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A few facts about lead acid batteries

Last update: 22 May 2002

1. HOW TO TELL IF A BATTERY NEEDS REPLACEMENT

It is not as easy as it seems. If e.g. the bike must be jumpstarted, chances are the battery needs replacement. However, you cannot be absolutely sure whether the starter is not functional: a starter (if you used a car for jumpstarting, remember that car batteries can provide higher current than those used on motorcycles).
A simple voltmeter is (almost) useless. A fully charged (but useless) battery will have normal no-load valtage but its internal resistance is very high.
I suggest first to thorougly examine for bad connections and cables (and starter button) in the starter circuit. If you are sure that all the above are ok, then the only correct method for identifying a defective battery is to charge it, then measure electrolyte density using a suitable battery densitometer. Density must be measured on every separate cell (there are six of them in a 12V battery). The correct value is 1.280 at 20oC. Of course, if the battery is the "maintenance-free" type, you cannot make that measurement. In that case, you must follow manufacturer's suggestions on how to detect the state of health of the battery.
A usual sign indicating that a battery has been reached the end of its life is the excessive need for distilled water filling ins (e.g. once per week or more). Sometimes it becomes suddenly absolutely dead (internal shortcircuit).

2. FOR HOW MUCH LONG A BATTERY CAN BE LEFT UNATTENDED (SELF-DISCHARGED)

The usual problems of self discharge and / or a "normal" current leakage (e.g. an alarm is installed on the bike) brings on a discussion about the battery's immunity to relatively "deep" (20-40% of rated capacity) discharging cycles. It is widely known that all types of lead - acid batteries do not tolerate a complete discharge (a permanent damage, called "sulphation" occurs). So if the permitted maximum "depth of discharge" (given as a percentage of battery capacity), the battery capacity and the leakage (discharging) current are known, one could derrive the time interval before battery become permanently damaged.
The depth of discharge depends on many manufacturing parameters. Common batteries do not tolerate a discharge in excess of about 20% of their nominal capacity (this value is a general assumption, it is not provided in the datasheets), at least when the discharge current is moderate or higher. Not exactly sure if the alarm standby current falls into this category, though. Let's assume the worst case, that is we have to avoid it. So let's make an example:

EXAMPLE: Suppose an alarm that consumes 5mA of stand-by current is installed on the bike. XL 600V Transalp (and the XL400V model also) are equipped with a 12Ah lead-acid battery. Assuming 20% maximum depth of discharge, the battery must not loose over 12Ah x 20/100 = 2.4Ah. That amount of energy (2.4Ah) is drawn by the alarm in 2.4Ah / 0.004A = 600h = 25 days. After that time interval (if the bike had been not used in the meanwhile), the battery must be charged immediately, otherwise its lifetime may be severely reduced. The above calculation does not take into account the self discharge which might be dangerously fast at high ambient temperatures (up to 1% per day).

CONCLUSION: If you plan not to use the above bike for more than (roughly speaking) 10-15 days, you must remove the alarm or periodically charge the battery or (better) leave it connected on a good trickle charger. Fail to do so and the battery performance might be severely degraded. Even if there's no alarm or other consumption, self discharge makes necessary to charge the battery once every month.

WARNING: YUASA warns against cheap trickle chargers that might overcharge the battery. Additionally, they strongly suggest to disconnect from the charger for 2 days, after trickle charging for more than one month.

3. HOW TO MAKE A SIMPLE AND ABSOLUTELY SAFE CHARGER

You need a cheap stabilised 13.8V-14.4V / 0.3A (or higher current) power supply. For lead-acid-calcium (maintenance free) batteries, use 14.5-15V. The higher the current, the faster the charging rate (a safe upper limit is about 1.2A for a 12Ah battery). You also need a diode capable of withstanding 1A or higher (e.g. 1N5401) and a suitable high power resistor. The diode is essential for preventing discharging the battery through power supply (reverse current), if the latter fails for some reason (e.g. mains power failure). Never omit it!
The resistor serves as a current limiter: do not depend on the limiting function of the power supply. Charging is indeed faster when using the current limiter of an electronically regulated power supply but it's not reliable, particularly if leaving it unattended.

charger

The resistor value and power rating can be calculated as following:
Assume the voltage of the battery to be charged is 10.8V (as I mentioned before, it is not allowed to be lower) and the power supply can give at least 1.1A without limiting its output voltage (say, 14V). So we can safely choose 1.1A as the maximum charging current, assuming the supply is able to provide this current. Then
R=(14V-10.8V) / 1.1A=2.9 Ohm. The nearest available value is 2.7 Ohm.
Power loss on that resistor will be (14V-10.8V)2 / 2.7Ohm = 3.8 W (max), so the resistor power rating must be at least twice that number. So we choose a 2.7 Ohm / 10W resistor.

A simple charger like this cannot charge very fast, because charging current decreases proportionally, as long as the battery voltage is increasing. But always remember: small charging current does not harm a battery.
To charge as fast as possible (without harming the battery, of course) an electronic current limiter must be adopted. We 'll talk about it later.
On the other hand, it can be left connected to a battery for prolonged periods (trickle charger). The final voltage will be that of the power supply. There's no need for automatic or manual disconnection when charging has been accomplished.

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