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Approximately. It all depends on the actual battery voltage. If the batteries are nominally the same you will supply 1/2 the current from each battery, thus increasing the battery life or charge by 2.
 

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Professor of makin stuff
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Discussion Starter · #7 ·
i am building nothing im just checking my theory against u guys i was darn sure i was right but a few guys at work insist that im on crack lol so yes i set up a thing to prove my theory is right to them on monday now ill leave it up to u guys to decide weather these guys work above or below me lol
 

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Nathan,

You absolutely can operate batteries in parallel. As has already been said this increases the run-time capacity (i.e. amp hours) available. For example: If you parallel two 110 amp-hour batteries, you end up effectively with 220 amp-hours of capacity.

There are caveats however. The batteries should all be the same size (in amp-hours), the same type (AGM, Wet, Gel, etc...), and be at the same point in their useful life-cycle. They absolutely must be the same voltage, but that should go without saying. But is it best if from the onset the batteries are new, of the same model, and from the same manufacturer lot. This is about the closest that the average person can get to a set of matched batteries so that they will all charge and discharge equally.

If size, type, or age are different then things get complicated. One battery might get worked harder than another, or one might get over charged. In either case its life will get shortened. Beyond that safety then becomes a big concern.

When operating in parallel like this it is wise to test each battery individually periodically. Should a cell go bad in one battery it not only could prevent the combination of batteries from being charged to capacity, it could become a safety concern that results in overcharging, overheating and discharge of gasious acid, pehaps even an explosion. The more likely you are to try and get maximum life out of each individual battery the more likely this is to happen. This is where charging the batteries separately (rather than in parallel) really becomes beneficial to you. If each battery is charged separately and tests good each time the battery is charged, you virtually eliminate the risk of the overcharge safety scenario caused by the bad-apple in the bushel. After the charge and if the battery tests good you can wire it back in for parallel load service.

You can lower the risks by charging the batteries separately (when a fast or bulk charge is necessary) but use a trickle/maintenance charger when actually wired in parallel. The trickle charger limits the amount of current that can be turned into heat if a cell does go bad.

Another thing to keep in mind is wiring the batteries together. Generally speaking, when bussing the batteries together, the gauge of the wire used to buss them should be much larger than the gauge of the cable used to pull load from the battery, perhaps even 4-times the size. This helps to ensure that when a load is pulled from the batteries all batteries contribute equally.

Hope this helps.
 

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Nother reason for bigger wire between batteries is if one battery is weaker than the other, the better charged battery will try to equalize the two. That is, try to charge the other battery to the better batteries state of charge.
The current that flows from the better charged to the weaker charged is called circulating current. Any type of mismatch between batteries results in a circulating current.
Many years ago, when pickups had campers in the bed, a dual battery system was usually installed. To interrupt this curculating current, a diode package was also installed so that the two batteries were electrically separate (in the eyes of current), but still supplied the 12 vdc.
In large over the road trucks, many times two batteries are run in parallel to supply the needed starting current. Many older (antique) trucks ran dual 6v. batteries for starting. The restoreres may have replaced the dual 6v. with a larger current 6v. In the case of dual 12's, replaced with one big (and heavy) 12v. Heavy means more money and physical space.
 

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I'd say to make sure you word the bet very carefully. As mentioned, any small variable will make the draw reading on each battery be not exact, even the connections which will always have some slight amount of resistance. That being said, a good example of your theory is diesel pickups. They use 2 batterys to double the reserve capacity, probably mostly because it is easier than fitting one large battery.
 

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Modern travel trailers use two 12-volt batteries wired in parallel. They work well for the loads encountered with lighting and constant drain and recharge. Two batteries last longer between recharges.

To add to the interest here - Full time RV'ers often replace the 12-volt batteries with two 6-volt wired in series to make 12 volts. the 6-volt batteries provide longer storage capacity than the 12's. I am not enough of an electrical engineer to tell you why, but I'm sure any old feller in an RV park who has them could provide you with endless discussion of the merits of 6-volt batteries. :thThumbsU
 

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Make Smoke, Boil Water!
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If your batteries are theoretically the same, then your load will be split between them. However, because of differences between batteries, one will always take a greater proportion of the load; all of this is covered above...

So since we're swerving off topic a bit, I might as well turn the wheel a bit further...

You asked about 6-volt batteries, perhaps I can help a bit. I'll simplify this as much as I can (but it won't be short) and I know someone else will jump in if I miss something.

It all goes back to battery design. The plates have to be a certain thickness to have charge-holding capacity. When you charge the battery, the plates give up a certain amount of their thickness to form a lead-based scabby material which builds up on the plates. When you discharge it, that scabby material comes off and falls down to the bottom of the battery case.

