using 24 v or 48 v system?

castilloj · February 2015

Hello, I'm trying to figure out which system would work better for me.
i currently have 4 435 a/h trojan 6v battery and I'm considering getting 4 more.
My question is would i get more power to work with if i do this? i currently use my battery from 12 pm- 5 pm then from midnight to 8 am.
in between those times I'm charging with my 20k diesel generator.
my loads during inverter times is 3 amps (per outback Mate)

westbranch · February 2015

What are your loads? Are you planning to put any heavy amp demands on the system once you add more batteries? If not stay at 24V and parallel otherwise you will need at least a new inverter.
How old are the current cells? Mixing new and old may deplete the new ones faster than expected.

castilloj · February 2015

i run a side by side fridge with the inverter i also run box fans in the windows, tv and satellite box.
The older batteries are about 2 years old.
also i do have the 48 v inverter its just not hooked up.

westbranch · February 2015

OK, is the wiring oversized to handle the 48v? if no, it might just be a toss of the coin. Do you plan more PV in the future?
Icarus' rule #1 is: Loads grow with time...

xsnrg · February 2015

If the wiring was sized for 24v, it should handle 48v without problem as the amps will be 1/2 of what they would be with 24v. Just need to make sure the rest of the system is rated for 48v. If you have the inverter already, then doing a single string of batteries will be less worry than doing two strings at 24v. Your usable KWh will be about the same either way, but losses slightly higher with 24v.

BB. · February 2015

Note--many common DC breakers and Fusees are rated for ~24-30 VDC maximum. If going to 48 volt system, you need to look for 48 volt or even 60+ volt rated protective devices.

-Bill

softdown · February 2015

Broad Rule of Thumb:
Under 1 Kw PV array: 12 volt system
1-2 Kw PV array: 24 volt system
Over 2 Kw array: 48 volt system

CALLD · February 2015

softdown wrote: »

Broad Rule of Thumb:
Under 1 Kw PV array: 12 volt system
1-2 Kw PV array: 24 volt system
Over 2 Kw array: 48 volt system

I'd go with this rule of thumb, but there are more things to consider:

1) As others have mentioned, above 30V DC you need to consider arcing issues with breakers. DC arcs don't self extinguish easily as any arc-welder will tell you.
2) Going with a higher voltage system is always better for efficiency as wiring need not be so heavy for a similar wattage. Power loss on the same wire gauge is exponential when comparing different voltage systems.
3) The only reasons people ever choose lower voltage battery systems is cost and simplicity. But the trade off is an inefficient system with very heavy wiring and fuses.

I run a 24v system but this is dictated by my inverter. My array is 1490watts and inverter is 2500watts. I cannot easily expand my system further without changing to 48V. I would have to buy a new inverter to do that, hence the problem with a lower voltage.

castilloj · February 2015

well, I'm currently using a outback VX3524 hooked up to a e-panel with a 250v breaker. I'm also using a morningstar mppt60, i also have 8 sonali 240w panels. But my main charger is my 20k diesel generator that i have to run 4 hours in the AM
and 6 hours in the evening.

BB. · February 2015

castilloj wrote: »

well, I'm currently using a outback VX3524 hooked up to a e-panel with a 250v breaker.

Is that a 250 Amp breaker?

I'm also using a morningstar mppt60, i also have 8 sonali 240w panels. But my main charger is my 20k diesel generator that i have to run 4 hours in the AM
and 6 hours in the evening.

A question on how you use the system... My guess is that you use the VX for night time/off peak load periods... And use the 20 kWatt genset primarily for lots of AC loads that would swamp a 3.5 kWatt AC inverter... And possibly a little "free" battery charging from the genset.

The primary load of your 20 kWatt genst is not charging the 24 volt battery bank/running as running a 3.5 kWatt inverter in AC pass through mode (20 kWatt genst ->24 volt battery bank would only be 10-30% of the load of such a large genset) would be fuel inefficient and light loads on the Diesel could cause mechanical/maintenance issues.

