Advice on Battery Voltage

[howardclark]

After gratefully receiving your very thorough advice on BSP fittings I can report my waterworks are now in order but now I need advice on the batteries! Bought the boat in April but don't know how old the original batteries are.
I have two original domestic 100A, one original engine starting 100A (all lead acid) and a new Optima which I installed in the forepeak for windlass/bow-thruster.
I've installed an Adverc system and Sterling charger for shore power. Last weekend was the first time I had everthing working properly and all the batteries received a full charge over 4 days. I left the boat for three days and on returning my first job was to check the voltages. The Optima read 13.3V, but the others 12.35V. Is 12.35V good enough or are they beginning to give up the ghost??

 

[halcyon]

RIP, very sorry there dead.
Take them out and get them load tested, just in case there is a wiring fault that is limiting there charging.
Traction/leisure batteries for domestic, starter type battery for engine, reason, traction batteries are designed to give a low current for a long time, starter batteries give you high current for short periods i.e. engine starter motor, batteries have differant plate designs. Though in honesty, it make little diferance as long as you do not allow the to drop below 50% capacity, and keep them well charged.

If the batteries are fully charged your looking at 13 volt plus on new, dropping to 12.7/12.8 with age, flat battery is approx 12.0 volt.


Brian

 

[howardclark]

I checked the charging voltages - the Adverc system is cycling up to 14.2V at the batteries and the Sterling is giving 13.8V (don't really understand that it should peak at 14.2V), so I think your right that they must be dead.

 

[Chris_Stannard]

If you are going to buy new batteries, check the space you have and then check on the web with Varta, Exide and other battery suppliers. Your domestics should be traction or semi traction but do make sure the poles are in the right place. You engine start can be smaller, 75 amp hours will do but should be a standard automotive battery. Check the battery's prices at a car exhaust and battery place, they are likely to be cheaper than your friendly chandler.

Chris Stannard

 

[Sinbad1]

Battery state of charge.

12.7v = full
12.4v = 70%
12.2v = 50%

 

[jollyjacktar]

12.7 they're in heaven [a state of great bliss but not dead batteries], 12.5 they're still alive, 12.2 charging due, 11.8 is much too late.

 

[Chris_Robb]

Re: Stirling Charger

I fiited one of these 2 weeks ago.

Make sure it is set for Lead acid. This will mean the voltages will clime to 14.8 for 1 to 4 hours depending on the switch settings you have chosen. Then it will go back to float voltage of 13.2.

Your batteries are nackered - go for truck batteries 100 amp to 130 amp very common - around £60 to £70. I take it your engine battery is lead acid? Gel and sealed batteries are not suitable and expensive. Read the article in PBO last month on batteries - very enlightening.

 

[bedouin]

Re: Stirling Charger

I think the article in PBO was deliberately provocative, and so should be taken with a large pinch of salt. All the organisations involved with safety at sea recommend using gel batteries on the grounds they are more resilient to inversion and running when submerged.

 

[vyv_cox]

Battery safety

Who are these organisations? I don't recall ever seeing these recommendations and I thought the PBO article to be sensible and logical. What was said must be correct - you cannot expect a gel electrolyte to accept the same levels of charging current as a liquid. And a sealed for life battery cannot be expected to gas off, which it will do with high charging rates.

If I was to be inverted for long enough for all the acid to run out through the cap breather hole I think I would be worried about rather more than getting the power back on. And as for submerged, how do I stop the seawater giving a direct short across the terminals?

 

[bedouin]

Re: Battery safety

It's certainly a recommendation of ORC and derived regs (e.g. RORC), and I think they are recommended in the RYA's publication; I've certainly seen it elsewhere, for the reasons mentioned above. Not having any practical experience, I can't really comment on the recommendations. Some car batteries I've owned would lose a significant amount of acid if only inverted for a few seconds.

In 'resistance to submergence' I think they mean the ability to survive a short immersion in water, rather than to operate submerged for a prolonged period - I don't know what the conductivity of seawater is - what do you think the resistance would be between the terminals of a submerged battery.

