How much safer is 24v vs 48v ?

[meetyg]

I'm thinking of changing my current DIY solar system, currently grid-tied with about 1kw of peak solar (4-5kwh a day during summer). I want to have some sort of battery backup for critical loads.

I thought of getting an all-in-one inverter/charger.
I have seen that the higher output ones (more than 3 kw) use 48v, but am reluctant because of the higher voltage. This installation will be in the house, as I don't have much choice (outside temps are too hot to store the battery outside the house).

So I have seen some nice 24v, 3kw inverters online and was wondering if I will regret going 24v. I am also going for low-voltage PV input (30-145v MPPT input).

So in short, how much safer is 24v as opposed to 48v Lifepo4?

Pros of going 24v in my case:
- Safer ?
- Sourcing Lifepo4 cells is difficult here in the Middle East, so going 8S makes things simpler and cheaper. I have 8 100ah cells on order, the price with shipping was already a large stretch for my budget.

Cons:
- Need larger wires
- Limits the installation to around 3kw
- Limits future expansion

Thanks.

 

[saen]

Safer vs productivity?
There is 96V battery, is there a reason for that?

 

[meetyg]

Of course each voltage has its uses. But my question is more along the lines of: "Will 48v shock you, and 24v won't? "

 

[Pim57]

A simple Google search:

48V is the practical and LEGAL definition of the maximum voltage to be considered "low voltage" and intrinsically "safe". Certainly 48V delivered UNDER your relatively insulating skin surface could kill you if delivered in the "right" place. But we are assuming people aren't walking around with subcutaneous electrodes exposed to accidental contact with "LV" wiring. 48V is reasonably safe for most people under normal conditions.

The original (and current) technology wired public telephone system operates on 48V for over 100 years. That old telephone technology is very likely the precedent for establishing 48V as the legal limit.

 

[saen]

Add to cons prices

 

[meetyg]

Add to cons prices

You mean 24v is more expensive?

 

[meetyg]

A simple Google search:

48V is the practical and LEGAL definition of the maximum voltage to be considered "low voltage" and intrinsically "safe". Certainly 48V delivered UNDER your relatively insulating skin surface could kill you if delivered in the "right" place. But we are assuming people aren't walking around with subcutaneous electrodes exposed to accidental contact with "LV" wiring. 48V is reasonably safe for most people under normal conditions.

The original (and current) technology wired public telephone system operates on 48V for over 100 years. That old telephone technology is very likely the precedent for establishing 48V as the legal limit.

Yeah, but a fully charged Lifepo4 could go as high as 56v. Does this make a difference?

 

[Lt.Dan]

I have worked with my 16s bank, and only once have I felt a little tingle when resting my arm across a bus bar on accident. It was 110* outside and I was very moist. The only thing I felt was a little tickle, and just enough to remind me to be safer.

I would not worry too much about safety of 24v vs 48v if the only thing you are worried about is getting shocked. Just be safe and wear PPE.

 

[John Frum]

I have worked with my 16s bank, and only once have I felt a little tingle when resting my arm across a bus bar on accident. It was 110* outside and I was very moist. The only thing I felt was a little tickle, and just enough to remind me to be safer.

I would not worry too much about safety of 24v vs 48v if the only thing you are worried about is getting shocked. Just be safe and wear PPE.

I got one on my sweaty arm in the same scenario with my 24 volt system.
Was learning across the inverter case which is grounded and working on the fuse block.

 

[meetyg]

Thanks.
So now I'm pretty much convinced that there not much of a difference safety wise.

Now I have to address some of the other cons of going 48v, but I guess that's for another thread (inverter related).

If cells were easily available, I wouldn't have any doubts. I was thinking starting small, with 24v. But on the other hand, if I will want more power, this is really limiting (as I mentioned, the highest output for 24v inverters I have seen is around 3kw).

