A 24v vs 12v charging question

[bluerain]

I have my liveaboard boat set up with 1000w of panels, through a tristar mpt60, 2 4xL16 6v batteries (24v system, 450 ah). I'm trying to compare efficiency vs my father's liveaboard boat, that has 800w of panels charging a 12v system, to be able to compare apples to apples basically.

For instance, right now my controller says it's putting out 24.5 amps to my 24 volt batteries. If my system had been a 12v set up with same equipment, it'd be 12v @ 900ah, would the controller be putting out 49 amps to the 12 volt set up in that condition?

[Mike90250]

yes, if you halve the voltage, the amps must double, for the same wattage. First savings is the cost of copper wire, you need much thicker wire to carry 50 A instead of 25A.

[Sunking]

I have my liveaboard boat set up with 1000w of panels, through a tristar mpt60, 2 4xL16 6v batteries (24v system, 450 ah).
I'm trying to compare efficiency vs my father's liveaboard boat, that has 800w of panels charging a 12v system, to be able to compare apples to apples basically.

For instance, right now my controller says it's putting out 24.5 amps to my 24 volt batteries. If my system had been a 12v set up with same equipment, it'd be 12v @ 900ah, would the controller be putting out 49 amps to the 12 volt set up in that condition?

Yes. Any more questions? MPPT Controllers are rated at Output Current where PWM is both Input and Output which is a bad thing.

With MPPT Output Current = Panel Wattage / Nominal Battery Voltage and that answers your question if you do the math. So your MS Tristar 60 Amp Controller has the following max power input vs battery voltages.

800 Watts @ 12 volts
1600 Watts @ 24 volts
3200 watts @ 48 volts


Now while that may not catch your attention right now, but higher voltage is always more efficient and less expensive to implement. This may sound crazy at first because you do not fully understand electric power, but higher voltage is also a lot safer and less prone to fires. From a safety point of view 12, 24, and 48 volts is all considered low voltage. Voltage is not high enough to really hurt you under normal conditions. 48 volts might make you jump an inch higher than 12 volts, but no real danger of burning you, or stopping your heart. The danger is current. Current running through resistance generates heat. Wires and connections have resistance. Example 100-Amps is a lot of current, and under the right conditions can melt connections, battery term post, and wires. A 12 volt 1000 watt Inverter requires 100 amps, at 24 volts only 50 amps, and at 48 volts 25 amps.

For most installations you do not ever want to exceed 100 amps for several reasons. Primarily expense and safety. On the expense side to conduct 100 amps for a short distance requires a minimum 4 AWG wire and that is only good up to a few feet before voltage and power loss become an issue. 4 AWG is expensive wire. But the kicker is DIY's do not have to tooling, knowledge, training, and experience to properly terminate even 6 AWG wire and cable. When you fail to terminate a wire correctly fire is waiting for you.

This may help you understand. It is a Table showing wire size required per a fuse size. You want to use the column for 3% or less voltage drop. Example look at a 20 amp circuit. Minimum wire size to just be safe is 14 AWG, but it is only good to 6-feet 1-Way distance. Same wire at 30 feet is still safe but you loose 10% of your voltage and power. So say 15 amps at 12 volts is 180 watts on a 12 volt battery, but at the load you arrive at 10.8 volts and lost 18 watts of power heating up the wire. At 30 feet you would need a larger 6 AWG wire which cost 400% more. At 48 volts the same 180 watt load only requires .3.75 amps. Much safer and less expensive.

[bluerain]

Thanks. I do understand most of it, I had just wanted to be sure in comparing the output I was getting that I was in fact getting it right. I have the ts-60 (so at 24v could have 1600w panels), currently with 1000 watts installed. My father's system is 12v with 800 watts, he's using 2 HQST mppt controllers ( a 30a and a 40a) and I wanted to see how the efficiency was stacking up between us. Most of my panels are angled well for the sun here, where his are not, and I do seem to have a fair bit more efficient power collection, I just wanted to make sure the math was right.