Hi All,

Back in March this year, the morning after our first night at anchor on a Bass Strait cruise, I was shocked to find my batteries were dead flat. This was confusing since I had a total battery capacity of 300AH which was fully charged during 10 hours of motoring the night before. Given that I had only the anchor light and fridge running (intermittently) overnight, this sort of load should not have been a problem for a 300AH battery system.

So, what was the reason?
In short, I found that the actual battery capacity was 45AH, not 300AH!
The reason for this extraordinarily reduced capacity was due to sulfation, in my case I believe due to incorrect charging over the years.

The symptoms of reduced capacity are not that easy to detect - the batteries will charge fully (but suspiciously quickly) and a voltage or hydrometer test will suggest their OK.
The effect of reduced capacity can be subtle - while you only have relatively light loads or maybe motor regularly, you may not even be aware of the problem. That is, until you really need extra energy to repeatedly hoist that anchor or start a sluggish engine.
So, how do you determine your true battery capacity?

For a start, battery capacity is usually quoted as C20 which means the "20 hour capacity". This tells you what current the (fully charged) battery can supply for 20 hours before it is flat.
Alternatively, it's the rate of current in AMPS that can be drawn from a fully charged battery such that in 20 hours its terminal voltage is 10.5 Volts (ie. it's flat).
Of course, this "20 hour current" (I20) depends on the battery size, but it's easily calculated. I20 = battery Capacity in Ampere hours (AH) divided by 20.
For example, for a 100 ampere hour(AH) battery, I20 = 100/20 = 5 Amps.
This means it can supply 5 Amps for 20 hours, at which time the battery voltage is 10.5 Volts (it's flat).

So to determine your battery capacity, you need to find out how long (in hours) your battery can supply its I20 before its voltage reaches 10.5 Volts.
To do this test, you need to be able to measure the battery current and voltage. If you don't already have the means to do this, bite the bullet and get one of these (or similar) from Jaycar (I have no association with them).
These meters are available from several suppliers, usually around the same price, under several different brands - same manufacturer. The're all you need for most boat electrical work and well worth having. They will measure, Volts, Amps, Resistance, Continuity, etc, etc.
One of the key features is that it's a 'Clamp' meter. To measure current (Amps) in a wire, you simply 'clamp' on to the wire of interest and read the display - so easy.
With other types of current measuring meters, you have to disconnect wiring to insert them into the circuit - messy and a potential for short circuits.

www.jaycar.com.au/400a-ac-dc-clampmeter/p/QM1563

Ok, now for the test:

CAUTION - you will be flattening the battery, be sure this is what your prepared to do!

Your aim is to find out how long (in hours) it will take for the battery voltage to reach 10.5V when discharging at the I20 rate.

1) Ensure you battery is fully charged.
2) Calculate your battery I20 (Ampere Hours divided by 20)
3) If you're using a clamp meter, simply clamp on to the main cable on the positive terminal of the battery.
4) Turn on enough loads (eg. anchor light, house lights, etc) so your battery discharge current (Amps) is close (doesn't have to be exact) to I20
5) Note the start time of this test
6) Monitor the battery terminal voltage, and stop the test when it reaches 10.5Volts noting the total elapsed time (Hours) of the test.
7) Calculate your battery capacity, AH = Elapsed Time (Hours) x I20 (Amps)

8) RECHARGE YOUR BATTERY!

Hope some find this useful.
I'd be very interested in the results anyone gets from this test - you may get a surprise as I did.

regards to all,
Allan

I used a couple of old headlight bulbs (low beam burnt out, hi-beam ok) for loads - gives you a nice steady load for free at around 5 or 6 amps per bulb.

I mainly us my smart charger on repair cycle this desulpurcatse the battery

Fix load as a half working car globe or 12 volts halogen down lights is good.
Don't need any amp meters bacause the load is constant. ( for our type of testing)
The only trouble is 10.5 volts cut off switch and timing, Who wonts to monitor 10 to 20 hours
till voltage drops to 10.5 volts.
In case you need to test more batteries and you need to be reasonably accurate and get consistent result,
you need a bit more friendly stuff.
Long time ago I build a few 10.5 v cut off relay switches, using LM741, mostly for caravans, and
for mine bat. test with dual relay to control $4 clock from 2 bugs shop.
When red led on cut off switch board lit, I new current and time....simple.

Select to expand quote

Charriot said..
Fix load as a half working car globe or 12 volts halogen down lights is good.
Don't need any amp meters bacause the load is constant. ( for our type of testing)
The only trouble is 10.5 volts cut off switch and timing, Who wonts to monitor 10 to 20 hours
till voltage drops to 10.5 volts.
In case you need to test more batteries and you need to be reasonably accurate and get consistent result,
you need a bit more friendly stuff.
Long time ago I build a few 10.5 v cut off relay switches, using LM741, mostly for caravans, and
for mine bat. test with dual relay to control $4 clock from 2 bugs shop.
When red led on cut off switch board lit, I new current and time....simple.

difficult to see how the average sailor can implement that
regards,
allan

Datawiz you are quite right that voltage tests themselves are hopeless for testing batteries because you don't know whats going on inside the battery. You can still get 12V across a 40 year old dry torch battery but it wont start an engine!

Expanding on what some others have written above, there is a simple rough way to estimate your battery capacity that can work, and a sailor can do it!! Given a $9.99 Multi-meter and a load. The load can be say your anchor light on or off.

1. Connect multi meter to battery on DCV range while battery is at rest (not charging or discharging).
2. Turn on anchor light. (It would be nice to know its exact wattage, say its a 12W bulb which is 1A current).
3. Wait couple of minutes then observe and write down the battery voltage and time. Say its 12.94V.
4. Wait 1 hour.
5. Observe and write down time and voltage again. Say battery is now 12.84V.
6. You have used 1A for 1 hour by turning on anchor light. The voltage has dropped 0.1V. So you can see battery is dropping 0.1V per hour at 1AH load.
7. That means at 1AH you have 8 hours of capacity to use until battery gets to 12V. Or 4 hours at 2AH, or 2 hours at 4AH.
8. Hopefully the volts will drop a lot less, indicating much better remaining capacity than my example.

The numbers aren't exactly right because the discharge curve of all batteries are different, and are different at different loads and temperatures and not linear. But its better than no estimate at all. To be more accurate, if you are using a lead acid battery, you can put your numbers on the graph attached to see what you have left.

Last 3 years I only use LED bar labeled like that / only 10 LEDs , without lower one/ using LM3914,
quite happy, any moment see what stage of the charge you in.
It was previously many discussions what voltage refers to stage of charge.
I don't care about 10% less or more , simply , ones I get down to 50% time to act.

You've got good batteries Charriot, some people wind up with batteries that indicate OK voltage wise but can't supply any amps due to various things going wrong inside them. If the batteries are good that tester you've got would work well.