Alternator voltage drops gradually

A few weeks ago I noticed that the battery was not charging properly, and after testing the output from the alternator, it was about 11.5 volts so replaced the alternator with a second hand one. I tested the voltage and it was around 14.5 at idle, but after taking it for a test run the battery was flat. So there is me thinking I needed a new battery. But the battery is only a couple of years old. I also checked all the earth points under the engine bay and cleaned them just incase I had a bad earth and they all looked good, tight and clean.

Anyway tonight I went out and tested the voltage off the new (second hand) alternator and it started at 14.5 volts, but as the engine warmed up the voltage output slowly kept dropping until it steadied around 11.5 volts. By this time the battery was flat. I tested the alternator voltage at the battery as well as at the alternator as well.

So, any ideas as to why this could be the case? Could I just be unlucky with the second hand alternator I have fitted as having the same problem as the one it replaced? Is there anyway this could just be the battery that has had it (seems unlikely if the alternator is not pushing out the right voltage)?

It is an M13A engine BTW with no second battery or split charging circuit added, it's all standard.

Cheers

Can you substitute the battery? A damaged cell could potentially cause a similar issue. Did you try it at idle or with some revs?

Perhaps the old alternator did not do the battery any good and it's had it.

Kirkynut

I dont have a spare Jimny sized battery, so I might just bite the bullet and get one and see. Out of interest, is it possible to fit a higher output alternator like off a Vitara or something?

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Might be able to shed some light on this for you as I've just done an absolute pig of a charging fault on one of our bikes with the same symptoms. Would charge ok when cold but when it warmed up it stopped charging.
Alternator function was ok thank god cos it's an engine out on one of these to replace it. As was the regulator / rectifier
Turned out 1 of the 2 wires from the output side of the regulator / rectifier was the culprit namely the earth wire. The resistance of the wire was altering due to the amount of current. You could measure the voltage and watch it steadily drop at the battery.
So I'd be looking at the wiring output from your alternator or main earths and see if the resistance alters from when it's cold / hot

skywalker65 wrote: Might be able to shed some light on this for you as I've just done an absolute pig of a charging fault on one of our bikes with the same symptoms. Would charge ok when cold but when it warmed up it stopped charging.
Alternator function was ok thank god cos it's an engine out on one of these to replace it. As was the regulator / rectifier
Turned out 1 of the 2 wires from the output side of the regulator / rectifier was the culprit namely the earth wire. The resistance of the wire was altering due to the amount of current. You could measure the voltage and watch it steadily drop at the battery.
So I'd be looking at the wiring output from your alternator or main earths and see if the resistance alters from when it's cold / hot

That is very useful as I too have a similar problem.
How quickly should the charging system get a battery charged assuming a battery just about turns the engine over when it is a cold start?

skywalker65 wrote: Might be able to shed some light on this for you as I've just done an absolute pig of a charging fault on one of our bikes with the same symptoms. Would charge ok when cold but when it warmed up it stopped charging.
Alternator function was ok thank god cos it's an engine out on one of these to replace it. As was the regulator / rectifier
Turned out 1 of the 2 wires from the output side of the regulator / rectifier was the culprit namely the earth wire. The resistance of the wire was altering due to the amount of current. You could measure the voltage and watch it steadily drop at the battery.
So I'd be looking at the wiring output from your alternator or main earths and see if the resistance alters from when it's cold / hot

I had exactly the same fault on my 400 Honda, with exactly the same solution as you

The voltage at the regulator was spot on, but at the battery just kept falling.
I rewired from the regulator directly to the battery (live and earth while I was at it) to cure it.
I think it is just a corroded joint somewhere in the wiring harness.

GuardianAngel wrote: I dont have a spare Jimny sized battery, so I might just bite the bullet and get one and see. Out of interest, is it possible to fit a higher output alternator like off a Vitara or something?

Sent from my SM-T705 using Tapatalk

I've fitted a larger alternator to mine can't remember which suzuki it was for but I'll check later.

Edit: found it 2004 SUZUKI LIANA 1.6 LITRE PETROL DENSO ALTERNATOR 31400-80G1 . It was more powerful than the one I took out.

helijohn wrote:

skywalker65 wrote: Might be able to shed some light on this for you as I've just done an absolute pig of a charging fault on one of our bikes with the same symptoms. Would charge ok when cold but when it warmed up it stopped charging.
Alternator function was ok thank god cos it's an engine out on one of these to replace it. As was the regulator / rectifier
Turned out 1 of the 2 wires from the output side of the regulator / rectifier was the culprit namely the earth wire. The resistance of the wire was altering due to the amount of current. You could measure the voltage and watch it steadily drop at the battery.
So I'd be looking at the wiring output from your alternator or main earths and see if the resistance alters from when it's cold / hot

That is very useful as I too have a similar problem.
How quickly should the charging system get a battery charged assuming a battery just about turns the engine over when it is a cold start?

Bit difficult to say really. Depends on how much is being taken out of the battery to power consumables etc. Assuming the battery and charging system is ok then I'd say possibly half an hour or so of steady driving.
If my battery was that flat though I would personally charge it with a decent battery charger. Charging systems really only exist to keep everything topped up so to speak rather than recover almost flat batteries

skywalker65 wrote: How quickly should the charging system get a battery charged assuming a battery just about turns the engine over when it is a cold start?

Charging systems really only exist to keep everything topped up so to speak rather than recover almost flat batteries[/quote]

Yes that would be my thinking. My commute is only ten minutes or so and it definitely does not top up a flat battery.

It may be time to re-activate this topic as it is similar to what is happening to my charging system!

Have you seen this YouTube video from Ivan?