How many domestic batteries?

[Nick Hudis]

I wonder whether someone would be able to help me assess whether I need more batteries.

 

My recently acquired boat, Out The Blue has a fairly standard 12v/240v set up with two domestic batteries, charger, shoreline and inverter. The main use of the 240v will be a small fridge freezer and charging phones.

 

She will be spending much of her time as a live aboard at a marina with a hook up, but we will be cruising from Loughborough to Bath during October and relying on battery power while moored during evenings.

 

My question is this: With the 12v supplying the usual domestic services of light and water/central heating pumps and supplying 240v to the fridge through the inverter, are two batteries going to have enough juice for a long October night, or would I be advised to get extra batteries installed? I notice that some boats seem to have ranks or three, four or even five batteries.

 

Thanks

 

Nick

[NB Lola]

Nick

 

The best way is to carry out a power audit.

 

However, from your post I would guess 3 batteries as that is what we have and your usage seem similar to ours, we also run a telly. I would also add that we carry a back up genny as a reserve

[BEngo]

First do a power audit. Tony Brooks website has a good guide to how www.tb-training.co.uk/MarineE05.html

 

Don't forget to include the use of 240V in the power audit and make allowance for the losses in the inverter.

 

This will give you a total daily usage in Ampere hours. Compare this with 50% of the capacity of your batteries.

 

Don't forget that battery capacity declines with use so if your batteries have been around a bit they won't still have the capacity it says on the label. As a rule of thumb knock off 20% for every year they have been in use.

 

If the usage figure is greater than 50% of the ACTUAL capacity of your batteries then you will be in the poo sooner rather than later. If it's less then for a short time you will be OK, provided you can re-charge the batteries every day.

 

Finally consider the re-charging arrangements. If you use, say, 110 Ampere hours every day, the engine alternator has to make that up and will need to deliver more than you use back into the batteries to top them up. To start with well-discharged batteries can accept a high rate of charge, but this soon tapers off and it takes a long time to put the last 20% in. A 70 A alternator will need to run for much more than two hours to fill the batteries. Therefore it is prudent to treat the batteries as though they are not as big as they are.

 

When you get to your new mooring the landline can deal with any shortfall and properly charge the batteries.

 

If you do a power audit and tell us the results of that and about your batteries ( what size, what type, how old, ) and the capacity of the alternator I am sure the forum will recommend whether you should buy some extras or otherwise.

 

N

[Nickhlx]

The fundamentals include how long you will be "engine running" each day, as each day you need to return the bank to 100% SoC.

 

If you don't then they will, over days, become ever more discharged and begin sulphating ( dying). If you are CCing down to bath, travelling every day, it is likely that you will have time to replenish the lost charge - A Smartgauge, or equivalent, would be a good bit of kit to instal and monitor things.

 

Whilst you can add extra batteries, there is no point if you don't charge long enough or have an alternator large enough to recharge them each day in the time available, although, batteries take T I M E to recharge, as they will only accept charge at "their rate", not at a rate you can "force" by using a "bigger alternator". A bigger alternator will help however when in bulk mode, but this is only for the first stage of charging, until the bank voltage comes up.

 

You should check that the alternator output voltage at cruise is set to the max that your intended choice of battery (chemistry) supports - if wet lead acid, this would be around 14.8 Volts.

Do this by checking at the battery posts when they are fully charged and with the engine running...

 

In general it would help the situation to use as little power as possible when the engine is stopped - i.e. do the most power hungry activities during the day when cruising. Some people "charge up" ice blocks in the freezer compartment and then switch off the fridge soon after stopping for the day, and open the fridge as little as possible until starting off the next day. If you do need to run the fridge overnight, it would use less energy by turning it down.

Obviously turn off any un-necessary power consuming devices ( lights ) and replace halogen lights with LED modules ( that will save a lot of power !).

 

Check the power consumption of your TV ( about 30 to 50 watts is good) and Laptops, and certainly charge them by day and don't leave them plugged in / on un-necessarily when the engine is off.

 

Hope that adds to your knowledge

 

Nick

[Paul C]

240V fridge/freezer will use more power than 12V because it will also need an inverter running, which has a current requirement of its own (over & above what it converts) plus it will be less than 100% efficient at converting the 12V to 240V. Having said that, they're significantly cheaper and many people successfully have a 240V fridge on their boat.

