Adding batteries for longer EOC
 

I'm having some problems with battery voltage dropping way too low in my morning commute to work with headlights on. I have two extra starting batteries laying around in the garage, and figured that I could just wire them in parallel with the existing battery in the Paseo. This should increase capacity, decrease peukert effect during discharge and recharge, and generally extend the life of the batteries.

The downside of this is, these batteries are all different. None are in amazing condition, and are very unbalanced as far as a battery pack goes. How is linking them all in parallel going to effect the other batteries? Are the stronger ones going to basically charge the weaker one? Is the weaker one simply going to waste this charge since it can't hold it? I'm not sure how they'll effect each other and any insight would be great.

Find a regulated 15v wall wart and a diode or two to make a charger.

Before connecting the batteries directly in parallel, equalize charge by adding resistance.

Can you elaborate on why this is necessary and what might happen if I don't do it?

Another way to ask the question:

If you wire two batteries in parallel with somewhat different capacities, what happens as they are discharged?

Will the larger battery "support" the smaller one, or is the parallel setup limited by the capacity of the weakest battery?

(In case it's not obvious, I don't know the answer.)

Early concensus seems to be (link 1, link 2) that the stronger battery will charge the weaker one as they discharge.

But I can't vouch for the quality of the info on those sites.

So I take it you prefer to coast with the engine off and that is why your alternator does not produce enough electricity for the 12 starter battery to run your accessories. . . .

The technically correct answer is to have one 'high discharge battery,' with the high cranking amps needed to start the car and a second, deep discharge battery to handle your 12 VDC load. Each is optimized for their specific missions and trying to use a 'starter' battery for deep discharge just makes it fail that much sooner.

One exception might be the Odyssey battery. They appear to have done some serious engineering in their batteries and claim to have both starter and deep discharge capabilities. I put one in my Prius, $170, based upon avoidance of a vent tube and their reported specifications. They have an 8-10 year projected life.

Do you have a measurement of how much current is drawn when your engine is off? This pretty well sizes the type of deep discharge battery needed. Although splitting the 12 V system is not a trivial task, it makes it easier to build the battery system you need. The 12V starter battery gets charged from the 12V deep discharge system using a current limited, DC-to-DC converter and/or a second alternator.

Have you looked at after-market options to reduce your electrical load? For example, LED rear lights or discharge head lights? Are there any other electrical loads to remove?

As for trying to get more use out of marginal starter batteries ... GOOD LUCK!
When a cell fails in one of them, that will be pretty much it. The question being do you want that to happen on the road?

GOOD LUCK!
Bob Wilson

If you connect two batteries with significantly different states of charge and not enough series resistance, it would result in a large current flow as the voltages equalize.

To equalize them, just connect the batteries in parallel with jumper cables (they have enough resistance, especially the cheap ones) and connect a charger to the batteries.

I have looked into LEDs and what not. The point is that I have 3 starter batteries sitting around right now not doing anything. I'd like to use them unless there are going to be big problems with it. Come summer, they won't be needed as my headlights won't be needed. This is more of a temporary fix.

before adding multiple batteries you might want to look at replacing it with a single bigger one... of course that's an extra investment, but at some point a battery will have to be replaced anyway, so that might be a good time to look at it. what you want to look at is the Ampere/hours rating wikipedia give's a good explanation of this

"
Ampere-hours (A·h) is the product of the time that a battery can deliver a certain amount of current (in hours) times that current (in amperes), for a particular discharge period. This is one indication of the total amount of charge a battery is able to store and deliver at its rated voltage. This rating is rarely stated for automotive batteries.
"

batteries for diesel cars or bigger vehicles tend to be higher rated... the advantage is that although it might be slightly bigger it could still fit in the original location, and you don't add that much weight your car...plus you don't have to do any extra wiring...

the average car seems to be between 50 and 75, but i've seen big batteries for like a range rover with has over 100Ah... they do come at a price but... multiple batteries will also have to be replaced at some point... so the theoretical cost would be about the same, and the resulting setup would be more stable

on the other hand, while perhaps more involving you could look at updating the alternator to a more efficient on, so that it tops off the battery much quicker...