need help figuring out Ah and watts of battery

wattwatt

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I picked up a handful of small 12V lead acid batteries (CSB HR 1221FW F2)and I'm having trouble figuring out the specs, namely the Ah and watts, so I can figure out what project they can be used in. The spec sheet says 12V (6 cells),21W @15min-rate to 1.67V per cell, 60/90A (5 sec) max discharge current, 249A short circuit current and 2.1 max charge current.

  • The "21W @15min-rate to 1.67V per cell" confuses me...does this mean each cell is 1.75 Ah (21W divided by 12V)? If so, then each battery is 10.5 Ah (1.75 Ah x 6 cells)? If not, how do I properly break down that verbage to get Ah?
  • When 8 of these batteries are in paralleldoes that mean this battery pack's maxdischarge current/rate is480A continuous,720A burst (5 sec)and the maxcharge current/rate is 16.8A???
 

Crimp Daddy

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From fully charged to 10v discharged, its about a 4-5 amp hour at most depending on the discharge rate

The 15 min rate is 12.5 amps per the spec sheet...
The 60 min rate is 4 amps...
 

DarkRaven

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http://www.makopower.com/site/assets/files/1123/hr_1221w_f2.pdf
Is this the specsheet you are referring to?

Specs of lead acid batteries are often given like this, it requires a bit more thinking if you aren't used to it. This is a C-rate, the concept is the same as it is with lithium batteries. The standard for lead acid batteries is usually C/20 which means the battery has the rated capacity at a discharge current which discharges the battery in 20 hours. 15 minutes means it's 4C, which is rather a lot for lead acid batteries, so this is a high drain discharge. This is likely done because of the intended use case for the battery, this looks like a UPS battery so a C/20 rating would be worthless. However, I think this is a mistake or I don't fully understand it as well. Have a look at the CC and CP tables on the bottom of page 1. It will give you a better understanding of the capacity in relation to the discharging current or power, i.e. usable capacity.

If you want a classic rating in Ah, this is probably close to a 4.5Ah battery. So six 4.5Ah cells in series. Capacity of cells isn't added in series.

For a parallel setup with eight of these you just multiply their specs by eight, except voltage.
 

LEDSchlucker

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In the spec sheet you linked is written 4.02A for 60min which is 4 amps for one hour (4Ah).

Ah doesn't depend on how many cells in series, that's only when you have cells in parallel.
 

DarkRaven

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We got it by looking at the CC discharge table. The problem is that this isn't rated capacity, this is usable capacity for given discharge scenarios. This is normal practice for lead acid batteries.
Look at the 1.67V row: 2.71A for 90min (1.5h) is 4.065Ah, 4.00A for 60min (1h) is 4Ah, 7.18A for 30min (0.5h) is 3.59Ah and so on. You see that the usable capacity gets lower while the discharge current is increasing, this is normal obviously. At the far end you get 43.1A for 2min (0.03h) which means only 1.436Ah of usable capacity.

The highest "stated" capacity is 2.83A down to 1.6V in 90min which means 4.245Ah.

And all these capacities are obviously for each cell and each battery alike. The difference is the stored energy which gets added up. A single 2V 4.5Ah cell stores 9Wh. Six 2V 4.5Ah cells in series gives a 12V 4.5Ah battery which stores 54Wh. And if you take this number and look at the CP discharge table you'll find 48.1W down to 1.67V in 60min which is 48.1Wh and the highest "rating" at the lowest CP is 33.9W down to 1.60V for 90min which is 50.85Wh.
 

wattwatt

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Oh okay, so I'm only getting roughly 32 Ah of capacity by throwing 8 of these in parallel?

Does the overall current jump to 480A, 720A (burst) with 8 of these in parallel or am I off on that too?
 

DarkRaven

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That's correct, 32Ah if you don't draw more than 32A continuous. Capacity will be significantly less if you draw more. And maximum possible current increases with each battery in parallel, so yes, 480A/720A(5s) is correct as well.
 

LEDSchlucker

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crashintoty said:
Oh okay, so I'm only getting roughly 32 Ah of capacity by throwing 8 of these in parallel?

Does the overall current jump to 480A, 720A (burst) with 8 of these in parallel or am I off on that too?

Yep, both correct ;)

It's always the same:
Paralleling, adds the Ah (And A), voltage keeps the same.
In series, Ah keeps the same, but V adds up.
Wh adds always up (paralleling or series).
 

wattwatt

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Okay. So if I used this parallel setwith an inverter do I still get 32 Ah of capacity on the AC side?

And, in the rare event I try to achievethat 480A/720A current from this pack (whetherwith an inverter or not), would Ineed to have cables rated for 5,760/8,640 watts?If so, wouldthe cablebetween each 12V battery have to also be rated for the aforementioned watts or just 1/8 of the watts (e.g., 720/1,080 watts)?I bet I'm off on thesecalculation, those numbersseem high.
 

DarkRaven

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crashintoty said:
Okay. So if I used this parallel setwith an inverter do I still get 32 Ah of capacity on the AC side?

You should check what the relation between Ah and Wh is ;)
If you do not draw more than 32A on the input of the inverter then you will get 384Wh out of the battery and into the inverter and ~326Wh (85%) out of it on the AC side due to losses while inverting DC to AC.

crashintoty said:
And, in the rare event I try to achievethat 480A/720A current from this pack (whetherwith an inverter or not), would Ineed to have cables rated for 5,760/8,640 watts?If so, wouldthe cablebetween each 12V battery have to also be rated for the aforementioned watts or just 1/8 of the watts (e.g., 720/1,080 watts)?I bet I'm off on thesecalculation, those numbersseem high.

You should check what the relation between A and W is ;)
If you would do something like that then you would need cables thick enough to carry this current. The power is not relevant. The cable from each battery up to the point where they are all connected in parallel will only carry 1/8 of the total current. From there on you need thicker cable to carry all the current.
 

wattwatt

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DarkRaven said:
crashintoty said:
Okay. So if I used this parallel setwith an inverter do I still get 32 Ah of capacity on the AC side?

You should check what the relation between Ah and Wh is ;)
If you do not draw more than 32A on the input of the inverter then you will get 384Wh out of the battery and into the inverter and ~326Wh (85%) out of it on the AC side due to losses while inverting DC to AC.

crashintoty said:
And, in the rare event I try to achievethat 480A/720A current from this pack (whetherwith an inverter or not), would Ineed to have cables rated for 5,760/8,640 watts?If so, wouldthe cablebetween each 12V battery have to also be rated for the aforementioned watts or just 1/8 of the watts (e.g., 720/1,080 watts)?I bet I'm off on thesecalculation, those numbersseem high.

You should check what the relation between A and W is ;)
If you would do something like that then you would need cables thick enough to carry this current. The power is not relevant. The cable from each battery up to the point where they are all connected in parallel will only carry 1/8 of the total current. From there on you need thicker cable to carry all the current.

Ahhh, I think I get itnow - watts is the variable that is the same value on both sides - it'sthe universal traversingmeasurement. For the sake of simplicity,384 wattsinDC is 384 watts in AC...? So pushing 32A at 12V gives me 384 watts into an inverter, but that 384 watts(again, sans conversion loss, for simplicity) on the AC side is only 3.2A at 120V...?
 
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