 

Number of batteries - as above - you'd need to do an accurate power audit to properly know how much battery capacity you'll need. However as a ROUGH rule of thumb, most boats have 3 leisure batteries, and it seems a popular trend on newer boats is 4 leisure batteries. I've seen some with 7 or more though.

[Arthur Brown]

Check what charging time you will actually have. If you are on shoreline then the batteries should always be in float and fully charged, mains to supply the loads and keep the batteries topped up. ON a cruise life should be easy with 8 - 10 hours of engine on time for charging. The problem comes live aboard with no mains or cruising -then you need to run an engine in the winter but may rely on solar in the summer.

 

With lots of living time away from mains you certainly will kill batteries (by overdischarge ) unless you have a good recharge regime.

 

LOTS of solar panels. will assist.

[bottle]

240V fridge/freezer will use more power than 12V because it will also need an inverter running, which has a current requirement of its own (over & above what it converts) plus it will be less than 100% efficient at converting the 12V to 240V. Having said that, they're significantly cheaper and many people successfully have a 240V fridge on their boat.

 

Number of batteries - as above - you'd need to do an accurate power audit to properly know how much battery capacity you'll need. However as a ROUGH rule of thumb, most boats have 3 leisure batteries, and it seems a popular trend on newer boats is 4 leisure batteries. I've seen some with 7 or more though.

 

Is that so, may be you could enlighten us with some figures.

[by'eck]

 

Is that so, may be you could enlighten us with some figures.

 

AC motors are generally less efficient than DC ones, see here. Then there is the inverter overhead when no shore/generator power.

 

Not much can beat the 12/24 volt Danfoss BD35/50 compressors.

[john6767]

We have just 2 x 110 batteries and don't have a problem, but we don't often use the inverter, the fridge and TV are 12v and most phone/laptop charging is via 12v to USB.

 

You don't say what type of lights you have, I recently changed ours to LED and that has made a big difference, and in October you are going to be having the lights on quite a bit.

[Charlotte Burnett]

BIG yawn on the 240/12v fridge debate - is it really helpful for a newbie?

 

Nick - the question is impossible to accurately answer over the internet. A power audit will tell you - in theory. With only 2 batteries, you are pushing it though. If you have a good inverter and are moving the boat every day, you have a chance of getting away with it. With something Chinese, on marginal batteries, mooring for the day because you are tired, probably not. Theory is exactly that, especially on a boat! To improve the situation, you might want to consider turning off the inverter (and therefore fridge) over night - a decent one will keep cool for those hours, especially as it gets cooler. I would echo the question of what lighting is on board though as incandescent bulbs and 2 batteries and a fridge will definitely be an issue.

 

You have a bunch of options but I'll break them into 2 categories; spend a bunch of money before you leave and more money when you break stuff as you move or just spend money when you break stuff as you are moving. With the batteries, I'd suggest that you might want to suck it and see - they might well be knackered anyhow and adding new to old unless you know the bank and set up is a game of roulette. BEngo has parroted the 50% rule of thumb - which may or may not be useful depending on what monitoring system you have. Perhaps, given your limited bank capacity a better rule might be NEVER drop below 10.8v because, in theory, at this point, batteries are unrecoverable. (My experience, however, has been pleasantly different from this).

 

You are doing a long journey, part of which after the clocks have gone back, make sure you have enough grain spirits on board, running out of electricity is far preferable to running out of booze!

[Paul C]

BIG yawn on the 240/12v fridge debate - is it really helpful for a newbie?

 

 

Yes it would be helpful, given that he/she is considering fitting another battery, which in addition to its own cost will need more cabling, a method of securing it, finding the space for it, etc. I don't have the figures to hand but others have confirmed that they tend to use more power once the inverter efficiency/losses are also considered. Another factor is that I know of at least 2 cases where an inverter left unattended, has caught fire. Of course, it depends how you use your boat. We leave our fridge on, and the boat unattended, quite often.

[Nick Hudis]

Well the boat is at Pillings Lock Marina and I'm in Newcastle taking ownership 21 September so I don't have the opportunity to do a power audit yet, but I've got a chap recommended by the marina to take a look and give an opinion and I'll report back. Thanks for all the replies. We are going to stick with the 240v fridge as she'll be spending much of her time on a shore line, but I want to make sure we are properly set up for the long trip to Saltford and for future cruising.

[Arthur Brown]

Long trips will cover your power needs, shoreline will cover you needs both, because you will have power coming into the batteries often. Where there will be issues is periods of being moored up without shoreline for which you will probably need more batteries and maybe a generator too.