Batteries serie / parallel

[Alec Swanepoel]

Need some more help please???

Theoretically speaking:

100Ah battery  = 100A for 1 hour???

4 x 100Ah batteries in serie = 100A for 4 hours?????

4 x 100Ah batteries in parallel = 400A for 1 hour??????

2 x Packs of 4 x 100Ah batteries in serie, packs connected in parallel = 200A for 4 hours????????

Regards. Alec 

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5 minutes ago, Alec Swanepoel said:

100Ah battery  = 100A for 1 hour???

Simplistically speaking, yes. In practice, no. Peukert's law. More about that later.

 

5 minutes ago, Alec Swanepoel said:

4 x 100Ah batteries in serie = 100A for 4 hours?????

No. In Series the Ah stays the same and the voltage goes up, so still 100A for one hour (but because you're doing it at 4 times the voltage, you get four times the Wh (watt hour)).

 

7 minutes ago, Alec Swanepoel said:

2 x Packs of 4 x 100Ah batteries in serie, packs connected in parallel = 200A for 4 hours????????

8 batteries, two strings of 4. When you series, the Ah stays the same. So each string is 100Ah. When you parallel, voltage stays the same, but Ah is added. So you have 48V, 200Ah. 200 amps for one hour (if you ignore Peukert) or 1 amp for 200 hours or 20 amps for 10 hours, etc etc. But once again, at 48V, you have 200 * 48 = 9.6kwh. If you had 4 strings of 2 batteries, you'd have 24V. Each string would still be 100Ah, but because you have 4 strings that would be a total of 400Ah, but at 24V that is still 24 * 400 = 9.6kwh. And if you took all 8 batteries and paralleled them, you'd have 8 * 100Ah = 800Ah, but at 12V, and 12 * 800 = 9.6kwh.

There ain't no such thing as a free lunch.

Okay, Peukert. Peukert basically says that the faster you discharge a battery, the less useful capacity you will get out of it. So if you attempt to discharge our hypothetical 100Ah battery at 100A, you'll get maybe 40 minutes, not to mention that you might damage it.

Most batteries are rated at C20 (over 20 hours), so the battery can give you 5 ampere constantly for 20 hours before it is dead. At 10 ampere, it will last slightly less than 10 hours (because Peukert says you lose discharge efficiency), and at 20 ampere you'll get maybe 4.5 hours. It's not advisable discharge at rates higher than C5.

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All this really reminds me that what we are concerned with, in the end is "work" (measured in joules, aka energy). Now as you might recall from those dreamy days known as school, Work equals Force times Distance.  Force, in turn, is mass times acceleration (Newton), and gravity is a kind of acceleration. So if you took a rock and dropped it off a tall building, the amount of work would be mgh, mass times gravity times height. So I can get the same work done by dropping a rock of half the mass off a building twice the height.

And that really is the same principle here. Drop half the amp hours off twice the voltage, and you get the same work done (once again, in theory, all sorts of inefficiencies come out to play too). So when you get confused, work out the total capacity for work (in kwh) in the simplest terms that makes sense to you. Then, you know you must always get the same answer when you start mucking around with things. When you count your cows and there is suddenly twice as many as you thought... better get off the Vodka there :-)

[Guest]

And then you have to think of not below 50% DOD for lead acids ...