Battery Questions

[aroos]

We have a 2014 C30 sedan and prefer to being at anchor rather than at a dock. We don't have a generator. While at anchor for more than 24 hours, we don't use the wine fridge, turn the main and cockpit fridges down to 2.5 and are careful with other usage (stereo and lights). We do have an Engel freezer which is kept at the lowest possible setting necessary to keep ice and food frozen.

We were at anchor for 3 days this past weekend. When testing the house battery voltage after 1.5 days I turned the house switch off for an hour in order to remove the load. I then turned it back on and forgot that doing so turns our nav screens on (I've gone into the screen menu and turned off the auto on option but to no avail). I had the covers on the screens and didn't realize they were on until 2 or 3 hours had passed and my house batteries were way down.

So here are my questions. As a result of my stupid mistake, my house system (while under load) went down to 11.5v by the 48 hour mark, at which point I went for a 1 hour cruise to partially recharge them. Despite the 11.5v reading, everything was working fine. How low can the house batteries go and still operate my devices? I know that depleting the batteries will reduce their life somewhat but that is a secondary concern for me. My other questions are with regard to the alternator. Will it recharge the batteries as effectively while idling as it will while cruising at say, 3000 rpm? The voltage output on the Volvo screen seems to say so. Finally, I believe the system always charges the starting battery first - as I always turn the starting battery off while at anchor, it's always well charged. If I want the alternator to concentrate on charging the house batteries, is there any way to accomplish that?

Thanks for your advice.

 

[ronnieg]

We have a 2014 C30 sedan and prefer to being at anchor rather than at a dock. We don't have a generator. While at anchor for more than 24 hours, we don't use the wine fridge, turn the main and cockpit fridges down to 2.5 and are careful with other usage (stereo and lights). We do have an Engel freezer which is kept at the lowest possible setting necessary to keep ice and food frozen.

We were at anchor for 3 days this past weekend. When testing the house battery voltage after 1.5 days I turned the house switch off for an hour in order to remove the load. I then turned it back on and forgot that doing so turns our nav screens on (I've gone into the screen menu and turned off the auto on option but to no avail). I had the covers on the screens and didn't realize they were on until 2 or 3 hours had passed and my house batteries were way down.

So here are my questions. As a result of my stupid mistake, my house system (while under load) went down to 11.5v by the 48 hour mark, at which point I went for a 1 hour cruise to partially recharge them. Despite the 11.5v reading, everything was working fine. How low can the house batteries go and still operate my devices? I know that depleting the batteries will reduce their life somewhat but that is a secondary concern for me. My other questions are with regard to the alternator. Will it recharge the batteries as effectively while idling as it will while cruising at say, 3000 rpm? The voltage output on the Volvo screen seems to say so. Finally, I believe the system always charges the starting battery first - as I always turn the starting battery off while at anchor, it's always well charged. If I want the alternator to concentrate on charging the house batteries, is there any way to accomplish that?

Thanks for your advice.[/quote

Hi Aroos,
First off your alternator will charge better at a higher RPM, than at idle. This being said it is not going to be something you would be able to really track on a volt meter as your alternator has a regulator built into it,.
Your 30 Cutwater has Automatic Charge relays, that open once the engine battery is open and allow the house and thruster battery to charge, If your engine battery is charged the relays open
Please feel free to call me

 

[NorthernFocus]

...my house system (while under load) went down to 11.5v by the 48 hour mark, at which point I went for a 1 hour cruise to partially recharge them. Despite the 11.5v reading, everything was working fine. How low can the house batteries go and still operate my devices?

If you look at the specs, most of your 12V devices likely say something like 10-15VDC. That basically means they'll function, not that they should be operated there. Virtually all of you electronics operate at 5V anyway and have internal voltage regulation. They also have low voltage protection and will shut themselves down. The things that are most subject to damage operating at low voltage are motors. As voltage drops, amps go up in order to supply the same amount of power. That means things heat up. Not good for motor windings etc. You should realize that once batteries get that low there is very little energy left in them anyhow. And they are deteriorating never to return to their former selves.

Ronnie mostly answered the rest. I'll add one more comment.

If I want the alternator to concentrate on charging the house batteries, is there any way to accomplish that?

Your alternator is controlled by a voltage regulator. It produces as many amps as necessary (or as many as it can) to maintain it's output voltage (nominally 14.5V). Once the automatic relays (ACRs) combine your batteries, then the current will flow where it is most needed. So basically the system as designed will accomplish what you want.

 

[SGIDAVE]

Hello Aroos,

The previous 2 replies give you good information.

What you want to know is called "State of Charge." It ROUGHLY correlates with the battery's voltage. Normally, you don't want to discharge your batteries below 50% SOC...this equates to ROUGHLY 12.2V. Each episode of discharge to less than 12.2V (or less that 50% SOC) damages the batteries...exactly HOW MUCH damage is usually determined later when you find shortened battery lifespan. Of course, if the battery/s fail to recharge then they are 'toast' and need replacing.

This chart gives guidelines if you monitor voltage alone:

dave