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It is a good and fair question that gets asked a lot, perhaps I should post a canned answer for a Sticky sometime.
Short story is there is more that can go wrong with parallel arrangements than serial arrangements. However I need to go over a few problems.
With a parallel arrangement you cannot equalize load or charge currents for a few reasons:
Internal battery resistance varies.
Wire lengths vary.
Connection resistance and workmanship varies.
What this does is cause one string in the parallel to do most of the work and causes significant wear and tear on the string, until the point it forces the other string to pick up the slack and cause premature failure on the second string. Most battery failures, not all result as high resistance or open circuit. For average Joe Home owner an open circuit is impossible to detect until both string have failed. The only way to detect it is with a hydrometer or well documented battery logs keeping track of individual cell voltages. I will bet you money in your biz you use 12 volt sealed batteries in which you cannot even take Specific Gravity or individual cell voltage measurements.
Second problem which is less frequent, but a real problem with sealed batteries is a shorted out cell. In a series arrangement is not much of a problem, as both load and charge currents easily pass through a shorted cell. It is also very easy to detect by the voltage check of loosing 2 volts, but still operational. However in a parallel arrangement, a shorted cell in one string is a major problem likely to cause a fire.
In a series arrangment load and charge currents are always equal no matter what. The worse thing that can happen and it will happen is a cell with go high resistance or open circuit like all batteries do.
So series affords the longest possible battery life, safer, and the lessor of the two evils.
Not to say parallel arrangments do not have their place because they do. I am forced to do it all the time with 48 volt 40,000 AH telephone battery plants. I have to use parallel arrangements because about th elargest 2 volt battery that can be had is 4000 AH, so it takes 10 parallel strings to get 40,000 AH. Bu tI use extreme measures like a custom built over head copper buss deisnged to math raesistance and use a lot of special equipment to balance all the resistances well below 15 micro-ohms. That is not possible for Joe Home Owner or Alarm contractors.
Bottom line is batteries are available from just a few AH like Alarm and Fire Panels all the way up to 4000 AH in about any increment you need. It is just smart buisiness practice to use best practice and in this case is also the least expensive. If you are going to take this thing solar, you are going to need a lot larger battery than the alarm cabinet can hold and will have to have an external box mounted. Two 100 AH 12 volt batteries are more expensive than 1 200 AH 12 volt battery.
Once you figure out exactly what the requirements are, come back and I will help you select components.
Short story is there is more that can go wrong with parallel arrangements than serial arrangements. However I need to go over a few problems.
With a parallel arrangement you cannot equalize load or charge currents for a few reasons:
Internal battery resistance varies.
Wire lengths vary.
Connection resistance and workmanship varies.
What this does is cause one string in the parallel to do most of the work and causes significant wear and tear on the string, until the point it forces the other string to pick up the slack and cause premature failure on the second string. Most battery failures, not all result as high resistance or open circuit. For average Joe Home owner an open circuit is impossible to detect until both string have failed. The only way to detect it is with a hydrometer or well documented battery logs keeping track of individual cell voltages. I will bet you money in your biz you use 12 volt sealed batteries in which you cannot even take Specific Gravity or individual cell voltage measurements.
Second problem which is less frequent, but a real problem with sealed batteries is a shorted out cell. In a series arrangement is not much of a problem, as both load and charge currents easily pass through a shorted cell. It is also very easy to detect by the voltage check of loosing 2 volts, but still operational. However in a parallel arrangement, a shorted cell in one string is a major problem likely to cause a fire.
In a series arrangment load and charge currents are always equal no matter what. The worse thing that can happen and it will happen is a cell with go high resistance or open circuit like all batteries do.
So series affords the longest possible battery life, safer, and the lessor of the two evils.
Not to say parallel arrangments do not have their place because they do. I am forced to do it all the time with 48 volt 40,000 AH telephone battery plants. I have to use parallel arrangements because about th elargest 2 volt battery that can be had is 4000 AH, so it takes 10 parallel strings to get 40,000 AH. Bu tI use extreme measures like a custom built over head copper buss deisnged to math raesistance and use a lot of special equipment to balance all the resistances well below 15 micro-ohms. That is not possible for Joe Home Owner or Alarm contractors.
Bottom line is batteries are available from just a few AH like Alarm and Fire Panels all the way up to 4000 AH in about any increment you need. It is just smart buisiness practice to use best practice and in this case is also the least expensive. If you are going to take this thing solar, you are going to need a lot larger battery than the alarm cabinet can hold and will have to have an external box mounted. Two 100 AH 12 volt batteries are more expensive than 1 200 AH 12 volt battery.
Once you figure out exactly what the requirements are, come back and I will help you select components.
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