Calculating battery requirements

[Nduetime]

He does one go about calculating the amp hours required when selecting a new battery? For example, I'm trying to figure out the requirements of running with stock incandescents vs switching over to LEDs. I've gathered stock incandescents would put me at ~6 ah for my 76 kz400 d3. I'm wondering how small of amp hours I can go if I just switch over to LEDs (turn signals and brake light) while still keeping the rest of the electrical components stock (electric start, etc.)

Much appreciation,

B.

 

[Sonreir]

There isn't a hard and fast calculation. If you're looking to go as small as possible, then a lot of your decision will have as much to do with your riding style as with your power requirements.

An alternator at higher revs will be capable of producing more power than one at lower revs, so if you do a lot of highway riding, you can go a bit smaller. More city riding (where you're in stop-and-go traffic and idling a lot) will require slightly larger capacity.

Kick only will also allow you to go smaller as the additional capacity and amperage won't be needed to turn the starter over.

LEDs won't make a big impact to your overall power requirements. The big three power drains on your system are the starter, headlight, and ignition coils. You can drop about 50% of your Ah requirement by going kick only. Low wattage headlight (e.g. LED) might drop you another 25%. I don't recommend messing with the coils too much.

6 Ah is usually suitable on a bike with no electric start.

All of the above assumes you already have (or will) upgrade to a newer regulator / rectifier.

 

[mydlyfkryzis]

The battery size is not based on the running load directly. The size is based on starting requirements and reserve requirement.

For instance, a CB750 requires about 200 CCA (Cold Cranking Amps). My CB360 requires 110 CCA. So if you have electric start, the CCA rating is the dominating requirement. A 110 CCA battery is usually about 12 Amp Hours.

If you don't have electric start, then the requirement is usually enough reserve AH to keep the bike running for a period of time without charging. For a cb360, standard lighting and coils, this is about 9 AH, or 25% less then the electric start version.

The battery also serves as a reference for the charging volts and as a place to dump excess charging current (On permanent magnet alternator systems). The alternator supplies all the electric needed for running the bike. However, at low RPM's the alternator usually does not put out enough current, so the battery supplies this while idling (at red lights for instance).

Too small a battery means the battery will discharge quickly at a light or long idle time. Low voltage is prone to make the engine misfire or stall.

While many have said they have installed very small batteries, I doubt the batteries last very long, and they probably would have problems (they may not admit it) if the bike idles much time.

Depending on your riding habits, the original size always works best. If you have reduced the lighting load, then the charging system will actually try to overcharge the battery. A smaller battery will suffer with the increased charging (Less load for lights means excess charging load). Sonreirs regulator will help that a bit, but since you stated you are keeping the electric start, the battery you use should have the same CCA as the original. For your bike, I would assume the 110/120 CCA rating is close to stock.