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Daimler 2.5 V8 Battery/Dynamo issues

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Location: Aberystwyth Wales

Re: Daimler 2.5 V8 Battery/Dynamo issues

Post by Sydsmith »

Just to clarify how a dynamo works in our cars.

The output from a dynamo in good condition will be more than 12 volts and is dependant on how fast the dynamo is turned by the engine, left to its self and run at speeds above idle for long periods, the output from a good dynamo will over charge and damage a battery and the dynamo, hence the need for a regulator.

The regulator controls the output from the dynamo so that the battery is not over charged and the dynamo does not overheat.

Because the regulator prevents the dynamo from over charging and the dynamo does not usually produce an output at idle speed, you are unlikely at idle speed to get a voltage reading at the battery much above the battery voltage when the engine is switched off. It will probably be lower because of the drain from the ignition system.

The most useful guide to dynamo health is the amp meter. As has been said, you should with the engine running at idle, and I am assuming around 800-1000 revs, probably see a discharge. When taken up to say 2000 revs, you should see a charge. The level of charge will depend on the state of charge of the battery (and the battery health). If the battery is low, the rate will be high, if the battery is fully charged then the charge rate in amps at 2000 revs will be low.

A good test is to start the engine, run it up to 2000 revs and check the charge rate, it should be in the positive but may only be small if as I said above the battery is fully charged, then keeping the revs up, turn on the headlights, the amp meter should flicker into the negative then quickly recover into balance or a small charge.

In simple terms the flicker indicates the regulator has kicked in, the charge, even if the amp meter is in the balanced position, indicates the dynamo is charging the battery.

The simple way to get an indication of the battery condition is to measure the voltage in a standing condition, with the engine off and nothing switched on on the car. It should be in excess of 12 volts, turn on the headlights without the engine running and measure again. A good battery should still show around 12 volts and hold it there with the voltage falling very slowly, a duff battery will show less and will very quickly drop in voltage.

A good dynamo will only provide sufficient charge for our cars in normal use, we used to say that if you start a car engine you need to dive for 10 minutes to replenish the battery. An Alternator will replenish a battery after a start in a fraction of that time because it will run up to a much higher charge rate with much lower revs, even at tick over an alternator will show some output, where as, as we have said, a dynamo needs several thousand revs to get cracking and because of the lower output, much more time to charge a low battery.

Best battery in my book is a Bosch, expensive but my SP250 battery is now 9 years old and going strong and the V8 battery is 10 years old. But keep them topped up with a GOOD QUALITY trickle charger in the winter. Syd

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Re: Daimler 2.5 V8 Battery/Dynamo issues

Post by Chris_R »

I bought a YUASA from Halfords in March 2017. The YUASA Silver comes with a 5 year warranty.
I have a better experience than Richard. If there is no current flow from the battery when the car is switched off it will maintain its charge for many many months with no issues. Anyway, if it fails before 5 years simply get it replaced and you get 2 for the price of 1!

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Re: Daimler 2.5 V8 Battery/Dynamo issues

Post by Kbeal »

Optimal gel cell batteries hold their charge but are expensive an modern looking

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Re: Daimler 2.5 V8 Battery/Dynamo issues

Post by AlanD »

After I posted the query I realised I had my battery charger on the "engine start" setting, rather than "high charge" setting so it didn't seem to be charging up. However I switched settings and it did become fully charged a bit later, so there is some life in it yet.

Many thanks to classiclife, Ian Slade and ShandyandLady for your useful advice. If I do need a new battery later I will probably go for a Bosch sealed unit with 4yrs warranty as suggested. Unfortunately there is no mains power in the barn where the car is kept, so the solar unit for trickle charging was a great idea. I've now purchased a Gunson Solar Battery Charger online for £24 which is arriving today. The barn is open at the front so there should be plenty of daylight for the solar unit to work.

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Joined: Sat Sep 21, 2019 1:24 pm
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Re: Daimler 2.5 V8 Battery/Dynamo issues

Post by AlanD »

I've just seen yesterday's postings. Thanks very much Sydsmith for your brilliant and detailed explanation of how a dynamo works; and Chris_R and Kbeal for great battery tips. I must say this forum is great - really impressed!

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