Battery Bank Resistence

Porky

New Member
I have a 48 volt battery bank consisting of 16 -12V batteries, consisting of four sets of 4 batteries in series and then the four sets tied in parallel to make a 48V battery bank. If one battery has a resistence of 2.7 ohms, what would be the total resistence of the entire battery bank as seen by my controller? Thank you.
 

Hedges

Photon Sorcerer
Combine in series/parallel.

Your 12V batteries are wired 4s4p (or 4p4s would have been the same), so 2.7 ohms x 4 / 4 = 2.7 ohms.

Except, that is probably 2.7 milliohms.

12V/2.7ohms = 4.44 amps short circuit current.
12V/0.0027 ohms = 4444 amps short circuit current, about right.

48V/0.0027 ohms = 17,778 amps short circuit current.

You should have one class T fuse for the bank, or four fuses (with at least 4500A interrupting capability at 48V) per string.
 

smoothJoey

Osprey giving flying lessons
Combine in series/parallel.

Your 12V batteries are wired 4s4p (or 4p4s would have been the same), so 2.7 ohms x 4 / 4 = 2.7 ohms.

Except, that is probably 2.7 milliohms.

12V/2.7ohms = 4.44 amps short circuit current.
12V/0.0027 ohms = 4444 amps short circuit current, about right.

48V/0.0027 ohms = 17,778 amps short circuit current.

You should have one class T fuse for the bank, or four fuses (with at least 4500A interrupting capability at 48V) per string.
There will be added resistance from the interconnects between the batteries.
 

Hedges

Photon Sorcerer
There will be added resistance from the interconnects between the batteries.

Yes but:

If 1' of 2 awg between batteries, only 0.16 milliohms. That's 1/20th of the battery resistance he probably meant to quote.

2.7 milliohms would be about right for 100 Ah 12V AGM.
At least, one source quoted 4000A short circuit current. The CCA rating of car batteries indicates about 3000A short circuit current.

0.27 milliohms about data sheet for one LiFePO4 cell, and about 0.17 milliohm about measured for one LiFePO4 cell.

So I stand by my short circuit current estimate, pending correction of the battery resistance he quoted.
 

Porky

New Member
Interesting. I have rechecked the info on the battery specs and it does say 2.7 ohms, but I believe it may well be a misprint. In keeping with other similiar batteries, 2.7 is most likely milliohms as was indicated by your responses. Thank you for the fast responses!
 

Porky

New Member
On the fusing, I was unsure. I atually ended up putting one 60 Amp fuse on each S string for a total of 4 fuses. So far nothing has blown.....:eek:)
 

Hedges

Photon Sorcerer
I was led to believe the resistance will be in the joinery.

That is correct, not sure how low a resistance we're supposed to get.
If 1 milliohm in the connection and 100A steady-state draw, that's 10W at the joint. It will spread into the battery and wire.
Easy to determine during operation with millivolt scale. 100A and 0.001 ohm gives 0.100V
 

Hedges

Photon Sorcerer
On the wire sizing, I used 4/0 stranded copper, in approx 5" lengths.

0.02 milliohms.

Can you draw a known current through that? Then measure voltage terminal to terminal of cable, also terminal of cable to terminal of battery?
 

Hedges

Photon Sorcerer
On the fusing, I was unsure. I atually ended up putting one 60 Amp fuse on each S string for a total of 4 fuses. So far nothing has blown.....:eek:)

Using 60A fuses is fine.
Question is, what voltage rating of fuse? What AIC rating at 48V? (what type/brand/model of fuse?)

Some fuses are only good up to 32V, e.g. for a 24V battery.

Some can open in case of a modest overload (200A), but can't successfully interrupt 4000A of a short, would just sit and arc for a while.
At least you have fuse per string, only needs to interrupt 4000A not 16,000A.
So a few choices of fuse which are good enough.

MRBF would have been OK with 24V battery, but not for 48V:


(Unless you can find enough additional wire resistance before the first plausible short circuit. But given you selected 4/0 even between batteries, not likely.)
 
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