Battery issues

[Peterangell]

Hi there, this is my first post... I have a problem with my batteries on my Beneteau Oceanis 34 which is 3 years old and hope someone can help (probably very simple...)

I have 3 batteries, 1 for starting the engine which I don't have a problem with. The other 2 are for the other systems on the boat, and I keep these on trickle charge whilst on the pontoon - they show 14v or thereabouts when on charge, and drop to about 12.9v once off the charger.

No problem when I am out for a couple of hours, but when I am on a long passage, say 10 hours or so, then the battery alarm goes off after about 3 or so hours, and shows about 10.9v on the display - I then run the engine for half an hour or so, it goes back up to 12.9v, and then they are fine for maybe another hour until the alarm is coming on again....

The display is showing that I am usually drawing about 6 to 9A whilst sailing (with the Fridge, radio, autopilot etc.), although in rough weather I have seen that undulate momentarily up to about 28A with the autopilot on.

The 2 batteries are 70Ah each - I am wondering if they are basically dead and need to replaced or should I revive them somehow... they have lasted 3 years and have been on trickle charge all that time - was this the problem?

Thanks for your help, sorry if I this is a silly question...

[Tayana42]

Peter, yes it sounds as though your house bank batteries are dead. Take them to a battery shop to be bench tested before your buy new ones though.

Next question should be, "why are they dead?" I'd take the battery charger to the shop to be tested too. You want a marine smart charger not a trickle charger to keep your batteries healthy as long as possible.


S/V B'Shert

[jcapo]

Agm, gel, or flooded? If flooded, are they watered properly?

What kind of "trickle charger"?

10 hours at 6 to 9A would be close to 50% discharge level for your 140AH (2 70AH in parallel) battery bank. Fairly new well cared for batteries would not be down to 10.9 at 50% discharge. The deeper you discharge the shorter the battery life.

They may be toast or they may just be showing reduced capacity. In either case I think you need you need a larger capacity bank with your load level.

[Peterangell]

Thanks both, when I say trickle charger I mean the charger that is installed on the boat, which is s proper marine one, sorry not sure what type of battery they are but will get them tested and replaced I think...


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[nigel1]

Peter,

Do you have, or have access to a battery hydrometer?

Check the water level in each cell (6 per battery), top up to the correct level with distilled water, and then fully charge the battery.

Then check the density of each cell, should be around the 1.300 mark. If way below that, or the cell readings are wildly different, then likely the batteries are toast.

When replacing, go for good quality deep cycle batteries, with the largest amp capacity which will fit the battery space.

Properly looked after, batteries should last a lot more than 3 years.

[Peterangell]

Thanks Nigel, I don't have a hydrometer (except for the homebrew!) but will check the water levels and top up (if they are those type) - the batteries still hold charge for 3 hours so I'm hoping there is still some life left in them...


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[DeepFrz]

You are way under batteried...you have a useable amount of about 30% of 140 amp hours or approximately 42 amp hrs. Say you average 12 amp draw. So just over 3 hrs. usage and your alarm goes off.

Are your batteries true deep cycle batteries? If they are the recreational "boating" type batteries then they are probably approaching end of life.

There is a lot of reading on this site about battery care and feeding. Also about the size of battery banks needed to feed a hungry modern boat. The Trojan Battery company also has some good information.

Battery Maintenance | Trojan Battery Company

[Peterangell]

thanks, will order a hydrometer and see what I can ascertain, but it looks like some new batteries for me...

[jwcolby54]

The first thing to understand is that a battery should only be drawn down to 50 percent of its total amp hours. So if you have two batteries in parallel, 70 amp hours each, you should never use more than 70 amp hours total without recharging.

The next thing to understand about batteries is that a marine battery is not a truly deep discharge battery. At least not in the same sense as a so-called golf cart battery. A marine battery is often used for starting outboard engines. And as such the plate structure is required to provide high current quickly, which is a fundamentally different plate structure than a medium current over a long period.

