quick battery question

[Arthur Marshall]

Can I use a battery charger to charge the boat domestic battery without disconnecting it from the circuit? Alternatively, would it charge OK if I just turned off the isolator switch or do i need to disconnect the terminals?

[nicknorman]

Oxymoron.

 

Anyway, it depends on the type of battery charger and/or the degree of supervision. The basic answer is yes, but if you are using an unregulated "cheap" garage type battery charger it won't have a regulated output and so once the batteries are approaching fully charged the voltage is likely to rise quite high. Some boat equipment, if still left connected, might not like that. So either use a regulated charger, or keep an eye on the system voltage and disconnect the charger before it gets to say 15v.

[Arthur Marshall]

I'll report you for calling me an oxymoron... thanks for the quick answer. I'm pretty sure the charger is regulated, but I'll keep an eye on it. I don;t want to just run the engine as my signwriter's working on the boat and the vibration might cause her to blow a gasket.

[Mitchellmoxo]

I heard that if you use a non marine battery charger and something goes wrong with it ther is a possibility of it sending 240v through your 12v system. However marine battery chargers are protected from this

[nicknorman]

I heard that if you use a non marine battery charger and something goes wrong with it ther is a possibility of it sending 240v through your 12v system. However marine battery chargers are protected from this

No I'd say that is an urban myth. With a non-marine charger one is accustomed to holding one or both 12v connections (crocodile clips etc) and if there was a penchant for them going to 240v there would be a lot of smoking bodies and court cases.

[Arthur Marshall]

Also, do you reckon it would charge faster with the charger or by running the engine?

[Alan de Enfield]

Also, do you reckon it would charge faster with the charger or by running the engine?

 

The battery is the controlling influence - NOT the charger.

 

If the Battery 'wants' 20 amps, then either a 75 amp alternator, or a 30 amp battery charger will only produce 20 amps.

 

If the battery 'wants' 10 amps then - ditto.

 

If the battery can take 70 amps then a 75 amp alternator will be 'quicker' than a 30 amp battery charger - HOWEVER, the batteries demand very quickly falls as charge goes in - you maybe will only be putting in the 70 amps for 20 minutes (or so), it will then gradually fall away to maybe 30 amps after an hour then there is no difference between a charger and the engine.

 

If you are not 'time critical' then a charger via a land line is much, much cheaper than running the engine.

[jay69]

If not time critical is it best to slow or fast charge(from a charger on a landline)?

[MtB]

 

The battery is the controlling influence - NOT the charger.

 

If the Battery 'wants' 20 amps, then either a 75 amp alternator, or a 30 amp battery charger will only produce 20 amps.

 

If the battery 'wants' 10 amps then - ditto.

 

If the battery can take 70 amps then a 75 amp alternator will be 'quicker' than a 30 amp battery charger - HOWEVER, the batteries demand very quickly falls as charge goes in - you maybe will only be putting in the 70 amps for 20 minutes (or so), it will then gradually fall away to maybe 30 amps after an hour then there is no difference between a charger and the engine.

 

If you are not 'time critical' then a charger via a land line is much, much cheaper than running the engine.

 

 

On the other hand if the battery 'wants' 100A and you only have a 12A charger, a 50A charger will charge faster (until the battery demand falls to 12A which will be quite a while).

[Tony Brooks]

If not time critical is it best to slow or fast charge(from a charger on a landline)?

 

Low rates of charge and low rates of discharge are better for the battery life PROVIDING the charge is enough to get them fully charged.

Low rates of disharg

[Arthur Marshall]

Thanks all - no mains at mooring so would have run the charger from a generator, which seems pointless! In the end I ran the engine.

[Robbo]

I heard that if you use a non marine battery charger and something goes wrong with it ther is a possibility of it sending 240v through your 12v system. However marine battery chargers are protected from this

Properly comes from marine battery chargers are more likely to use a isolation transformer where others won't. I think the likely hood of going wrong is extremely small in this area

"A battery charger intended for marine use must have an isolation transformer in which the incoming AC power flows through one coil and transfers power magnetically to a totally separate coil. This second coil, which is electrically isolated from the incoming AC side, supplies power to the DC side. All marine battery chargers use isolation transformers, but some automotive chargers do notthe neutral AC wire may be common to both sides of the transformer ( Figure 6-13A ). In this case, should the incoming neutral and hot leads get crossed, the common line will be hot! This can happen quite easily with improper wiring, or by inserting a two-prong plug the wrong way around, resulting in a reverse polarity situation in which the entire negative side of the boats DC system can sometimes be charged at full AC voltage, creating a severe shock hazard for those aboard and for swimmers (see Chapter 4 ). Even without reverse polarity, an automotive-type common neutral connection will bypass any galvanic isolator and encourage stray-current corrosion . Only marine battery chargers should be used on boats." from "Boatowners Mechanical and Electrical Manual 4/E" by Nigel Calder

http://amzn.to/1rLNeqJ Edited May 9, 2016 by Robbo

[jay69]

Thanks Tony. Jay.