Solar for permanent boondocking?

[harryn]

As noted, think in terms of "watts" and "watt - hours", not amps and volts.

Example:

T105 battery = (about 200 amp - hours) x 6 volts = 1200 amp - volts - hours = 1200 watt - hours.

It doesn't matter how you wire it, it's "capacity" is like a glass of water - doesn't change just because you set 10 of them side by side, or stack them on top of each other.

You still have the same amount of water (or electricity capacity)

Of that capacity, since it is a lead based battery, you can routinely use about 50% of it. Sort of like having a glass of wine, but half if it is soaked up in a sponge so you can only drink 1/2 of what is in the glass.

LiFe batteries are more like heavily taxed vodka, you can drink the nearly the whole shot (other than what sticks to the walls), and the effect is strong, but paying the tax is painful to your pocketbook.

[harryn]

BTW - there is a misunderstanding of how 48 Volt to 12 volt converters are wired and work internally.

Suppose you need 200 watts of 12 volt power from a 48 volt pack.

You wire the DC - DC converter to the ( + ) and ( - ) of the 48 volt pack, not anywhere in between.

The converter will take in 200 watts of 48 volt DC and convert it to 200 watts at 12 volts.

(yes, there is a small loss, but it is not bad).

These are routinely used on golf carts. The battery packs in golf carts are often 36 or 48 volt and the converters are used to provide power for 12 volt radios, lights, etc. This is not new technology - it has been around for decades.

Like anything electronic, the better ones are more efficient and reliable, and the cheap ones are junk.

[brulaz]

Originally Posted by Natebert

I suppose that using a 24v system (6x12v) and a battery balancer like this:

https://www.victronenergy.com/batter...ttery-balancer

Spec sheet:
https://www.victronenergy.com/upload...alancer-EN.pdf

Would help keep the batteries in check?

I actually have one of these things on order. It was only US$75.
Not sure if I'll need it for my 24V system but was worried that there might be problem as my two 12V banks (of two 6V GC2s) in series are different in age and manufacturer. So we'll see.

Others have cleared up the confusion about Ah capacity vs Wh capacity.
In short, when I configure my 4 GC2 batts as a 12V system, I get 440Ah, but only 220Ah when configured as a 24V system. But the Wh remains the same.

You cannot have six 6 V batteries in a 24V system. Just groups of 4, 8, 12 ...
And with 12V batteries in a 24V system, you can only have 2,4,6,8 ...

24V components tend to be cheaper then 48V components, but that's usually because they can't handle as much power. Not sure if you can even get a measely 2000W inverter for 48VDC. And I suspect that most 24V and 48V components are higher quality (and more expensive) than 12V components as they're designed for heavier usage.

Some info/opinions that I've gleaned from other sites:

12V vs 6v FLA (flooded lead acid) - comparing similar capacities (like 2x12V in parallel vs 2x6V in series, same Ah), the 12V typically have less internal resistance so they have less V drop with high loads. But they also typically have shorter # lifecycles. As a rough example, you could draw down a pair of Trojan 6Vs to maybe 25% state-of-charge over a 1000x before killing them. But you could only draw down a pair of similar Trojan deep-cycle 12V's down to 50% and expect them to last 1000 cycles.

AGM vs FLA - AGM's are clean and neat, can be installed inside and in different orientations. They usually also have less internal resistance than FLAs, so they have less V drop with high loads. Compared to my 6V FLAs, the 12V AGMs I've seen reports on have very low voltage drops. But AGMs like to be fully charged on a regular basis. You cannot abuse them (partial charge over long periods) like FLA's and expect them to easily recover. Daily generator top-ups seems to be the norm. You can equalize FLAs with high voltages to de-sulfate them after abuse; that's more difficult with AGMs.

[mrgrayaz]

it seems to me that running a 12V battery bank with your bank and inverter right next to each other - wire size isnt a big deal . 2/0 runs that are a foot or two long will hold a LOT of current. 12V inverters are readily available at least to 3000watts (xantex)

Really I think the key is using a MPPT controller and a much higher voltage for the solar panels is the way to go - keeping the current and wire size to the roof to the minimum.

Mike

[harryn]

Originally Posted by brulaz

You cannot have six 6 V batteries in a 24V system. Just groups of 4, 8, 12 ...
And with 12V batteries in a 24V system, you can only have 2,4,6,8 ...

24V components tend to be cheaper then 48V components, but that's usually because they can't handle as much power. Not sure if you can even get a measely 2000W inverter for 48VDC. And I suspect that most 24V and 48V components are higher quality (and more expensive) than 12V components as they're designed for heavier usage.
.

There is one way to squeak by with 6 batteries and get 24 volts - by using 12 volt batteries rather than 6 volt ones.

I actually do have some 48 volt, 2000 watt inverters, and lower capacity ones are available, but you are right - they are oriented toward telecom use and are optimized for sine wave quality rather than for heavy lifting on compressor starters. Definitely more expensive than 12 volt models.

The decision of bus voltage might seem complicated, but it actually isn't for your goals:

- 12 volt isn't likely to succeed in the power range / air conditioning aspect

- 24 volt will definitely work and you can buy all of the parts yourself with some serious searching, but they are out there. The way that I would approach it is to use 2 each, 24 volt, 3 kW inverters and split up the loads. It might make sense to have 2+ battery banks.

- 48 volt parts are available to consumers for "part" of the system, but not "All" of the parts needed. There are some components that are either limited to professional installers or have to be custom fabricated.

While I am a big 48 volt proponent for systems 3 kW and above, if you want to go that route, you really should partner with an installer / supplier that will work with you rather than attempting it completely on your own.