Dear All, I am addmiting my ignorance but---

I have a pretty simple arrangement

2 batteries with a switch with

No 1 barrery

No`s1&2 batteries

No 2 battery

and off.

Usually I start on both batteries and charge No1 battery going out under engine and charge No2 battery on returning to the mooring.I use no2 battery for domestics.

I am wondering if I should have a dedicated engine battery and use that alone to start the engine..

If I start on both batteries and the No2 is low does this compromise the power that I can get to start the engine? Will the batteries try to equalise themselves?

Regards David

I too have this set up; nice and simple! two batteries , same capacity.

Re using two batteries for starting. If they are in good condition, of the same type and capacity should be no problem.

Where it gets a bit dodgy is when you have one good battery and one poor battery. Instad of adding up the Amps available the poor battery tends to rob the power from the good one, even before you start.

If you charge batt 1 and batt 2 together, In Parallel, the poor battery takes most of the charge and prevents the good one getting fully charged.

I tend to use one battery for starting and then run on that to recharge. Next time use the other one  etc. One benefit of this set up is that you tend to get early warning if one of your batties is giving trouble; then you only have to replace one.

On a long motoring journey, I try to swap batteries every hour or so and find this works really well and gets them both well charged.

Bottom line is that there's no real place for a suspect battery on a boat!

I and many others have exactly same arrangements and IF you are good at remembering change-overs etc. - then despite various claims to contrary is still oe of the best set-ups.

I personally do not like dedicated start batterys. I prefer having possibility to choose battery for what job, be able to boost one with other etc.

The switch itself is pretty low tech ad YES the batterys will equalise across it if one is low. But as soon as engine starts - alternator kicks in and over-rides this and charges more to low battery bringing them both up to similar state. You will read various stories that only VSR / Diode control etc. does this but that is not true. A low battery has low resistance to charge and will accept a higher rate than a near full charged battery. The area that VSR's / Diodes come in are that they split the batterys and prevent one discharging to other.

Now onto the trouble with charge systems : Diode splitters require that charging is battery sensed as the diode block drops charge voltage approx. 0.7V = enough to never charge batterys properly if as normally the case alternator is engine charge sensed. Not an easy matter to change as some would tell you. I have a diode splitter fitted to my engine box and is disconnected - I tried to get my alternator to battery sense, but failed. I'm not good enough in alternator design to cheat it as some books illustrate by a diode in the regulator senser.

VSR's don't have the drop of voltage that diodes have and have in recent years become dependable systems. They isolate batterys so that engine starts on one, once charging the VSR operates and charge gores to one most needing it etc.

Both once working are self regulating and require no input from operator. OK fne - but I know of 2 yachts that were near disasters from diode systems that went awry and cooked batterys. One in fact suffered extensive damage to the surrounding structure from the battery literally exploding.

Sorry if I wax on too long, but its a pet subject and I am not a fan of the wallet financed answer to boat charging / systems. Simple logical sensible steps taken to ensure adequate battery size, sensible regime with the switch, and no requirement for boosted alternators or add-ons.

I'm lucky in that my boat is a motor-sailer and so my engine is running more hours during use than a cruiser/ racer etc. But anyway .... I have a Maplin splitter (Kemo Module at about £9) fitted into mains charger circuit. This then supplies both medium large batterys - #1 = General duty Leisure Battery (quoted for Engine start duty also) usually put to domestic service 90 A/hr, #2 = Heavy Duty Diesel battery usually for engine starting 55 A/hr (temp fit as the 90 A/hr after 8 yrs decided to go to Battery Heaven !). Both batterys via standard 1 - both - 2 - off switch and standard alternator on Perkins 4-107 engine.

I find with above system ... I usually start on #2 ... leave charging for 1 - 2 hrs, then switch to both till engine switched off. If #1 is used a lot when tied up / sailing and no charging, then I would usually start on #1 and leave charging instead of #2.

I have a batt check switch, simple 3 position non-locking toggle switch to meter showing voltage of each battery. I also have a Draper Battery / Alternator tester which can indicate if battery or alternator is faulty.

10 yrs of above system on this boat has proven to work, in that time I have only purchased 2 replacement batterys. 1 of those was ex breakers yard and lasted the above 8 years !!

I have a two switch system. One for domestic and one for starting (inherited with the boat).

It works well and the real benefit is that the starting battery cant be used for domestic by mutinous (usually female or "helpful (to the females) male" crew when they have flattened the domestic battery.

(I have however a pair of jump leads hidden away should I need to start the engine from the domestic battery)!

It seems to work ok so far,

Sorry girls im not an MCP - but its just the way it happens

Oh Dear Richard !! Don't you have your switch hiddena way like mine ? Mine is under the companionway steps and can't be seen easily, once you know where it is - it's easy to locate and use.

I sometimes carry an emergency batt pack - £30 out of Halfords - just in case. Beauty is - it charges of 12v source when engine running - meaning I always have a start capability !

Nigel ....

Unfortunately my crew is far too resourceful ... once used by me and the secret would be out.

