Basic battery question

[FairweatherDave]

As a novice to boat electrics this is a basic question wanting a little guidance. (I have only just learnt how to use my multimeter a bit).....

I have 2 batteries on my Centaur with the original engine. Battery 1 starts the engine, battery 2 is a deep cycle leisure battery (85mAh) for everything else. Battery 2 was new in June. Last weekend with everything off Battery 1 read 12.51v and Battery 2 12.81v. So to my knowledge those are normal good readings. Anchored overnight with Batt1 isolated and 10watt (conventional not LED) anchor light on and a little use of internal lights, nothing else. In the morning battery 2 reads 11.48v and depth guage instrument not functioning unless both battery 1 and 2 switched on. (Due to access I did not read battery 1 in the morning but it fired up the engine fine).
Q1. Is that amount of voltage drop on battery 2 normal for one nights use? (Surely not). From my understanding of Amp hours and battery capacity (learned off here) battery 2 should have about 40 useable amp hours and I should have used about 9 Amp hours.
Q2. Motoring back I read the charging rate from the alternator (ie multimeter across the battery terminals again but with engine chugging along). Both read 12.21. I am guessing that means undercharging and I need to checkover the alternator set up more thoroughly. (However I am guessing this is an unrelated question to the question 1 issue.)
Thanks for any pointers and/or recommendations to basic small boat electrics books. Dave

 

[howardclark]

Alternator output voltage

Don't read the alternator output at the battery - you need to read it at the alternator. This can be awkward - I normally use crocodile clips so as not to get too close when belts are whizzing round. The alternator should be giving out 14V+. If not then the alternator is not right, but i rather suspect that it will be OK and that your current is leaking somewhere else.

 

[howardclark]

PS

the adverc or sterling website usually give a good summary of battery charging and available 'intelligent' systems - i've used an adverc system for about 10yrs

 

[pvb]

Don't read the alternator output at the battery - you need to read it at the alternator. This can be awkward - I normally use crocodile clips so as not to get too close when belts are whizzing round. The alternator should be giving out 14V+. If not then the alternator is not right, but i rather suspect that it will be OK and that your current is leaking somewhere else.

Quite the opposite! You should read the charge voltage which is actually reaching the battery.

 

[FairweatherDave]

Thanks both. I can guess measuring at the alternator diagnoses the alternator and at the battery diagnoses effective charging level. But what about my question 1. Do I assume I "have a leak" and what would be my first stepthere?

 

[emearg]

I had a similar problem to what you seem to have and what I found was that checking the batteries/alternator by measuring voltages is not accurate enough and does not give you the amperage usage understanding that you want.
Get your self one of these…http://www.maplin.co.uk/dc-ac-current-clamp-multimeter-629713

or if you are really flush.....a FLUKE 336

Then……..
1. Measure the amperage of all your electrical items with the engine off, one by one…this will tell you your demand on the house battery(clamp meter over the wire going to the house "bus bar")

Chart plotter 0.4
VhF Radio receive mode 0.2
VhF Radio transmitt mode 3.5
FM/AM radio 0.4
steaming lights 1.2
Nav lights 3.6
two fore peak lights (10 watt each) 1.3
Two main cabin lights (12 watt each) 0.9
Instruments 0.2
Auto pilot 0.4
Coolbox 4. 0
spot light 8.0

(you will always know what your demand is if you do this)


2.Then turn on your electrical appliances with the engine running (clamp meter over house battery positive wire) and always make sure that your amperage going into your batteries is greater than your demand, if the demand is greater than your amperage supply to your battery then you have a loose connection somewhere. Your connections have to be absolutely spotlessly clean. If for example you lose 0.5 volts because of a voltage drop then that is 5% of your voltage supply.
3. Also make sure that your wires going to the batteries are not needlessly to long as it MIGHT cause problems with battery isolators (I need to get clarification over this point) as on my batteies I had a charging deficit and one wire coming from the battery isolator to the house battery was 300mm to long and when I cut it to the minimuim length required then my charging deficit disappeared. or was it that I then had nice clean terminals..........never test the amperage coming from the alternator as I addressed each point, so will never know!!!!!!!

