Battery overcharging.

[Ian Wegg]

Please excuse (another) novice question.

My battery appears to be overcharging. The +ve terminal becomes corroded very quickly and yesterday battery fluid appeared on the casing. With the engine running, the ammeter shows 12 amps.


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I'm assuming the voltage regulator is faulty. I can't see any adjustments on it, is this something easily fixable?


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Any hints on what to look for, or replacing the unit if necessary, would be appreciated. (The car is an AG tourer but most of the mechanicals are 1935 Ruby (I'm told)).

[Howard Wright]

Hi Ian

Yes that’s way too high an amp reading.  The ‘regulator’ you indicate is actually a cut out and doesn’t control the current going to the battery.  The current from the dynamo is controlled by the third brush on the commutator.

The easiest way of explaining this is to look at one of the many ‘how tos’ on the internet

https://www.austin7.org/Technical%20Arti...refresher/

Even one on this forum


https://www.austinsevenfriends.co.uk/for...p?tid=3710

Cheers

Howard

["Slack Alice" Simon]

That is a lot on half charge.

I suggest the resistor in the system has been shorted out.

The original resistor looks like a spring, and may be located in the base of the control box, or in a mounting on the dynamo.

On half charge, it is in series with the dynamo field coil, and so reduces the output.

The main control of the output is by moving the third brush on the dynamo, as I guess those articles will explain.

My point is, if you are getting 12 amps on half charge, the circuit needs to be investigated

Assuming you have a standard dynamo.

[Andy Bennett]

And of course that means battery acid is squirting/bubbling out of the fill caps. I had that once with a problem charge and acid literally squirted up about 18 inches through the small air hole in the cap. Pure luck I wasn't looking down on it at the time otherwise I would have had a face/eye full. So, apologies if stating the obvious but whatever you do don't get close when it's charging 12A.
Andy

[Dave Mann]

As Howard says that is far too high a charge rate, the maximum output of a C35 dynamo is 8 amps and continued use at such a high rate will burn out the armature necessitating a rewind. I assume that you have a C35 dynamo like this      for which this is the wiring diagram 
  Generator_type_4.jpg (Size: 27.64 KB / Downloads: 363) . The output is controlled by the third brush which is narrow than the other two and can be rotated in the direction of rotation to increase output and vice versa it is shown here in dismantled state      in the high charge position. It can be moved with the dynamo assembled in the car, if you are unsure of things find a knowledgeable owner to help.

[AustinWood]

The Austin dynamo does tend to overcharge the battery. A fully charged battery should be good for 400+ miles so it's better to have the charge low rather than high. Using headlights will reduce the range. We have LEDs.
Our dynamo doesn't work and we use a battery conditioner when the car is not in use so we always start with a fully charged battery.

[Ian Wegg]

Thank you, everyone, for the advice and the links. I see better now how it should work.

My dynamo is a type C35A.


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It doesn't have a box on top just the exposed terminals, which are shorted across with a wire? That's not something I've done, I would have thought that would have created quite a different problem but it has been working that way for 3 years!

Because the car is a "bitza" it's quite possible I have a mismatch of dynamo, cut out and/or dashboard control (I notice the charging rate doesn't seem to change between "Summer" and "Winter" settings).

Thanks again for the help. Please don't hold back on anything that might seem obvious to you as it probably won't to me, the squirting acid warning is greatly appreciated.

[Reckless Rat]

I would guess that you definitely need to back off the 3rd brush - you really don't need or want to be shoving that amount of current into the battery. For the summer/winter to function you need a resistor across those two terminals of about 3 ohms. (assuming the rest of the wiring is correct.) - I'd also treat myself to a new bottom hose before it disintegrates as well!

I run with the metal dust covering strip removed as it helps to cool the dynamo. You can also adjust the third brush easily.

[Ian Wegg]

- I'd also treat myself to a new bottom hose before it disintegrates as well!

Yep, I noticed that as soon as I posted the photograph - another Winter job. It's funny what you can spot in a picture that you hadn't seen when you took it -  but it is dark in the engine compartment!

[Dave Mann]

That wire linking the dynamo terminals is the cause of there being no difference between summer and winter charge, it is effectively on winter. If that wire is replaced by a resistor of about 3 ohms and a high wattage it will give you summer charge as in the wiring diagram I posted. That box on the dynamo contains the resistor and field fuse as shown in this photo      .