Charging ancillary battery on boat.

[windlipper]

I have a motorcycle battery that I use for powering my dinghy inflator.
When this needs charging is it OK to simply connect it in parallel to my boat's engine or domestic battery? Thus charging it either from the boat's AC battery charger or charging it while running the engine.
I would have thought it was Ok but an "expert" tells me one should never connect different size batteries in parallel for any reason.
Don't supppose it matters too much if the m/cycle battery doesn't gat a full charge as long as it's enough to inflate the dinghy.

Thanks in anticipation

 

[prv]

The only reason I can see not to do that is if your boat battery is a basic lead-acid and you charge it at the relatively high voltage that these can accept, and the portable battery is a gel or AGM with a lower charging voltage. Other than that, I believe each battery will draw the current it needs without any trouble.

Pete

 

[windlipper]

Thanks, you make a good point, being a m/cycle battery it will be gel and should therefore receive a trickle charge. A zapping from the boat's charger may be too much for it.
Cheers

 

[noelex]

If its a gel battery the best way is to get a 12v hobby charger.
Anther simple way is to put a diode and wire wound resister in series with the battery, but you need to experiment a little to get the correct resistor value, and even then the result is not ideal.
Without these tricks the flooded charging voltage and charging current will be a bit too high. The battery may still have an acceptable life, try to connect them when the main battery is on float, keep an eye on the battery, it's temperature and no naked flames, (if you want to just connect them together)

 

[nimbusgb]

Thanks, you make a good point, being a m/cycle battery it will be gel and should therefore receive a trickle charge. A zapping from the boat's charger may be too much for it.
Cheers

Perhaps not. My BMW M/C has a traditional lead acid battery. Several Hondas before that also ran on standard lead acid. No reason to go gel, we try not to roll bikes on their sides if we can help it!

 

[William_H]

If its a gel battery the best way is to get a 12v hobby charger.
Anther simple way is to put a diode and wire wound resister in series with the battery, but you need to experiment a little to get the correct resistor value, and even then the result is not ideal.
Without these tricks the flooded charging voltage and charging current will be a bit too high. The battery may still have an acceptable life, try to connect them when the main battery is on float, keep an eye on the battery, it's temperature and no naked flames, (if you want to just connect them together)

It is not clear if your main battery charging is the type where there is a high voltage bulk charge. ie 15v or so or if it is a crude one voltage regulator ie 14v. If it is higher voltage then as Noelex says you need a silicon diode between the charger and the small battery and a resistor to limit charge current. Even then an amp meter would be good to know exactly what current you are charging the gel battery at. They are easily damaged by overcharge. If it is a simple charger or simple alt regulator at 14v then forget the diode. For a resistor you can use a car head or stop lamp.
I would start with a 2 ohm resistor that is likely to limit charging to around .5 amp. Then make sure you don't leave it on too long. ie 3 or 4 hrs max. (or about 80% claimed AH rating) good luck olewill

 

[noelex]

Perhaps not. My BMW M/C has a traditional lead acid battery. Several Hondas before that also ran on standard lead acid. No reason to go gel, we try not to roll bikes on their sides if we can help it!

You could be right, a lot of motorcycle batteries are just flooded, but for a battery that carried around on deck and stored in a cupboard a gel battery would be a Much better option, so I hope it's a gel battery.
There also some small AGM batteries available. These are completely sealed and would be a close match for the flooded battery in terms of charging voltage so may be the best option when replacment is needed. This kind of battery could probably be safely parralled with the main batteries to charge and would still be safe if tipped over.