3 kw should be enough for some critical loads, but it might be more difficult wiring this into my existing load panel. I currently have 16a breakers in the panel, going to various loads (3 phases in total, 25a each, split into many 16a breakers, going to sockets and lighting around the house).

If I had say 5.5kw, I could just rewire one of the 25a phases (220v) from the panel, to the inverter output.

 

[Ozark Tinkering]

I got on my sweaty in the same scenario with my 24 volt system.
Was learning across the inverter case which is grounded and working on the fuse block.

I used to maintain batteries in a powerhouse. one system was 130vdc that took care of all the process controls and the other was 277vdc that took care of control room lighting and T/G turning gear. Got my sweaty arm across the 130vdc once and it hurt like shit. I made contact above my rubber glove at the elbow. Thing about dc is it grabs and don't turn loose. I was lucky it wasn't the 277vdc.
Another thing you never want to do is get across the leads of a 2500hp 5kv. synchronous motor after saturating the windings during a 30 minute time study with a 5kv meggar. That will jack you up quickly!

 

[John Frum]

Her

Thanks.
So now I'm pretty much convinced that there not much of a difference safety wise.

Now I have to address some of the other cons of going 48v, but I guess that's for another thread (inverter related).

If cells were easily available, I wouldn't have any doubts. I was thinking starting small, with 24v. But on the other hand, if I will want more power, this is really limiting (as I mentioned, the highest output for 24v inverters I have seen is around 3kw).

The question is how much power do you need?
For stationary systems I would say 48 volts nominal is the default now and picking another nominal voltage is based on specific requirements.

Here is a cheat sheet for beginner systems that demonstrates scale based on inverter power.

easy button systems
    overkill bms
    eve 280ah cells
    simple inverter with hard coded low cutoff of 2.5 volts per cell
    or inverter with remote switching and a victron battery protect for configurable low voltage disconnect
    or inverter/charger with built-in configurable low voltage disconnect
4s 120
    120 amps * 10 volts low cutoff * .85 inverter efficiency = 1020 inverter watts
    120 amps * 12 volts low cutoff * .85 inverter efficiency = 1224 inverter watts
8s 100
    100 amps * 20 volts low cutoff * .85 inverter efficiency = 1700 inverter watts
    100 amps * 24 volts low cutoff * .85 inverter efficiency = 2040 inverter watts
16s 100
    100 amps * 40 volts low cutoff * .85 inverter efficiency = 3400 inverter watts
    100 amps * 48 volts low cutoff * .85 inverter efficiency = 4080 inverter watts

 

[meetyg]

Her


The question is how much power do you need?
For stationary systems I would say 48 volts nominal is the default now and picking another nominal voltage is based on specific requirements.

Here is a cheap sheet for beginner systems that demonstrates scale based on inverter power.

easy button systems
    overkill bms
    eve 280ah cells
    simple inverter with hard coded low cutoff of 2.5 volts per cell
    or inverter with remote switching and a victron battery protect for configurable low voltage disconnect
    or inverter/charger with built-in configurable low voltage disconnect
4s 120
    120 amps * 10 volts low cutoff * .85 inverter efficiency = 1020 inverter watts
    120 amps * 12 volts low cutoff * .85 inverter efficiency = 1224 inverter watts
8s 100
    100 amps * 20 volts low cutoff * .85 inverter efficiency = 1700 inverter watts
    100 amps * 24 volts low cutoff * .85 inverter efficiency = 2040 inverter watts
16s 100
    100 amps * 40 volts low cutoff * .85 inverter efficiency = 3400 inverter watts
    100 amps * 48 volts low cutoff * .85 inverter efficiency = 4080 inverter watts

I guess I forgot to take inverter efficiency into account. Thanks for the reminder.

Decisions, decisions... :-(

The problem with going 48v (in my case) is that there is a large investment in the beginning, mainly for the cells... But I'm starting to think that maybe it's the right choice. Just wasn't planning on the large investment up front. Most inverters recommend at least 100ah batteries.

