My 3.0kva UPS for computers with additional batteries is a 72v setup.

Bobdown wrote:

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 Just interested as to why 36V Mike?

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Hello Bob

Good question.

My experiments to date lead me to believe the optimum battery capacity for my situation is around 3.5kWh to 4kWh. A 120Ah lithium battery has a usable capacity of around 1.3kWh so three of them would be ideal but, of course 3 x 12 = 36V - hence my interest. I could go for four 12V batteries (48V) but this adds $1k, 15kg and extra room to the situation. Another alternative is to go for 12 individual cells, which is probably a better solution.

Going to a higher voltage has the significant advantage that it reduces current; eg. a 1500W microwave requires 125A at 12V but at 36V only 42A. This has the advantage that it makes the electrical installation much easier, reduces the fire risk and improves reliability.

Unfortunately 36V is not a common DC supply voltage so I may end up with 48V anyway but I thought I'd put feelers out to see if anyone is using it.

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Edit:

To expand; I'll leave the caravan at 12V as it's far too much trouble to change all the caravan internal stuff (radio, lights etc) to anything else and will continue to do as current which is to let the original caravan roof mounted solar panel do its thing but, via a time switch, use the external system to provide 240V for two hours (whatever?) per day to the caravan to top up its 12V system. This is a little wasteful of energy but, generally, I have enough solar not to worry about that.

-- Edited by Mike Harding on Friday 27th of January 2023 06:26:02 PM

Hi Dick0

>Run the van 12V through a 36V to 12V step-down converter.

I would not do that; a 36V/12V power supply will have losses at least equal to those of an inverter and is an unnecessary expense and complication. Better simply to use 240V to run the whole van, as I do currently, and allow that to charge the van's 12V battery.

>Have the solar panels in Series Array to solar controller at 36V.

I prefer panels in parallel for my style of bush camping.

>The question being... What is the overall benefit over a complete 12V Parallel system?

12 volt systems have a number of downsides: 

High current with consequent power losses in cables

Increased fire risk due to high currents

Very low tolerance to voltage drop caused by those high currents or sagging batteries

To be honest Dick0 it's hard to thing of a single benefit to a 12V caravan system and I'm sure the only reasons cars/caravans run on 12V is historical and the fact it only needs one battery. Most trucks, I think?, run on 24V or 48V.

I'll continue to examine the 36V route but I'm wondering if it may not be worth absorbing the extra battery dollar/weight/space cost and going to 48V which is a more common DC supply voltage. Watch this space :)

Thank you Les, but unfortunately neither Google not I have any idea what you mean?

12, 24 and 48 is well covered with equipment, other voltages, not so much. I have a 5000VA system running on 24 volts and can't see any technical reason for going to 48V

Sorry Mike try this oz X pro

Yep thats it Mike and I also found it to be an odd voltage .

I can't see the logic in going to 36V for a basic DC run to the inverter. Unless there is some serious space constraints. Pretty easy to achieve only 1% power losses and man handle 4/0 cable in a single cab setup. Paralelling 12V batteries is dead simply and allows one to quickly isolate just one in the event of a problem with the battery or with any maintenance. Lithium has basically no peukert effect, you just have to worry about the IR, which will not be any issue if the battery is sized sensible, that means the voltage to the inverter should never drop below 12V. What ever the IR of the new battery is, multiply it by ~1.5 to 2 times to get a ball park of it's aged IR down the line, factor in a cold day (10-15C) and at just 40%SoC OCV which gives about 1v to play with. Plenty! The higher cost of thicker cables will be a one time affair, I can't see any benefit with just ~5M of cable, even 10M it's just not worth worrying about in real life. The reduced fire risk is more valid for poor crimps and practices. The danger of loose/ bad connections is still largely present with 36V. You have to use professional crimping tools and periodically check for signs of high resistance in all your connections, fuses etc. 36V won't change this at all. In fact at lower currents of 20-30amps is enough to start a fire with a loose lug. I use a little IR temp gun to look for the smoke n gun under high cont load, about 10degrees C above ambient is what your looking for. Also the insulation blackening usually happens ahead of a bigger problem. Basically with high performance lithium you must watch your system like a hawk, I'm constantly checking voltages, IR, impedance, and tightness of connections, among others. Others will say mate your thinking about it too much... Ahh yes, the old saying you are what you eat springs to mind, I'm also the one who never has to ask for help with my system. The only area where high voltage works wonders for me is portable solar panels, since my cable runs are 20to 40meters, here 18V panels are impractical. I'm still waiting for someone to make 100v modules... I don't want to brag, but it's very easy and practical to run 2500va inverter loads off a 12V system. What with nowadays all the kinks being ironed out and all the lithium mysticism folded in on it's self.

Mike is still a member but does not come back here much. You could send him a PM. He has established his own forum.