48v Multiplus Invertor with 12v batteries

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ironmancole asked Jan 4, '22

48v Multiplus Invertor with 12v batteries

Hi,

I'm fitting out my boat and have bought a 48v Victron Multiplus 8kw Charger Invertor. I went big as I'm looking to reduce my reliance on gas and got fed up with a 3kw invertor and the limitations of it when I want to run a lot of items. I also bought 4 x 205ah 12 volt batteries to satisfy the 48v, which I believe have to be wired in series.

I am told that if I wire in series then I effectively lose the 820ah potential of 4 batteries and would only in effect have the ah of just the one battery, or 205 ah.

Can anyone advise if this is correct and how I would go about accessing the 800+ amp hours I thought I'd got access to? Do I need 16 batteries to achieve 820 ah or can I in fact wire in parallel whilst keeping the invertor happy?

Thanks for any guidance here, I'm out of my depth :)

Multiplus-II battery victron

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Answer 1 · 2022-01-04T16:11:07Z

seb71 answered · Jan 04, 2022 at 04:11 PM

You don't "lose" anything. You just have wrong knowledge.

If you wire those four 12V 205Ah batteries in parallel,

you will have a battery with

205Ah x 4 = 820Ah,

but at 12V.

The energy stored is 12V x 820Ah = 9840Wh.

If you wire those four 12V 205Ah batteries in series,

you will have a battery with

205Ah

but at 12V x 4 = 48V.

The energy stored is 48V x 205Ah = 9840Wh.

As you can see, the energy stored is the same.

Notes:

12V / 48V are nominal voltages. The actual voltage varies.
You can't use the entire capacity.
For using that 48V invertor, you must wire the four 12V batteries in series.

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Answer 2 · 2022-01-04T16:12:09Z

billknny answered · Jan 04, 2022 at 04:12 PM

You have been told correctly.

If you wire the batteries in SERIES you increase the voltage, but the available Amp-Hours does not increase over the single battery rating. Four 205 Amp-hr, 12V batteries in series can supply 205 Amp-hrs at 48 Volts.

If you wire the batteries in parallel you do get 820 Amp-hrs, but only at 12 Volts. The inverter will not work.

The amount of ENERGY available is the same in both cases. Four 12 volt 205 Amp-hr batteries in series, or in parallel, in any combination, supply 9840 Watt-hrs of energy.

To have a 820 Amp-hr, 48V battery bank using 12V/205Amp-hr batteries you would need to have four parallel strings of four batteries in series (16 total). That would be a 39.4 kiloWatt-hr battery bank.

If these very basic electrical calculations are a mystery to you, I suggest you not go about designing this system yourself. Seriously.

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Answer 3 · 2022-01-04T16:14:15Z

kevgermany answered · Jan 04, 2022 at 04:14 PM

You have lost nothing. As the batteries are going to be in series, you have 205AH at 48V. It's the same amount of power, but as the voltage is 4* higher, you use one quarter of the amps.

Power = Volts * Amps.

So

205 * (12 * 4) = 205 * 48 = 9840W

But... you're maybe undersized on the batteries if you're going to really use the 8KW.

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Answer 4 · 2022-04-09T14:56:40Z

sharpener answered · Apr 09, 2022 at 02:56 PM

Actually @kevgermany the units are wrong. The capacity is 205Ah and the stored energy is 9840 Wh.

If the OP is really planning to have 8kW of load, this size battery is going to last a little over 1 hour before it is flat. How is he intending to recharge it in a reasonable time, from the engine? from solar PV? from shore power?

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kevgermany ♦♦ commented · Apr 10, 2022 at 01:42 AM

Read op again. He said he installed 8kW for future expansion. He did not mention actual load. Agree the batteries are undersized.

Yes, thanks I missed the hours out.

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