Any way to get 48v and 12v out of the same battery bank?

[Sunder]

Guys,

I'm pretty sure the answer is no, but there are some smart people here.

I've offered to build a 12kwh battery bank and inverter for my dad's farm house, which is to be totally off grid. There was some miscommunication there, as I designed the system based on the assumption of a 48v system, to have less loss coming from the solar panels, and less loss converting from battery to 240v. This will be using 4 x 12v deep cycle batteries @ 250Ah.

However, he assumed I was using a 12v system, and has wired up a whole lot of 12v low voltage lighting all around the house. Here's what I see as my options:

1. A single step down transformer from 48v to 12v. Pros - only one needed. Cons - inefficient and needs to be pretty big
2. Use the normal 240v to 12v transformers that they come with and hook it up to the 240v circuits. Pros - Already got them. Cons - very inefficient
3. A separate 12v system with a separate solar voltage regulator. Pros - efficient. Cons - lots of extra $$


4. Here's the question - is there any way I can use diodes or some other technology to draw 12v from all four battery banks evenly (So I don't end up with 1 battery being dead, and 3 others being over charged)? Essentially, this is the question of "Can I have a 4s1p and a 1s4p configuration at the same time?".

Pretty sure it's not possible, but I thought I had to ask before looking into the other options. Thanks!

[ZOMGVTEK]

No way to parallel and series cells at the same time. Technically you could set up a rig that would switch the 12V load between the 4 series 12V banks, but that would be a bit complicated and silly.

Your best bet would be to set the battery up in the config thats going to draw the most power, and run a DC-DC for the other voltage. It's probably easier and less costly to get 12V out of a 48V battery. It's not that inefficient or expensive, as long as you only need a bit of power on 12V.

The house is being lit with 12V? Is this LED stuff?

[ashwright]

How many amps do you need to draw on the 12v?

I have seen some small DC-DC 48V -> 12V converters which are around 90% efficient and can handle around the 10-50A mark.

For example something like this:
http://au.mouser.com/ProductDetail/Mean ... TarlfTs%3d

There are plenty more versions on that site.

[Sunder]

How many amps do you need to draw on the 12v?

I have seen some small DC-DC 48V -> 12V converters which are around 90% efficient and can handle around the 10-50A mark.

For example something like this:
http://au.mouser.com/ProductDetail/Mean ... TarlfTs%3d

There are plenty more versions on that site.

It's going to be about 15 lights of 10-15w each. So let's say 200W to be safe, or around 13 amps.

I'm thinking this way may be the simplest solution.

Thanks for the help, both of you.

P.S. ZOMGVTEK, it's a combination of low power down lights, LED lights and 12v Compact Fluoro.

[999zip999]

A little expensive. So I know 12v is your desire, but what is the pro side of 48v solar storage and 12v output ?

[heathyoung]

Well, if you run your 48V as close to the load as possible, you reduce your transmission losses in wiring.

48V is a common panel voltage and MPPT reg voltage as well.

[Sunder]

A little expensive. So I know 12v is your desire, but what is the pro side of 48v solar storage and 12v output ?

The Inverter is a 6kw model = 125A @ 48V, or 500A @ 12V. I was hoping to use 4 AWG, even though 4AWG is only rated to ~100A, but the cable is only going to be 1m long, and actual time above 100A would be very short.

Also, the closer the source voltage is to the desired voltage, the more efficient the inverter tends to be, but in this case, we're talking 1-2%.

[teklektik]

It seems like your selected inverter (6kW)is considerably larger than you projected need of 200W.

One possibility is to use a few more small DC/DC converters - for instance these Green Galaxy units from ThunderStruck Motors...

Here's an ES thread about re-purposing switching power supplies. The little Toshiba ADP-60RH seems to work well at 48v and can be had for about $10 a pop (eBay – ‘toshiba ADP-60RH'). Nice little waterproof package with easy hookup.

Using a few smaller inverters would also give you a better soft-fail situation...

[Sunder]

It seems like your selected inverter (6kW)is considerably larger than you projected need of 200W.

One possibility is to use a few more small DC/DC converters - for instance these Green Galaxy units from ThunderStruck Motors...

Here's an ES thread about re-purposing switching power supplies. The little Toshiba ADP-60RH seems to work well at 48v and can be had for about $10 a pop (eBay – ‘toshiba ADP-60RH'). Nice little waterproof package with easy hookup.

Using a few smaller inverters would also give you a better soft-fail situation...

200w is the 12v draw.

We'd peak at about 4kw on the 240V draw, but I'm selecting a slightly oversized inverter so we don't shorten its life.

[dnmun]

is it possible to convert your 240V AC applications over to 48V DC? i assume this is pumps and refrigeration so if there is a source for that equipment you might wanna look at the capital costs and amortize it.

the 12C DV lighting circuit may work at 48V DC too if you can use flourscents that will ignite from the 48V DC source. i think AW did something similar on his crazybike.

that would get you to a simpler wiring solution and instead on dealing with an inverter then just deal with a BLDC motor for the pumps and refrigeration. if it exists. BOL.

[dogman]

Is there that much difference, wiring wise, between 12v and 120v? Could he just change the panel and then run led lights at 120v?

Or does he already have $$ in fixtures, bulbs, and the wire itself would need changing?

Run compact flourescents at 48v?

Seperate, smaller 12v power storage hooked to just a few of the panels? Add some 12v panels? Won't be that expensive to do the lighting needs seperate.

I don't have answers, but maybe somebody else does, to my questions if you haven't thought of those options.

[TylerDurden]

Some communities regulate the local electric code to include a low-voltage range from 0V - 50V (or similar). The low-V installations are then not restricted by the regs that typical household elec installs require.

I concur with the suggestion for DC-DC switching convertors.