minimizing battery on a honda xbr 500

Hey all!

I'm building a café racer -inspired minimalistic street bike from a Honda XBR 500, you can read the full build thread here: https://www.thumperclub.com/smf/index.php?PHPSESSID=89e37673a6cf5a1169036a21abba98a7&topic=4853.msg31972#msg31972 . Now I'm at the point of designing the electrics on the bike - I'm building a wiring loom basically from scratch to minimize the amount of wires and also relocating some components.

The modifications electrical-wise are:

• All but the headlight will be replaced by LED's, including the indicator lights on the gauge cluster
• The electric starter is removed, this bike will be kickstart only.

I've set my mind that I need a smaller battery since the power requirements drop drastically. The original battery was 12V 12Ah, and I intend to replace it with a 12V 2,9Ah battery.

The problem here is, of course, the alternator and the charging cirquit. I'm going to use the original alternator and regulator/rectifier with the new battery. Here are some specs:

Alternator:

Alternator capacity: 12V, 170W/5000rpm

Regulator/rectifier:
Type: Transistorized, non adjustable
Regulated voltage: 14-15V

Charging current:
Charging start rpm: 1000-1200rpm
At idle: (+) 8A/Above 14V (light OFF)
At 3000rpm: (+) 12,5A/14-15V (light OFF)
At 8000rpm: (+) 13,5A/14-15V (light OFF)

The new battery:

Nominal voltage: 12V
Capacity: 2,9Ah

Charge voltage @ 25°C:
Cycle use: 14,5-14,9V (-30mV/°C), max current 0,9A
Float use: 13,6-13,8V (-20mV/°C)

The battery's datasheet can be found here: https://www.cps.fi/Datasivu/CT/CT2.9-12.pdf

So, I essentially want to build a cirquit that monitors and regulates / limits the amount of current (and maybe voltage as well) that flows to the battery while charging. I've heard this kind of devices are called "Charge controllers" and are very common amongst wind turbine enthusiasts and the likes.

My question is, has anyone here done this, or knows a good schematic/design I could use? Also, my knowledge regardin the charger cirquit is pretty limited, so feel free to educate me on the matter - If you feel that what I'm trying to do is impossible, please inform me and tell me why!

Looking forward to getting this project started!

-kaff-

It shouldn't matter since vehicle electrics are a voltage based system (endeavor to maintain a constant voltage) based on battery voltages.

And since all 12V lead acid batteries have the same voltage characteristics, no change is generally required.

The exception may be for some AGMs (which may require a lower voltage limit)...
And current limit for the battery as you rightly recognise.
However vehicle batteries often exceed their specified limits but that is not considered a problem.

General requirements are charging between 13.8V & 14.4V where 14.4V is the generally accepted upper limit for acceptable (non-)gassing levels. (Especially for AGMs - they get hot instead of boiling off liquid.) Above 14.4 is acceptable for short durations only, except when equalising which is generally an occasional maintenance thing only (as in 4-stage battery chargers where only 3 are used for AGMs, or some alternators that allow (say) 14.4V to ~14.7V a short while after initially starting, though usually only for cranked engines).

So it's really only current limiting you may want, but yet again I'd suggest a suck & see approach. If the smaller battery doesn't last long, then see if that's a current limit issue, or too much current drain for its size due to brake or indicator lamps; engine demand, etc.
A smaller battery should never last as long (lifespan-wise) as an equivalent larger battery as it will experience higher recharge and discharge currents relative to its AH capacity, and have a greater discharge depth. Plus lower thermal inertia/capacity etc.

oldspark wrote:

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It shouldn't matter since vehicle electrics are a voltage based system (endeavor to maintain a constant voltage) based on battery voltages.

And since all 12V lead acid batteries have the same voltage characteristics, no change is generally required.

---

This is exactly what I've been thinking, along with most of my fellow students  professors at the uni. In my head, an alternator won't "force feed" a battery more amperes than it takes, and that depends on the internal resistance and potential difference. Although the alternator has capacity to feed over 10amps, it won't do that if the electrical system requires little wattage and the battery is in good shape. The only way the alternator could rise the amperage to the battery is if the voltage would rise as well, but it doesn't since it's regulated. And of cource some wall chargers that are current regulated need to be noted, since they can force feed more current than the battery needs.

oldspark wrote:

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General requirements are charging between 13.8V & 14.4V where 14.4V is the generally accepted upper limit for acceptable (non-)gassing levels. (Especially for AGMs - they get hot instead of boiling off liquid.) Above 14.4 is acceptable for short durations only, except when equalising which is generally an occasional maintenance thing only (as in 4-stage battery chargers where only 3 are used for AGMs, or some alternators that allow (say) 14.4V to ~14.7V a short while after initially starting, though usually only for cranked engines).

So it's really only current limiting you may want, but yet again I'd suggest a suck & see approach. If the smaller battery doesn't last long, then see if that's a current limit issue, or too much current drain for its size due to brake or indicator lamps; engine demand, etc.
A smaller battery should never last as long (lifespan-wise) as an equivalent larger battery as it will experience higher recharge and discharge currents relative to its AH capacity, and have a greater discharge depth. Plus lower thermal inertia/capacity etc.

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If the battery costs under 20€ it's well worth a shot just plugging it in and seeing what happens. An amp- and voltmeter might be in order to monitor the first charge to see if they stay in reasonable limits.

Bingo! on what your educated colleagues think. Telll them to get on some of those other forums!

As I analagise it - it's like having a glass joined with a tube/pipe to a HUGE dam. If the glass water level is the same as the dam's (voltage), then the dam cannot force water into the glass - now matter how big (amperage/capacity) the dam is. But raise and lower the glass's voltage (surface level)....

The dam & glass can be a alternator & battery or vice-versa, or the car loads etc.
(Of course, car loads will simply absorb whatever water, er... current, they can until their pipe or valve (current demand) restricts or their voltage equalises. (It's not a good analogy for loads, and whether its current demand or back- or reflected-EMF = voltage is king of the same or interrelated or cause-effect etc.))


And you share my practicality re battery cost versus the trouble or concern or cost of current limiting.
Mind you, all the above is/was worth confirming - especially if any safety issues, or damage that could arise to other equipment.

There can be many issues - eg, smaller battery is less filtering of the AC alternator (a battery is like a capacitor) - but they should rarely present problems. Lots of ifs, buts, shoulds & coulds....

Probably the best suggestion is to check the battery temperature now & again after installation. Warmth is normal, but hot is not. (Both relative to ambient temperatures.) Batteries heat once fully charged if continued to be charged (not a problem in your case - ie, voltage the same) or from excessive charge current.


A later confirmation of how your small battery seems to go could be good.

And maybe a small AGM battery? Or at least sealed... (Must be for cyclic use and handle currents. Small AGMs are often cheaper than equivalent wet/flooded batteries, and can be more robust (or less!). And they don't wreck chrome after tipping over!)


But gotta go now... maybe more later if needed.