Battery advice requested

[alanporter]

I have two series 27 deep-cycle lead acid batteries on my sailboat that I use for both engine starting and domestic purposes. Being a belt-and-braces kind of guy, I keep a series 24 cranking battery on board, not wired in, so that I can temporarily connect it up in case I need additional help in engine starting. I charge this battery once a month to counterect the natural degradation. My question is, does it do any harm keeping a battery fully charged without ever draining any power from it ? My battery is four years old and never been used. As regards using the deep-cycle batteries for engine starting, my 25 hp engine starts easily at the first turn of the key.
Helpful criticism will be most welcome.

 

[alan54j]

i`m a great beleiver in use or lose ! its obvious not to run a battery flat and leave. keeping the third battery topped up is good, but i would switch the third batt. with one of the other batteries every now and then monthly if you use the boat regularly.
hope this is helpful ?
alan54

 

[Ships_Cat]

I wouldn't worry about it.

Cranking batteries are essentially always fully charged even in use if just used for cranking as the AHr drained during cranking is only a very small fraction of total capacity and they go straight onto charge as the engine runs. In between use they may slowly discharge but that is the same as with your stored battery now so no difference.

When charging the stored battery though you should ideally ensure that it does get fully charged ie part of the charge cycle should be at 14 point whatever volts.

John

 

[William_H]

It sounds like you are a belt and braces and a piece of string for a standby belt man. I reckon your spare battery will suffer old age just as fast as the batteries in use.
If you were to husband your 2 installed batteries properly as in keep one isolated from domestic discharge and test start the engine periodically and each of those batteries then you shouldn't need any more batteries.
Still if you have the battery and can spare the extra weight then you should wire it in and use it occasionally to start the engine (alone) and charge it often. olewill

 

[Anonymous]

Some deep-cycle batteries are good for engine cranking and some aren't. Clearly yours are, since they are still OK after four years!

You won't degrade this reserve battery by leaving it disconnected provided that you give it a regular boost to 14.4V every month or so. But it's not just the voltage; with a big enough charger the voltage can quickly get to 14.4V while the current is quite high - you must leave it charging at that voltage until the Amps have dropped and then leave it charging at 14.4V for an hour.

The big weakness in your scheme is that your emergency battery might have gone high resistance with a defective cell ages ago and you would never know without trying it at high cranking currents. Secondly, you are missing the amenity value of that huge lump of battery just sitting there. If you wire it in, you will be able to check regularly that it can provide a starting current and you can use the extra capacity for domestic load.

 

[Pye_End]

I agrre. You are better to use it - for two reasons -

You will constantly assess the condition if you use it.

A cranking battery is the better one to be used by the engine. The deep cycle may be fine for turning over the engine, but one day the engine may cause the deep cycle to end its career. You would have to look at the numbers to see how suitable they are, but in general a deep cycle does not have good cold crank.

A simple changeover switch will sirfice - start engine on one, run for 10 minutes to re-charge, and then change to deep cycle bank.

You will not de-grade a lead acid by not using it, so long as you keep it topped up as you are doing.

 

[tobble]

Seems bonkers to be lugging around a superflous battery. Three options:

1. wire the 2 deeps cycles in as one (or even sperately) bank and the cranker as dedicated to actually cranking.

2. get rid of the cranker all together if you are happy using a deep cycle - but be careful when it comes to replacing the deep cycle as a different one might not manage the cranking

3. get rid of one deep cycle if you don't need it and have one domestic and one cranking.

another thing to bear in mind is how big is your engine? if its small ( less than 10hp ish ), a) a deep cycle battery will have a much beter chance and b) once one has the knack, they can be started pretty easily by hand! thinking of doing away entirely with a cranking battery entirely to start my 8hp inboard!

 

[andy_wilson]

Better solution

Wire the two deep cycle in parallel for domestics and reduce the discharge depth and hence increase the life of them.

Use the cranking battery for what it is meant for so you know it is fit for the job.