 

[Plevier]

Can I suggest you're both right (pvb and howardclark)...
Yes the voltage at the battery terminals is what matters for charging but assuming it's not an externally sensed alternator - which sounds unlikely in the context of this installation - only by checking at the alternator terminals will you know if the alternator is OK and drop occurring in between.

To the OP - yes both your conclusions make sense.
Either something is flattening your battery or it isn't getting charged properly.
12.5V is not enough to show fully charged. Should be about 12.8V after standing with no load for a while and you need an absolute minimum of 13.8V to charge it.

 

[pvb]

Q1. Is that amount of voltage drop on battery 2 normal for one nights use? (Surely not). From my understanding of Amp hours and battery capacity (learned off here) battery 2 should have about 40 useable amp hours and I should have used about 9 Amp hours.

No, it's not normal. Either the battery is faulty, or there's something else draining current from it.

Q2. Motoring back I read the charging rate from the alternator (ie multimeter across the battery terminals again but with engine chugging along). Both read 12.21. I am guessing that means undercharging and I need to checkover the alternator set up more thoroughly.

That's not undercharging, it's not charging at all. Your alternator may be faulty, or there may be a wiring fault.

 

[concentrik]

. Last weekend with everything off Battery 1 read 12.51v and Battery 2 12.81v. Dave

You said you measured with everything off, which may be misleading, since a load would reduce the terminal voltage you read. By how much is dependent on the battery condition, so it would have been good to try it with the anchorlight on.... although at that point you had no idea what was to happen!

As others have said the charge voltage from the alternator is too low - it should be 13.8 to 14.2 depending on.... well that's another story, but between those two would charge a good battery.

Can I suggest something about meters? I encourage you to buy a 'clamp ammeter' - they can be had on ebay and you want one which will measure ac and dc current along with the usual resistance, ac/dc voltage. I bought one of these -

www.bit.ly/On09qi

it saves you guessing about the effect of voltages present by measuring the actual current through the wire. So in your case you could clamp it round the battery positive and measure the current with everything off, then with your anchorlight on, then with all off but the engine running to see if there's a charge (I don't think there will be from what you've said)

I see emearg has sugg this already.... I concur!

 

[Chalker]

The OP measured the charging voltage 'chugging along'. Alternators need the engine to run at more than tick over. Try measuring again at at least mid rpm before blaming the alternator.

 

[TopDonkey]

Below is a helpful extract of understanding battery voltages, it wasnt written by me, i copied it from someone else ages ago


The open circuit voltage, is measured when the engine is off and no loads are connected. It can be approximately related to the charge of the battery by:
Open circuit voltage Approximate
charge Relative
acid density
12 V 6 V
12.60 V 6.32 V 100% 1.265 g/cm3
12.35 V 6.22 V 75% 1.225 g/cm3
12.10 V 6.12 V 50% 1.190 g/cm3
11.95 V 6.03 V 25% 1.155 g/cm3
11.70 V 6.00 V 0% 1.120 g/cm3

Open circuit voltage is also affected by temperature, and the specific gravity of the electrolyte at full charge.

The following is common for a six-cell automotive lead-acid battery at room temperature:

Quiescent (open-circuit) voltage at full charge: 12.6 V
Fully discharged: 11.8 V
Charge with 13.2–14.4 V
Gassing voltage: 14.4 V
Continuous-preservation charge with max. 13.2 V

After full charge the terminal voltage will drop quickly to 13.2 V and then slowly to 12.6 V

Open circuit voltage is measured 12 hours after charging to allow surface charge to dissipate and enable a more accurate reading.

All voltages are at 20 °C, and must be adjusted -0.022V/°C for temperature changes (negative temperature coefficient - lower voltage at higher temperature).

 

[pvb]

The OP measured the charging voltage 'chugging along'. Alternators need the engine to run at more than tick over.

No they don't. The output current is limited to some degree by revs, but the voltage is controlled by the regulator.

 

[wotayottie]

Dave

I really dont know why so many posters feel the need to bicker over minor technicalities. Anyway:

Last weekend with everything off Battery 1 read 12.51v and Battery 2 12.81v. So to my knowledge those are normal good readings.

The 12.51 is maybe half charged and the 12.81 is if anything a little bit above normal for a fully charged battery which is about 12.7. But you need to tell us more about the circumstances to make a useful comment. For example, had you just finished running the engine to charge battery 2 but not battery 1.

