Low Voltage Disconnect for Fridge

[RuK]

Hi,

 

I've bought a Red Flash LVD10 low voltage disconnect to install on my fridge wiring to safeguard the batteries from being damaged. I run the whole boat on a 245W solar panel.

 

It was sold as 'programmable' and I foolishly took this to mean I could program whatever disconnect voltage I wanted, which I was going to make 12.6V to stay on the safe side of protecting my batteries. Now unfortunately I find that of the preset programs this switch will disconnect at the lowest is 12V.

 

I've phoned DMS technologies who sell the unit and they insist their programs will protect 12V batteries and I'm now wondering if my caution with my battery voltage is naive and over the top. Does anyone out there have views on this?

 

I would expect the fridge to take the battery voltage below my low voltage setting at night, and then reconnect in daylight when the solar panel starts feeding it 13V or so. I could buy another switch I've seen which has a 12.5V disconnect setting, but I wonder whether I should just use the one I've got

[cuthound]

12 volts (open circuit rested voltage) is about 40% SoC for a typical wet lead acid battery, so is a sensible emergeny cut off point, whereas 12.6 volts is around 80% SoC, which means it will be cutting off frequently unless you have a large battery bank or very light load.

 

You shouldn't be relying on a low voltage cutoff, and only use it as a last resort. Ideally you want discharge a battery below 50% SoC.

[Chewbacka]

But as this is going to disconnect the fridge it wont be a rested and unloaded battery, it will be a battery powering the fridge when it hits 12V, so 12V is probably about right. However to avoid the unit turning on when the battery has rested a bit and then turning off again, I assume it can be set to turn off at say 12.0V and not turn on again until say 13.0V.

The concern I would have is you may end up with a warm fridge for longer than you realise and that depending upon what is inside could be risky. But if just beer, then fine

[Phil Ambrose]

(shakes head) again a thread about turning fridges off. We have both a 12v under counter fridge and 12v under counter freezer...... never had to turn either off in 15 years living aboard, maybe you need to rethink your charging regime and your energy storage.

Phil

Edited June 16, 2016 by Phil Ambrose

[cuthound]

But as this is going to disconnect the fridge it wont be a rested and unloaded battery, it will be a battery powering the fridge when it hits 12V, so 12V is probably about right. However to avoid the unit turning on when the battery has rested a bit and then turning off again, I assume it can be set to turn off at say 12.0V and not turn on again until say 13.0V.

The concern I would have is you may end up with a warm fridge for longer than you realise and that depending upon what is inside could be risky. But if just beer, then fine:clapping:

I agree that if the fridge is connected the battery will be under load, but you can't quote accurate SoC figures for a battery under load, and wanted to demonstrate to the OP that 12.6 volts is not anything like a flat battery.

 

You make an excellent point about food safety, which the OP should take seriously.

[RuK]

Thanks for your replies.

 

It just makes me uncomfortable the idea of taking the batteries right down to 12V. I remember someone telling me if you have your batteries below 12.4V then you're damaging them. The switch will disconnect at 12 then reconnect at 13. I'm getting the impression though that disconnecting at 12 under load is not nearly as bad as the batteries sitting at 12V SoC? If I used the switch like this in reality it wouldn't switch the fridge off much I don't think, i've never seen it go down to 12V while I've been checking it with the fridge on. The batteries are 3 x 110ah. I'd rather lose some food/milk than damage the battery bank of course..

[Sea Dog]

There's an adjustable low voltage cut out as standard on some Danfoss fridge compressors, factory set at, iirc, 10.4 volts. I guess that's to protect the fridge itself rather than the batteries, but it may have been handy in situations such as the OP's if it had a big enough range. Has anyone made use of (adjusted/modified) this circuit for battery protection purposes?

[spadefoot]

Perhaps just getting yourself more solar panels (assuming you have space), would be more sensible. You could get another 250w panel for £100 or so.

[Robbo]

Thanks for your replies.

 

It just makes me uncomfortable the idea of taking the batteries right down to 12V. I remember someone telling me if you have your batteries below 12.4V then you're damaging them. The switch will disconnect at 12 then reconnect at 13. I'm getting the impression though that disconnecting at 12 under load is not nearly as bad as the batteries sitting at 12V SoC? If I used the switch like this in reality it wouldn't switch the fridge off much I don't think, i've never seen it go down to 12V while I've been checking it with the fridge on. The batteries are 3 x 110ah. I'd rather lose some food/milk than damage the battery bank of course..

 

Using batteries damages them, but that's what they are therefore. Only charging to 100% after discharging them (every day) will prolong their life to anything reasonable, but if using engine power then you'll be wasting diesel and money. If charging by engine then 40%-80% daily and then a 100% weekly is prob the best compromise for battery life / diesel money. Only solar breaks that routine in summer!

[ditchcrawler]

Thanks for your replies. The batteries are 3 x 110ah. I'd rather lose some food/milk than damage the battery bank of course..

or get food poisoning if you are not careful. With care you will be fine, we never had a fridge when I was a kid and lived.