Tuesday, 17 January 2012

Assault and Battery (6)

A few replies to questions that have been raised about batteries that I ought to share. I am no expert in battery technology. But I am an interested amateur with a good knowledge of electronics and prepared to delve.

When messing around with batteries there are several dangers to be aware of. One is the battery acid which can be very harmful. The second danger is the risk of shorting out the battery terminals with metal tools. This can cause molten metal to splash around. I always wear safety goggles when working on the boats batteries. I have also made up a set of insulated tools (exposed metals bound with insulating tape) that I use. The tools I use are shorter in length than the distance between the battery terminals. When charging batteries produce hydrogen gas which is explosive. You must try and avoid any sparks being made. Try as far as possible to work in a well ventilated place.

Q1) Can I mix battery types in a battery bank.

The simple answer to this is no. As far as practical all batteries in a battery bank should be replaced with a new set of batteries. The batteries should be capacity sized at the same ampere hours and manufactured by the same maker. Mixing batteries of different ages will cause the newer batteries to age prematurely. Its a false economy to do a mix and match.

Back in the days when I used to maintain large battery bank systems. Part of building a new battery bank was that the batteries would be tested and selected to match as far as possible others to be included in the bank. But we are talking about life support systems in an emergency situation.

If you have a bank of four batteries in your boats leisure bank and one fails prematurely. Remove it and run on the three remaining batteries. Do not add a new replacement into the system. There are people who do this and they swear that it works OK. Most of them are also adding different capacities, different manufacturers and also second hand batteries into the bank. Not a scenario for me.

Q2) How can I test my batteries for performance.

You can test the whole battery bank, a single battery or a single cell in a single battery. However, I always recommend starting by checking for the simple faults first, like a loose battery connection or a malfunctioning battery isolator switch. Problems arising with batteries tend to be gradual, if it is a sudden change the problem may lie elsewhere.

When batteries are used in a bank together they tend to age at a similar rate. As batteries age capacity reduces. However if you are having charging or discharging problems, then one battery in the bank may have failed. The test is best done by charging each battery in your battery bank separately. If you are using wet cell batteries (not the low maintenance type) top up the batteries with distilled water before charging.

Charge the battery with a good battery charger until the battery reaches a full charge. Disconnect the battery from the charger. Allow the battery to sit disconnected for 24 hours to see if there are any internal problems causing the battery to self discharge. At rest the batteries should give a terminal voltage above 12.23 volts. Any that indicate less than 12.23v you could take the batteries to be tested at an auto electrical centre. They do this "drop test" using a special load testing device. The battery 20 hour discharge rate is used to give an idea of serviceability.

You can also test the specific gravity of the battery acid in each cell on a battery using a very cheap specific gravity testing tool. Checking in this way will identify any cells in a battery that are failing or failed. Like the charge test, wait 24 hours before testing the specific gravity in each cell.

Q3) What is the best way to monitor your battery bank.

This depends on what it is that you want to monitor. If it is the voltage, then a simple volt meter placed across the battery terminals will suffice. The voltage reading will vary depending on several factors. If any electrical item is drawing amps out of the battery for instance. Then the "terminal voltage" that's the voltage measured across the battery terminals will vary.

If you want to monitor the current (ampage) going into and out of the battery during the charge or discharge part of the cycle. Then a simple ammeter with a suitable shunt will suffice. Now whilst the two items above give some useful information. Monitoring a battery bank really means measuring the battery bank capacity (state of charge).

This only way to monitor a battery bank for capacity performance and state of charge is by measuring the voltage across the terminals. However, this can't be done during the charge cycle or anytime soon after the charging has finished.

Batteries take some time to settle back to the quiescent voltage state. (A quiescent state is one in which a battery is considered stable and unlikely to change its terminal voltage.) To reach this state can take a number of hours. I would not take a reading in less than 12 hours. It could take as long as 24 hours to reach the fully quiescent voltage state. It all depends on the temperature and the type of battery.

There are two problems with measuring the capacity in this way. One is that it is not practical to wait up to 24 hours to be able to measure the capacity. The other problem is that the capacity or state of charge of a battery can vary between 100% and 50%  with only a tiny difference in reading of 0.64 volts measured at the battery terminals.

Q4) I have seen several battery monitor system advertised which one is best?

such as the Peukert number) that may be required to be input. They also vary in price and ease of installation. Monitoring a battery bank is a black art. At best most monitoring systems will require a periodic resets to be performed as your battery bank ages, if only to maintain some accuracy. They may not be the simple plug-in and forget monitoring systems you expect. Look at the functions provided by the monitor and look at the documentation provided. If the functions are what you want to monitor and the documentation is explanatory and understandable. That's the one for you and me!

Peukert Law gives a value for the internal resistance and recovery rate of a battery. A value close to one indicates a type of battery with good efficiency and minimal loss. A higher number reflects a less efficient battery. Peukert Law readings for lead acid are between 1.3 and 1.4. As the battery ages this value will get bigger as efficiency decreases and losses build.

Q5)  What is the correct disposal method for a lead acid battery?

One that does not effect the environment. Many scrap yards will give you good money for a battery usually calculated on total battery weight. The money raised in this way will help to off-set the cost of new batteries. I sold a duff 110ah leisure battery about a year ago for £6.
Q6) What can I do to increase the number of battery cycles?

Simple, do not discharge the battery. However that is not a very practical solution. This is why you should try and calculate the power budget for your boat.

The theoretical capacity of my battery bank is 500 ah.
I want to be able to draw an average of less than 12 amperes over a 24 hour period. (in theory 288 amps but it will be lower)

This gives me a theoretical peukert number of 1.2219993327851 which is lower that the more usual 1.3 to 1.4 20 hour figure. So that gives me additional capacity. Which in turn means extra cycles.

I have produced a chart for my own battery bank based on information provided by the battery manufacturer. This gives me a state of charge at a quiescent terminal voltage and the estimated number of cycles available if I don't exceed a specific discharge value.

The reality is that I am giving myself the best opportunity to have a leisure battery bank, that will have a long and undistinguished life cycle.


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