24 volt to 12 volt converter

OldSmokey wrote:

BFL13 wrote:
"- battery V drop under load is halved on a % basis at 24v"

Is that true? It might be halved or doubled as such, but I would think on a Percentage Basis it would be the same. Please elaborate--my brain quit :)

assume T105RE with internal resistance of 0.001 Ohms per cell and 1200 Watt load

for 12V we have 6 cells so 0.001*6*100 A = 0.6 V
0.6/12*100% = 5 percent drop

for 24V we have 12 cells so 0.001*12*50 A = 0.6 V
0.6/24*100 = 2.5 percent drop

That's the theory, and it's what I've found in practice. With the same inverter load I get about the same V drop, whether the batteries are 12V or 24V.

It was about 1V drop both times in my test case. So 12.5V down to 11.5V on the old, or 25V down to 24V on the new.

Different inverters have different lower V limits so can't really compare, but the new Kisae claims that it will alarm at 22.4V and shut off at 21V. But haven't heard it yet.

To be honest not sure I understand how the internal resistance of batteries works in this situation ...

BFL13 wrote:

2oldman wrote:

BFL13 wrote:
In fact, as Morningstar keeps saying, some amount of over-panelling is a good thing with an MPPT controller, since it does have the self limiter feature (unlike a PWM controller that will fry if heated up by over amping it)

What does self-limiting mean? Kinda sounds like throwing away power.

It means the MPPT controller will not go over its rated amps so as not to over-heat and fry if over-panelled. This comes from the way a buck converter works whether in an MPPT controller or as its own gizmo, AFAIK.

They do this by having a wattage input limit in its specs, but it might really be an amps limit, I am not sure. Morningstar talks about their amps limit in their specs, but that might be a watts limit, I don't know.

I don't have any info on how long it is wise to stay at max amps with a self-limiting controller so as to avoid heat damage. Just how long that would be in any day "depends" of course. Too many factors. I imagine most scenarios would be with just a short time mid-day, but you could end up with more of the day at that. Night-time shuts it down for however long that is.

the limit is normally defined by the power output stage on the low side ( the 12,24,48 V side) i know this having smoked quite a few in various designs.. :o

600 watts in my book is right on the limit of 12 volt systems.
now, your 40 A controller is too small for a 12 V system unless you intend to throw those watts away.. 40A*12V=480W and you waste 120W ! not an ideal situation.

What are you thinking?
You don't charge at 12v you charge at 14+v
14*40 =560w if he can get it, there is next to zero loss
14.4 or higher is even better
When the batteries are low he will use every amp the solar can make
He probably won't get 600, but he sure will Not be throwing away 120w

2oldman wrote:

BFL13 wrote:
In fact, as Morningstar keeps saying, some amount of over-panelling is a good thing with an MPPT controller, since it does have the self limiter feature (unlike a PWM controller that will fry if heated up by over amping it)

What does self-limiting mean? Kinda sounds like throwing away power.

It means the MPPT controller will not go over its rated amps so as not to over-heat and fry if over-panelled. This comes from the way a buck converter works whether in an MPPT controller or as its own gizmo, AFAIK.

They do this by having a wattage input limit in its specs, but it might really be an amps limit, I am not sure. Morningstar talks about their amps limit in their specs, but that might be a watts limit, I don't know.

I don't have any info on how long it is wise to stay at max amps with a self-limiting controller so as to avoid heat damage. Just how long that would be in any day "depends" of course. Too many factors. I imagine most scenarios would be with just a short time mid-day, but you could end up with more of the day at that. Night-time shuts it down for however long that is.

BFL13 wrote:
"- battery V drop under load is halved on a % basis at 24v"

Is that true? It might be halved or doubled as such, but I would think on a Percentage Basis it would be the same. Please elaborate--my brain quit :)

assume T105RE with internal resistance of 0.001 Ohms per cell and 1200 Watt load

for 12V we have 6 cells so 0.001*6*100 A = 0.6 V
0.6/12*100% = 5 percent drop

for 24V we have 12 cells so 0.001*12*50 A = 0.6 V
0.6/24*100 = 2.5 percent drop

MEXICOWANDERER wrote:
Compressor air tank has 140 lbs. Tires need 90? Common sense is needed when choosing and selecting.

This is relevant.. I guess.

Compressor air tank has 140 lbs. Tires need 90? Common sense is needed when choosing and selecting.

OldSmokey wrote:

BFL13 wrote:
"now, your 40 A controller is too small for a 12 V system unless you intend to throw those watts away.. 40A*12V=480W and you waste 120W ! not an ideal situation"

I am not understanding this at all. I posted earlier how his 600w array would work out. (goes here after I do some computer stuff)

you don't understand this because your estimate of panel watts assumes panel heating. I can tell you that there are situations/locations at full sun insolation that will max a panel out. been there, done that. now, I'm not saying this is common, but it does occur and should be taken into consideration.(certainly in my business). one other thing, it's also not a good idea to run a CC at max I too long and too frequently.

Not trying to disrespect your posts over-all, but I am in a nit-picky mood on some aspects. Trouble with that is if I get something wrong, then I am in for a double roasting! :)

Anyway, I am getting the notion that we mostly agree on things, once clarified. The OP seems to know what's what, so he can wade through all this ok.

BFL13 wrote:
In fact, as Morningstar keeps saying, some amount of over-panelling is a good thing with an MPPT controller, since it does have the self limiter feature (unlike a PWM controller that will fry if heated up by over amping it)

What does self-limiting mean? Kinda sounds like throwing away power.

BFL13 wrote:
... and he would be crazy to buy a new controller or inverter or anything.

I don't think we need to go that far!

BFL13 wrote:
"- battery V drop under load is halved on a % basis at 24v"

Is that true? It might be halved or doubled as such, but I would think on a Percentage Basis it would be the same. Please elaborate--my brain quit :)

Posting instead of editing--so what is the low voltage alarm set at with a 24v inverter? With a 12v inverter it is say, at 11v. From that info--when provided-- we can see what the percentage voltage sag might be?

Not sure I am getting this straight, but maybe.