Are separate service and starting batteries really necessary?

[Anonymous]

Lead acid batteries ("battery" means "battery of cells") all consist of a number of 2.2V cells in series. The voltage of 2.2V is due to the laws of physics and is a characteristic of the materials being used, and unchangeable for lead-acid. If all the cells are packed into one container (typical car battery) then any cell can fail internally, ruining the battery. This is a common failure mode. If the cells are individually packed (2.2V) then you need to wire 6 of them in series. If any one of them fails, you can replace it so it becomes practicable to carry a few spares. However, as commonsense suggests, it takes more space to package the 6 2.2V cells than it takes to package one 12V battery. Certainly you will always get a more Ah for a given volume if you use 12V batteries. However, most of us have to consider the practical size of battery box. In some cases, the high Trojan profile can give you more Ah with the same 'footprint' on the floor of the battery compartment - it depends on whether there is a height limitation.

Trojan batteries are 6V and just happen to have a brilliant reputation with yachtsmen around the world. They are not good *because* they are 6V, they are good because the manufacturers set out to design and make excellent batteries albeit at a price. The size of them is presumably to suit their primary market - golf carts - rather than yachts.

Reason for edit:- In the original post I talked about 1.1V cells which is, of course, the voltage of a NiCd cell. I was thinking 'NiCd' while writing lead-acid. Another poster picked this up (a few posts down from here) and I have revised this post. Sorry for the confusion for those of you who have been puzzling over the original unedited post /forums/images/graemlins/smile.gif

 

[Guest]

Many advise against .... but simplicity wins IMHO

We hear about fancy better batteries, fancy wiring systems to keep 'em in top condition. But it's great till it goes wrong.

Batteries : I believe in simple 12V car Heavy Duty jobs - sold for Diesel starting ..... I also believe in Leisure caravan battery's. These two types supply reasonable power as long as simple regime followed.

Wiring and charging .... again off shelf simple cheap and able to be fixed .... OK - so I stick with the 1-Both-2-off switch, but have a Maplins splitter for charger to go to ... then to each battery - which are a) HD diesel start battery, b) HD truck battery.

Now if anything goes wrong - you can always find a car battery somewhere. You can bypass switches etc. ....

KISS - it's a Boat .... not NASA Space Shuttle. I agree that Live-aboards / ocean crossing jobs need a bit better and they also have people on all time to look after it all ....... but a boat like mine - sits for ages doing nothing - then expected to start up and go ... KISS.

 

[pvb]

Oh dear, no!

[ QUOTE ]
Lead acid batteries ("battery" means "battery of cells") all consist of a number of 1.1V cells in series. The voltage of 1.1V is due to the laws of physics and is a characteristic of the materials being used, and unchangeable for lead-acid.

[/ QUOTE ] Although your post sounds as if you know something about the subject, the fact that lead-acid batteries actually have a cell voltage of 2.1v, not 1.1v, makes me wonder.

 

[Guest]

Re: Oh dear, no! Hee Hee !!!!

I think he's getting mixed up with plate to plate against cell partitions !!

Trying to remember back to my long ago Physics lessons .... a single pair of plates in a beaker produced just over 1V .... but a Lead-Acid battery uses cells with more than one pair of plates in each cell - giving rise to just over 2V ??

But of course it's easy to verify truth of the 2V statement ... count the plugs in the top .... 6. Or with older style HD batterys count the bar links connecting the cells ... 6.

6 x 2 = 12.

QED

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[Anonymous]

Lead acid cell voltage

You are quite right, of course, I was writing about lead acids and thinking NiCd. If the system will let me so long after the posting, I will edit that out.

Thanks for picking it up and pointing it out /forums/images/graemlins/smile.gif

 

[alant]

What about the systems where there are no seperate Engine/domestic isolators? Arn't these much the same thing, with both banks dropping together.
I much prefer ability to switch off engine battery when not in use.

 

[William_H]

Batteries are made in 6v form simply because for the same capacity in 12 volts they would be too heavy to lift. I had some 220AH 6volt which weighed in at 32 kg i certainly wouldn't want a 12 volt 220AH at something over 60Kg. olewill

 

[Anonymous]

That is undoubtedly true in some cases but Trojans are very light. You could easily lift several at a time. Maybe golf trollies and carts are 6V? Dunno, but it isn't a weight issue.

 

[kengill]

This is cheese rind shaving at its very worst.

 

[awyatybw]

I used to have a pair of identical 105Ah jobbies with the One-Both-Two switch. I was always nervous about electricity consumption, especially as we like to sail and to anchor when cruising and therefore may not run the engine every day. The capacity was clearly insufficient but the lack of a dedicated engine battery was the real problem - I could have designated "Battery Number One" to the engine but somehow that doesn't work out, especially give the limited total capacity.

Last winter I added a separate engine start battery and a split-diode charging regime. The engine battery has an isolator switch, primarily so that I can avoid overcharging it when running the engine for long periods. For the same reason it is a low-maintenance type, rather than maintenance-free, so that I can top up the acid from time-to-time. It is a completely standard type that one might find in a Land Rover etc.

I replaced the 105Ah units with a pair of Trojan 130Ah units. I chose them not especially because of Trojan's excellent reputation but because they afforded me the maximum capacity that I could fit in the main battery box. Very irritatingly, it is 5-10mm too shallow for several standard battery types. Out went the One-Both-Two switch to be replaced by a simple on-off isolator.

There is also a third isolator switch which can parallel the engine and domestic batteries. The three switches can be used to achieve a variety of operating conditions but the normal practice is that both domestic and engine are "on" whilst aboard and the parallel switch actuating lever is detached and in the chart table. Messing with battery switches while cruising has gone, peace of mind has been substantially improved and I no longer berate the crew quite so heavily each time they switch on a light. I do still keep a close eye on the battery monitor.

With SWMBO and two daughters aboard, my nervous twitch is now only related to water consumption.