Additional service batteries

[timturner]

I have two 70ah service batteries (as fitted by Jeanneau) in my 36i and I am adding an additional one. I have found that I could get a 100ah which is in the same size case and would fit in the space that I have for it. I am getting conflicting advice about this - some say you must not fit varying "ah size" batteries in parallel because the lower rating will run out first and then the battery still with charge, will transfer charge to them? Has anyone any experience of this dilemma? Naturally, I am trying to achieve as much capacity as I can but I do want to do it properly. Thanks Tim

[MartyB]

Tim,

Assuming one ea of the batteries is a house and starting, and you will end up with 2 house and 1 starting battery, then yes, the two house batteries should be the same size, and to a degree, the same age etc. If you run the the two house bats seperate, then the fact that one is probably a size 27 vs a 24 or some such thing, then it is not a big deal, ie you charge them seprately off of the AC charger, they discharge at different times, ie you have to switch them over when one dies if you will.

Otherwise, you would be better off to buy 2 new 100 ahr batts, and trade the smaller one in as best you can in the process. Also look at 6V batteries, and run them parrallel. may times two of the in equal sizes will run/last longer. I found that to be true with a Travel trailer I had. I still only have the two size 24's in my 30' boat. Have not needed more power than I have currently.

marty

[sailingpages]

Hi Tim,
I have a 36i with 2x 70Ah House batteries and 1x 70 Ah Engine Battery (same type as house) which were fitted by Jeanneau. To increase the capacity of my house bank, I intend to add an AGM Lifeline GPL-1400T (55 Ah, 850 CCA) as the engine battery. This means I can then use the 3x 70Ah batteries supplied by Jeanneau as a 210Ah house bank.

Although Lifeline batteries are expensive, I am opting to go this way to have a very robust engine battery. It can also be easily installed adjacent to the battery charger in this compartment as it is sealed, non-spillable. You may also want to consider adding something like this for the engine battery as an option.

Regards,
Geoff.

[Trevor]

Hi Tim,

I was looking for more house capacity on our SO36i and fitted the start battery below the floor in the locker behind the shower.

You already have 3 x 70 Ah batteries under the aft stb bunk. Simply remove the red cable from the start battery isolator switch and connect it to the existing house bank. You now have 3 x 70 Ah batteries in parallel.

Now install a new start battery in the location in the locker and connect a ground cable to the ground isolator and the positive cable to the location on the start isolator which now has nothing on it. You need to purchase two battery cables about 1 metre long.

You end up not having to change any charge wiring because you have replaced the existing start battery with another and very simply paralleled the old start battery to the house bank. They are all 70Ah batteries and all rated the same.

When the time comes to replace them you may consider deep cycle batteries for house and start batteries for the start function. The ones that come with the boat are just normal car batteries from what I can see.

The notion that paralleling batteries of different capacity is bad because the one that goes flat first will flatten the other is not really true. Consider two water tanks paralleled as an analogy and you will see that any extra water tank increases capacity and does not absorb water from the other tank. Paralleling batteries of different capacity is more a problem of charging them at an optimum rate.

I hope this helps,

Trev

[Don Reaves]

The real problem with running batteries in parallel, even if they are the same size or age, is that if one fails, it can destroy the other one.

If one battery fails because the material on the plates sloughs off and ends up at the bottom of the cell shorting out the plates, it effectively shorts out that cell. This causes the voltage to drop correspondingly, and the other battery attempts to charge it. Of course, the battery won't be able to be charged to the normal voltage, since it now has 5 cells instead of 6. So it runs the good battery flat, which is often fatal for an older battery.

All in all, I intend to replace the two original parallel batteries with 6V golf cart batteries in series as soon as there is any evidence that the originals are getting tired.

Don Reaves

[Trevor]

Hi Don and Tim,

While the shorted cell scenario is true, the most common cause of battery failure is positive plate corrosion, meaning the battery cell has gone open circuit, not short circuit. That means that two 6v batteries in series are more likely to cause problems due to open circuit cells than two 12v batteries in parallel suffer through short circuit failure.

Given that the start battery is 12 volt, having 12v house batteries gives you somewhere to go to get the motor started if you have battery problems of one sort or another. If you had a flat start battery and a failed house battery, having a really good 6v battery would be of no help at all to start the engine.

Given the energy density of a 6v battery is no greater than a 12v battery, the use of 6v batteries is interesting but of no practical value.

Happy sailing, ;D

Trev

[Don Reaves]

Well, it happened. One of my 12V batteries developed a shorted cell, effectively becoming a 10V battery. I will be replacing them both with two 6V batteries in series at the beginning of next season. At 240 amp hours, they will be able to store much more energy than the two group-24 12V batteries that were delivered with the boat.

Don

[davideso37]

I read all this advice earlier in the year and paralleled up the three existing 12 Volt 70 ah batteries on my SO37 and put the new battery in as the starter. As Trevor says this gives three batteries of the same age and specification on the house bank. The new starter battery holds a higher voltage charge and since the change we have not run out of house bank battery capacity overnight. Of course when the first house bank battery fails then we will need to change all three. This we will do with sealed electrolyte which are now permitted in the sailing regs. We have had a bit of spill from the vented electrolyte batteries possibly with a bit of thumping through the waves off shore. I think the unsealed batteries should be banned.

Regards

David