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Old 21-02-2015, 21:17   #16
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Re: Is it OK to combine different size batteries in the same bank?

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Originally Posted by wesevans View Post
NO! all the batteries in the same bank need to be the same size and type. If not the batteries will not recharge properly. Also if you have more than one bank it is best to charge them separately unless you have separate charging circuits.
WHY?

Sorry, Wesevans, you are another victim of this often repeated misconception. It is important for batteries in SERIES to be the same size and type and matched as close as possible but it does not apply to batteries in PARALLEL.

Do you have some scientific basis for the statement or did someone mis-advise you too and you feel obliged to pass it on?
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Old 21-02-2015, 22:13   #17
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Re: Is it OK to combine different size batteries in the same bank?

My understanding is that in charging the batteries, if mis matched, they will only charge to the holding of the smallest battery in the bank.
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Old 22-02-2015, 01:11   #18
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Re: Is it OK to combine different size batteries in the same bank?

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My understanding is that in charging the batteries, if mis matched, they will only charge to the holding of the smallest battery in the bank.
Yes, that misunderstanding carries over from SERIES charging where the same current flows through all the batteries. But the current when charging PARALLEL batteries does not divide 50/50 for two batteries, or 33.3/33.3/33.3 for 3 batteries. In fact each battery only takes current in proportion to its capacity.

In parallel they are all at the SAME voltage. If one gets to fully charged at say 14.2 volts, the other battery has to be at 14.2 volts too.

Think of it as filling a 100 gallon (amp hour) tank connected in PARALLEL at the bottom to a 10 gallon (amp hour) tank that have the same height (voltage). As you fill (charge) them they both remain at the same depth (voltage) until eventually both are full even though one needed 10 times as much CURRENT as the other.
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Old 22-02-2015, 05:59   #19
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Re: Is it OK to combine different size batteries in the same bank?

Depends.. The difference of the internal resistance of the batteries will ruin the battery bank. If the difference is minor and you have less duty cycles in the time you expect batteries to live then it doesn't matter so much. But be aware..
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Old 22-02-2015, 06:25   #20
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Re: Is it OK to combine different size batteries in the same bank?

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Depends.. The difference of the internal resistance of the batteries will ruin the battery bank. If the difference is minor and you have less duty cycles in the time you expect batteries to live then it doesn't matter so much. But be aware..
How can you justify that statement? How will it "ruin" the bank? Batteries don't even "know" that they are in parallel with another, they each respond individually to the voltage on its terminals and carry a load, or charge, at a level corresponding to their internal resistance. They don't get lazy or change capacity just because there happens to be another battery in parallel.

As any battery gets older its internal resistance increases. Corresponding to that its capacity decreases. Eventually you have to make a decision about when to replace it due to loss of capacity. But so long as it is contributing significant capacity you don't replace it because that capacity is less than others in parallel.

A legitimate criticism of batteries in parallel is it is more difficult to know which of the batteries is no longer supplying sufficient capacity to justify its weight and space so a means of isolating them for testing is good practice.
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Old 22-02-2015, 06:49   #21
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Re: Is it OK to combine different size batteries in the same bank?

RUDIMENTARY Visualization of parallel v series connection
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Old 22-02-2015, 07:17   #22
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Re: Is it OK to combine different size batteries in the same bank?

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RUDIMENTARY Visualization of parallel v series connection
That's pretty cool but not correct. The parallel drawing is correct but series is not.

When current passes through batteries in series (charging or discharging) it is an equal amount in ALL the batteries. It has to be because there are no exits, what goes in one end must come out the other since there is only one path.

So if you put 50 amps into 3 batteries in series for 2 hours, each battery adds 100 amp hours to its capacity. THIS is where matching of capacity is important because the 100 amp hour battery will become fully charged, the 245 amp hour battery will be only 41% charged and the 65 amp hour battery will be overcharged and gassing excessively as current is forced through it on its way to charging the other two.

The situation is even more critical on discharging.

The 65 amp hour battery will be fully discharged before the other two. When the remaining two continue to force current through the discharged battery you end up reversing the polarity on its weakest cell with -34 volts and rapidly destroying it.

By matching you "hope" they will all discharge uniformly so that they will all be fully discharged at the same time and no cells have enough charge left to destroy another. This NEVER happens and that is why you should never discharge batteries fully so the weakest cell in the chain is fully discharged. When the first cell in the chain is discharged on a 12 volt battery there is 10 volts left to destroy it. But when that happens on a 24 or 36 volt battery there is 22 or 34 volts still available to kill the weakest one.

The chance of matching all 18 cells in a 36 volt battery is much less likely so following the 50% maximum discharge rule to get maximum life is much more important for higher voltage batteries.
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Old 22-02-2015, 07:21   #23
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Re: Is it OK to combine different size batteries in the same bank?

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Originally Posted by Andina Marie View Post
That's pretty cool but not correct. The parallel drawing is correct but series is not.

When current passes through batteries in series (charging or discharging) it is an equal amount in ALL the batteries. It has to be because there are no exits, what goes in one end must come out the other since there is only one path.

So if you put 50 amps into 3 batteries in series for 2 hours, each battery adds 100 amp hours to its capacity. THIS is where matching of capacity is important because the 100 amp hour battery will become fully charged, the 245 amp hour battery will be only 41% charged and the 65 amp hour battery will be overcharged and gassing excessively as current is forced through it on its way to charging the other two.

The situation is even more critical on discharging.

The 65 amp hour battery will be fully discharged before the other two. When the remaining two continue to force current through the discharged battery you end up reversing the polarity on its weakest cell with -34 volts and rapidly destroying it.

By matching you "hope" they will all discharge uniformly so that they will all be fully discharged at the same time and no cells have enough charge left to destroy another. This NEVER happens and that is why you should never discharge batteries fully so the weakest cell in the chain is fully discharged. When the first cell in the chain is discharged on a 12 volt battery there is 10 volts left to destroy it. But when that happens on a 24 or 36 volt battery there is 22 or 34 volts still available to kill the weakest one.

The chance of matching all 18 cells in a 36 volt battery is much less likely so following the 50% maximum discharge rule to get maximum life is much more important for higher voltage batteries.

+1 Bingo!!!
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Old 22-02-2015, 09:42   #24
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Re: Is it OK to combine different size batteries in the same bank?

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Originally Posted by Andina Marie View Post
That's pretty cool but not correct. The parallel drawing is correct but series is not....................
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Old 22-02-2015, 11:21   #25
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Re: Is it OK to combine different size batteries in the same bank?

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Originally Posted by Andina Marie View Post
How can you justify that statement? How will it "ruin" the bank? Batteries don't even "know" that they are in parallel with another, they each respond individually to the voltage on its terminals and carry a load, or charge, at a level corresponding to their internal resistance. They don't get lazy or change capacity just because there happens to be another battery in parallel.

As any battery gets older its internal resistance increases. Corresponding to that its capacity decreases. Eventually you have to make a decision about when to replace it due to loss of capacity. But so long as it is contributing significant capacity you don't replace it because that capacity is less than others in parallel.

A legitimate criticism of batteries in parallel is it is more difficult to know which of the batteries is no longer supplying sufficient capacity to justify its weight and space so a means of isolating them for testing is good practice.
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