Battery Qestion Revised

[Jean]

Thanks for answers so far but I’m not sure any of them have explained why not! They have though given a lot of food for thought.
Perhaps I can revise the question:
If a yacht has 2 different types of battery fitted (normal car type as number 1 and caravan leisure type as number 2) and the standard 1,2 or both selector switch, could it harm either battery if starting the diesel engine on both? If so, why?
A comment regarding one of the earlier responces (“Never charge with selector switch on both”) is “Wont the battery back emf’s (which increase with battery charge) reduce the risk of overcharging one?”.

 

[Chris_Robb]

Main point I am not clear about is the Don't charge on BOTH.

I have 1 x 130Ah engine battery, and 2 x 130Ah domestics, paralleled as one.

I have a solid state charge splitter, which effectively connects the engine to the domestics once the voltage has risen on either set above 13.5V. and an Adverc Regulator monitoring the voltage on the Domestics.

When the Splitter is operating this is surely effectively the same as using the 'both' switch setting which parallels the batteries.

If as would be usual, the domestics are in need of charge, and the engine is almost full, the act of putting the batteries in parallel will allow the current to flow from the full battery to the domestics. The Alternator will then charge both batteries according to the relative amount of charge they need. with the domestic batteries controlling the point that charging stops.

I have volt meters on each battery set, and the engine battery never goes above the 14.4 V charge limit.

So I don't understand why this is not a good thing to do. But it seems to work!

 

[colin_jones]

I would not worry too much. For the past 10 years, I have run 1 normal car start battery and 2 traction batteries. Because I start my engine and run it most days in the Summer, I leave all 3 ganged up, so that all 3 work hard to cope with the fridge. Sometimes I charge them separately on either 1 or 2, but it is mostly on 'both'.

I have about 450 amps of power and the alternator is only rated 75 amps. This breaks the 3 - 1 ratio rule, but it seems to work.

My batteries generally last about 3-4 years of this hard work and abuse.

 

[ccscott49]

The leisure battery is not designed to deliver the high amperage required to run a starter motor on the engine, the plates of the leisure battery are likely to distort, with the possibility of an internal short. The engine start type of battery is designed to deliver a high currnet for a short time, then be recharged quickly when the engine starts. Leisure batteries are designed with a different charge regime in mind, that of the initial boost, then lower current charge then float to top off. The normal engine alternator does not provide this. The reason for the smart chargers, adverc, sterling etc. A charge splitter will not allow one battery or bank discharge into the other. I'm not sure about the back EMF but I get your point, but! the charge regime for the two different types of battery makes them incompatible. I'm sure you will get away with it for a while, but in the long run?

 

[keplero]

Avoid to put select switch on both. Do it only in case of emergency.
If the batteries are not "twin" (perfectly the same) one will discharge the other, resulting both in the worst condition.
In normal condition, the engine must be able to start under one battery.

Massimo

 

[ccscott49]

I have thought more about the "both" switch position, once the engine is switched off, the switch must not be used on "both" especially if one battery is lower than the other, a charge splitter would solve this problem, it would prevent one battery discharging to the other, but you can charge on "both"! The batteries are in parallel, not series, so the batteries will take what current they require, the alternator is a constant voltage unit, not constant current, each battery is essentially a seperate circuit! I'm sure somebody else will verify or dispute this statement so I will wait for another answer, but this is how it seems to me.

 

[Guest]

Jean
Many of the answers don't seem to be addressing your questions. I will attempt to do so assuming that you have a very simple system consisting only of two different battery types and a selector switch.

Original question. Which switch position to use for engine starting?

Answer. Whichever position that the high current battery is connected to.
My starting battery is connected to position 1 and from previous experience and reading the replies it seems that it is usually position 1. If you have any doubts carry out a physical examination to see which positive terminal of which battery is connected to position 1.

Subsidiary question 1. Why use the high current battery for starting?

Answer. I think you probably understand this now - because starting an engine (particularly diesel) requires hundreds of amps for a short period.

Subsidiary question 2. Is there any problem with starting the engine on the leisure battery?

Answer. No, providing it has the necessary power. A fully charged battery of 100Ah (no matter what type of battery) is quite adequate for starting a small to medium yacht engine, without damaging the battery.
If the engine fails to start on the high current battery (position 1?) then switch to the other position and try again. If the engine still fails to start then try again on position Both.

