Sorry, meant to say battery wire size
L/Battery - go large or go lithium?
- Thread starter Chris1983
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- dc - dc charger victron vsr
With 6mm cable, you expect 0.0031 ohm/metre, so say 0.08 ohms assuming a 5m run. So at 0.1A you'd expect a voltage drop of 0.008V, i.e. negligible. You're getting more like a 0.1V drop so in relative terms it seems big. However in absolute terms it's still small and so potentially swamped by other sources of measurement error and variation so I wouldn't worry about it for now, and just have another look when some meaningful current is flowing just to verify that you don't have an issue.
Meaningful, as in a decent amount of current. In practice that will occur when the sun is out and the battery isn't too full.
If you mean 6mm2 then the calculated voltage drop is going to be about 0.1v at 5amps 0.02v at 1amp
No you are not risking kaboom so long as they are fused at the battery end.
I note also the MPPT has switched to bulk now.
I think you're just seeing natural variance of calibration and measurement.
Of all the equipment you have I'd trust the smartshunt figures the most - so long as they occasionally get a deep discharge and then full charge they track pretty well and they are close to the battery electrically.
No you are not risking kaboom so long as they are fused at the battery end.
I note also the MPPT has switched to bulk now.
I think you're just seeing natural variance of calibration and measurement.
Of all the equipment you have I'd trust the smartshunt figures the most - so long as they occasionally get a deep discharge and then full charge they track pretty well and they are close to the battery electrically.
Urgh, I've slipped a decimal point on that current, that should be more like 1A, so a voltage drop of 0.08, i.e. pretty bloody close to what you're getting...
Conclusion the same though, probably nothing to worry about, verify when some current is flowing!
Double urgh, I somehow messed up multiplying by five on the 0.0031 too. Sorry!
Total resistance 0.0031*5 = 0.0155ohms, so expect the same drop in volts at 1A current flow. So back to the original conclusion that it seems slightly high in relative terms but small in absolute so affected by measurement error and verify again when some decent current flowing.
Apologies for the incompetent maths...
All fused
Shunt right next to battery, I’ll run battery low with fridge etc, then post some screen grabs on sunny day
Yes that may explain the difference but only if the cable to the rear is a little undersized and causing voltage drop. My LB and shunt are under the passenger seat and MPPT in the rear wardrobe but the readings are exactly the same. I’ve used 16mm2 cable to the rear with a short section of 4mm2 from a busbar to the MPPT. Your drop is well within the recommended 3% max so I wouldn’t worry.
Your shunt shows starter battery and mine does not, is that a wiring difference?
Yep. Second small cable into the other aux connection. The shunt should have come with 2 thin, fused red cables. I connected that to the input to the DC-DC from the SB. The other is the positive from the LB.
Yeah so looks like one small positive wire to positive side, no feed in from SB.
Having just peered under seat, I may just leave all as is. Not sure I have the fortitude today (considering all is working)
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