Wiring a Battery Charger with a diode isolator

jonrarit

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Having just fitted shore power, I'm now looking to fit a battery charger but how do i wire it in to the batteries circuit?

Current set up is single alternator running through a battery isolator diode (sure power 702R) to two batteries. I understand the diode not only seperates the batteries, but directs the appropriate alternator charge to whichever battery needs it.

If the charger only has one outlet do I wire it in above the diode so the diode directs the charge to the approriate battery (like it does already from the alternator) or would the current drawn by the diode "fool" the charger into thinking the batteries need more charge than they actually do and thus the charger keeps pumping out a higher charge current instead of switching to "trickle" mode?

On the other hand if I wire it to one of the batteries (after the diode) then would the charger not just charge that one battery as the diode would prevent the charge getting to the other one....or does the surepower diode have some clever trickery that opens to both batteries when it senses a charge to one?

Thanks in advance

JR
 

RAI

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There's no short answer. A long answer can be found in Nigel Calder's book on yacht mechanical and electrical maintenance.

Personally, I prefer a voltage sensitive relay (VSR) to diodes and even more preferably and expensively the 3/4 stage charger approach with separate starter and domestic battery outputs.
 

philip_stevens

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If you fit a Sterling battery charger, you will find that there are 3 outlet terminals on it. As on my boat, one terminal is for the engine start battery, and the other two are connected together to charge the service batteries.

The charge wiring goes directly to the battery terminals, bypassing the splitter diode unit.
 

VicS

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A charger with two or more outputs is probably the best solution.

The trouble with putting the charger output through the diodes will be the voltage drop

A VSR may be a better bet that the diodes for splitting the alternator output, no voltage drop being the main reason unless your alternator is battery sensed. The VSR could also split the charger output.
Not everyone agrees that a VSR is better than diodes though.

You should not normally have to worry about charging the engine start battery. Starting the engine will take very little from it and it should be recharged again pretty quickly by the alternator. Recharging the house battery is likely to be the main reason to be using a mains charger but it is understandable that you might want to be able charge the engine start battery.

A simple solution might be to use a 1,2, both, off type battery switch to direct the charger output to which ever battery needs charging or perhaps better a simple, but heavy duty, single pole double throw switch.

I like my last suggestion best of all if you only have a single output charger! A SPDT switch.
 

jonrarit

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Good points from all thank you.

Vics you are quite right of course. I hadnt really thought about it until you mentioned it but as you say it's unlikely the engine battery will need shore-powered-charging and it's not difficuly to temporarily do if ever it did.

I think I'll just wire the charger to the domestic battery only .....sorted :)

JR
 

john_morris_uk

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Good points from all thank you.

Vics you are quite right of course. I hadnt really thought about it until you mentioned it but as you say it's unlikely the engine battery will need shore-powered-charging and it's not difficuly to temporarily do if ever it did.

I think I'll just wire the charger to the domestic battery only .....sorted :)

JR
And its how Bavarias battery chargers are wired if I remember correctly.
 

macnorton

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I have two power take off points (like the car cigarette lighter socket) one for each battery bank, these are next to a 3 pin mains outlet.

I have a cheap battery charger with a power take of plug fitted in place of the crocodile clips, pick the one you want to charge and there you go.

I normaly leave two solar panel connected to these and use the charger if I need to top up one or the other..
 

Tranona

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Even the most basic staged charger will have at least 2 outputs. My simple 25amp Synergex has 3 and each one can be set individually for the type of battery it is connected to so that you get the right charge regime.


The size and type of charger you get will depend on what you want it for. If it is simply to "top up" a smallish house bank then a 10 amp will be sufficient, but if you want it to power your domestics when you are moored up and on shorepower you will want a more powerful charger. For topping up between visits to the boat a solar panel is probably adequate if you run the engine when returning to base.
 
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