Voltage drop at fridge / heater

Tim Good

Tim Good

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Ok so I was my webcasts heater gave a low voltage warning and I've recently installed a 12v fridge.

Before startup the voltage in both was 12.65v on batteries only and now shore power. At startup each shot down for a split second and then settled out at:

Fridge: 11.5v
Heater: 10.2

The heater went into shutdown due to low voltage but fridge continued. So as a solution I clearly need to wire decent gauge wire to each directly rather the routing to the big switch panel as it is now.

However I clearly need to switch each one on or off so what's the general solution to this? Have a relay which is switched by the current 12v supply?
 
David2452 said:
Your heater should never have been cabled up to a switch panel anyway, connect direct to the battery bank with decent cable and an adjacent inline fuse.
Click to expand...

Why ever not? If the switch and cable are up to it. Whats going to happen?, the heater puts out less heat because.... the wires are heating up.. same end result.
 
David2452 said:
Your heater should never have been cabled up to a switch panel anyway, connect direct to the battery bank with decent cable and an adjacent inline fuse.
Click to expand...

Ok so I wire it directly. But then how donyoubswitch it on an off? Via a relay which is switched at the panel?
 
David2452 said:
No.......... well not the heater anyway, doubly so as it's a TT.
Click to expand...

The penny has only just dropped as to what sort of heater he is talking about!

dont understand the question "Ok so I wire it directly. But then how donyoubswitch it on an off? Via a relay which is switched at the panel?
 
Last edited:
VicS said:
The penny has only just dropped as to what sort of heater he is talking about!

dont understand the question "Ok so I wire it directly. But then how donyoubswitch it on an off? Via a relay which is switched at the panel?
Click to expand...

My interpretation of that post is somewhat confused too Vic, possibly the OP is looking at an arrangement like that on TRS (dangerous goods) applications. This would overcome the possibility of accidentally leaving the heater on when leaving the boat, in this the main supply would be direct from the battery with no possibility of interruption, however the trigger cable would be routed through the isolator so switching that off would allow the heater to go through a cooling cycle and shut down without the possibility of a restart until the isolator is "made" again. I think we need a bit more from the OP.
 
Ok so at the moment the main 12v feed comes via the switch panel and an appropriately sizes circuit breaker in the switch. There is then a separate heat control switch with flashing light. You can turn this off and the heater still goes through its cooling cycle. I'll rewrite direct from batter temporarily and see what drop I get.

The fridge doesn't come with an on/off switch apart from the thermostat so in this case then I suppose I'll need to use a relay and rewrite a decent feed directly from the battery.

Is there any chance that these voltage drops I am seeing are coming from slightly knackered batteries rather than the wiring?
 
Why use relays? I understand David's point about not powering off the heater before it has run through the cooldown sequence, but on Vivere, everything, less the auto bilge pump, goes through a master battery switch (last thing switched off before we leave the boat). The (makuni) heater is wired directly to the master switch, and the fridge is wired via the switch/breaker panel with a 10A switch/breaker. I don't see the need for a relay.
 
My eber is powered through the 1,2,both switch but I am very careful to switch the heater off at the control panel and give it time to run it's power down cycle before swithing the batteries off.

The reason for this is If I wire it direct the led stays lit telling me the room temp and over time will drain the battery.

Back to the original question, either you need bigger cables to the heater and fridge or the current ones are corroded.
 
pvb said:
Easy to check with a multimeter on the battery terminals. Certainly worth checking before you start re-wiring.
Click to expand...

How can a multimeter do a decent test other than just measuring voltage? Surely a drop test is the only way of really tell if a battery will perform correctly under load as opposed to just swing it it holds a voltage?
 
I suppose I should have pointed out (for the OP anyway) that his heater, unlike the more modern ones will, assuming 12v, pull about 20a for a short period, but more importantly will continue to draw about 7a constantly under normal running conditions Even for the average marine install where the heater is within 3m or so of the battery bank this requires not only decent cable and low resistances in all connections but quite a decent battery capacity as you can imagine, even running the thing for 6 hours or so is going to cause a material depletion of the average cruising yacht's battery bank, even a healthy one, generally speaking it's over three times the average properly specified modern blown air heater's power usage.
 
northcave said:
How can a multimeter do a decent test other than just measuring voltage? Surely a drop test is the only way of really tell if a battery will perform correctly under load as opposed to just swing it it holds a voltage?
Click to expand...

Maybe I should have explained it more simply. As I understand it, the OP has found that the voltage reaching his heater at start-up is only 10.2 and has inferred that the wiring may need upgrading. Subsequently, he asked whether the low voltage might be a result of worn-out batteries. I said it would be easy to check with a multimeter - to do this, it would be necessary to measure the voltage at the battery terminals, then turn the heater on and see what the voltage at the terminals is when the heater is starting up. If the voltage drops close to the 10.2 he's measured at the heater, this would indicate that there isn't a wiring fault, but that the problem is with the batteries.
 
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