Two 12v in series or two 24v in parallel?

[scott harris]

I am designing a mobile off grid 24 volt system that has a fairly constant 1500 watt load that will run for 7 hours a day while the sun is shining (hopefully shining). There will be some times when the battery will need to supply almost the entire load, but only for 10-15 minutes, so massive amp hours is not important. I plan on using BB 100 amp hour batteries. Which is best, two 12s in series, or two 24s in parallel? or doesn't it matter?

 

[Craig]

I honestly do not think it matters. I would just use the 24v but that's just a personal preference. What configuration will the batteries be
12v 100 Ah ?
24V 50 Ah ?

 

[scott harris]

Thanks for the help. yes. 12v 100 Ah and 24V 50 Ah. I did not make that clear.

I was wondering if one configuration would be better at balancing, or better at flowing the 62.5 amps required by the load, or even charging a bit faster?

 

[Craig]

well with the 24V batteries you will have no problems balancing at all. with the 2 12v batteries balancing could become a problem but unlikely. Since they have a built in bms neither should get too high or too low . I just do not know exactly what happens if one BMS turns battery off while the other is still active.

 

[Max Bender]

Two 12v 100ah batteries wired in series will sustain a 1500 watt load for about an hour.

 

[Craig]

Two 12v 100ah batteries wired in series will sustain a 1500 watt load for about an hour.

Shouldn't it be 2400 watts both ways whats the difference?

 

[Max Bender]

Why is BB charging so much for their 24v 50ah battery? Doesn't it contain the same number of cells as their 12v 100aH battery just wired to make it a 24v 50ah battery? Plus there isn't that much of a cost difference between a 12v and a 24v BMS, is there?

 

[Max Bender]

Shouldn't it be 2400 watts both ways whats the difference?

The common denominator is 1500 watts. The variable is either two 12v 100ah batteries wired in series for 24 volts at 100 amps or two 24v 50ah batteries wired in parallel for 24 volts at 100 amps. The length of time that each battery configuration will sustain a 1500 watt load is the same - no difference. However, the question brought to the table turned out to not be one of volts and amps but rather a question balance and charge - that is, which configuration would best for the batteries in terms of longevity and efficiency. In that regard, there might not be a difference between which configuration is best if it's true that the battery doing most of the 'work' is the battery that's 'first in line'.

 

[nosys70]

if the battery capacity is close to the load, it means you will heavily drain your battery each time they are in use.
so parallel battery would be better, since the Amps will be split between batteries.

 

[Max Bender]

well with the 24V batteries you will have no problems balancing at all. with the 2 12v batteries balancing could become a problem but unlikely. Since they have a built in bms neither should get too high or too low . I just do not know exactly what happens if one BMS turns battery off while the other is still active.

If Scott used two 24v batteries and one turns off, could the remaining battery run his appliance at least for a little while?

 

[Supervstech]

I think the 12v is cheaper, because they sell more of them, consequently the bms gets a higher volume price.
24v makes more sense to me though.

 

[Craig]

If Scott used two 24v batteries and one turns off, could the remaining battery run his appliance at least for a little while?

I would think so but if they were in series it would break the series.

 

[Craig]

The common denominator is 1500 watts. The variable is either two 12v 100ah batteries wired in series for 24 volts at 100 amps or two 24v 50ah batteries wired in parallel for 24 volts at 100 amps. The length of time that each battery configuration will sustain a 1500 watt load is the same - no difference. However, the question brought to the table turned out to not be one of volts and amps but rather a question balance and charge - that is, which configuration would best for the batteries in terms of longevity and efficiency. In that regard, there might not be a difference between which configuration is best if it's true that the battery doing most of the 'work' is the battery that's 'first in line'.

Im still a little confused here both batteries are 100Ah at 24 volts in the end so why only 1500 watt hours for an hour why not 2400 watt hours in an hour?

 

[Bob142]

Max is referring to the OP's statement:

I am designing a mobile off grid 24 volt system that has a fairly constant 1500 watt load that will run for 7 hours a day while the sun is shining

 

[SCClockDr]

12V x 100AH = 1200 watt hours
24V x 50AH = 1200 watt hours
Watthours are the same.

 

[Supervstech]

12V x 100AH = 1200 watt hours
24V x 50AH = 1200 watt hours
Watthours are the same.

However, two 100Ah 12v batteries is 100Ah 24v

 

[Max Bender]

I would think so but if they were in series it would break the series.

You're absolutely correct. But we don't yet know if Scott will go for two 12v batteries or two 24v batteries.

 

[Max Bender]

I think the 12v is cheaper, because they sell more of them, consequently the bms gets a higher volume price.
24v makes more sense to me though.

That makes perfect sense. And in hindsight, I see that the difference in cost for two of either battery isn't that much - 'bout a hundred bucks is all.

 

[Max Bender]

Shouldn't it be 2400 watts both ways whats the difference?

No difference as far as duration goes with a 2400 load. What we're working with tho is the 1500 watt load of whatever appliance that Scott is using.

 

[Max Bender]

if the battery capacity is close to the load, it means you will heavily drain your battery each time they are in use.
so parallel battery would be better, since the Amps will be split between batteries.

You're absolutely right, and that would be something for Scott to consider if he's thinking about going with one 24v 50amp battery instead of two. Although he could get by with only one battery (and save $999), he'd have to stay very close to the 10-15 minute window of battery use. In that situation, an adjustable low voltage alarm would help.