PUR 40E On 24V to 12V DC to DC Converter, starting amperage

[botanybay]

I have a PUR40E from my last boat which ran on 12V, the new boat is 24V, I am looking to use one of the Victron DC to DC converters to take the 24V down to 12V right near the watermaker.

The watermaker claims to use 4amps at 12V, however, I don't know if that is a nominal 12.6V, 12V even, or even at 14.4.

So two questions,

1) what is the peak amperage of the 12V unit needed to get it started

2) what is the optimal voltage for the watermaker (some of the converters can be adjusted as high as 15V)

Available units (not isolated output, those are not as efficient (these are ~92% efficient))
continuous / peak / voltage
5A / 5A / 12.0V
12A / 20A / 12.0V
17A / 25A / 12.0V
25A / 35A / 10V - 15V
40A / 55A / 12.0V
70A / 85A / 10V - 15V

My guess is that the 12A / 20A version would be quite sufficient although the 17A / 25A is almost the same price.

The 25A / 35A version would allow boosting the voltage up to normal charging voltage of 13.6A and provide a backup for the SSB converter although I will probably use an isolated version for the SSB.

I may have to go and drag out a scope and look at the inrush to see what is happening and remembering the sound of the unit I doubt that the 4A is truely continuous.

If anyone has looked at this or has thoughts I would most appreciate the feedback!

David

[MARC D]

Wouldn't be simpler to tap 12 vdc in your battery bank and run that pair thru a fuse to supply your watermaker?

[Sailmonkey]

I've never measured the starting current, but with an analog ammeter I can tell you the 4 amps is an average. From memory, the current varies from 2 amps to 8 amps depending on where in the stroke the plunger is. I would bank on 16+ amps initial inrush.
The current does drop dramatically when the engine is running and the voltage is near 14+ volts.

[botanybay]

Originally Posted by Sailmonkey

I've never measured the starting current, but with an analog ammeter I can tell you the 4 amps is an average. From memory, the current varies from 2 amps to 8 amps depending on where in the stroke the plunger is. I would bank on 16+ amps initial inrush.
The current does drop dramatically when the engine is running and the voltage is near 14+ volts.

Very interesting, thank you very much for the information. I am thinking that the right path forward is a DC to DC converter with a settable voltage which would allow me to set the input voltage to 14.4V (should be within design due to acceptance voltage for wet cells) to keep the amperage down. The smallest Victron DC to DC converter with a settable voltage is the 25A Continuous and 35A peak which should easily handle the required power levels.

I might be able to use the 17A continuous 25A peak unit or the 12A continuous 20A peak but these are fixed voltage units at 12V.

I will let you know what I find when I start getting these units up and running.

David

[botanybay]

Originally Posted by MARC D

Wouldn't be simpler to tap 12 vdc in your battery bank and run that pair thru a fuse to supply your watermaker?

That would not be a good idea.

The reason is that your battery bank would be perpetually fighting becoming "un-equalized". Specifically, the lower half (because of where ground is you would have to use the lower half) would have energy pulled off. So let's say the watermaker uses up 10% of the available amp hours of the lower half of the bank, then when charging the voltage is applied across the whole bank. The result, the lower half of the bank never gets fully charged and the upper half of the bank consumes excess water because the voltage of the cells in the lower half of the bank have lower voltage allowing the individual cells in the upper half of the bank to see excess voltage.

A far better solution is to use a voltage converter, they are now very inexpensive and are quite reliable. As a backup, the generator starting battery happens to be 12V and so I could run off that as a backup.

David

[botanybay]

Originally Posted by Sailmonkey

I've never measured the starting current, but with an analog ammeter I can tell you the 4 amps is an average. From memory, the current varies from 2 amps to 8 amps depending on where in the stroke the plunger is. I would bank on 16+ amps initial inrush.
The current does drop dramatically when the engine is running and the voltage is near 14+ volts.

It turns out I have a spare 18-30V -> 13.6V converter which is rated at 12A continuous and 18A peak, it is a fully isolated model so there is no ground connection which might actually be a good thing from a corrosion standpoint as the watermaker is floating with respect to ships ground. However, the fully isolated models are a bit less efficient (88% vs 94% efficient).

The upshot is that with a 12A converter I am only seeing about 0.1V of change from the highest current to the lowest current in the cycle. So, based upon this it would appear to be sufficent. I am running with a 5A fuse on the 24V side and it is working fine so that suggests the peaks are not over perhaps 10A.

Thanks for the feedback on the observed current. I will keep the list appraised as to how the system works out.

[MARC D]

To David:

Thanks for the explanation!
I thought the current flowing within the entire bank would ''equalize'' the voltage, sort of.
Is it the inner resistance of each battery that will cause problem you are talking about?

Marc

[botanybay]

So far the DC to DC converter is running the watermaker very nicely. The DC to DC converter is making a bit of heat but just slightly warm to the touch. The stability of the voltage is really nice as the speed of the motor on the watermaker is very stable.

Currently running the watermaker at 13.95V with fluctuations down to 13.80V

I have run the unit for two hours straight this way and it seems to be working very nicely.