Gell Batteries/Common Starting

[sirockin]

Hi

I'm rewiring a 60' sailing yacht which is to be used both for long-haul blue-water cruising, and short charters. It currently has five separate battery systems:

> 2 switchable 24V bank for domestic/service
> 1 x 12v bank for some instruments/radio
> 1 x 12v battery for engine starting
> 1 x 12v battery for genset starting

To me, this seems unnecessarily complex, and in the case of the two service banks, undesirable, so I'm proposing to combine the 24v banks and also to use one larger battery for engine & genset starting.

The owner wants to change from wet cell to gell cell batteries as the battery compartment is neither well ventilated nor easily accessible when at sea.

As we're replacing the battery chargers, we can cope with the reduced charging voltages here, but we may have problems getting appropriate regulators for the alternators. If we can't get them, I'm proposing to use diode(s) to drop the voltage.

I'd be interested in everyone's views on:
> Using one battery for starting two engines
> Using a gell battery as a starting battery (manufacturers claim they're fine but they would!)
> Alternator charging of gell batteries

Thanks in advance,

Dave

 

[TigaWave]

one battery for both engines is fine but......if its for charter work and its good practice two are better.
An engine battery should be in duplicate and connected so that with the use of a switch either battery can be used to start the engine.
MCA blue code, not the exact wording I'm sure, but if the boats going to be used for charter the owner should have a copy you can refer to.
If the 1st battery fails you want a quick reliable method of connecting a second fully charged battery in its place. Linking two is not the best option as if ones flat then as soon as you link them you wont have a fully charged battery any more.

 

[pvb]

Makes good sense to combine the 2 x 24v banks.

There shouldn't be any problem in using one battery to start both the engine and the generator, and a suitably sized gel battery won't be damaged by engine starting.

As far as alternator charging is concerned, I'd imagine there's a smart regulator fitted (if not, there should be). Most of these can be adjusted to give the correct charging regime for gel batteries.

As an alternative to gel batteries, had you considered AGMs? Lots of similar qualities - sealed, maintenance free, low self-discharge, etc - but not so fussy about charging voltage.

Ideally, the battery compartment should be ventilated in some way. It's easy enough to achieve with a little computer fan wired so it only operates when the batteries are being charged.

 

[Anonymous]

While what you are proposing sounds, in principle, fine, I wouldn't want to make specific comments without knowing the sizes involved - genset, charger (A), solar?, expected dc load, size of existing and proposed batteries. Further, the owner has presumably had problems or he wouldn't want to change to gel - what were the issues? What is the feedback from those who last operated the yacht - what changes would they make?

 

[Oldhand]

Had no problems using a gel battery for engine starting on our last boat.

 

[sirockin]

Thanks for the replies everyone.

Tigawave:
Yes I agree about having a backup method of starting. My intention was to use a selector switch from the 12v Instrument battery (providing it could deliver the requisite cranking amps). I'm looking at using 4Ds for both the starting and the Instrument battery.

pvb:
There's a suitable smart regulator on the 24v alternator. The problem is the two 12v alternators have built in regulators. AGMs sound like a good idea and, surprisingly the quote I just got for them was less than for standard gells. How are the settings on modern battery chargers suited to AGMs? The ones I looked at had a max charging voltage at warm temps of around 14.0 volts. I know newer chargers now have settings for gell or wet cells - AGMs seem to fall somewhere in the middle.

Lemain:
Well I've done the load calculations - it's just a matter of getting the appropriate batteries, charging systems etc. I'm looking at around 400 Ah on the 24v and around 170 Ah on the 12v instrument. I reckon around 170 Ah for the engine and genset should do it. As there's a genset on board we'll just use alternators and battery chargers. The boat's just changed hands. The previous owner reported no problems, but then again, the wet cells were dead! The battery compartment is under a stowage area which in turn is under a cockpit floor. At the best of times it's a pain in the a*** to access. At sea with spray etc, I reckon it's a definite no-go. Plus, the new owners feel the boat may well be left unattended for months at a time - another reason for avoiding wet cells.

Thanks again everyone for the feedback.

Cheers,

Dave

 

[Anonymous]

Re: Gel Batteries/Common Starting

Without details of the actual load I can't comment on the sizing of your batteries although I am surprised that you need such a large cranking battery. I'm sure that you are aware that 'gels' are lead-acid batteries. The difference between gels and wet cells is that the electrolyte is stored in a gel, allowing you to operate the battery in any orientation. There aren't any other advantages, unless you see the inability to top them up as an 'advantage'. When lead-acid cells get near to the end of their charging period they release hydrogen and oxygen gasses. In open cells, these are vented off harmlessly and the water is replaced. In gels and sealed cells, there is a chemical mechanism that allows the hydrogen and oxygen to combine into water, so you don't need to replace the water and you don't have to provide for hydrogen to be vented safely. The problem comes when the cells are charged too far or too fast and the mechanism cannot keep up with the rate of production of gasses. This causes a build up of pressure which is vented off and the water cannot be replaced since the cell is sealed.

I am not convinced that all fast chargers (mains powered and alternator 'smart chargers') are safe for gel batteries. I have personally experienced failure of gel batteries at an age far lower than the manufacturer estimates to be the design life and I have heard a similar comment from another forum member, a well-known and seemingly knowledgeable contributor to this forum. If I wanted to use gel cells I would ensure that the charge rate was kept at a low level. For myself, I have instead gone for flooded lead-acid batteries so that I can top them up as required, without damage.

 

[pvb]

Specific answers...

Dave, the 2 x 12v alternators could have smart regulators added relatively easily. This wouldn't be necessary for charging the starter battery, but it could help speed the recharge of the instrument battery.

Most modern marine battery chargers now have an AGM setting, so it shouldn't be difficult getting the right charging voltage.

I wouldn't worry about fitting sealed batteries (either gel or AGM). Most of the tales of "premature failure" don't come with a history of how the batteries were treated by previous owners! If you start off with new batteries and treat them properly, they should last for many years. My domestic bank has sealed maintenance-free batteries (not gel or AGM, just ordinary types) and it works fine.

As you're aware, one of the biggest threats to batteries is excessive depth of discharge, so big battery banks tend to survive better. For this reason, I'd wonder whether your planned 400Ah 24v bank is big enough for a 60ft yacht, especially with charter guests on board.

 

[Anonymous]

Re: Specific answers...

[ QUOTE ]
Most of the tales of "premature failure" don't come with a history of how the batteries were treated by previous owners!

[/ QUOTE ]Just to put the record straight since I was the one who commented about premature failure, in my case the previous owner had set the charger to a setting for 'diode splitter' when there was no splitter in the system. HOWEVER, the Mastervolt charger is set by a small dip switch and it is completely ambiguous whether a switch is said to be 'off' or 'on'. That is the problem with most dip switches. I have also seen dip switches on another well known marine charger. It isn't always easy to set the things up correctly but if you have flooded lead acid batteries you can always add water if you have been overcharging them. Little or no harm done, whereas with sealed batteries, you are finished.