Two different batteries

[Slinky Spring]

My boat has a two battery system. Currently both leisure batteries. Engine starting is still a problem and begining to think low battery might be the problem. Can i replace one of the leisure batteries with a conventional car type which i would keep for engine starting and run the lights etc off the leisure.
I remember reading somewhere here that both batteries should be the same or they would damage each other.
Any advice welcome

 

[l'escargot]

If you have a two battery system where each is isoleted from each other to their respective circuits, you should have a starting battery for the engine. It isn't what leisure battteries are designed for and they won't last as long, they have trouble producing the high amps for a short period.

 

[Billjratt]

The 'feeling' that both batteries should be the same is if they are connected in parallel for significant periods. Each will attempt to charge/drain the other and both will eventually end up at the lowest common denominator, so a bad one will drag the other down. If you have a regime where they are separately used and charged (diode splitter) they can be different specifications, occasionally paralleled for starting, and then switched apart again.

 

[VicS]

Both leisure and starter batteries can be wet acid, AGM or gel. If they are to be connected in parallel (for charging perhaps) the they should be of the same type.

If you fit a starter battery of the same type as your leisure batteries there will be no compatibility problems.

If your leisure batteries are OK why not combine them into a domestic bank and fit a starter battery with a suitable CCA rating in addition (assuming you have the space of course).

 

[sarabande]

IIRC if you have two batteries in parallel or series, and they are of different capacity or quality (even with a different number of charging cycles), then as soon as one is fully charged (and therefore needing just maintenance) the other will be asking for a full charge rate - which doesn't do the the "full" one any good.

The M&S emergency power room next to work, last month replaced ALL of its standby Yaesu commercial batteries (6v unfortunately) in the genny starting bank for this very reason. They could not afford to let one battery in the bank be a bad one.

 

[VicS]

[ QUOTE ]
They could not afford to let one battery in the bank be a bad one

[/ QUOTE ] Thats true if there is a bad battery in a bank depending upon whats wrong with it, it could drag the rest of the bank down with it. That's the reason for not leaving a starter battery in parallel with the domestic battery. One fails and then you may have nothing.

Two batteries of different capacity in parallel are no problem as long as they are the same type when it comes to charging.

 

[sarabande]

"Two batteries of different capacity in parallel are no problem as long as they are the same type when it comes to charging. "

Sorry, but I can't see how that works.

If Batt A reaches its full charge, while Batt B is only at n%, then the charger will keep shoving power into both batts until Batt B is full, as the charger sees the paralleled battery as having (100% + n%)/2.

I only got O level physics, so may be GCSE is a bit different. /forums/images/graemlins/grin.gif

Happy to learn if/how I am wrong !

 

[Tranona]

It is not clear whether you have a split system with a diode or a VSR. If you have then you can replace your engine battery with a dedicated start battery and then use the surplus battery to boost your service supply. If you don't have a split system this is perhaps your first priority.

You might also then consider using a micro start battery such as a Red Flash or an Odyssey as these are much smaller and can be installed in any orientation.

 

[Loginname]

[quote
Happy to learn if/how I am wrong !

[/ QUOTE ]

Once the two batteries are connected in parallel they will end up at the same level of charge. You won't have one at 100% and one at n%.

 

[VicS]

[ QUOTE ]
If Batt A reaches its full charge, while Batt B is only at n%,

[/ QUOTE ] That's not how it will work out.

I will attempt to explain using the water flow analogy that you may remember was sometimes used to explain electrical things, but only if no one labels me "expert " in inverted commas

I will represent two different sized batteries as two different sized water tanks, both are the same depth and when they are empty the water level is equivalent to 11.7volts and when then are full the water level is 12.7volts, representing fully discharged and charged states. We will start with one with more charge in it than the other
I'm going to regret this.

Now we then connect the two tanks together and to a water supply that comes from a higher level, equivalent to 14.4 volts from the alternator or a battery charger.

When they are first connected together some water will flow from the fuller one to the less full one but once we connect the supply pipe and depending upon the flow rate from the supply it will start to fill the more empty tank faster until both tanks are at the same level

I am regretting this already

Then as we continue to fill them the water will tend always be the same level in the two tanks until they are both at the full level when the automatic control (not shown) shuts off the supply or we disconnect it manually.

Well I tried. Maybe someone else can do better. If you think you can please post it on the Boats Yachts Marinas forum /forums/images/graemlins/grin.gif

 

[William_H]

I like your sweet little pictures Vic and agree with the theory.

I have a theory that a batteries cranking capability is greater for a start designed battery than a leisure battery Relative to its size.

So I have always thought that a great big leisure battery will still give a good crank or at least as good as a tiny engine start battery. Of course this is really relative in itself. I am assuming he has a big leisure battery and relatively small engine. Does using a leisure battery for cranking damage it? again I think not if it is a big battery.

What I am getting around to is that possibly the leisure battery is dead. Possibly also the other is near death. Probably from old age.

I would assume he has a "off,1,2,both" switch which might explain 2 identical batteries.

I always feel that the separate circuits and a VSR is the way to go with provision for emergency7 start from both batteries. In this case a new engine start battery would be cheaper considering he might need a new leisure battery as well. olewill

 

[VicS]

I have always been under the impression that leisure batteris are just modified starter bateries and are quite Ok for engine starting and also for some not very deep deep cycling. My last leisure battery was even marked with a CCA figure and I often suspected that the modification may only have been the label that said "Leisure battery". The new on is not marked with a CCA figure though!

Glad you liked the "sweet little pictures" and agreed with the explanation. /forums/images/graemlins/smile.gif

 

[alan17]

Well done Vic.

 

[sarabande]

electrical enlightenment ! All is now clear, thanks VicS.

/forums/images/graemlins/smirk.gif