Anchored overnight with Batt1 isolated and 10watt (conventional not LED) anchor light on and a little use of internal lights, nothing else. In the morning battery 2 reads 11.48v .................. Q1. Is that amount of voltage drop on battery 2 normal for one nights use? (Surely not). From my understanding of Amp hours and battery capacity (learned off here) battery 2 should have about 40 useable amp hours and I should have used about 9 Amp hours.

At 11.48 volts the battery is effectively flat so either you were using more leccy than you thought or the battery capacity is a lot less than 85aH. Or maybe after reading the first 12.8, you left the mooring and used your instruments etc for a day before anchoring. In which case you may have a duff alternator as suggested below. So what did you do between taking the 12.8 reading and the 11.48 reading?

2. Motoring back I read the charging rate from the alternator (ie multimeter across the battery terminals again but with engine chugging along). Both read 12.21. I am guessing that means undercharging and I need to checkover the alternator set up more thoroughly. (However I am guessing this is an unrelated question to the question 1 issue.)

Maybe not unrelated. Seems likely that the alternator is not charging fully and certainly it and the battery wiring should be checked. Assuming that is that your meter reads correctly and you are getting good contact on the battery posts with the meter leads.

When the engine is running at reasonable revs then the voltage at the battery terminals should rise as the batteries become charged to about 14.4 volts. When the engine is turned off, the lead acid batteries will initially have some excess voltage on the plates but this will decay gradually and without any load down to the 12.7 or thereabouts of a fully charged battery.

P.S. If you want a clamp on ammeter, forget the Maplins l;ink and look on Ebay for the same thing at half the price here for example

 

[FairweatherDave]

Thanks all. Really appreciate the posts and amount of detail.
The frustration of course is it is no easy thing to get to the boat and play around, and of course I'd rather be sailing, but now I know things are not right so will be getting onto this. This is our first season with the boat and on this occasion we were sleeping six so keeping the family happy and stuff in some kind of order was top priority....
To answer Wotayottie the only thing I did between the engine off readings overnight was to dry out. The tickover/chugging (apols I can't remember, tide ebbing fast and not much water left) readings were taken on the mooring having used the engine for 20 minutes maximum at the end of the weekend.

Seems to me step 1 is get a clamp on ammeter and multimeter gets the thumbs down and step 2 is to look for "leaks" (a subject which I need to research but is presumably what an ammeter would be particularly useful for).

 

[FairweatherDave]

Wotayottie :To reveal my ignorance I often isolate battery1 when motoring for fear of not being able to start the engine when I need it, so presumably I have then not been charging it ...

 

[Bodach na mara]

I see that the most sensitive DC current range for the clamp meter is 0 - 40 A. Is this sensitive enough to measure the leakage current or the small currents when only electronics such as wind meter are in use?

 

[ex-Gladys]

I see that the most sensitive DC current range for the clamp meter is 0 - 40 A. Is this sensitive enough to measure the leakage current or the small currents when only electronics such as wind meter are in use?

Beware the so called clamp on meters. I bought one from Maplins, it claiming to do DC current, when i opened it up and read the manual, it was clamp for AC only, probes for DC, so just a multimeter really. A pound to a penny the evil bay one is the same. I gave up in the end as you need to spend at least 70 quid, probablly more.

 

[Richard10002]

Beware the so called clamp on meters. I bought one from Maplins, it claiming to do DC current, when i opened it up and read the manual, it was clamp for AC only, probes for DC, so just a multimeter really. A pound to a penny the evil bay one is the same. I gave up in the end as you need to spend at least 70 quid, probablly more.

I bought a UNI-T clamp meter from eBay for about £30. It came from Hong Kong, but works fine wit AC & DC currents.

I had previously bought one from Maplins and, as you say, it was AC current only, not DC. I returned it and got my money back without question. However, the Maplins one refered to above says it's DC current, so it will be, I'm sure.

 

[Searush]

Check the alternator belt tension too. May not be an issue, but it's a dead easy to check & fix if it isn't right.

 

[nigelmercier]

Thanks for any pointers and/or recommendations to basic small boat electrics books. Dave

There is only one book you need: Boatowner's Mechanical and Electrical Manual by Nigel Calder. It will pay for itself time and time again.

Howard, your profile picture is very annoying! Please make it smaller.