Can I charge the boat battery while driving my car?

[LionsDen]

I have an Audi estate where the battery is in the spare tyre well in the boot. Assuming I can tie the boat battery down, can I charge it whilst driving using my jump leads? Any tips/concerns/comments?

Why would you want to do this in the first place

 

[oldharry]

Deep cycle boat batteries are not compatible with a car charging system!

Many boats also have car alternators with simple internal regulators often set as low as 13.8 volts. Deep cycle batteries need a multi stage regulator set to their gassing voltage of 14.4 volts or higher otherwise their is a high risk of sulfation. Batteries need to gas slightly to reduce sulfation and to stir up the electrolyte to stop damage due to the acid settling at the bottom of the battery - stratification

So can you use the car to charge the boat battery - yes - but it's not best practice, batteries will die sooner if not charged correctly - but I suspect your boat alternator is the same type with no external multi stage charger added.

Caravanners do it all the time, whenever they are towing their vans. They have Leisure batteries in the van which is automatically connected to the cars charging system whenever the van is being towed. And it works fine.

The important thing is to ensure that the starter battery cannot be discharged in to the auxiliary battery, and this is acheived either by a volt sensitive relay that only cuts in when the alt raises the voltage above the static battery level, or by a pair of blocking diodes - exactly the same arrangement as protecting the starter battery from discharge on board.

Easily obtainable are quick release battery terminals which lock down on to the terminals, gicing them fully insulated protection in use. As said elsewhere dont risk crocodile clips!

 

[Stemar]

I would question how long you can put up with manhandling your battery to and from boat to car etc.

Been there done that.

Assuming you have no choice as no shore power you are much better using solar power to maintain charge.

For the last three years we have used our boat most weekends and do not have the benefit of an inboard with alternator charging our outboard only contributes 6 amps flat out.

We have two 10w flexible panels and a controller that maintains charge and we usually find that the batteries have recovered to full charge between weekends.

Longer boat use we usually get some shore power for our Aldi charger to give them a boost if the sun has not done its job.

Your choice......... I have seen what battery acid does to a car carpet.....

+1 for the solar panel - and the battery acid on the carpets, which stays around and eats SWMBO's best coat next time you go out

I have 40w of fixed panels & a dual battery charger and, during the summer, they keep up with our battery usage at anchor as long as we leave the (amp-hungry, peltier effect) cold box off. In winter, both batteries are fully charged when I come down after a week earning the sailing tokens. You should be able to get the same for around £100, which is a lot cheaper than an Audi boot carpet, and has the bonus of not having to lug the battery about.