dynamo or alternator

[bmcecosse]

Worth looking at bad connections - and is anything draining the battery when the car is standing??

[morrisminormonkey]

Hi all and Ray.

Seems like it is a failing battery after all!

A neighbour of mine couldn't help but knock my door last week after seeing the moggy on the drive and asked about it, long story short, he put his battery from his MGB onto my car as they are the same (not sure they should be) and it started first time so seems the battery indeed doesn't have enough in it to supply cranking power.

He attempted to charge my battery to see if it was just flat from all our stop starting on the journey but said 4 of the cells bubbled the moment he switched it on which he thought a bad sign so looks like i'm in need of a battery, any advice welcome

thanks again for all your patience and advice

[Trickydicky]

Hi,

I fitted this to mine, http://www.burtonpower.com/dis-car-nect ... WbRf=GBase

When the car goes in the garage I take the brass screw out as a matter of course not just as a security feature but in case you leave a door open for example which would flatten the battery,after all its only a 37amp hour battery so does not take long to drain.
As mine does not get used daily it also helps prevent discharge over time, you only need to completly flatten a battery a couple of times and you will find it may damage it and not hold a charge.

[rayofleamington]

If a few of the cells give a LOT of gas when charging, then that really does sound like battery is at end of life.
There will normally be some slight gassing-off when charging, but on a standard car battery in good condition it will be only light and should be even on all cells.

The charge regulator (big black thing on bulkhead) for the dynamo is a bit crude compare to the more modern (sensitive) electronics in an alternator, but you should see up to 14 / ~14.5 volts at the battery when engine is running at higher revs.
The charge regulator is just a switch, and will turn off the charging if/when the voltage goes above the set point - therefore if the voltage is sometimes ~14ish* and sometimes lower (off charge), this is normal. The max charge voltage is set by the regulator and usually best left as it was made - however if it is over 15v then that's not ok. 18v and over will kill a battery fairly quickly, and you end up with the same symptoms as a flat battery.
*For a dynamo, the actual number will depend on how much power the dynamo can deliver at that engine speed and how charged the battery is.

For an alternator the charge voltage will be constant** and almost all are set between at 13.5v to 14.5v and you should not see more than ~0.1v fluctuation on the volt meter (**except a few limited cases such as when battery is flat and engine is at idle speed, the alternator will be charging the battery as much as it can but may not have enough power available to push the voltage up to the set point)

A good battery will show 12.5 to 12.8 volts when left standing for a couple of hours. A good and poor battery can both show higher voltage if they have just been disconnected from charger, so that can easily confuse people.
A good battery that needs re-charging will show less than 12.5, but so will an old battery that is failing..

Therefore in this case your volt meter is only going to tell you if the charge circuit is working when engine is running (running well above idle speed to test a dynamo). Checking that this is ok is a really useful thing to do!

To do a good test on your battery, leave it on charge 10 hours or more, then it needs a 'high-load tester'. If you're lucky, a local 'back-street' battery supplier or garage might do that for you. This test shows what happens to the battery voltage when delivering a lot of current - on a poor battery, the voltage drops a lot more than a good battery.

If you can't get someone to do the test (and vehicle charging circuit seems ok, charging voltage is ok etc.), then I'd be inclined just to go for another battery anyway.