 

[VMALLOWS]

Re: Battery safety

Vyv,
I'm with you all the way. This is the safety police ............ and the marketing people at it again.
The best battery you can buy is the most amp/hr for the least ££ that will fit in the space you have. It should also be unsealed so you can top it up. Well kept, it will typically last at least 6 years.

 

[vyv_cox]

Conductivity of seawater

Your question started me on a train of investigation that has turned out to be very interesting, but not necessarily because of the original question.

Conductivity is measured in Siemens/metre, where a Siemens = the old mho unit, i.e. 1/ohm. The conductivity of sewater is somewhere around 50 micro-siemens/metre. I know from other applications, i.e. increasing the conductivity of lubricating oils to avoid static charge accumulations, that the threshold figure between conducting and not conducting is 25 pico-siemens/metre, so seawater is 100,000 times more conducting than that. So seawater is a conductor. However, the conductivity of copper is somewhere around 60,000 siemens/metre, or a hell of a lot more than seawater.

Whether seawater is enough to short out a battery remains a question that I am not qualified to answer. However, what this does show is that ideas widely promulgated regarding anodes are total nonsense. I have said here many times that there is no way that an anode on the outside of a hull can protect an engine inside it. These figures demonstrate that to be true. In the engineering of cathodic protection of structures it is widely accepted that anodes on tubulars can only "throw" their protection for six pipe diameters. Conductivity of seawater is the reason. So if you expect seawater to complete a circuit from an anode, along the hull, up the engine seawater inlet, through the engine and back down a cable to the anode connecting bolts, you are extremely optimistic.

 

[ean_p]

Re: Conductivity of seawater

Hi Vyv....no seawater won't short out a 12v battery....I have a small sealed unit in a diving torch that is often pluged and unpluged under water with no problems....to my knowledge the biggest problem with seawater and batterys(wet ones) is the chlorine gas it will make if only slightly contaminated and this also I think renders the battery useless.....

 

[hugh_nightingale]

Re: Conductivity of seawater-trying to reply to as

many points as possible at once.

Be very careful of battery voltage. By itself it is a poor indicator of battery condition. Just a few other things are acid condition, temperature, yes, and age. The only sure way is a complete battery conditioning charge/recharge cycle.

By the way, gel cells can be quite good. Look at Gates cells. Even the small ones can give a discharge current of 250A and recharge characteristics are good as well. However, in "our" experience at least one manufacturer of gel type batteries used to insist on giving a charging regimen that did not work, the charging voltage was too low and the cells never achieved full capacity. Luckily not a battery normally associated with us.

Be assured most, if not all, cells will gas on recharge especially on overcharge, even so called sealed NiCd cells. Note that the explosive ratio mixture of hydrogen is 4% to 70%. DO not charge in sealed containers. Slight overcharge is used to even up the individual cells of a battery pack.

Sea water is VERY conductive, otherwise sea water batteries just would not work. In my experience it is equivalent to 100 Ohm between two conductors. Why this figure given the bulk resistivity? It seems to be due to something along these lines. When the conductors are close there is a short direct path. As the conductors are moved apart a greater volume of bulk sea water comes into play. This does not really take into account area: the resistance between two large plates must be small, again otherwise sea water batteries would be useless. You can make a very good leak detector out of two strips of PCB or two bits of veroboard connected to a TTL gate. Mind you, the copper does not last too long once the sea water gets on it.

 

[bedouin]

Re: Conductivity of seawater

Unfortunately I couldn't work out how to calculate the resistance between two points in seawater, given the resistivity - I suppose it must be similar to calculating the magnetic flux between two poles in an infinite medium.

However making some very rough approximations, it suggests that the resistance is measured in hundreds or thousands of ohms - if I've got the calculation right the resistance of a 1cm cube of seawater is c 200 ohms.

Indeed my handheld electronics (VHF, GPS, EPIRB) is all 'waterproof' although the connections between the battery and the instrument itself is not watertight (of course in this circumstance the cross section of water that could conduct is very small anyway).

 

[hugh_nightingale]

Re: Conductivity of seawater

You're about right (see other post) but its more complicated than that. You can put two wires 1cm apart but around them is an ever increasing volume of water. Its the same as you suggested - electric flux between two wires in an infinite medium.