Banking Batteries HELP Needed

[acdbox]

 

Me too, I also have the feeing that a lot of background knowledge & theory needed to make an informed decision is missing.

 

 

I know you meant converter but using transformer tends to indicate a rather poor level of technical knowledge so I am concerned that you may be making decisions based on not a lot.

 

To give equal ability to run a given electrical load you need the same number of batteries be it a 12 or 24 volts system. they are just wired differently.

 

We have no idea how well charged the batteries were a month ago and we also have no idea about their actual capacity. If one moth ago they were at a rested off load voltage of 12.7 to 12.8 (fully charged) then they have discharged a 1/4 of their labelled capacity. This implies they are either suffering excess self discharge (other may disagree) or their capacity is severely reduced by sulphation. The point is we do not know the rested off load voltage a month ago so this is all conjecture. Personally I would not buy them unless they came from a mate who I trusted to tell the truth.

Ok. Thanks Tony

[acdbox]

 

If I may suggest, some reading on 12 volt systems and battery construction, use and terminology may be helpful to you.

 

Just to try and put it simply (it is a little more complex than this, but hope it helps) :

 

You have a 12v battery that (when new) had a capacity of 110Ah.

 

110AH is (in theory) the ability to run a device using 110 amps for one hour, or 55 amps for 2 hours, or 1 amp for 110 hours

 

As a battery is used it begins to degrade ( you will find in battery specs the number of charge / discharge cycles - maybe 300 cycles) and will no longer have a capacity of 110 Ah.

 

As you continue to use the battery ( and how much use had an unknown second hand battery had) it will continue to loose its capacity and may be as low as 20Ah.

 

It will still have 12 volts (or 12.4 in your example) but it will no longer have the capacity to run a device for very long - example - the 110 amp device you could originally run for 1 hour will now only run for 10 minutes (or so).

 

A battery has two characteristics :

 

1) Voltage

2) Capacity

 

The voltage of a 12 volt battery will vary depending on its state of charge (see table below), the capacity (Ah) will depend on its age, usage history and how well it has been looked after.

 

 

Being left at 12.4 volts (about 70% charged) will have done the batteries no good at all. If a battery is left then it should be fully charged, and regularly trickle charged to keep it topped up.

Alan, thank you. Makes it clear... Very surprised seeing the diagram. Kind regards.

[Arthur Brown]

Batteries need keeping charged, BUT most recreational narrow boats will sit idle for a month once or twice a year.

 

Chose your battery voltage by the big fixed needs! Starter motor first then alternator. If you can run everything on 12v as spares and accessories are much more easily available and often cheaper.

[ditchcrawler]

Personally I would not buy them unless they came from a mate who I trusted to tell the truth.

Agree 100% or they were only a tenner each

[SteveS]

A little of topic, but to MTB; why wouldn't you put your batteries in series for the 24v and feed the batteries through diodes (fused) with 12v? That way you have the whole boat run on 12v except for the starter circuit.

[ditchcrawler]

A little of topic, but to MTB; why wouldn't you put your batteries in series for the 24v and feed the batteries through diodes (fused) with 12v? That way you have the whole boat run on 12v except for the starter circuit.

Uneven discharge

[John V]

seen it done and seen umpteen variations. I have 24 system throughout on the main boat, good with long cable runs as it helps with volt drops but you have to accept that most of the gear will cost more.

using unequal batteries in a single bank to give you 24 volts is only sensible if each 12v battery has its own independent charging system then you avoid the problem of unequal discharge/charge but then you double the number of alternators / chargers so cheaper to use same batteries with same age.

I have seen one boat that was built with a 24v overall supply with individual 24 to 12 converters for each bit of 12v kit.......messy!

 

Best bet is, as already said, ......don't

 

edit to add.......with the double charge system on a 24v bank you CAN then use the centre tap as a 12v supply without introducing too many problems for ordinary batteries

Edited August 25, 2015 by John V

[SteveS]

not if you feed the 12v out in parallel. or have I missed something?

[John V]

not if you feed the 12v out in parallel. or have I missed something?

 

If that was at my post then I think you have ....could you explain what you mean ?

[SteveS]

No, sorry John I was responding to Ditchcrawler. What you said is correct.

[John V]

No, sorry John I was responding to Ditchcrawler. What you said is correct.

 

(sorry if my reply sounded a bit on the grumpy side)

[ditchcrawler]

 

If that was at my post then I think you have ....could you explain what you mean ?

Same question then

[SteveS]

OK, I see what you mean...but it would be a heck of a sight cheaper if you fired the starter circuit with 12v and put a heavy duty solenoid between the batteries to give 24 v and a lightweight solenoid to disconnect the parallel feed from one battery during start up.(This could all be done off the starter switch) It would mean all the complexity is in one location. however I do agree 24v is lot better over a long distance and would be a pressing argument if you have a heavy draw at the other end.Steve

[Tony Brooks]

OK, I see what you mean...but it would be a heck of a sight cheaper if you fired the starter circuit with 12v and put a heavy duty solenoid between the batteries to give 24 v and a lightweight solenoid to disconnect the parallel feed from one battery during start up.(This could all be done off the starter switch) It would mean all the complexity is in one location. however I do agree 24v is lot better over a long distance and would be a pressing argument if you have a heavy draw at the other end.Steve

 

I think some older Mercedes vans had a ready made solenoid assembly to do that, Maybe worth looking into.