-Bill

castilloj · February 2015

Thats exactly how i use the system. Because i don't have enough panels to completely charge my system , I always used the genset to do it. also when the Gen is running i turn on a little 12,000 btu AC.

and yes thats a 250 amp breaker on the E-panel.

castilloj · February 2015

When i spoke with Trojan for the charging parameters, they suggested i put my low battery cut out at 24v. the problem is that it doesn't give me much power usage .

westbranch · February 2015

Castilloj, setting that LVD at 24 means you are saving your battery from failure/death, and when you disconnect you should see a rebound in the voltage... that no draw voltage should be near a 50% DoD...

castilloj · February 2015

I understand why the LVD should be that. thats why I'm wondering if i need to add more batteries or switch to 48 v system.

BB. · February 2015

But, under heavier loads, your cutout voltage, perhaps, should be closer to 23.0 Volts--That will give you more usable capacity and not have starting surges knock your inverter off line.

Also, check the specific gravity when you have your shut down programmed to 24/23/etc. volts--And see what the battery is really at.

Also, having very short/heavy cables from the battery bank to the inverter will help reduce "phantom" workings/errors (i..e., something like 1 volt drop due to battery to inverter cable voltage drop is not unusual).

The 24.0 volt setting is usually a bit more relevent for measuring a battery after ~3+ hours of resting (or very light loads)... It may be less useful for you if you have (relatively) heavy loads/surge loads.

Some devices allow you to use ~24 volts for 4 hours or 23 volts after 4 minutes type thing... You have to go through your equipment manuals to see if your hardware can do that.

Also, do you have any Battery Monitors? They can display, and a few can give an alarm contact, your battery's estimated capacity--Useful for non-technical folks (i.e., 75% or less, cut back on power usage--especially if poor weather is forcast; if 50%, start the genset; if below 50% and genset is running, or you cannot start the genset, call "Castilloj" now, turn off all non-essential loads, etc.).

Battery Monitors have "their own charm" and can be miss-leading or wrong--Specific Gravity/monitoring voltage/loads/charging is still needed to make sure the Battery Monitor is not drifting.

Don't mean to turn this into a battery monitor thread--Unless you want too.

-Bill

castilloj · February 2015

Can you recommend a good battery monitor? right now I'm relying on the Mate. I also have a HydroVolt meter that will be here next week.

and going thru my stuff i notice i have a [FONT=Helvetica, Verdana, Arial, sans-serif]Xantrex Link Power Monitor Part # 220-1067-01-01- all i have is the Shunt and the monitor. I'm missing the wiring and the fuses and temp sensor.[/FONT]

BB. · February 2015

Just from what I have read...

Battery Monitors from NAWS:
http://www.solar-electric.com/batter...smonitors.html

Trimetric--Not too expensive, a bit like programming a VCR clock (12:00AM blink). Some folks have a bit of issue the first time through
Midnite Voltage Monitor--Voltage monitors may not be as accurate (but may not have "drift" issues that sometimes plague shunt type monitors). Midnite version has a red LED if the batteries have not been fully charged in the last 7 days

Others that I have read about:

Victron--Very nice reports. At least one model has alarm contacts and may have a computer interface (somebody here recently posted from a Victron--As I recall)
Smartgauge--Another voltage only monitor out of UK--They have a nice website for technical details about their monitor and batteries--Something to research
Xantrex LinkPro--Similar to Victron unit, has programmable alarm contact. I don't know about availability

And, next time, Midnite makes two controller families with integrated battery monitors. The larger Classic, and the smaller (30 amp Kid). Just need a $xx shunt and a bolt on-remote $45 ADC board.

Outback makes an integrated monitor--I don't really know anything about it.

If you describe what you would like from a monitor--I am sure some other members can chime in with their recommendations.

-Bill

castilloj · February 2015

Im usi g 4/0 cables between my batteries. Also i ordered the Victron 700 meter.

using 24 v or 48 v system?

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