24 volt to 12 volt converter

I have the Renogy 40amp controller. Which has a 100volt max input from the panels. Going with 2 strings of 36 volts.

tdevery wrote:
.. my battery bank has to be 24 volts. So i need to convert this to 12 volts for my inverter-charger

You need to buy a 24v inverter ( which I have for a spare), and a 24->12 converter for the coach loads.

tdevery wrote:
Thanks. I read that the MPPT controller would just exclude the extra amps. But was not sure and did not want to wreck it. My panels will be flat on the RV roof. So I will be producing less amps. Presently have 2 100 amp Renogy panel's on a stand. At the outerbanks this summer i was getting 12.5 amps. Panels wirrd for 24 volts.

What can wreck the controller instantly--no second chance-- is going over its Voc limit. Say you have four panels with Voc of 36v. Two in series is 72 Voc and the controller's limit might be under or over that. If the limit is 100Voc you are good at 72. Some days your sunlight will get you more than rated Voc so you need some margin.

Put the other two panels in series for 72 Voc and parallel the two pairs to stay at 72.

If the controller's limit is 60 Voc, then you must put all the 36s in parallel to stay at 36, under that 60 and with some margin.

Thanks. I read that the MPPT controller would just exclude the extra amps. But was not sure and did not want to wreck it. My panels will be flat on the RV roof. So I will be producing less amps. Presently have 2 100 amp Renogy panel's on a stand. At the outerbanks this summer i was getting 12.5 amps. Panels wirrd for 24 volts.

tdevery wrote:
You do not understand. I want to increase my controller presently at 40 amps output, to 80 amps. With my controller output at 24 volts i can do this. But then my battery bank has to be 24 volts. So i need to convert this to 12 volts for my inverter-charger

Amps at 24v is half the amps at 12v.

Your output amps come from the output watts. You divide the output watts by the battery voltage to get your amps.

Here is how it works 24v-12v. Your 600w array might be putting out 540w on a nice day. ( You lose about 10% due to panel heating in the sun--those panels get hot.) You lose another percent or two on the wires going to the controller's input. So input watts might be 540 -11 = 529w

So you have 529w on the input with the controller doing say 30v at Vmp ( example) so the input amps is 529/30 = 17.6a. (wire gauge for that) The controller efficiency is say 96%, so output watts is 508w.

Now your batteries are starting in the morning at say 12.5v, so they will get 508/12.5 = 40.6 amps --wire gauge for that-- (but the controller rated at 40 will keep the amps at 40, not letting the amps get higher.)

Later after the batteries are recharged some, their voltage is now 13.5v rising, so amps now (assuming same watts--actually with higher sun at noon watts might be higher--depends if panels are flat or aimed at sun all day) anyway 508/13.5 = 37.6a

As battery voltage rises, amps go down with the same watts.

With the 600w you are slightly "over-panelled" for that 40a controller, but that is a good thing for your over-all daily AH haul since you will get to use all the array can do earlier and later in the day when the sun is lower.

Or i spend $500 on a new charge controller

You do not understand. I want to increase my controller presently at 40 amps output, to 80 amps. With my controller output at 24 volts i can do this. But then my battery bank has to be 24 volts. So i need to convert this to 12 volts for my inverter-charger

Your MPPT controller already has a buck converter in it that takes the "24v" solar array as input and puts out "12v" on a "12v" battery bank. (as above make the 4-6s into a "12v" ) Connect the controller to the batteries FIRST so the controller can see it is on 12v, then connect the input wires from the array.

BTW the MPPT controller will pick an input voltage from the array depending on what the array can do at the time with the sunshine available and the state of the batteries on being charged. In "bulk" the controller will pick somewhere near the panels' rated Vmp. After that during absorption and float it will pick a higher voltage. That 36v mentioned is perhaps the Voc of the panels.

The voltage on the wires from array to controller will be at "24v" so they can be thinner than the wires from the controller to the battery bank which should be shorter and fatter to carry the "12v". Place the controller as close to the battery bank as you can.

Your batteries should be in series/parallel with 2 6volt in series and tied in parallel to the other 2 6 volt in series for 12 volts not 24 volts