I have 8 x 100ah cells on order from Gobel Power. Cost me 850 USD including shipping (by sea). If I were to go with Air freight, shipping would cost me almost as much as the cells themselves.

I guess that in the U.S. you guys have it easy, as many dealers have local warehouses these days. Here in the Middle East, it's a nightmare trying to get cells (not to mention taxes, customs and local bureaucracy).

 

[Lt.Dan]

Don't forget even though your 24v inverter is only 3kw, most of the AIO (MPP or Growatt) can be stacked for more output. This way you can keep adding cheaper 24v batteries/inverters as money allows.

 

[MattiFin]

Electrical shock wise not that huge difference.
but electric arcs are nastier with 48 volts and you NEED to have proper fuses in 48v system. Some fuses are rated for 32v but not 48v.

24v system is somewhat more tolerant for stupidity(accidental short-circuits) than 48v system. (12 volt system just makes sparks but not much of an actual electrical arc as anyone who has attempted to weld with car battery knows)

 

[MarcoWasRight]

Did you ever make a decision on ordering 8 more cells? I was going to go 24V because my first set of batteries is a bunch of NMC I haven't arranged in series yet. The more time I thought about it, the more I realized I was going to go LiFePo4 as soon as the housing market crashes and I can snatch up a foreclosure. Looking at all in one units its pretty obvious that 48V gets the most choice whereas 24V seems like an afterthought to some manufacturers.
I just think it would be a big waste of money (for me...but probably you too) to spend money on a 24V system when 48V is where everyone with a stationary setup ends up (mostly).

 

[meetyg]

Yes, I ended up going 24v.
I'm still in the process of installing. I got a MUST PH1800 3kw 24v inverter, but as with many recent Chinese/Taiwan inverters (including some MPP models) it doesn't have ground-neutral bonding internally.
I'm pulling my hair to try to solve this issue.. really don't understand how they release an AIO without a bonding relay (or other solution).

As for the battery, I'm also in the process, waiting for some thicker bus bars to arrive. The ones I got with the cells are ridiculously small for 105Ah cells.

So I can't really tell you if I regret it or not, yet.
But I agree that 48v systems are a bit more common, at least on the forums.

 

[Zapper77]

I can confirm that while sitting on a concrete slab and touching the positive battery terminal (when charged at 54 volts) you may feel a tiny zap like sticking a 9v battery to your tongue. (Well I did, your results may vary). Now I can confirm that 370v solar will tingle even when isolated with rubber shoes.

 

[meetyg]

Yeah, those HV solar arrays really scare me. That's why I opted to go low-voltage (~100v).
This limits expanding my solar array, but I feel more comfortable with it. Although 100v is still alot, it's less than those 250-500v arrays.

 

[fafrd]

Yes, I ended up going 24v.
I'm still in the process of installing. I got a MUST PH1800 3kw 24v inverter, but as with many recent Chinese/Taiwan inverters (including some MPP models) it doesn't have ground-neutral bonding internally.

Where did you purchase from, what did it cost you, and how long did it take?

I’m also looking for a 24V hybrid and didn’t know about the offering s from MUST prior to this thread.

Very interested in your impressions of build quality and performance as you get things working…

I'm pulling my hair to try to solve this issue.. really don't understand how they release an AIO without a bonding relay (or other solution).

Because it’s not an inverter that was designed for the US market. EU is single-phase 230VAC…

As for the battery, I'm also in the process, waiting for some thicker bus bars to arrive. The ones I got with the cells are ridiculously small for 105Ah cells.

So I can't really tell you if I regret it or not, yet.
But I agree that 48v systems are a bit more common, at least on the forums.

I also have a string preference for 24v because of the fact that a 24V batters can be charged by a 1S string. I have severe shading that moves across my array in the morning, so I get +15% daily output from a parallel 1S array versus what I’d get from a 2S or higher series string,

How did you learn about MUST? What do you think about the build quality?