Is it safe running shore power with battery charger direct to one of two batteries

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joyfull

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I have shore power to battery charger and then alligator direct onto terminals of one of the two batteries (engine and domestic) whilst working on the foreshore (bilge keeler). I have mini fridge running off the 12v circuit continuously.

Is that ok to leave charger on all the time (it is a new charger so hopefully quite smart).

Will it also charge the engine battery (they are wired in parallel - negative to negative with the positive running through the isolator switch).
 
I have just installed a cool box & i noted that with the cool box working off the batteries it tricks the smart charger into shoving a high voltage into the batteries. this means that although I know that the batteries are fully charged they are receiving 14.7 volts & 3.5 amps. The cool box drawing nearly 3.5 amps acording to the specs. I am worried that might cook the batteries
If I turn off the coolbox, the charger drops back to about 13.4 volt trickle charge.
So i suggest that having the charger supplying fridge & batteries together may be pumping an excessive amount into a fully charged battery & should be checked during operation. Of course loads of people have fridges & they cut in and out, unlike my cool box that runs continuously. You suggest that your fridge is running continuously.
My solutions was to install a 240 V spur & then a 240V/12V transformer to the coolbox & byepass the charger when in port.
As for charging the 2 batteries from one i would have thought that Ok because I have 2 batteries in my domestic bank & only one is connected to the charger. so that is , I assume, what you effectively have although one is a starter.
 
If your fridge is taking 3.5 amps and tyour charger is putting in 3.5 amps, where is the problem? You will likely find that with the engine running the voltage in the battery circuit rises to near 14.7 volts all the time because thats what most alternator regulators are set at. With a proper plumber in battery charger ( ie not one that uses crocodile clips) there should be no issue in using the 12 volt system whilst the charger is permanently on. I have been doing just that for literally decades. And by the way I also use the mains circuit at the same time for the boats immersion heater or kettle.

What makes me nervous about the OPs post is the reference to crocodile clips. No way would I want to leave that sort of charger operating full time.
 
birdseye said:
If your fridge is taking 3.5 amps and tyour charger is putting in 3.5 amps, where is the problem? You will likely find that with the engine running the voltage in the battery circuit rises to near 14.7 volts all the time because thats what most alternator regulators are set at.
Click to expand...
My engine charge drops back to under 14 V ( about 13.4/13.7 from memory)after a few minutes of starting & amperage balances the autopilot etc. Have not tried it with the cool box yet though. It is not a "smart" alternator.
 
I have always left the on board battery charger going as it is designed for that purpose,

However if in doubt about the adequacy of the system on the boat it would be better to switch off the charger and isolate the batteries while the boat is not in use .
 
What makes me nervous about the OPs post is the reference to crocodile clips. No way would I want to leave that sort of charger operating full time.
Click to expand...
These 5A are croc clips
F1037249-01.jpg


so are these 20A according to RS
F7148266-01.jpg


I assume the OP is using the 20A ones!
 
Last edited:
Norman_E said:
For battery maintainence as opposed to recharging after deep discharge it is hard to beat one of these CTEK MXS 5.0 Fully Automatic Battery Charger 12V, 5 Amp - UK Plug 744759183057 | eBay but do not use croc clips, connect with the ring terminals supplied. The Ctek charger comes with a plug and socket in the low voltage circuit so that you can disconnect it when not required.
Click to expand...
For the OP's problem (if indeed there is one) a charger with a supply mode might be a better idea:
MXS 7.0
 
It should work fine, Most boats with a built in Batter Charger are wired this way and fridges / cool boxes run away happily on Shore Power for days on end! The charger should be appropriately sized. It should have excess capacity to replace the max drain on the batteries + a safety factor . Check to see what your cool box is rated at and then the Charger. Normally what happens is that the cool box/ fridge cuts in and draws power from the battery. The Battery voltage reduces as power is drawn out. The Charger senses this and after some time clicks in and replaces the power that was used. It will work hard for a while at 14. .5 Volts approx to bring the battery voltage back up and then reduce its output for a while before repeating the cycle.
Dawdream beliver , i wonder if your Domestic Battery is beginning to fail. If you have for example an 80Ampere/Hour battery then there should, be enough Capacity for the battery to supply 3.5 A for a few minutes before the Charger senses the drop and kicks in. If its happening immediately and the charger stays on continuously It sounds like there is not much storage capacity in your Battery bank.
Kinsale 373
 
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