Battery charging

[Sandpiper]

My Bav 32 (2003) is fitted with a "smart" battery charger and two batteries, one domestic and one engine. (Sorry don't have the tech specs to hand). I believe the charger is original factory fitted equipment.

On charter boats I have been on in the past, the domestic and engine batteries had separate isolator switches, so that when you are under sail or at anchor you can turn off the engine battery and be sure that the engine is going to start next time. I was surprised to find that my boat has only one isolator - under the chart table - which means (I assume) you can have both batteries on or off.

In the user manual for the charger it claims to preferentially charge up the engine battery over the domestic one - which is good - but is the circuitry smart when you are discharging the battery?

So if I go sailing and forget to turn the fridge off for example, or lying at anchor overnight with lots of lights on, does the domestic battery discharge first, followed by the engine battery? Or perhaps they are on separate circuits with no crossover? Or perhaps I will be stuck with no way to start the engine!

It would be good to know the answer to this one, as otherwise the tendency when lying at anchor is to annoy the rest of the crew by obsessively turning off lights, and perhaps worse, causing them to drink warm beer when it's not strictly necessary!

[Tony Barlow]

I had a situation where I was unable to use the engine on my 2002 32 due to a cooling problem so all my domestic and navigational equipment went dead. My engine battery remained fully charged. My sugestion is that you conduct a practial exercise while you are in the marina of discharging, logging the results and sharing them with us. By all means find out what should happen but compare what actualy happens with the theory.

[Sandpiper]

Thanks Tony, that sounds like a plan.

I will do a test as you suggest and update this post with the results.

[Moodymike]

I have the same B32(2003). On mine the engine battery is on a separate circuit so cannot discharge. As I understand this is the standard set up. What you will find is that the battery charger is only linked to the domestic battery, there is a spare terminal on the charger to wire the starter independent.

[Nigel]

In the user manual for the charger it claims to preferentially charge up the engine battery over the domestic one - which is good - but is the circuitry smart when you are discharging the battery?

When discharging, the shore power charger is irrelevant. In order to isolate the batteries when discharging, you have to split the feeds from the alternator using one of the following, in order of likelihood:

• A blocking diode type charge splitter (example)
• A Voltage Sensitive Relay (VSR) (example)
• Twin alternators

[Sandpiper]

After a couple of weeks cruise up and down the NE coast I can now confirm from experience that the engine start battery is indeed isolated from the domestic stuff. After several nights at anchor, the engine start battery never dropped below about 13V whereas, after anchoring overnight with the anchor light and the Chartplotter on to provide the anchor alarm, the domestic battery was occasionally down as low as 10V.

Many thanks for all the good advice on this topic.

[Nigel]

... domestic battery was occasionally down as low as 10V.

You want to keep discharge that low to an absolute minimum, 12V is about as low as you should go, and it should be fully charged when you leave - that can often mean 24 hours on the charger!

Perhaps you need warm beer and no lights after all 

[Brian]

http://www.mmbalmainauto.com.au/PDF/State_of_charge_12_volt_batteries.pdf

This chart gives a good guide to the state of your batteries. Check the voltage without any load as this will give a truer state of charge.