Its its like "emergency supplies" everything but everything domestic (they know better than to mess with boat stuff) gets reorganised and restowed according to an impenetrable logic with the comment "we dont need that it can go here" and hours of fruitless searching trying to find my emergency choccy biiccies(usually eaten or taken home)

oops sorry to go off topic but I think its an ownership thing we havnt been doing it long enough to reach the comfortable compromise yet.

I love them to bits nevertheless

Richard

As I understand it, a dedicated engine start battery (some say)  should not - be a deep cycle battery.  Using a deep cycle unit to start evrytime will seriously shorten its life because of the "damage" done to plates by the short demand for high current.  On the other hand, a specialist starting battery will not dupolicate as a domestic battery because it is not designed to give low current over a long period!

I feel a compromise coming on!  Apparently modern gel or carbon fibre batteries can double in both roles and are about half the size of their predecessors.  What I am having trouble with is finding out the optimum Ampere Hour rating and Cranking Ampere specification for what will be the primary starting battery - but have to double at times to provide domestic services - for a 35hp diesel.  Any advice will be welcome.

Regards

David

Hi David and welcome to the forum.

Truck batteries seem to be the best answer these days - granted they will only do about half as many deep cycles as a dedicated deep cycle battery but they cost less than half as much.

Any truck battery will start a 35hp engine with ease.

Cheers

Chas

Any start battery ... car / van / truck will start a 35hp boat engine.

The one battery to avoid is the true leisure battery that has soft thick plates, designed for caravan deep discharge only. You may be lucky to get a start out of it - but most likely plates will buckle and short.

Best all-rounders are Truck batterys designed to do both starting large engine and also run the hydraulic tail-lift. Why someone claims their life is less than standard battery ? I run a Mercedes 814 truck with 6 litre engine and hydraulic tail-lift. Batterys don't fail noticeably earlier than my car or my inspectors cars !

The fancy Gel / carbon batterys were designed to fit ever smaller spaces but IMHO if you can it conventional - you will save money. I would even suggest a trip to local Car Breakers and check out their possibilities. You could get a good 80 - 100 A/Hr ex crashed van for little more than a tenner. I've done this and every battery this source have lasted very well ... last one 8 years !

I have a three battery system-all 120ah truck batteries-one dedicated to engine with relay operated split charge to two primarily used for leisure.I have recently put together a new switching board such that it will be possible to divert power from any/all of batteries to start.I have also added an earth isolator switch as is already fitted to existing system.Luckily have loads of space and heavy displacement.

Know full well that batteries are far more than I need for a 40hp engine but at least there is spare capacity if working starter to bleed fuel system and like.

Price  of batteries have risen-one I bought two years ago was about £60 -the one last month almost £80-that replaced a 110ah deep cycling/starter battery that came with boat and packed up at about 5 years old!

Phil

Hi David, I think your system sounds a very sensible one. Does it mean you can austensibly run it as if you have a dedicated engine battery as long as you remember to throw the switches?    I think my system may be very similar. I have just a few thoughts….some a bit off the point yet hopefully related.

I bought Betty, my GRP Twister 28, immediately after the previous owners’ short lived intention to sail her across the Atlantic, He had however begun to prepare her for the crossing.

I don’t have significant electrical knowledge but I have added a few additional electrical things. Betty came with three batteries, two for the domestic and one solely for the 16HP Vetus engine. He had also fitted a Rutland 913 wind generator on the stern quarter deck with a Marlec regulator which also gives a voltage readout of a) both battery banks, b) the charge in amps to each bank.  It also has a switch where the windvane can be 'braked' if it is too noisy or not required. The Marlec also has input terminals if solar panels are to be added and a check on their input can also be made. Although other owners I know have used the smaller Rutland generator, they don’t seem to get nearly as much charge as I seem to and from published tests of wind generators the 913 seems overall the best value for money. e.g. some have higher charge rate but also costing a lot more and which often are extremely noisy.

The electrical items that I have added to the original GPS include a chartplotter, navtex, AIS, tiller pilot and a large DSC radio. At present I have all the standard nav’ lights. I don’t have any LED’s and run 8 internal 20w 12v bulbs for cabin lighting etc.     I have never had even remotely flat batteries and I never hesitate to use all the electrical items or lights on the domestic side.       My domestic batteries are standard 80AH as typically used on diesel cars and vans, which means I could jump lead to them to start my engine if req'd.  I have considered getting one of the 'battery packs' as/Nigel above which seems extremely good value.

My recent long cruise in a friends boat which had reasonable sized solar panels fitted,  still relied on the engine to be run around 4 hours a day to keep the batteries charged as he had a fridge and an autopilot and no windvane.  I dislike engine noise and turn my engine off at every opportunity so these hours were not a pleasant experience. I also have a windvane which obviously requires no electrical power unless run in conjunction with a tiller pilot. 

In future I’m considering building in a small fridge and fitting a diesel heater so the Amps and required voltage will undoubtedly change significantly and I may have to be a bit more economic with my use of some items or fit some more modern gismos to improve the battery charge.    

Even more reason to have a dedicated engine battery to prevent me getting stuck and unable to start up.    Hope this is of use, Scotty