cooking batteries

[Tatenlyle]

Hello, and thank you for accepting me into your group. I have been trying to start my box Seven, a 1933 RP. Battery was flat, charged it, would hardly turn her over. Tested at 5.9v. charged it again, all the red lights came on the charger, and wouldn't fully charge, (4amp charger). Took the top off, and the plates are swollen and buckled. Its an Exide, 8 or 9 months old. The old one also has buckled plates, again 5,9v. on my dynamo, D and F are linked, which may not be the cause. Can anyone please give me a clue where to start, before I cripple another battery

[Chris KC]

I may be imagining this but I don't think batteries are lasting as long as they used to - something to do with the squeeze on heavy metals I wouldn't wonder. Keeping it on a trickle charger is the only preventive measure I'm aware of.

[andrew34ruby]

Your dynamo will not work correctly with D and F linked. It might overcharge, or it might not charge at all if the dynamo is faulty.

Start with a good battery, then check the charging before you destroy the new battery. Overcharging won't destroy the battery in minutes, you will have plenty of time to sort out the charging.

[eddiefarrell]

I am not too familiar with the Austin 7 charging system being a new person to acquire one but i remember from my apprentice days on 12 dynamo cars, you could test a dynamo by removing the wiring from the dynamo, joining the F and D terminals and starting the car, using a volt meter on that bridged wire and earth, you could get up to 90 volts by revving the engine. This was used for test only. The regulator joins the two connections until the voltage reached 14 volts and then kicked out but came back in and out to hold 14 volts. I suspect the 6 volt system regulator would cut the link at about 7 volts, so you will cook batteries and blow bulbs as well as eventually burning out the dynamo by leaving the link on.

[Tatenlyle]

Thanks for your replies. I will remove the link firstly, then check the voltage output at charging revs, I understand this should be 7.2v

["Slack Alice" Simon]

The battery acts as a voltage limiter.

If it isn't in the circuit, you will get lots of volts.

If you get lots of volts, the dynamo is at least working a bit.

It sounds like your battery is no good, so you need another one anyway.

Wire everything up as it is supposed to be,  and start your diagnosing from that point.

[Stuart Giles]

With the D and F linked on the dynamo, it will give full charge even when you set "summer half" on your dashboard; the "half charge" setting should switch a resistor into the field connection. Charging at 9 or 10 amps won't do the battery any good once it's fully charged.

[Tatenlyle]

With the link, charges 2a, 2a discharge when ticking over. I removed the link and replaced the wire from F to the dynamo. Again a 2a discharge and 2a charge when revved. Not running, , it shows continuation between D and F, and from D or F and any terminal on the regulator, and earth. I couldn't get a voltage reading from the dynamo, my multi went wild when the probes approached it. Funny, but the voltage reading at the battery terminals when running was only 0.7v.

[John Cornforth]

Hi Tatenlyle

When the engine is running, Austin Sevens have one of the most "noisy" electrical systems known to man.  Digital instruments, even quite good ones, can be upset by all this noise and read nonsense.  One way round this is to use an old fashioned analogue (moving pointer) instrument.

Linking D to F on the generator puts it permanently into "Full Charge" mode.  Not the end of the world for an hour or so but not recommended if you are going to drive around all day. It sounds like there may be a fault somewhere and the link was added in a vain attempt to counteract it.

There is no dynamo voltage regulator, the charging arrangement simply relies on the battery to control voltage, so the voltage will vary a bit (Typically 6 to 7 volts) depending on the state of the battery and the current going in or out of it. When working properly (no lights on) I'd expect the charge current to be typically 8 amps on Full/Winter and 4 Amps on Half/Summer when motoring above 25 mph.

As others have commented, modern batteries do not last very well for some reason.  Leaving them discharged for more than a short time will kill them off.  I leave mine on a maintenance charger at 6.9 Volts when not using the car, and keep an eye on the acid level.

Cheers, John.

[Tatenlyle]

Thanks John, I'm removing the link and reconnect the F wire to the dynamo, then keep an eye on things