Battery overcharging.

[Ian Wegg]

Have you tried it with the switch in the winter/ full charge position ?

Yes, I have and there is no difference between the two positions. i.e. since removing the wire, there is now no charge in either setting. 

I had thought that the Winter position might still give me a charge (albeit too much) but I guess this is indicative of something else awry.

[Dave Mann]

Ah that means:
1) either there is a wire missing between the dynamo and the switch panel or
2) there is a pair of rollers in the switch panel which are operated by moving the left switch to winter or the right switch to put the lights on      which close contacts between D and F to give winter charge. This suggests that the contacts are dirty or the rollers are missing.

[Tony Griffiths]

Hello David,
Might you have a link to the resistor you bought?

The resistor should be about 1.7 to 2 ohms. I got a suitable power resistor from ebay.

I now can't remember, but a similar resistor can be found on ebay. The seller is 'Bright Components' and  Part is described as '50 watt Aluminium Clad Power Resistor' The item number is 291418948986. They supply them in a range of resistances.

Thank you, David. That's appreciated.

[Ian Wegg]

The eBay link is: https://www.ebay.co.uk/itm/125417012347

...or you can save a few pennies on the Bright Components website at: https://store.brightcomponents.co.uk/pro...w-2r-100w/
(they're 1p cheaper and the postage is less).  I also have a 10% discount code valid until the end of the year: HK28FPC3

I ordered at the weekend and it arrived yesterday (Tuesday). I haven't fitted it yet.

BTW: I noticed from his post in 2019 ( Dynamo Field Resistor ) that David bought the 100W version so I've gone with that.

[John Cornforth]

Hi Ian

The 100 Watt rating for this type of metalclad resistor applies when it's bolted to a heatsink. If you choose to leave it sitting in free air the rating is more like 20 Watts. That should still be fine though, the reduced field current through it when it's in use will be roughly 1 amp and the dissipation will only be about 2 watts. This means the surface temperature won't be too high and you can mount it wherever it's convenient.

[gezparton]

I was led to believe that an ammeter shows the current being drawn from the electrical system.

[Robert Leigh]

The ammeter will show the nett current in to or out of the battery, not the current drawn. This will be dynamo output less whatever is being drawn by electrical equipment.

[Ian Wegg]

I was going to wait until I had time to do the job properly but curiosity got the better of me (concours judges and purists look away now!).


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With the 2 Ohm resistor temporarily in place, the ammeter is now reading a maximum of 4 amps at fast revs. At idling, it's going down to -2 amps and the ignition lamp lights up. I'm happy with that, at least there's no risk of my passenger getting a squirt of acid through the seat. I'll use the battery conditioner when garaged.


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I'll do the job properly (i.e. make it look less like a terrorist device) later. Once I've made the 3rd brush adjustments I might experiment with a 1.5 or 1 Ohm resistor if necessary.

Many thanks again for everyone's advice, it's much appreciated.

[John Cornforth]

Hi again

I have a traditional 6 volt battery with the usual small vent holes in the filler caps. To deal with the slight acidic fumes coming from these, I place a piece of "battery mat" on top. This stuff is claimed to neutralise acid, so presumably is impregnated with something alkaline. It's primarily intended to be placed under the battery to protect the mounting tray. Only downside is that it's bright yellow !

[AustinWood]

If you use a battery conditioner the battery will be fully charged when you set off. The 3-brush dynamo will then be very likely to overcharge it.
We use a battery conditioner but our dynamo doesn't work so no problem.