Rods 'n' Sods - UK Hot Rod & Street Rod Forums banner
1 - 5 of 5 Posts

·
Registered
Joined
·
267 Posts
Discussion Starter · #1 ·
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.
 

·
I'm Not Jed Clampett
Joined
·
2,725 Posts
Well I did a lot of stuff about lead acid battery science at college; however, as that was an awfully long time ago I've forgotten almost all of it.

But, as all the cells in a lead acid battery are charged the coating on the lead plates changes its chemical composition. The coating continues to build and the SG (Specific Gravity) of the suphuric acid electrolyte increases as the battery reaches full charge. The coating changes back again as the cells in the battery are discharged, the SG of the electrolyte also falls again.

Can't see the issue with a big battery and a small alternator except that the battery will take longer to bring to full charge, batteries will become damaged if left completely flat for a long period of time because a chemical change occurs to the lead plates which prevents the surfaces' chemical change to the charged state.

Can't really see a damage issue with a small battery if your regulator is correctly set, ultimately this is what sets the charge rate whether you have a dynamo or an alternator. Batteries will be damaged by overcharging, this is because the particles of the charged plate surface (lead peroxide IIRC) vibrate off, then the particles fall to the bottom of the cell and ultimately short the positive and negative plates out.

From a practical point of view, unless you have loads of extra electrical equipment on your ride, The dynamo output should be enough to maintain an already charged battery in a fully charged state. I would check the output of the regulator, it should be around 14V, Most of the later small Lucas dynamos (if that's what you have) push out a maximum of around 22A, but it will only do this if the voltage regulator is correctly adjusted. This is easy if you have the earlier two coil reg. You need a special tool to adjust the later three coil regulators.

HTH
 

·
Registered
Joined
·
267 Posts
Discussion Starter · #3 ·
I think I might have answered my own question on another forum. They said the same Stu, it won't damage it but it won't charge it up fully. Someone asked the question what the charging system was and I mentioned that it only put out 10A at about 30mph. On the flathead it simply doesn't get revved much unless I'm on the motorway, so in this weather when I'm using the lights it's just not going to get charged.

I could put a smaller pulley on the dynamo but I can't see it lasting long with the added revs.

So I think the only answer is to swap to an alternator really.
 

·
Registered
Joined
·
140 Posts
Here's an easy way to think about it. The battery is only there to start the car, once it is running the dynamo or alternator provides the power. if you tot up what the total electrical load is, then the alternator needs to be able to provide at least that at idle. if not, the battery will make up the shortfall, but will eventually become flat, normally the next time you try to start the car.
 

·
Registered
Joined
·
727 Posts
Jonny,Jonny,jonny Thers not only 1 answer ( putting an alt....alt..altenator thingy on it) You gotta remember buddy the Flathead was used as a daily engine for many a year and the dynamo and regy box also served duty on daily driven stuff since Pontchus Pilot was an air caddet, both reliably. My Consul pickup has a 2l pinto with an alterthing on it but only cus it was hung off the side when I ploped it in and worked. If it was buggered I'd of slaped the dynamo of the Consul engine on it, I shit you not. That dynamo was on the Consul when I got it 15 years ago never let me down. only changed the engine cus a pinston went up the bore sideways.
I've allways driven old stuff as daily cars curently a Moggy with mild A series + dynamo, the original dynamo did go south on me once but managed to make it charge again with some match sticks to get me home 150 miles. Try that with an alterminator!
My Flathead T used a lucas 12 volt dynamo and mini battery worked tip top.
If you start sliding down the slipery slope of puting a a a thing on your car before you know it there will be radials and disk brakes and seat belts and juke boxs in churches and girles wearing skirts half way up ther arses.
Any way I think I've let you have my opinion I've got to rap the kids pressies (a stick and a hoop) they had a ball in a cup for birthdays.
Happy Christmas
Martin.
 
1 - 5 of 5 Posts
Top