24 Volt Solar

[Danjo]

At what point does it make sense to go to 24 volts for a PV array and battery storage? Ive been watching a lot of YouTube. There’s a guy named Will Prowse that has a Chanel that seems to have a lot of good info. There’s a video on 24 volt systems and he makes arguments for cheaper cabling and better battery usage because batteries are happier in series rather than parallel. But at what point does that make financial sense? The refrigerator is the largest demand at about 400Wh a day. But all the runs in my little short bus are going to be, well, short. The one thing that keeps me going on thinking there’s something to this is the idea that batteries are happier and live a longer life when they are in series and so in the case of 24 volts it would be easier to have that.

[brokedown]

What you're really combating is the rise of amps. More volts means less amps for the same amount of wattage, and equipment and cabling costs go way up around 50A or so. I'm maxxing out my 40A charge controller with 600W of panels goig to a 12v bank, my next upgrade would be to a 24v battery bank and the accessories required to let me draw 12v fromit.

[TheHubbardBus]

https://www.solarpaneltalk.com/forum...considerations


One big factor to consider is your panels. Charging a large 12V bank requires a lot of amps (because you don't have the volts). Most charge controllers you'll find are 80A or lower, and the 80s are expensive. So with a 12v system, you may find yourself buying expensive controllers and/or needing to parallel 2 controllers. With a 24V system, you can run 1000+w of panels off a single 50A controller no problem.



I've gone back & forth more times than I can count deciding between 12v & 24v for our short bus build. In the end, we're going w/ 24. FYI - 24v to 12v stepdowns are stupid efficient.

[john61ct]

Note that with a modern MPPT controller,

Victron in particular gives a lot of flexibility and value in sizing against the most powerful best value panel types

exactly the same panels are fine for either 12V or 24V bank / loads system voltage

Only with cheap PWM type controllers does the choice of panel voltage relate to that of the bank being charged.

With MPPT, you can take a given solar setup and just power it down, swap the 12V bank for a 24V one, and power back up, no other changes required.

[john61ct]

For a large rig with very high power load devices to feed, especially if lots of input comes from alternator charging, 24V will be more efficient, beyond the minimal savings in copper from thinner wire gauge.

But for those running mostly native DC setups, avoiding inverters as much as possible, mant device types are more expensive in 24V versions

or even unavailable, so you need buck vonverters to run 12V circuits anyway

added cost and complexity.

I would recommend sticking to 12V unless you have a compelling reason to go to 24V

especially without a 24V alternator option.

[Mountain Gnome]

This has been one of the most simple yet informative threads I've read on this subject.


Thanks Y'all!


It got me thinking, is there a "switch" that can be purchased that will convert my battery bank from 12V to 24V? In one position, 12V (AGM likely for me) batts will be connected in series (to 24V), in the other position, the batts will be connected in parallel (kept at 12V)?


It would be especially nice if you could buy or build this switch using high-amp continuous use 5-pin relays, so when you turn the key for the bus motor to on, the deep-cycle batt bank switches to 12V to charge off the alternator. With the key off, they switch to 24V.


The trick would then be to figure out how to use your interior RV accessories (rated for 24V) while driving, but maybe that wouldn't be a problem (for me at least). Or maybe you could just use a 12-volt stepdown for a fridge or something that must run when it needs to, run 24V to the back of the fridge from the batt bank, and step it down only 1 foot or less away from the fridge. The line out of the stepdown for said fridge would also go into another 5-pin high-amp continuous relay switch controlled by the bus motor's ignition switch. When the bus is running, the relay powers the fridge off the alternator, but when the motor is off, the fridge grabs power from the stepdown.


That would then allow a better solar panel setup.


Or am I still under-educated?

[john61ct]

There are indeed "serial parallel switch" devices, commonly used with trucks and race cars, and some designed for boats. Yandina Trollbridge good example, by the inventor of the combiner / VSR / ACR concept.

Idea is charge sources are 12V, 90% of the time bank is 12V, but a certain device used that needs 24V can be fed that for short periods

KISS is a better approach, 24V system, with 12V output from DCDC buck converter(s) as needed.

But only if there is a strong reason for the 24V system in the first place.

[Mountain Gnome]

Originally Posted by TheHubbardBus

One big factor to consider is your panels. Charging a large 12V bank requires a lot of amps (because you don't have the volts). Most charge controllers you'll find are 80A or lower, and the 80s are expensive. So with a 12v system, you may find yourself buying expensive controllers and/or needing to parallel 2 controllers. With a 24V system, you can run 1000+w of panels off a single 50A controller no problem.

Seems to me this IS the reason for 24 volt batt banks.

[john61ct]

Only if you think one big expensive SC with multiple panels is a good idea.