Other questions which have arisen:

Two batteries at different states of charge - engine NOT running.
It is a typical situation on a yacht that the starting battery is often in a higher state of charge than the domestics battery. In this condition, switching to position Both means that the battery in the higher state of charge will charge the lower battery until the voltage at the terminals of both batteries is the same. This voltage will be lower than the fully charged voltage and it is possible that you would only be able to start the engine on position Both, or not at all.

It is a situation to be avoided.

Two batteries at different states of charge - engine running.
The basic charging system (ie alternator regulator only - no 'smart' charger)is 'Constant Voltage'. This means that the output voltage from the alternator is kept constant at about 14.5 volts. no matter what the current(amps) demand is (up to the current rating of the alternator). The voltage at the battery terminals will depend on the state of charge of the battery. Over a period of charging time, the voltage at the battery terminals will increase, and it is the voltage at the battery terminals which determines the amount of charging current taken from the alternator.
ie As the battery is charged, its terminal voltage increases and the amount of charging current it demands from the alternator decreases.

The charging current tapers off over a period of time because the battery itself regulates the amount of charge it takes.

This means that if two batteries, whose initial charge states are unequal, are charged from the same alternator at the same time, then the battery having the lower initial state of charge will draw a higher charging current than the battery having the higher state of initial charge. Eventually, both batteries will reach their fully charged state, although it will take longer for the battery starting with the lower initial state of charge to get there.

So what happens to the battery which reaches its fully charged state first?
It just carries on being charged, but at a trickle rate. Just as in your car.

I have heard talk about cooking the batteries, but have never experienced it. I believe that that could only happen in a 'constant current' charging system.

There is no problem with leaving the selector switch in the Both position while the engine is running.

As a point of interest, I have read that, due to advances in battery technology, the difference between starting batteries and leisure (cyclic) batteries is diminishing.

Hope this helps.

Achilles heel.

 

[Guest]

unless one battery is almost completely discharged, the idea of one flattening the other when they are charged in parrallel is theoretical rather than practical. once the charging voltage has risen above 12.7 there can be no discharge anyway, and the charging voltage from your alternator will rise to this level pretty quickly unless it is undersized in relation to the battery capacity or the battery is almost dead.
have used the parralell system (domestic and engine batteries are put in parralell when a relay senses alternator output) without problem for 10 years.

 

[LadyInBed]

The definitive answer. . .

It should be laminated and put in every boats battery compartment!

 

[ccscott49]

Do not use the caravan liesure battery for starting the engine, it was not designed for this job! Unless in an emergency, then who cares if the battery is knackered afterwards! You have a engine start battery. Do not, at sea flatten both of your batteries trying to start the engine, unless you have a fully charged hand held VHF, you may need some battery power, to call for help! In my experience, if a diesel engine does not start with a fully charged battery, a power station will not start it!

 

[ccscott49]

Re: The definitive answer. . .

You reckon! You are advocating flattening all your batteries!

 

[LadyInBed]

Re: The definitive answer. . .

That’s not how I read the post I replied to.

 

[charles_reed]

The most sensible answer so far NM

NM

 

[charles_reed]

The most sensible answer so far NM

NM

 

[charles_reed]

Battery Life

Sounds very short to me - are you using the standard, in-alternator regulator?

 

[ParaHandy]

Jean

There are inherent dangers in mixing battery types. The explanation is long (& tedious and given that you can buy v good 110ah "leisure" batteries for £60 not worth the effort). This site, which is not in German, is as near to the horse's mouth as those thirsting for such knowledge would want!

http://www.uuhome.de/william.darden/

 

[Jean]

Re: The most sensible answer so far NM

Thanks again everyone who has responded, all with some interesting points. The question arose at a recent club meeting. As an electrical engineer (although not a battery expert), my response was “no”, particularly with the mix of batteries and unknown relative charge states. Besides, if you have a good high current engine start battery, why should you need to parallel it with a battery not primarily intended for this purpose! As a point of interest, if you have a flat start batter, I suggest it should not harm your leisure battery if you use it to start your engine in an emergency (in this situation, no point in paralleling, with the flat normal start battery as it will reduce the power available to the start motor anyway).
Earlier today, I dug out a copy of the PBO June 1977 supplement “ ON BOARD ELECTRICS”. If you have access to a copy, its well worth reading as its all in there.