[C320-list] Looking to setup temporary 12v supply while batteries are removed?

[Jeff Hare]

Hi Scott,

Don't do the battery charger thing.  It doesn't work like you're thinking it works.  

Chargers are not usually fixed voltage devices.  They vary their supply voltage in order to hit a target 13.8v across a connected battery and monitor the current flow to determine state of charge.   Say the battery is at 12.5v.  A car battery charger hooked to that battery will increase it's supply voltage (across the battery clamps) in order to induce enough current flow so that the battery now reads 13.8v across its terminals.   It'll continue to vary the actually voltage needed to induce the amount of current needed to keep it at 13.8 volts until the battery itself is near 13.8v (which can be calculated by the battery drawing very low amount of current at this point).  This is when a car battery is considered charged. (essentially)   I'm not being super precise, here, just conveying the concept.

When there's no battery hooked to a charger, battery chargers won't know what voltage to produce on the output since even if it went to 200v, there wouldn't be any current draw. So it will simply think it's not hooked to a battery.  Suddenly putting a non-battery load on it will provide a completely unregulated voltage on the load and the current will be all over the map.  It probably just won't work, or could damage the load depending on your car charger.

A 12v supply, like used for Ham radio gear, etc. is designed to produce and hold a fixed REGULATED voltage and supply whatever current the load needs up to the rating of the power supply.  Totally different process.  When you hook a charger to a battery, the battery "regulates" or smooths out the (usually) modified sine wave voltage the load will see. 

So, I wouldn't don't it on my boat.  😊  Buy an inexpensive AGM Car or Lawn tractor battery, hook up a trickle charger and your bilge pump.  Or use your starting battery for this with a trickle charger/maintainer plugged in.

My $.02
-Jeff Hare



 -----Original Message-----
From: C320-list <c320-list-bounces at lists.catalina320.com> On Behalf Of Scott Westwood
Sent: Monday, October 29, 2018 7:22 AM
To: Catalina List <c320-list at catalina320.com>
Subject: [C320-list] Looking to setup temporary 12v supply while batteries are removed?

I want to remove my batteries and provide a "temporary" 12v supply only to run the bilge pump over the winter.
My batteries are dying. Plans are in the works to replace both LA with AGMs before spring.
Concern: Shore power charger cooking the last dying battery and releasing caustic fumes and possible fire?  The charger dried up (boiled) the other/second battery and the fumes were real bad.
I want to remove the last battery and shut off the battery charger and run a 12v power supply if/when the bilge pump would need to run.  Right now the bilge is dry and no water leaking in but you never know.
I have one of those rolling battery charger/starter for cars and I can maybe turn it on "always on" and leave it hooked up in place of the batteries???  As long as I have shore power then the boat would have 12v (if needed).  I am not near the boat now but it is a Jabsco pump in the port cabin seat near chart table. Can't be more than 5-8 amps??
High end related second question:   Is there a permanent  battery charger that can also provide emergency crank/starting current when connected to shore power - if batteries are dead?  I know you can have a dedicated start battery but it seems silly to not have the capacity to use shore power as an emergency start power.  Asking too much?  Overkill? Thanks, Scott Westwood scottwestwood at bellsouth.net H (919)-362-8538    C (919)-618-7185