Friday, 28 September 2012

Battery woes!

I had an interesting chat with someone who was disposing of a large leisure type battery in a skip. My first comment was "why don't you take it down to your local scrap merchant as they will give you a few quid for it. It will help to offset the cost of a replacement battery." Then I had a thought, I said is this your only leisure battery or is it part of a leisure battery bank. Doing a pick and mix with leisure batteries that are different ages is not a good idea. 

It turns out to be a small boat with just a single battery. However, the battery has to be replaced every year or so. We continued with our chat and I went on to explain that the life of a battery is controlled by the number of discharge cycles and how deep the discharge is.

Lets suppose that we have three identical boats. The example given below is by way of illustration only. There are many other factors that can effect the performance. 

In boat (1) a battery is usually discharged down to 25% of its capacity in ampere hours and then recharged.  The battery in boat (1) might give a hundred charge and discharge cycles before reaching the end of its useful life 

However boat number (2) only ever discharges the battery down to 50% of its ampere hours before recharging. The battery in boat (2) would give a larger number of charge and discharge cycles lets say for this example three hundred and fifty cycles before reaching the end of its useful life 

At the same time boat number (3) only ever lets the battery discharge to the 75% capacity point before recharging the battery.However the battery in boat (3) might give nine hundred charge discharge cycles before reaching the end of its useful life.

In other words the deeper the battery is discharged the less charge and discharge cycles the battery will perform before reaching the end of its useful lifeI also explained that the longer a battery is left in a discharged state before being recharged the number of charge and discharge cycles will fall even more.

My new found friend explained that the battery does not go through many charge discharge cycles in a year. It seems that when the boat in question is left in the marina, it is left on charge at all times. So I asked what kind of charger he used. It turned out to be a car battery charger. This then rang another alarm bell with me. 

So armed with my trusty multi-meter we went for a look. There it was a brand new 80ah battery. Attached to the engine alternator in the usual way. It also had the automotive battery charger attached with a couple of uninsulated crocodile clips that they come supplied with.

First thing is, for doing the odd charge crocodile clips are fine. For leaving a battery on charge for long periods its better to use a more secure quick release fittings to attach the charger. I use IDE "Y" power cables which I adapt for use. The female connector is attached to the battery. They are available at most computer stores for a few pounds.

When I looked at the charger it was very basic. It had an un-calibrated voltmeter to measure the voltage. However, the meter display was a needle against a coloured background going from red at one end to yellow in the middle to green at the other end.

In the words of David Dickinson, this charger was "as cheap as chips" obtained a few years previously at a car boot sale! Of unknown vintage but good for occasional use to charge your car battery when it's been flattened by leaving the lights on overnight.

It was however, the worst battery charger in the world for leaving connected to a battery for long periods.

The charger consist of a unregulated output that would deliver seven or eight amps in the usual thirteen to fourteen volt charging range. However when the battery starts to reach its full charge capacity and the current starts to taper off, the charging voltage will rise. 

Because there is no ammeter on the charger only a simplistic colour scale measured against the battery voltage. There is nothing to give any indication of the true state of charge.

The battery is also at risk of over charging and not undercharging. Now, you will often hear the phrase "trickle charge(provide a maintenance charge) and most people assume that any battery charger will also trickle charge  a battery. 

Good quality battery chargers have a regulated output and the way the charger performs depends upon the state of charge of the battery. As the battery nears full capacity and the current tapers off. A regulated charger will also limit the voltage so that the battery is not over charged. 

All batteries have a point where the charge causes "gassing" there is no single point at which this occurs. It varies depending on the different type of battery and the temperature of  the battery. By way of illustration, gassing in a sealed lead acid battery as rule of thumb would be at around 14.2 volts or higher. When the battery is almost fully charged and in typical ambient temperatures we have in the uk. 

The battery charger colour gradient meter was almost at the top of the green scale. Indicating to any lay person that the battery was almost fully charged. When I measured the battery voltage with the charger connected the voltage was 17.5 volts! When I measured the battery charger output when not charging the voltage was 18.3 volts!

I suggested that the battery charger should be chucked in the skip with the old battery and replaced with a better quality one. It would work out much cheaper than a new battery every year.

* Gassing occurs by electrolysis of the water used to make up the battery acid. This is not a problem with non sealed batteries as long as you check the battery acid level periodically and top it up with distilled water whenever needed. Sealed lead acid batteries cannot be topped up with water lost through electrolysis. Modern sealed lead acid batteries can recycle the gasses produced to prevent damage to the battery as long as the charge rate is slow.


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