JustinSchoeman Posted August 13, 2020 Posted August 13, 2020 I would just like to confirm wire sizing for my battery. I am used to aviation specs where the minimum wire size for 200A would be 110mm^2... But I see the inverter manual says 55mm^2 and the SANS spec seems to say this is OK. But you are still looking at 50W loss into the cable at max draw. 55mm^2 is obviously a lot easier (readily available cable and lugs) - and obviously guaranteed to fit in the inverter cable glands. What is the general feeling about battery cable sizes? Quote
Stanley Posted August 13, 2020 Posted August 13, 2020 According to SANS, 50mm^2 is just barely enough for 200A when using 2 cables horizontal, flat spaced in free air. I guess 70mm^2 would be better. Quote
JustinSchoeman Posted August 13, 2020 Author Posted August 13, 2020 Thanks. May be best to wait for the inverter, and see what the max size cable is I can fit through the glands and/or fit on the terminals... 55 really does seem a little small. Quote
Gnome Posted August 19, 2020 Posted August 19, 2020 On 2020/08/13 at 12:39 PM, JustinSchoeman said: Thanks. May be best to wait for the inverter, and see what the max size cable is I can fit through the glands and/or fit on the terminals... 55 really does seem a little small. Which inverter is it? Also 200amps sounds like max draw... For a 5kW unit @ 48volts, 50mm should be ok, 70mm ideal if you spend a lot of time at high power. Quote
___ Posted August 19, 2020 Posted August 19, 2020 On 2020/08/13 at 9:51 AM, JustinSchoeman said: What is the general feeling about battery cable sizes? Ideally you want around a 1% voltage drop, but iirc the recommendation is <3%. So on a 50V battery, that's 0.5V. Generally the cable runs are fairly short from the battery to the inverter. If we assume 10 meters total length (5 meters on either side). 50mm^2 cable has a resistance of 0.35Ω/km, so for 10 meters that's 0.0035Ω. 0.5/0.0035 = 142A. That's for ten meters, which I deliberately chose to be on the long side. Put the inverter closer, and 50mm^2 should be good for 200A or 10kW. If you need to go bigger than that, then use 35mm^2 and use two cables (double up). Quote
JustinSchoeman Posted August 19, 2020 Author Posted August 19, 2020 Total cable length (positive and negative together) is around 2m. So I suppose 50mm^2 is actually more than sufficient. Although I would still want to use 95mm^2 for joining the two layers of the battery pack (to reduce voltage errors to the balancer). But I suppose it makes sense to take the end-plates off and add extra terminal bolts and do 2x 50mm^2 - that way I get half the contact resistance too... ___ 1 Quote
Stanley Posted August 20, 2020 Posted August 20, 2020 12 hours ago, plonkster said: 50mm^2 cable has a resistance of 0.35Ω/km I think this information (and how to get it) may be useful for some other people, so I thought that this is as good a place as any to teach others how to find the resistance of their cables. A 1mm^2 copper wire with a length of 1m has a resistance of 17mΩ (i.e. 0.017Ω). This is the only number you need to remember. For any cable, of any cross-sectional area and length, you simply divide the 17mΩ by your cable's area and multiply the result by the length in m. So to use @plonkster's example above for 10m of 50mm^2 cable. (0.017Ω / 50mm^2) * 10m = 0.0034Ω ___ and Louisvdw 2 Quote
JustinSchoeman Posted August 20, 2020 Author Posted August 20, 2020 So much easier with aviation wiring... Take the fuse rating, look it up in the table, and get the minimum wire diameter. No room for choice/acceptable losses/etc. Quote
___ Posted August 20, 2020 Posted August 20, 2020 Just now, JustinSchoeman said: So much easier with aviation wiring... Take the fuse rating, look it up in the table, and get the minimum wire diameter. No room for choice/acceptable losses/etc. I disagree. The 17mΩ rule is much easier to remember . Of course tables are available for this sort of thing too, just google for it. There are tables for both copper and aluminium. This is usually what I do, I just google it. Which is how come I'm 0.01Ω off on my anwer above. @Stanley's rule is an awesome little tool that is definitely going into my bag of tricks, along with the 345 rule (Pythagoras) and my Afrikaans Sints/Cosas/Tanta trick to remember how the trig functions work Quote
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