Batteries - you live and learn

[tome]

It was time to replace my domestic batteries. The current arrangement was 2 x 6V batteries in series giving a total of 240AH.

When I went to my normal supplier, they tried to talk me out of 6V batteries on the basis that they were not so popular and I'd be better off using 12V batteries in parallel. However, the biggest batteries I could fit were 105AH giving me 210AH in total, and the price was £60 each. I cannot go for the cheaper low-maintenance as I have a Sterling regulator and need to top up the cells from time to time.

I didn't want to lose 30AH capacity, so went into Lucas just in case they had anything. They had 6V 240AH batteries in stock which were direct replacements so I fitted these at a cost of £74 each. Moral of the story is that they told me you always get better capacity with 6V cells, and I wasn't aware of this. Thought I would share this little gem.

See Trojan batteries which are stocked by Lucas.

 

[boatmike]

Actually there is no such thing as a 6 volt cell. The cells in a 6 volt and 12volt battery have the same output there are just twice as many.
You are right though that often in the 6 volt babies the cells are bigger making 2 6 volt ones higher in capacity than I 12 volt one but the physical size will be bigger accordingly.

 

[tome]

Yes, typo

 

[Anonymous]

Trojan batteries have an excellent reputation - arguably the best. I had wanted to fit the 6V Trojans but they were too high for my battery box and it would have been very difficult to alter things. You should be fine with those for years and years. Like you, I wanted flooded lead acid so that I can top up and I went for the Varta semi traction 12V.

 

[VicS]

Re: Batteries ... capacity

Just a point I thought it worth clarifying.

If you fit two 6volt 240AH batteries in series you still have 240AH (but at 12 volts)

If you fit two 12 volt batteries in parallel you double the capacity.

I think that agrees with what you are saying. I'm not making a fool of myself am I but the processor speed is getting a bit slow these days and the memory is getting a bit temperamental

 

[tome]

Yes, they are taller than the 12V batteries which is a pity for some applications. My battery box is a fibreglass moulding with lid and designed to take them. There's space for longer batteries (about 6 inches spare) but couldn't find a longer version which would fit. Glad to hear that Trojan have a good reputation. Previous ones were Deta (German) which I believe were about 5-6 years old.

I knew it was time to replace them after the Eberspacher wouldn't last a night on a full charge.

 

[tome]

Re: Batteries ... capacity

Quite right Vic. Batteries in series increase the voltage, in parallel increase the AH.

 

[BigLes]

I've just increased the capacity of battery power, from a single 50AH battery to a pair of 110AH batteries (plus of course the starter battery). These are below the bunk in the after cabin, and with the extra weight of the new batteries I find the stern sits lower and the bow sits higher in the water - obviously the extra weight.

I assume this will impair the windward performance of the boat as she will not 'bite' into the water so well? Any thoughts? I'm going to try her for a while but have the funny feling I may have to re-locate the batteries further forward?

So again a lesson - condider the weight of the batteries also!

 

[VicS]

Re: Batteries - effects of weight

You could always send the crew forward

 

[Anonymous]

Put an extra water tank forward?

 

[LadyInBed]

Increase your anchor chain scope.

 

[tome]

Hi Les

A quick look suggests that a 50AH battery will weigh about 16kg whereas a pair of 110AH will weigh just over 50kg. Hard to think that this weight difference(equivalent to a child in the after bunk) makes that much difference. Could this be the onset of paranoia?

Tom

 

[William_H]

The reason they make 6 volt batteries is that as batteries get bigger in capacity you just can't lift or move 12v ones by hand. If you want an even higher capacity battery you end up with 2 volt individual cells simply to keep size and weight manageable. I guess in the end it is a question of what will fit and what is available at the best price as to whether you go 2x6volt or 2x12 volt except you can isolate 2x12 volt ie use one save one. will

 

[Birdseye]

I suspect that when you add 2no 6 volt batteries in series you will not get the amp hours that is stated, because the current not only flows through the circuit but also through a second battery So some power will be used up.

Whether this accounts for the difference between the ratings of otherwise identical 12v and 2 x 6v batteries or is a totally trivial effect, I do not know.

 

[tome]

[ QUOTE ]
because the current not only flows through the circuit but also through a second battery So some power will be used up

[/ QUOTE ]

Would you care to explain what morcel of physics this little gem is based on?

 

[pvb]

Glad you said that...

I thought it but didn't dare post it! /forums/images/graemlins/wink.gif

 

[VicMallows]

Bad connection between the two batteries in the middle? /forums/images/graemlins/smirk.gif

 

[Sailfree]

If you fit 2x12V in parallel what happens to the charging alternator. If you charge 2x6V you get R(total)= R1+R2. But while the resistace of a 12V may initially be higher for charging in parallel from memory you get 1/R(total) = 1/R1+1/R2. My understanding is that charging 2 batteries in parallel effectively halves the resistance and you can risk blowing the output diode/regulator pack on the alternator unless it was designed for this.
My knowledge is only schoolboy physics can someone more qualified please explain the actual situation.

 

[pvb]

It\'s not a problem...

There's no risk of damaging the alternator when charging 2 or more 12v batteries connected in parallel. Don't worry about the physics, just be assured it's not a problem.

 

[tome]

Yes, any poor connection anywhere in the system (whether parallel or series) would introduce resistance and therefore a power loss through heat dissipation. It's not an inherent weakness of connecting 2 batteries in series, rather it's a weakness of poor installation.