Motorcyle Tech: bad battery or bad alternator?

Factual Questions
1

Oh, how those words from the guy who sold me the motorcycle haunt me now.

OK, so it’s not really all that dire. But in the two or three weeks since I bought the bike from him, I have had some starting/running issues from time to time. On occasion, after having the bike out for a long run, I’d come to a stop light and it would die on me. Sometimes I could start it up again right away, and other times I’d have to let it sit for a while before it would start again. On the latter occasions, during the unsuccessful start attempts, the engine might turn over, but the battery would very quickly drain (lights would go out while holding the starter button). As soon as I’d take my finger off the starter, lights and turn signals worked again, but I wouldn’t have enough juice to actually get the engine started.

So, last Tuesday, after leaving my bike at the train station all day as usual, I got back and couldn’t get it to start at all. It exhibited the same symptoms as above, but no amount of waiting allowed it to build up enough juice to start again. I should mention that there was definitely NOT a lack of fuel here as I had just filled up 9 miles prior to this happening.

At any rate, I got the bike towed home, and this past weekend, I went out and got a trickle charger. The battery charged up in about two hours, and the bike started right away. I spent maybe half an hour on it without incident Saturday afternoon. I took it out again Sunday morning, also without incident.

So I’m wondering if this is enough information for someone here to figure out whether this might be an alternator or battery issue (or perhaps something else altogether). I am, of course, hoping that it’s a battery because quotes from two different Honda dealerships place the cost of replacing the alternator at about $700 with labor. I only paid $750 for the bike itself. It is, for the record, an '83 Honda Nighthawk 450.

Your thoughts would be greatly appreciated. (And mods, if this belongs in IMHO, I apologize and humbly request relocation.)

2

The easiest thing to do is to just buy a new battery and see if that works. On my bike ('86 Honda Shadow) the battery doesn’t charge when the bike is at idle so I don’t think that means anything. The headlight dims abit at idle - that is what leads me to believe the battery is not charging.

It might be your starter and not your alternator or battery. I had similar issues on my bike. It would only start if the battery had a full charge. The starter was going bad and was taking too much juice. With all the power going to the starter, there wasn’t enough for a sufficient spark and it wouldn’t start. It cost me $200 for a new starter and a couple of hours to replace it, but I haven’t had a problem since.

I would start with a new battery. You should be able to pick one up for about $20. Even if that is not the problem, it never hurts to have a new battery.

3

Does this happen when you pull in the clutch? I’ve had my bike die on me when pulling in the clutch to switch gears, that usually means the idle is not set high enough. Other then that I would say a bad battery. Mine is doing the same thing, not always starting, got enough juice to have lights but not turn over, not always starting even when warm. I’ve even charged the battery over night and had problems a day later. I’d start with a new battery and let it go a few months, unless it dies on you in a week or so. I’ll be doing the same thing, but not until spring since it’s cold enough here that I know the day before when I’ll be riding and can just charge it.

4

On the two or three occasions that it has died on me during or after a longer excursion, yes, it died when I pulled in the clutch while coming to or at a stop. It idles right at about 1,200 - 1,300 RPM when it’s warm. However, because of the issues where I haven’t been able to start it at all when I haven’t ridden for a while, I assume that it’s still a battery issue (or a starter, as brewha mentioned).

If batteries really can be bought for as cheap as $20, then I will definitely start there. I need to take it in to a shop for a couple of other things, though (replacing a tire and having new brake shoes put on), so I’m going to have them go over it either way and give a proper diagnosis.

I assume it won’t hurt to ask them to clean out the carbuerator as well? I don’t know if the previous owner ever did this, and he had it about two years. I can’t see what the downside would be outside of labor cost.

5

You might check on the regulator, or the rectifier.

If the regulator is dodgy, then it depends which way it goes down.

One fault is not keeping the voltage down at 12-12.5V, and it overcharges your battery, which can boil dry.

The other way, it just doesnt charge the battery enough.

If its the rectifier, then you find that the lights go fairly dim as the revs fall, and goes up to full brilliance as they rise - to a certain extent it happens lsightly if all is well, but the differance will be quite a drop.

Other things you might check, ensure your terminations are all good, it sounds like its not a new machine and you never know if perhaps someone has just botched some wiring together, loose crimp connectors and burned wire ends are possible.

With is being a Honda, I’d expect the alternator to be pretty good, and the starter motor, but if they are not, it can cost plenty unless you can find a wrecker.

6

This information is not correct. Fully charged a 12 volt battery produces 2.2V per cell, for a total of 12.72 volts. When you go to recharge it, there is internal resistance in the battery that has to be overcome. That internal resistance is 0.2V per cell, for a total of 1.2V. 12.72 + 1.2 = 13.92 Volts from the charger to fully charge the battery. If your charging system only puts out 12-12.5V you battery will always be dead.
If the diodes (we don’t call them rectifiers in the transport business :slight_smile: ) in the alternator are bad, the bike may act the way you describe or then again it may not depending on the just how the diodes failed. If a diode is shorted, the battery will not charge very well if at all.

Question for the OP. Do you have access to a digital volt ohm meter?

7

Not that I’m aware of, unfortunately. Do you think it’s worth buying one for this? Or should I just go ahead and buy a new battery and see how things progress?