Whereas Victron's model lines allow for much greater efficiency, more flexibility and better value, enabling a 1:1 ratio between SCs and panels.

So 36+Voc panels, up to 65V, rated 240-280W (up to maybe 300W if you find a super bargain) pairs perfectly with a SmartSolar 75/15, at only ~$120 each.

Each pair optimizes its own MPPT independently of the others, so greater conversion efficiency, especially with partial shading, slightly differing angles to the sky, bit of bird poop etc.

Yes you'll save a few hundred just on the SCs going to 24V, but that would not justify the decision on its own, unless the other factors discussed above were in alignment.

All this assumes energy efficiency is a priority. Those running mains household appliances off big inverters, I agree no big difference, any savings lost in the noise.

[Mountain Gnome]

OK. I think I get it. It almost goes over my head, though
At any rate, I have room for two automotive-starter-sized AGM batts in the batt box, and room for 2 or maybe 3 "average" sized panels on the roof (since I have 2 roof AC units, a 2'-square hatch/sunroof, and I want space for a platform/deck/luggage rack also, all within about 20' long of roof-space.) So maybe I really don't need to worry about heavy-duty charging stuff.


If I could find the cash and time to get the AC units up and running and see how one alone works when it's 95°F+ outside, maybe I'll ditch the second (rear) one (it's not in the optimal position for my inside build, anyway). I bet one's fine for just me, a dog, cat, and if I ever get lucky and find one who likes to hike and travel, a good woman.

[PNW_Steve]

I didn't have any trouble finding reasonably priced accessories to run from my 24 volt battery bank. The only thing that I have that I will need a 24/12 converter for is my Webasto. It was available in a 24v version but got such a rocking deal on the 12v.

[TheHubbardBus]

Originally Posted by john61ct

But for those running mostly native DC setups, avoiding inverters as much as possible, many device types are more expensive in 24V versions

or even unavailable, so you need buck converters to run 12V circuits anyway

added cost and complexity.

I would recommend sticking to 12V unless you have a compelling reason to go to 24V

especially without a 24V alternator option.

One step-down converter seems both less expensive and less complex to me, vs one SCC per panel.

[MambaJack]

When I was researching this. The following was recommended to me.

12 volt with 500 watts or less
24 volts 500 to 1500 watts
48 volts 1500+

However, my system is 24v 2000w

[MambaJack]

With only 2 batteries. I cant imagine not just going 12v

[Danjo]

Thanks for all the great replies! It sounds like the 600w system that I’m planning would benefit from being 24v. Now onto choosing components!

[john61ct]

Originally Posted by MambaJack

When I was researching this. The following was recommended to me.

12 volt with 500 watts or less
24 volts 500 to 1500 watts
48 volts 1500+

However, my system is 24v 2000w

To me that's very odd, old PWM tech maybe?

I suppose if all the power cited were being sent over one pair and a long distance, to save on the expense of lower gauge wiring?

These days, 60+ volts from the panel to the SC is the way to go, and again, has nothing to do with the voltage of the bank and loads.

The SC of course needs to be as close as practical to the bank.

[PNW_Steve]

Originally Posted by MambaJack

When I was researching this. The following was recommended to me.

12 volt with 500 watts or less
24 volts 500 to 1500 watts
48 volts 1500+

However, my system is 24v 2000w

Looks sensible to me.

I cringe every time I see someone buy one of those junk Chinese 6000'watt 12 volt inverters. I guess that there is one born every minute......

[john61ct]

Originally Posted by MambaJack

With only 2 batteries. I cant imagine not just going 12v

Well there are single batteries with thousands of Ah capacity.

I'd call a little bank under say 400Ah.

But the need for higher voltage, really has to do more with the power consumed by loads.

A 24V winch for example will be much more efficient, lower amps less heat.

Good DC aircon units, a strong motor needed for a 30 gph RO fresh water maker, 3+kW inverter for running wasteful mains appliances, that sort of stuff.

Or on the other side, if you plan on getting a lot of power out of a large frame high output alternator, 24V is more efficient once you get past say 200A.

I'm not saying going 24V isn't a good idea, just that there really isn't any compelling need other than the above examples.

[nanoplane]

I'm going with 12V battery system for the following reasons:

- Using an MPPT controller, I can use a series/parallel arrangement on the solar panels to limit current on any string to about 15A or so. using 8Ga wire, 40' bus. I have 20 - 100W panels.

- On the battery side, yes the wires are LARGE (4/0 for the 'master" ones)... but they're short.

- I can use a battery to battery charge controller to let my 12V alternator charge my house battery system when I'm driving. I'm using the Sterling BB1260.

- I get 12V easily to support my standard appliances and USB charging ports...

[john61ct]

Only that last part pertains to 12V specifically.

But can be somewhat significant for those using mostly DC native devices.