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My bike won't hold charge / has a flat battery
Originally posted by @supamodel
Motorbikes have small batteries and charging systems which are significantly closer to their limits than cars. Fortunately, they are simple and there's a few things to check to make sure everything is healthy.
Nearly every motorcyclist should have a multimeter. You can buy small ones from places like Jaycar for $5, it isn't like they are a huge expense. Ultimately you need to be able to measure AC up to about 100 V and DC up to about 15 V to determine if your charging system is healthy. Most cheap ones can do that stuff just fine.
1. The battery.
Bike batteries cop a lot of abuse. In Canberra, they are subject to huge temperature variations. Cold, in particular, is hard on batteries. It's very common to discover a bike that was fine in summer just can't start in winter. The colder a battery is or the older it is, the more it struggles. So, the first port of call if your bike has a flat battery is to see if it's a bit old - anything more than 5 years is generally good running for a bike battery though some last much longer.
However, sometimes batteries appear to discharge and then someone buys a new one. They then discover that the new battery goes flat too... this indicates another problem. It's either going to be that there is a drain somewhere, such as heated grips permanently wired into the battery, or there's a problem with the charging system.
Checking for a drain is pretty easy. With everything off (i.e. the key off etc), pull the main fuse or the negative lead off the battery. Then connect the multimeter in series with the negative lead (or plug each prong into each side of the fuse holder you just pulled the main fuse from). In current measuring mode, you want to see a figure pretty much 0A worth of current draw with everything off.
If that seems ok, well, you need to look at your charging system. Without getting too complicated, the vast majority of bikes charging system consists of a stator (generates the power) and a regulator/rectifier (turns the AC volts from the stator into DC volts, and regulates it so that it's a pretty much constant 14V which is perfect for charging the battery).
2. The regulator/rectifier
The regulator/rectifier or reg-rect on most bikes is a separate item to the unit that provides the charging. They often fail on motorcycles because they get very hot and they aren't located in places with a heap of airflow. Some bikes, e.g. some Hondas, seem to be more prone than others to reg-rects failing due to some poor design choices, too.
Generally a regulator/rectifier problem shows itself as overcharging the battery which boils away the fluid inside the battery, causing it to have no charge. Sometimes they can not provide enough voltage or stable enough a voltage, but you need to do the fault-finding chart below to work out what's the issue.
3. The stator
The other component of a bikes charging system is the stator. This generates an electrical field and is generally behind a case on the side of the engine. Lots of people neglect this when they replace the reg/rect but a faulty reg/rect can short out the stator, burning out some wires and causing it to not produce enough power. They then replace the reg/rect and battery and discover the bike still isn't charging properly. There are easier ways of finding out what's the problem, hence the fault-finding section below.
4. Fault finding
I could get into a great huge fault-finding chart but I don't need to, as a supplier of motorbike charging system components from the US provides a better one than I could ever come up with: http://www.electrosport.com/media/pdf/fault-finding-diagram.pdf
It's more common people blame the reg/rect when it is the stator than vice-versa, and sometimes you have to replace both, but why bother replacing things you didn't need to when it doesn't actually fix the problem.
5. Fixing stuff.
Fixing things is relatively easy. If you don't know what you are doing, take it to someone to get it fixed.
If you want to do it yourself, there are a few simple things you might want to consider.
* Sometimes you can replace regulator/rectifiers with units from other models and they might have more cooling or last longer. Generally they have similar inputs and outputs but the wiring colours might be a bit different and the mounting systems might be a little bit differently. But most work pretty much identically.
* Stators don't need to be fully replaced. Some places can 're-wind' them which is where they wind new copper wires on them and add new insulation.
* There are some aftermarket replacements for things like stators etc; their quality varies. Generally OEM is 'best' unless there's some known problems with the OEM stuff (e.g. early blackbirds seemed to suffer from troublesome reg/rects; early CBR1000RR fireblades had some stator issues).