Batteries – There are many bikes from the 1960s which used 6 volt batteries and modern equivalents are still available. However, most bikes from the 1970s and newer use batteries nominally rated as “12 volt batteries”. I’ll focus on the issues around the 12 volt batteries here and I’ll add a 6 volt section soon.
Modern developments in battery construction have taken us from the original design – lead-acid batteries (still being produced) – to high output maintenance free designs that last for years with good care. Lead-acid batteries are still available and still useful, but the acid is very corrosive, and the acid level must be periodically checked and topped up with distilled water. A good lead-acid battery will last three seasons with good care. But now we have maintenance free batteries, that can last for a decade.
A modern “12 volt” motorcycle battery, for a motor about 750 cc’s, given proper maintenance, will have about 13.5 volts when new and for about the first year of it’s life. For the next two years, it will be around 13 volts. When it gets older and the voltage drops to about 12.7 volts, it is only marginal for day-to-day use.
Even if your bike will start and run with such a battery, your charging system will work hard to charge the battery while running the ignition and accessories like lighting. Eventually the bike won’t run well at higher RPM because the ignition is not getting full power. Worse, you may end up damaging the stator and/or regulator & rectifier. So do yourself a favor – make sure that the charging system on the bike is in good working order and do NOT rely on that charging system to make up for a poor condition battery. You’ll be money and time ahead by including the battery in your refurbishing / restoration plan.
The RIGHT way to Charge a Battery
All small batteries must be charged with no more than 2 amps. Any more power will quickly shorten the life of the battery. This applies to the old style lead/acid batteries and the newer maintenance free types.
Testing Batteries
Please note – if you charge a battery, remove the charger and connect a battery tester or a multi-meter set to “DC volts”, it may show that you have full voltage – but in fact it’s wrong! If you repeatedly do a volts test on an older battery that’s just come off the charger, the volt reading will slowly decline for a few hours before stabilizing, often at a reading far below the initial reading. And so the situation comes all too frequently that someone charges a battery, checks it’s voltage “yup, 12 volts!” and then is disappointed that the motor hardly turns over. This can lead to people replacing starters etc. because they wrongly think the battery is fine.
Volts or Amps?
Though batteries are basically referred to by their voltage, i.e. “a 12 volt battery”, it’s the cold cranking amps that is a better measurement of it’s capability. An inexpensive battery tester gives you the opportunity to view the battery’s voltage, simulate a load (like cranking the starter) and you can watch the meter to see how quickly the battery recovers (or fails to recover) from the load test. This is the way to ensure your battery is truly good or not.
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