Electrical Question

[Turtle59]

G'day to the brains trust among us that know a **** load more than I,

I'm wondering if someone can solve my issue for me.

I'm trying to run my 12v fridge on my truck. So here goes. My FG 649 is a 12v system with 2x12v batteries running in parallel.

How can I run an isolator switch on the batteries to isolate one battery to run the fridge but still have both batteries available to start the truck??? Is there such a result.

Or do I need to run extra batteries to run the fridge?

If I were to run extra batteries would the alternator on the truck be sufficient to charge more batteries?

I look forward to your views.

Cheers

 

[SkiFreak]

You shouldn't isolate one of the start batteries. Having a second system for the fridge is a much better and safer system.

The way dual battery systems normally work is that there is a voltage sensitive relay (VSR) that is connected between the start and house batteries. When the vehicle is running the alternator will charge the start batteries and when they reach a specified voltage the VSR will engage and allow charging of the house system. So, in response to one of your questions, the alternator in your truck should work just fine for this type of system.
On the flip side... when the truck is not running the the VSR will isolate the start batteries from the house system, so you will not drain the start batteries while running your fridge.

Google is your friend... just search for dual battery systems.

 

[gait]

I can't picture a way of automatically separating the batteries for fridge and connecting for vehicle start. If you must then a manual isolator switch would work.

While it can, and no doubt is, done, its not recommended, its inherently better to separate vehicle and house.

The conventional arrangement is a "dual battery" system. Worth a google.

Apart from manual isolator the simplest is a probably voltage sensitive relay. Charges house when vehicle running and volts sufficient.

The alternator will typically be more than sufficient to charge both vehicle and house.

 

[LeishaShannon]

How big is the fridge, do you have the power specs for it , and how long are you wanting it to last between drives?

 

[Turtle59]

How big is the fridge, do you have the power specs for it , and how long are you wanting it to last between drives?

The fridge is a new Waeco CFX 65 DZ. Power consumption is 1.9ah/h. I would hope to be able to get 2 days out of the battery but I do have a folding solar panel rated at 180watts to plug into the truck as needed.

Thanks for the feed back so far. Very helpful.

 

[SkiFreak]

Power consumption is 1.9ah/h.

If you have no power input for 2 days, a 100A/H deep cycle battery should easily meet your requirements for running that fridge. Assuming a 50% duty cycle of the fridge compressor, that would be 22.8 amps per day. In the "real world" the current draw would likely be less than that, unless you were accessing the fridge quite frequently.

 

[Bris31]

If you have no power input for 2 days, a 100A/H deep cycle battery should easily meet your requirements for running that fridge. Assuming a 50% duty cycle of the fridge compressor, that would be 22.8 amps per day. In the "real world" the current draw would likely be less than that, unless you were accessing the fridge quite frequently.

Bit confused here - 1.9ah/h in 24 hrs will be 22.8amps per day or it is the average consumption (after stop/start cycles). But it seems much efficient than Primus mammoth 65L stated 4.6/Ah.

 

[SkiFreak]

Bit confused here - 1.9ah/h in 24 hrs will be 22.8amps per day...

If you assume a 50% duty cycle (on for 30 min each hour) then you arrive at the figures I gave.
In reality, the compressor will only work this hard (on average in a 24 hour period) when it is hot outside or if you are opening the fridge door a lot. I normally use a 50% duty cycle as a "baseline" when working out power requirements of fridges, but you should use less power than this in the real world.
My logic is that it is better to over estimate power usage in the design phase than it is to under estimate it and not be able to get the usage you are after.
Putting additional insulation around the fridge or including a fan at the back of the fridge compartment, to remove hot air, can also give you much better efficiency. The power to run the fan is normally negated by the efficiency increase.

 

[Alastair D(Aus)]

Turtle,
as has been said the best way to go is to have a vsr and a second house battery bank. This leaves your start system untouched and aids reliability. If you were to separate one of the start batteries for running the fridge and then combine them for starting you have potential to really shorten the life of both batteries. It is highly likely that on occasions the battery that has been running the fridge will be well discharged and then you ask it to help start a truck. This means the other battery is probably going to take the bulk of the load. In the long run both batteries will suffer.

