DC power supply

[Dan McCosh]

I would like to use a battery charger as a DC power supply to run a refrigeration unit. Anyone know if there is a problem with this? There is plenty of power, but I don't know what the effect of the unfiltered rectified current might be on the motor.

[gary porter]

Dan, if you have a very good charger you can use it but it should be connected to a batter as well. Chargers generally don't put out clean enough voltage to safely run an appliance. By putting a battery across it you cause the output to be smoothed if you will. You might also go to an electronics store and find what is called a battery eliminator. This puts out proper clean DC voltage.
Good Luck
Gary

[ion barnes]

Since you must already have 120vac (to run a battery charger) I would suggest using a power inverter or transformer. It will avoid the problems that Gary mentioned.

[carioca1232001]

ion barnes, your suggestion above is NOT APPLICABLE

His fridge runs off DC (direct current).

Power inverters convert DC into AC (alternating current).

Likewise, transformers are able to step-up (or step-down) AC voltage - not the case here.

A viable option is to connect electrolytic capacitors (cylindrical-can-type as used in power supplies) across the output of the battery-charger and then connect up to the fridge - but this depends on a series of factors.

Give the charger/fridge manufacturer a tinkle to find out :

1. Charger-type:

if standard type using 60 Hz transformer(110V AC-to-say 18V AC) followed by full-wave (or half-wave) rectifier bridge, plus some regulating circuits (primitive ones have none!);

or the more modern type, where the 110V AC is immediately rectified (to 110V DC), followed up by a 110V DC-to-12/15V DC high-frequency power converter;

2. Maximum charger current output at rated output voltage (this data is likely to be available in the charger manual); this is to ensure that the charger can match the fridge motor´s starting current , which it probably can anyway as it handles uncharged batteries;

3. Rated voltage for fridge, permissible voltage "droop" (could be 2-4 Volts below rated voltage) before fridge motor cuts-out, as well as fridge motor starting current;

4. The peak voltage, as well as the peak-to-peak "ripple" voltage on the unloaded charger output;

the standard-type charger can have considerable "ripple" depending on the capacity of the internal capacitor bank (if installed at all)and a primitive one have no voltage regulation at all;

the modern type of charger is virtually "ripple" free ("ripple" = 0V)and it´s peak unloaded output voltage would be close to battery voltage (13-14 V DC), plus it´s regulating electronics quick to act.

Get back to me if you have the data - and I could help you to size the capacitor, if you are interested.

To measure peak unloaded voltage and peak (or rms) unloaded"ripple" on your existing battery charger with an Avometer or DVM:

Switch your DVM meter to DC volts, connect it to charger output and register the value displayed - you now have peak output voltage at no-load.

Without altering the connections, switch your DVM to AC - you will register RMS or PEAK "ripple" depending on your selection of the pushbuttons

[Klaus]

IMO, it would be much wiser to have the charger charging a battery and running the fridge of this battery. This is how these fridges are designed to be used off 12Volts (at least all of the one's I've come across).
Battery acts like a giant reservoir for startup power peaks and absorbing charger ripple.
If the charger output matches the fridge's running Amps then the battery need not be very big.
Do set the charger Voltage so the battery won't get overcharged when the fridge cycles off.
Klaus

[carioca1232001]

Agree, as long as he has room for that battery.

Perhaps he could use a pair of compact motor-bike (6 V DC) batteries in series.

[Frank Wentzel]

Dan the battery idea is fine for short term use but over an extended term the batteries will die from excessive overcharge. The capacitor idea is the best bet for using an inexpensive battery charger to run your refer. However they can be quite expensive in themselves - almost negating the savings on a cheap battery charger. But this is your lucky day! I have a mess of large capacitors around and would love to get rid of some of them to a beneficial use (rather than tossing them in the trash - I find it almost impossible to dispose of "good" stuff). Send me your mailing address by e-mail and I will send you all you need to flatten out your DC (no charge, of course).

/// Frank ///

[carioca1232001]

If it is a cheap charger, consisting of a transformer, rectifying bridge and practically nothing else, it´s output is going to swing quite a bit when the motor cycles on and off.

I don´t believe the capacitors alone will take care of this voltage excursion, that could exceed the max voltage rating of the motor and thereby damage it. You will probably need a power-zener-diode working in unison with the capacitors.

It will therefore be necessary to obtain further data on the intended setup, as I have outlined above.

[Dan McCosh]

It seems the motor operates at 12-25 volts, and it is recommended that you use the higher voltage wnen operating at AC. This makes the battery charger a bit iffy, so I think I'll go with a new power supply. Thanks for the input.

[carioca1232001]

If the fridge motor automatically adjusts itself for voltage variations between 12 - 25 V DC, it probably has a high-frequency power converter up front.

[hikingchrs]

Originally posted by Dan McCosh:
It seems the motor operates at 12-25 volts, and it is recommended that you use the higher voltage wnen operating at AC. This makes the battery charger a bit iffy, so I think I'll go with a new power supply. Thanks for the input.do you mean that this can run on AC or DC if so get a 120v to 24v transformer and run it on AC.

[EVANS Wilson]

Yes, I am thinking of installing it the other way around...

Increasing my Battery capacity a bit, installing a 3000W invertor to convert 12v. to 120v.Then installing a small freezer (3.7 cu. ft.).(Wrapping it with more insulation,cooling fan etc...).

The idea was to fill it completly with Ice\water bottles, and some meat,frozen stuff in plastic containers, etc. I was thinking I could rotate the Ice Jars to the cooler every couple of days.

I thought that a combination of periodic use of the motors alternator, (while cruising)or limited use of a generator while at anchor would maintain a low enough temperature.

Shore power of course is availabe while at home.

I am looking at a 3000w invertor from Canadian Tire, price 500.00$ +tx Cdn.

There are other models available that I will check out during the Winter.

Just wondering what you opinions are on my rather home spun project of freezer\ice box power supply.

I live in North Eastern Canada, Quebec (Harrington Harbour). The summer time Temp rarely gets above 25 celcius (70ish F).

Any advice is appreciated.

Wilson

[Dan McCosh]

As I understand it, the central issue is that the DC compressors are more efficient than the generally available AC compressors for home-style units. The inverters likewise aren't very efficient, and the net result is that running an AC compressor from an invertor is a high battery drain. We used a shore--powered unit for a number of years, and it was fine but didn't affect the needs while anchoring out. We are aiming at increasing our offshore capacity to about two days, with much of the time not cruising spent at the dock with an AC power supply (which supplies 24 V DC) running the cooler.