I did order myself a Clymer manual today so that, after the initial shop visit, I’ll be able to do at least some maintenance myself.

8

regarding DVMs

Harbor Freight is your friend here. $3 on sale, and plenty accurate enough for most things. I buy a couple when I notice them on sale. Have one in each bike (4) and car (2) in each tool box etc. No pain felt when it slides off the fender onto the driveway, so they see a lot of use. Only on rare occasions will I dig out one of the “good” meters.

I have had a couple of these that came with bad leads though. Look for < 1 ohm when you short the two probes togethor.

9

Diodes and rectifiers are two different things. Diodes are individual electronic components which allow a current to flow one way. Rectifiers are devices that convert AC voltage to DC voltage. A rectifier will have diodes, although other devices can have diodes as well.

10

If you put 6 diodes together, you have a diode bridge which I assure you will convert 3 phase AC to DC. Look at the very bottom of the link for how they are set up in an alternator.
Diode bridges are also called bridge rectifiers, but not in when used in vehicles.

11

In the future, I’d appreciate it if you’d all tell me not to try to fill a dry-charge battery myself. I was very cautiously working on it in apparently inappropriate lighting, so I couldn’t tell how full the cells were getting, and I managed to overfill four of the six. Now I’ve got to get a syringe or something to get out the excess before I can charge the battery. :frowning:

On the plus side, I’ve managed (so far) not to burn myself with acid. :rolleyes:

12

The reason god made kick starters… :wink:

13

You know, I’ve had a couple of folks say to me, “Well, does it have a kickstart?..No?..Well, that’s a shame.”

Lesson learned. :slight_smile:

14

I’ll just point out that internal resistance is not measured in volts, the cell volt drops are measured in volts, and volt the drops themselves depend significantly upon the current passing through the source impedance. If you have a fast charger, you’ll find these volt drops are greater than when using a trickle charger.

What happens is that as the battery achieves full charge, the terminal voltage falls from the initial charge output, to that of the voltage of a fully charged battery and its one of thw ways intelligent charger units recognise when to switch to trickle, or maintenance charging rates.

It would depend upon the age of the machine as to wether the rectifier and the alternator are seperate parts, however nobody I know in the motor trade would describe a rectifier as a diode.

You mention that you had to top up the battery, if this is well down it will also cause the symptoms described.

15

Your OP sounded just like my story, so I share it with you and you can see if it helps:

When I bought my bike, the previous owner had installed two fog lights on the crash bars. I thought they were a great idea because they made me more visible. However, as a novice rider, I seemed to be having a lot of trouble taking off from a standstill. I thought I was having to rev it up way too much, but I just wrote if off an being inexperienced.

After a month or so, I got caught in a traffic jam and had to sit idling and creeping forward on the freeway for about an hour on a 95 degree day. The bike died and it was obvious that the battery was nearly dead. I let it cool down and checked the battery cables. They were okay and I managed to get the bike to start, barely. I got off the freeway and took the backroads home. The next day was Saturday, so I started investigating.

After charging the battery, I disconnected both fog lights, hooked a volt meter up to the wires of one the the lights: 13 Vdc. I plugged in the other light with the bike idling: 11.5 Vdc. I had to rev it back up to 3500 RPM to get the voltage back up to 13 Vdc. I figured that with both lights on, I was only charging the battery when I was going 65 MPH or faster. I took both lights off and have been happy ever since (until two weeks ago). I also noticed that the problems with RPM from a standing start went away.

The morale of the story is: some bike alternators are not meant to power a bunch of accessories, like additional lights.

Now, if someone can tell me why my starter is dragging when I first hit the switch. New battery and it still starts, it just worries me a little.

16

I was too busy griping to properly phrase the situation – sorry about that. The old battery was filled to the proper levels. I went out and bought a new battery last night, since it was only $27. It was dry upon purchase, so I had to fill it myself, at which point I ran into the issues I described above. Sorry for the miscommunication.

17

A motorcycle battery in general will not last as long as a car battery, more than a couple years is pushing it, apparently. I bought a battery-tender “smart” charger and a pigtail connector so I can have the battery fully charged all the time. Incidentally, you want to charge a new battery for a full 24 hours, at least this is standard for auto types.

18

With your username, Common Tater , I would have expected you to address the alternator issue.

19

Well - it’s far more likely the battery is at fault, esp. with a used bike. Speaking of batteries, there is an excellent website devoted to all things about batteries - lead-acid types most often found in auto and marine applications anyway - maintained by what can only be described as a fanatic - http://www.batteryfaq.org

20

Yes you are correct. However the voltage drop is a direct result of the resistance. If you sub in voltage drop for resistance in my post, my point still stands.

This is incorrect. As the battery charges the voltage output from the charger increases. When the voltage reaches a preset level the charger decides that the battery is charged and it switches to maintenance mode. At this point the voltage does not instantaneously fall to a fully charged 12.72 V but will stay higher than this due to surface charge.

I think we are with a hood/bonnet problem here. I see you location is in the UK. Over on this side of the pond in over 30 years in the business I have never heard or read anyone calling them anything except diodes.
Cite "the alternator output is fed into what are called diodes, which convert the AC into DC. "
Cite #2 Diagram labels them as diodes.