If you don't want to go for a dual battery system then the best would be to leave the start batteries joined and run the fridge via a low voltage cut off switch which will disconnect when the batteries have reached a voltage that leaves sufficient capacity to start the truck. This may leave your fridge warm! Remember that lead acid batteries of all types do not like being discharged below 50% as it reduces their life significantly.

On the scale of things the cost of doing things properly by installing a house battery and vsr is not large. you will keep reliability and not reduce the life of your start batteries.

 

[gait]

Bit confused here - 1.9ah/h in 24 hrs will be 22.8amps per day or it is the average consumption (after stop/start cycles). But it seems much efficient than Primus mammoth 65L stated 4.6/Ah.

welcome to the murky world of fridge comparison. Its marketing that's confusing.

The units used by Waeco (1.9 Ah/h) are technically correct for energy consumption. Assuming 12v.

If the volts aren't known the better unit is Wh/h

The 4.6/Ah for Primus is technically meaningless and requires a bit of interpretation to make sense of.

22.8amps per day is also technically meaningless, though we would probably interpret it as 22.8 Ah/day at 12v with the help of the context.

Fridge energy consumption varies lots with ambient temperature. Today in Brisbane the Waeco would probably consume a lot more than 22.8Ah. It also depends on what temperature the fridge is set to.

Here's a A$13 wattmeter which is useful for measuring fridge energy consumption.
http://www.ebay.com.au/itm/LCD-Digi...793708?hash=item339841fdac:g:5dgAAOSwNSxVA54X

 

[dlh62c]

Try not to mix battery types, different battery types often require different charge rates. Some Gel Cell and AGM batteries may require special settings or chargers.

 

[Turtle59]

Thanks blokes for the intelligent conversation and learning here. I am sure I am not the only one who will benefit from some of this.

We are all truly very lucky to have so many people on this forum with such great depth of knowledge in so many areas.

OK, so here is the play at the plate. I have ordered a battery cradle from the truck wreckers and will pick it up this afternoon. I have also ordered 2 105ah deep cycle (maintenance able) batteries to pick up today also. This will be mounted to the chassis on the opposite side to the starter batteries.

I have about 10m of 16mm2 cable, a hell load of assorted lugs and a Projector Isolator Switch. So I know what my weekend will consist of. Between that and fitting a new GPS to my motorcycle and looking after my 3 little ladies. A busy man.

Any suggestions welcome.

Best wishes to you all.

 

[SkiFreak]

With 2 x 105A/H house batteries you should get 4 to 5 days use of the fridge without any battery charging, assuming that nothing else is connected to the house batteries (which is unlikely).

What I would add is to keep your cable runs as short as possible, as voltage drop can be a real issue when it comes to charging batteries. You should also run a positive and negative cable from the start batteries to the house batteries. Don't be tempted to use the chassis as you earth connection.

An isolation switch is good to have between your start and house batteries, but it is not an ideal dual battery solution.
An isolation switch is a manual device that you can "forget" to use. A VSR is a much better option, in my opinion, as it will automatically engage to charge your house batteries and disengage when the truck is not running, ensuring that you do not flatten your start batteries. Remember... when the isolation switch is engaged all 4 batteries will act as one.

When wiring from your start batteries, connect the positive lead to the same battery terminal as the cable coming from the alternator and connect the negative cable to the negative terminal post on the other battery. Do a similar thing on your house batteries; positive on one battery and negative on the other, with interlink cables between + & + and - & - (for a 12v paralleled system).

 

[SkiFreak]

While I am at it...
If you want to do things "properly" then you should also include a fuse/circuit breaker between the start and house batteries. Instead of making multiple connections on your house battery terminals it is better to run single positive and negative cables into your camper and connect them to individual buss bars. This will give you easy access to 12v power inside the camper. Naturally, all individual circuits in the camper should be fused accordingly to the device being powered.
In my opinion you should not skimp on electrical safety. It's just not worth the risk.

 

[Alastair D(Aus)]

terminology

There may be a terminology issue here. Projecta seem to use the term isolator broadly.

this is called an isolator but is an auto vsr.