On the subject of car batteries...

Jonny69

Jonny69

Jonny69

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Who knows anything about the science of car batteries, like how they hold charge, voltage etc? I've heard in the past if you put a high output alternator on a battery that's too small you'll overcharge it, or at least charge it too fast and damage it. On the flipside I've heard that if you use a battery that's too big for your charging system you'll also damage the battery.

Which puts me in my current position (see what I did there) which is that I have a car with a dynamo and I would argue maybe the battery is too big for it. The problem is the dynamo will rarely fully recharge it if I'm doing a lot of stop-starting or if the lights are on and it eventually goes flat, but I'm wondering if it's because the battery is too big. The battery spends most of its life partially charged.

What happens when a lead acid is only partially charged? Does the voltage drop right off or does it hold full voltage until right near the end? Would I have been better off with a much smaller battery which although doesn't hold as much reserve would always get fully charged and therefore have the voltage to crank?

I think ultimately I'll have to swap it to an alternator like I normally do on my old motors, but that's a way off yet.
 
What sort of electrical equipment does the vehicle have? What is the A/H output of the dynamo?

With that info we can look at the current draw of the car and the charging rate of the dynamo and choose a suitable battery. It may be better to go for a smaller car battery or a bike battery if the electrical load is low (lights, horn and stero, spark and some relays if the car is on carbs).
 
Charging too small a battery with a high current source (e.g. alternator) is quite likely to damage it, the plates can warp from the heat generated.

Charging a large battery from a low current source will not damage the battery or the source, it will simply take longer to reach a fully charged state from a discharged state. Taken to extremes the battery will never charge as there is always some internal leakage current, but this should never be an issue in a car.

If the large battery in your car never gets to a fully charged state, i.e. the dynamo is incapable of putting back what has been taken out, then fitting a smaller battery will make things worse. Also note that a larger battery will typically have a lower internal resistance i.e. a higher CCA rating. This is exactly what you need to start a big engine in cold weather.

As an interim measure would it be possible to plug in a battery conditioner overnight to help maintain the charge on the battery?
 
Ok, so it looks like the size of the battery won't make any difference but it will get killed by the dynamo not charging it properly, sitting discharged a lot of the time.

Dureth, it's a 4 litre flathead V8 with a monster of a vintage starter motor, so it needs quite a lot to crank it compared to a modern motor. No electrical equipment to speak of except for the lights which are 55W halogens. Charging system is a Bosch dynamo and control box which is off at idle and puts out about 10A when running along at 30mph in 4th. At revs it puts out significantly more, but most it's used is in town so it doesn't get a good chance to charge back up :(

Alternator conversion in the new year is in order basically. It'll upset the purists but that's too bad.
 
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