One way or the other, if you are using only 6 amps for 10 hours, you have drawn down 60 amps out of your available 70 amps and so your alarm should in fact be going off. You have in fact used almost all of the power that you should be drawing out of your battery. If you are in fact using 10 amps for 10 hours, you are way over the amount you should be drawing out of your batteries.

I have to agree with another poster that your batteries are simply too small for your actual usage. You would be better off with a pair of so called golf cart batteries. They are designed specifically for drawing large currents for long periods of time without damage.

And you should probably have batteries which will provide 100 to 120 amp hours. 70 amp hours just seems too small for your requirements.

I also highly recommend a solar panel to continuously charge the batteries. This will give you a longer usage without starting the engine, or perhaps even never having to start the engine to recharge the batteries. Highly recommended.

[Peterangell]

that is useful, thank you - at this time I'm not sure what type of battery they are - they came when I bought the boat new

I went to France a couple of months ago, and didn't use shore power as I was only staying 1 night. overnight I plugged in a 150W inverter. I then left the next morning and it was very rough, drawing about 28A for the autopilot as I mentioned - then the battery alarm went off after a couple of hours (not a surprise in hindsight) - and since that time I have had this problem with the alarm going off after about 3 hours of sailing

when I get to the boat next weekend I will check the water level and have ordered a hydrometer so I can test that too - if I need to get new ones I will see if I can get some higher Ah ones in the space I have...

[nigel1]

Peter,

If the battery's came with the boat, and the boat was new, they are most likely lead acid batteries, and most likely Leisure batteries, which while some manufactures claim the are deep cycle battery, they are not true deep cycle.

One other thing to check, which may not be an issue with the Oceanis 34, but was with mine, was the battery switch set up.
On the 473 there are 3 swtiche, one for the common negative, one for domestic battery positive, and one for engine start battery positive.
When I was checking out all the wiring, I found that beneteau had set up the switches so that when both positive switches were ON, both banks of batteries became common, which is not a good set up. I removed the link, but keep the cable handy just in case the starter battery fails.

If you are looking for batteries in the UK, then this supplier offers some good deals.
Check the Trojan 6 V batteries, you connect 2 of these to make a 12V battery and these are proper deep cycle.

https://www.tayna.co.uk/Trojan-Marine-Leisure-Batteries

[Peterangell]

Thanks Nigel that is a great help... will try to work it all out next weekend when I can get down to the boat

[hellosailor]

Peter-
Stop into a chandlery, or go on Amazon. Get a book on basic 12v systems for boats and before you spend money on new batteries, get an idea of how the whole system (alternator, charger, batteries, regulator, cables, other options) must be integrated and balanced to perform properly AS A SYSTEM.


If not, you may find the new batteries are also dead in under three more years, when they could have lasted six to eight, if other parts of the system got some attention too.


Plenty of recommendations here and elsewhere online, shouldn't take more than a weekend (or borrowed time during the week) to read the gist of it and figure out what parts of the system may need your attention.

[edsailing]

I would also check out your charging voltage, it's not unknown for the the alternator to be low on volts on beneteau electrics - should charge at at shade over 14v - the batteries won't be fully charged if it's not hitting this, I believe there is a tweak than can be done to the voltage regulator if there is a problem.

That said, I agree with the others - get as much good quality capacity as you can - it'll give you a lot of peace of mind!

[Peterangell]

thanks again for the input all... when charging on shore power it is over 14v, i can't remember what it is when the engine is running but i think it is the same...

Nigel - i think you are correct, i have 2 red switches and one black switch (common negative) - i remember being told during handover that they are linked so when they are all on, they are shared - i think that is to ensure the engine battery remains charged enough to start - you mentioned that you removed the link, but my batteries are in different parts of the boat (the 2 consumer batteries are in the engine compartment and the engine battery is in one of the rear cabins), so i'm not sure without looking at the weekend whether they are linked or not or how easy it will be to remove the link...