Robinson avatar image
Robinson asked

Off grid battery voltage

Off grid project.

What is prefered 48 or 24v? Plan to use 4x FLA 48v 750Ah batterys, 2nd hand FLT 600mm tall type, fit well in brick bunker - won't freeze, cheap too. Had one of these batteries for 2 yrs on 3 solar panel. Just 24v for now, testement Victron MPPT abilty manage battery life still working well, 150/30 MPPT is very robost unit.

So is 48v is better for battery and inverter, but I like 24v for control circuits, have 5m cable run to bunker with 350mm2 is lots.

Shortly add 24 PV panels and 800W wind turbine, just bought 150/60 MPPT for extra PV.

When running Wind turbine on my 150/30 MPPT, it just won't settle!, but with a basic 24v charge controller just works, charge controller will short the turbine when battery is full, I also use step up transformer to boost turbine voltage at low wind speed. Will watch Victron products for wind compliant MPPT.

Do use BVM to switch in 20A water heater above 27.5v. This uses 24v relay.

Current thinking is to add small 24v system 2x12v car battery, charged by wind turbine when 24v is low. Just another relay to switch 3 phase into either 24v or 48v charge controller.

Probably Quatro 8k inverter, no plans to have genny, prefer hydro system. Some concern about limited winter sun, and my 5kW GSHP drain battery, will burn some wood to supplement heating.

Many challenges.

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3 Answers
Alistair Warburton avatar image
Alistair Warburton answered ·

Higher voltage = smaller cables and copper is not cheap...

Have you considered putting the inverter in the bunker? Short DC run, longer, but way smaller AC run.

Have you considered charging the 24V from the AC output of your main system, That simplifies things and gives you stable battery backed control voltage.

I am about to run a new 24V control supply bus/loop but it will be derived from a 12V battery system, via a boost converter, or two. The 12V will be on continual float, suppyed by by the main AC output. No as efficiant but way easier than a bucket full of high current relays.

MPPT on a turbine, wind or water, needs to be a very slow loop so that the decion to change the terminal voltage on the turbine, and thus the speed, is based on an average output over several minutes, if not tens of minutes for wind.

You may be better measuring wind speed, again an average, and setting the terminal voltage from a lookup table.

You may want to think about installing a small PLC to manage this stuff, again something I am about to do.


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Robinson avatar image
Robinson answered ·

Bunker is built, has 7x 70mm2 2core cable to power room, bunker is damp not good place for inverter. Major headache is space in barn conversion, battery systems are bulky, can produce noxious gasses, so bunker was solution, FLA battery boil less at 2 to 5C.

I think 48v battery is just better, same 96 cells in 4 strings. But do I need 23 cross links, or can I rely on 4 separate strings to share current. 24v just doubles issues here.

I do accept 240v 24v charger could be simple, not in spirit of design.

Monitor mid battery volts is good idea, which BVM does well, but which string? Or add 4 way selector switch, use my voltmeter.

Think 2nd BVM channel could be more useful to monitor wind turbine current, color control will record this value, until I can buy a Victron MPPT to do wind turbine job.

Will watch web, Victron pride their ultra fast response of MPPT, light changes fast too. You are right time lag on wind turbine should be longer, maybe 1min, it still needs to respond to gusts, but time response is simple variable in MPPT app, just add it in next version update.

I work with PLCs and AC drives every day, be nice to keep this simple. So use less power in January, do washing up by hand - and enjoy view, run wood stove alot, simple off grid existing!

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JohnC avatar image
JohnC answered ·

Hi Doc. 96 large cells? You're in 48V+ territory now, please forget 24V. One saving grace is your mppt's now have double their 24V capacity.

I hope they come with auto-watering. With 4x strings, take care balancing your wire lengths to each too.

Monitoring them could be onerous, and you could consider batt balancers. For those (in Victron blue) you'd need 3x units (12V blocks), all the cross-wiring, etc.

Call me a doomsayer if you will, but this isn't worth doing any way but properly. This is a substantial batt system, and deserves good treatment.

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