Out of curiosity, when I go to wire a pair of batteries in parallel, do you guys think it would be fine if I used one group 27 and one 24? I want to have a little more juice throughout the day, but I don't really want to go out and buy another group 27 right now if I don't need to.

Does not make a difference

The batteries need to be the capacity. You will over discharge the smaller battery and ruin it.

The batteries need to be the (same) capacity. You will over discharge the smaller battery and ruin it.

This is an incorrect assumption but very common. When batteries are in parallel you can't discharge one more than the other because they are at the same voltage = they are CONNECTED TOGETHER.

To discharge the small one more than the big one would mean it would have to be at a lower voltage but that can't happen.

Same when charging, you can't overcharge the small one without overcharging the big one. If the small one gets to full charge = full voltage, then the big one must be at the same voltage and also fully charged.

Glad Ann-Marie chimed in on this one  ;D!

Don't think I could have received a better answer than that.

Think I need to pick up some cable and wire mine together.  Hopefully I'll get to do some fishing in the morning.  Already redid my TM mount, now I just want some more power....

The batteries need to be the (same) capacity. You will over discharge the smaller battery and ruin it.

This is an incorrect assumption but very common. When batteries are in parallel you can't discharge one more than the other because they are at the same voltage = they are CONNECTED TOGETHER.

To discharge the small one more than the big one would mean it would have to be at a lower voltage but that can't happen.

Same when charging, you can't overcharge the small one without overcharging the big one. If the small one gets to full charge = full voltage, then the big one must be at the same voltage and also fully charged.

If you have two batteries in Parallel. One is 50 AH and one is 100 amp hours. Lets say you are drawing 50 amps of current. that is 25 amps from each battery. How long do you think the smaller battery will last? after two hours you have used up the smaller battery. But you will still try to draw 25 amps from it. I am not sure I would want to do that to a battery. Now every time you remove the load the batteries will equalize voltage and the bigger battery may even recharge the little battery. But, you will discharge the bigger battery even more.

as for charging the smaller ones voltage will rise faster possibly messing up the charger keeping the bigger battery from getting a full charge. or the bigger battery will keep the voltage lower by draining the little one keeping the voltage artificially low causing the little one to be over charged.

from optima

http://www.optimabatteries.com/product_support/faqs.php#misc

this website says the same

http://boatsafe.com/nauticalknowhow/marine_battery.htm

Do what you want I wouldn't do it.

Your basic assumption is wrong. You say

If you have two batteries in Parallel. One is 50 AH and one is 100 amp hours. Lets say you are drawing 50 amps of current. that is 25 amps from each battery. How long do you think the smaller battery will last?

But you won't draw 25 amps from each battery. You will draw 66.6% from the 100amp hour battery and 33.3% from the smaller one. This is because the amount of current is governed by ohms law, current = Voltage / Resistance. The voltages have to be the same because they are in parallel but the resistance of the small one will be TWICE the resistance of the large one so it will only supply half as much current. The resistance is inversely proportional to the surface area of plates in the battery and the 100 amp one will have HALF as much as the 50.

Same happens when charging, twice as much current will go into the 100amp-hour battery as will go into the 50 amp hour so the process is self regulating for charge and for discharge.

The reason some advise matching size and age is to get maximum CAPACITY out of your battery installation and maximum battery sales. If you have a 50 amp capacity that is brand new and a 100 amp capacity that is 50% dead, then your total capacity is only 100 amp-hours. But if you are content with the final capacity it is no reason to throw away an aging battery. If you need full capacity, then replace them frequently.

Also with batteries in parallel, if one dies it won't damage the other one but it will drag the charge and voltage down so you have minimal remaining capacity. If you want maximum time between failures then that is a reason to use matching batteries and replace them both as soon as they get old.

I prefer to get the maximum I can out of my batteries.

one word..... impressive. 

I took two batteries. one was a 12V 105AH, the other was a 12V 12AH battery. The load was both filaments an 1157 bulb.

Both batteries were fully charged. 12.6 volts for the big one and 12.4 for the little one. Terminal voltage of both batteries together was 12.6 volts. I hooked the bulb up to the big battery. The current draw was 2.5 amps. The current draw on just the little battery 2.5 amps. Hooked both batteries in Parallel. Surprise the current draw was 2.5 amps. measured the current coming from the big battery. it was 1.28~1.32 amp. Measured the current from the small battery the current was 1.22~1.26 amps.

to me that is the same current from batteries under load. Please feel free to experiment yourself.

I could not test the charging at this time.

IRRESPECTIVE OF THE THEORY, LOOK AT THE PRACTICAL SIDE.

1. A battery is fully charged at rest at about 12.8 volts. If your BIG battery and your SMALL battery are in parallel at 12.8 volts then both are fully charged.

2. You apply your lamp load to both batteries and pay no attention to how much current each supplies. Voltage starts to drop over time until it gets down to 12.2 volts.

3. DID the smaller one get discharged more than the big one? IF it was discharged MORE it would have a lower voltage.

4. BUT THEY ARE IN PARALLEL. This means they are at the SAME voltage. How are you going to discharge the smaller one (to a lower voltage) WITHOUT discharging the big one? You can't have the smaller one at a lower voltage (discharged more) than the big one too, they are tied together.

The same applies to charging.

When they are in parallel and get to 14.2 when a charger would cut back to maintenance stage, HOW could the smaller one be OVERCHARGED without overcharging the BIG ONE TOO? To over charge the small one its voltage would have to get above 14.2, but so would the big one and the charger will cut them both back to maintenance.

ATTAGIRL!!!

Great info.  Somebody needs to give Ann-Marie a Battery Guru title or something.