaggy D.A.]

[stinkwheel]

Aye Luckyfish, hadn't thought about it that way. Does rather leave it as "suck it and see" mind. It ought to need to be charging at 2/3 capacity to hold its own with the lights on. I think I'll aim small in terms of battery. The bike has an ammeter so I'll be able to see if it's charging or not.

Mind you. Would it not be the case that if it's not, on average, putting enough power out to keep up with the constant losses (plus a bit extra to charge the battery), it doesn't matter what size battery is fitted, it will eventually go flat. So I have to assume it's doing enough to run the headlight or I'm wasting my time and need to go back to the drawing board. So the battery only really needs to take up any slack caused by adding in flashers/brake/horn.

There is an option for using AC lighting and a diddy/no battery to run LED flashers. Thing is the AC output isn't enough to run a full power H4 bulb (it was designed for a 35w headlamp) and it remains unclear if the LED H4 bulbs will play nicely with AC. Could prove an expensive experiment if they pop. Dual stop and tail lamps definately don't work with AC but single ones do.

I'm not a huge fan of HID and I'm pretty sure it wont work on 12v AC either (well, they do, they run on AC but not directly, I suspect AC to AC ballast is something you'd have to home fabricate).
____________________
“Rule one: Always stick around for one more drink. That's when things happen. That's when you find out everything you want to know.”
I did the 2010 Round Britain Rally on my 350 Bullet. 89 landmarks, 3 months, 9,500 miles.

[stinkwheel]

[Hong Kong Phooey]

I'm pretty certain the LiFePO4 battery in my CBR is mainly air, and the box casing was just to make it fit nicely into the traditional place.

Could you consider dismantling the innards of one and repackage to suit?

Not sure if they're flat or cylindrical cells.
____________________
'81 CG125, '97 FZS600 : '99 CBR600F4, '09 KTM RC8

[Luckyfish13]

Yeah if the output doesn't stay above the 30W constant for the headlight then it's gonna drain the battery for sure...probs need to keep it above like 4k most of the time....reminds me when I had to hold revs high on a old carby...at the lights...people used to think I was on day release...

Still without the electric start there's no need for the big battery...you'd defo be good with 2.3Ah as long as u keep the revs up...if u don't the batteries gonna lose charge anyway....a higher rated battery will just last longer before it needs to be charged...
____________________
Daytona 650
XT 125R