So now you have only so much space where a battery has to fit and it has to satisfy a certain power capacity. So you build it with just enough 'beef' in the plates to do that. But because of the space the top of the case takes up with a 12-volt, you can only make the plates so thick - and they have to be kept apart, otherwise they'll short out to each other when the battery is charged (when the scabby stuff builds up).

But a 6-volt battery has lots of space inside for beefy plates, because it only has to fit half as many cells into the same physical space. Plus you can make them really stout inside, by adding lots of extra bracing. So you have the advantage of lots of power capacity plus ruggedness in the same battery.

Now if you make the battery extra tall, you can have lots of space in the bottom for the scabby stuff to fall into, and remain harmless. This, plus big chunky plates, and lots of bracing, and you have a golf-cart battery. Use a little less of everything and you have a 6-volt RV battery.

What typically kills a battery is to allow it to be discharged too deeply, thereby knocking all the scabby stuff off the plates; and once that's gone, if discharge continues, a fuzzy sulfate material begins to build up on the plates, which prevents the plates from forming new scabby stuff. An unrecoverable battery with this condition is called 'sulfated'.

Also, a battery cell can be shorted out if the scabby stuff builds up so deeply in the bottom of the battery that it shorts across the plates. The battery then self-discharges (how fast depends on how bad the cell is shorted).
 

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Steamguy, I can relate to your terminology - "scabby stuff" and "fuzzy sulfate material", plus your very understandable descriptions. Thanks for the explanation.
 

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Modern travel trailers use two 12-volt batteries wired in parallel. They work well for the loads encountered with lighting and constant drain and recharge. Two batteries last longer between recharges.

To add to the interest here - Full time RV'ers often replace the 12-volt batteries with two 6-volt wired in series to make 12 volts. the 6-volt batteries provide longer storage capacity than the 12's. I am not enough of an electrical engineer to tell you why, but I'm sure any old feller in an RV park who has them could provide you with endless discussion of the merits of 6-volt batteries. :thThumbsU
The two batteries last longer that one 12v battery because there is more lead in the two 6v batteries that the 12v one.

A good rule of thumb: The heavier the battery, the higher the CCA rating, and the longer it will take to discharge.

Great thread guys.:thThumbsU:thThumbsU

Now, I went the cheaper route and replaced my six 6v batteries in my golf cart to three 12v ones. I'm only running the cart at the campground on the weekends and I have yet to use a full charge on just one weekend of driving around. However, I found that the charge light stays full longer and then when the level starts to drop, it discharges pretty quickly. We had it running for one season like this with minimal problems. The campground is small, so the cart won't completely be discharged when we return to charge it.
 

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It all goes back to battery design. The plates have to be a certain thickness to have charge-holding capacity. When you charge the battery, the plates give up a certain amount of their thickness to form a lead-based scabby material which builds up on the plates. When you discharge it, that scabby material comes off and falls down to the bottom of the battery case.

So now you have only so much space where a battery has to fit and it has to satisfy a certain power capacity. So you build it with just enough 'beef' in the plates to do that. But because of the space the top of the case takes up with a 12-volt, you can only make the plates so thick - and they have to be kept apart, otherwise they'll short out to each other when the battery is charged (when the scabby stuff builds up).

But a 6-volt battery has lots of space inside for beefy plates, because it only has to fit half as many cells into the same physical space. Plus you can make them really stout inside, by adding lots of extra bracing. So you have the advantage of lots of power capacity plus ruggedness in the same battery.

Now if you make the battery extra tall, you can have lots of space in the bottom for the scabby stuff to fall into, and remain harmless. This, plus big chunky plates, and lots of bracing, and you have a golf-cart battery. Use a little less of everything and you have a 6-volt RV battery.

What typically kills a battery is to allow it to be discharged too deeply, thereby knocking all the scabby stuff off the plates; and once that's gone, if discharge continues, a fuzzy sulfate material begins to build up on the plates, which prevents the plates from forming new scabby stuff. An unrecoverable battery with this condition is called 'sulfated'.

Also, a battery cell can be shorted out if the scabby stuff builds up so deeply in the bottom of the battery that it shorts across the plates. The battery then self-discharges (how fast depends on how bad the cell is shorted).
That,s mostly true, but nowadays ( the past 10 years or so ) the positive plates in the golf cart batteries ( and some other types too ) use "envelope" seperators to keep the "scabby stuff" contained. This mostly eliminates the shorting out of the cells that was the leading cause of battery failure. :fing32:
 

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I ran dual batteries for years in a 69 AMC AMX. Had to have them for hot starts as the single just could not crank the engine over. Lesson to be learned, measure your crankshaft throw before installing. 13:1 compression is bad. 12:1 isn't much better on stock starters.
 

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All bets are off if you hook up the positive terminal to the negative terminal on the 2 batteries. POOF!

Whatcha making Nathan??


Not always. If you hook one pos on one batt to the neg on the other batt, you can then use the other pos an neg to power something that is 24 volts
 
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