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Old 22-08-2015, 11:03   #1
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runningdc 24vdc system at lower than design volts.

Hi,

I have an academic question.

Consider a 24v system on a boat. What would be the issues to be faced if the system was run at a nominal 22.5v, with low being 21v and the high of 24.9v.

Here are my thoughts.

1. higher amperage, perhaps 18% higher on average. So wires would need to accommodate. Assuming they are ok in size, other issues related to amperage?

2. Inverter might not like it so much.

3. cabin fans, hella and the like, would it wear them prematurely?

4. Windlass might not like this, voltage drop over wire + lower voltage to start might be an issue.

5. ???


Yes, this question relates to our boat. I am not filling in the blanks to attempt to keep the discussion on the issue presented above. Please try to resist guessing why I am asking for now, I will share later.

Thanks!
Chris
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Old 22-08-2015, 11:23   #2
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Re: runningdc 24vdc system at lower than design volts.

You will murder your batteries by undercharging them.
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Old 22-08-2015, 12:00   #3
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Re: runningdc 24vdc system at lower than design volts.

You aren't going to damage any of the DC peripheral devices. Very few things actually require a full 24v and will operate normally on lower voltages. Most electronics accept a wide input range, and inductive loads such as fans and the windlass will just have less pulling power than otherwise.

Amperage isn't going to be an issue. If you're running 24V on a cables originally sized and designed for 12V, you should be fine as far as amperage is concerned, as higher voltages have lower amps for the same wattage. Obviously you can't run 12VDC devices on 24vdc unless they're specifically designed and rated for it (a lot of marine electronics can run on either, you have to check each device's feed power requirements).

HOWEVER, batteries, combiners, chargers, and inverters can be very picky about feed power depending on chemistry (batteries) or technology. You'll just have to check them.

Are you going to tell us why you're running lower than normal voltage? Otherwise I'm going to have to start guessing...
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Old 22-08-2015, 12:12   #4
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Re: runningdc 24vdc system at lower than design volts.

I'll tell soon. For this discussion assume that the batteries are a non issue.

The system was designed for 24v initially, not converted for a 12v system.


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Old 22-08-2015, 14:44   #5
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Re: runningdc 24vdc system at lower than design volts.

I'm going to assume you're looking at an alternate storage technology/chemistry that doesn't match exactly with lead-acid 24V systems.

It will all depend on each piece of equipment you have. Our fridge compressor, for instance, will run down to 21VDC so would probably work on your system as long as you don't have too much voltage sag. Incandescent lights would work but be dim. Fluorescents and LEDs may or may not depending on their power supply configuration. The windlass will likely work, but current draw will go up.

A lot of the heavy-use items (windlass) are really designed to operate at 26-27 volts (voltage with the alternator running) so if you try to run them on your lower voltage system when not charging the drop from design voltage is actually greater than what you are expecting (it's not a drop from 24 to 22 but actually a drop from 26 to 22).

Another alternative for smaller loads is to use a DC-DC converter and use it to create 13.8V circuits for things that like a good, solid voltage.
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Old 22-08-2015, 16:10   #6
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Re: running dc 24vdc system at lower than design volts.

Yes, alterative chemistry. We already have a 24>>12 voltage converter that powers the computer, some nav equipment and such.

We are working towards 100% LED, and they are all wide voltage, so thy are fine. We do have one florescent light left, but I could change that over pretty easily.

Anyone hazard a guess as to how the cabin fans might fare? We have a couple of 24v hellas, and two reversible 2 speed fans of unknown origin in the heads.



Chris

Quote:
Originally Posted by Dsanduril View Post
I'm going to assume you're looking at an alternate storage technology/chemistry that doesn't match exactly with lead-acid 24V systems.

It will all depend on each piece of equipment you have. Our fridge compressor, for instance, will run down to 21VDC so would probably work on your system as long as you don't have too much voltage sag. Incandescent lights would work but be dim. Fluorescents and LEDs may or may not depending on their power supply configuration. The windlass will likely work, but current draw will go up.

A lot of the heavy-use items (windlass) are really designed to operate at 26-27 volts (voltage with the alternator running) so if you try to run them on your lower voltage system when not charging the drop from design voltage is actually greater than what you are expecting (it's not a drop from 24 to 22 but actually a drop from 26 to 22).

Another alternative for smaller loads is to use a DC-DC converter and use it to create 13.8V circuits for things that like a good, solid voltage.
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Old 22-08-2015, 16:57   #7
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Re: runningdc 24vdc system at lower than design volts.

We would also be charging at 24.9, so the windlass remains my biggest concern. it is an old SL Francis horizontal, 24v. Given the cost to replace, I certainly would not want to do anything that would damage or affect it's motor life.


Chris
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Old 22-08-2015, 17:41   #8
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Re: runningdc 24vdc system at lower than design volts.

For all your fans, I suspect they will be fin just run slower than what you are used to.

For your windlass my advice is to get the motor data plate information and research what the motor is designed to handle. In a worse-case scenario you could have an additional battery or just a few cells mounted at the winch that are charging and can be switched into the circuit in series when the windlass is energised. An example that is not the best but probably the simplest is to just have 2 12V batteries up there in parallel with a charger and they get switched via solenoids to series to energise the windlass. However, first thing I would do is check the motor data and then measure what the actual loaded voltage is when it is operating and see if it is workable as it might still be in spec just slower.
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Old 23-08-2015, 09:15   #9
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Re: runningdc 24vdc system at lower than design volts.

Trying to simplify here...

Some items will run slower and dimmer (generally motors and DC lamps). They will draw fewer amps, and will be running on fewer volts, so it's sort of a "squared" effect on the amount of power they will be drawing. Thus, you'll get lower useful results out of them (less water pumped, lower light output.)

Some items won't care. Mostly electronics with a wide input voltage listed on the back of the unit. If a product that consumes 24 W operates at 24V, it draws one amp. But if you drop the voltage to 21V, it might draw 1.15 amps to compensate.

I don't know if there are "constant current" loads, but if so, they would also be operating at lower power input.

Mostly, I don't think it would make much difference.

Chuck
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Old 23-08-2015, 11:00   #10
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Re: runningdc 24vdc system at lower than design volts.

Quote:
Originally Posted by witzgall View Post
Hi,

I have an academic question.

Consider a 24v system on a boat. What would be the issues to be faced if the system was run at a nominal 22.5v, with low being 21v and the high of 24.9v.

Here are my thoughts.

1. higher amperage, perhaps 18% higher on average. So wires would need to accommodate. Assuming they are ok in size, other issues related to amperage?

2. Inverter might not like it so much.

3. cabin fans, hella and the like, would it wear them prematurely?

4. Windlass might not like this, voltage drop over wire + lower voltage to start might be an issue.

5. ???


Yes, this question relates to our boat. I am not filling in the blanks to attempt to keep the discussion on the issue presented above. Please try to resist guessing why I am asking for now, I will share later.

Thanks!
Chris
Don't try to reinvent the wheel. There have been and are 24, 32 and 36V systems but the end user, fan, lights, etc.. May put you in the poorhouse.
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