How can I test my batteries

[BrendanChandler]

I suspect that my 5 year old batteries need replacing as they do not seem to hold a charge as well as they might.

They are sealed batteries.

Is there a simple means of testing them.

I would hate to replace them only to find that there was no need.

If they are defective, what is the best choice of battery for my needs. I use both batteries as house and cranking batteries in one bank and keep a separate portable battery pack as a reserve in case I accidentally allow the pair to run down.

Brendan

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[Talbot]

take them to a garage and ask them to test. They have a simple gadget that they put across the terminals and it checks the status of the battery.

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[ParaHandy]

you need a voltmeter - digital - to check battery condition. decent one at halfords for £35 ...

charge the battery, leave for a few hours, put a reasonable load on it for a few secs, measure the open circuit voltage (disconnect it). if 12.7v the battery is 90% charged, 12.6v 80% etc ...

anything less than 12.4v is dubious ..... they reckon 6yrs for a battery

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[freedom44]

I am fortunate to have a friend who operates a battery centre supplying both cars and boats. His advise is that you need sophisticated bench mounted equipment to test the capacity of a battery, especially if they are gells. I think he said it takes about 2 hours for each battery.
I would have thought that any reputable battery centre would offer this foc if you agreed to buy the replacement thro them .
He also would be the person to discuss suitability. I use gells for house and a normal lead acid one for engine cranking.

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[Joe_Cole]

Talbot,

If they are ordinary "car type" batteries then the garage tool will be helpful. However, if they are Deep cycle batteries then I don't think that the garage tool will help. The readings would be entirely different. (I think!)

Joe

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[MedMan]

There is only one sure way to be certain - do a run-down test.

Batteries are usually rated on a 20 hour basis. i.e. the rated capacity is obtainable if you run the battery down over 20 hours. [If you run it down more quickly you will get less Ah out] You need to apply an appropriate load and time how long it takes before the battery voltage takes a sudden, dramatic dip to 10v or less. If your battery holds up for 15h or more it is still pretty good; if it holds up for 10h it is down to half its rated capacity and you might feel it's time to change it; if it lasts for 5h or less you should have changed it long ago!

So what is the 'appropriate load'. It is the rated capacity in Ah divided by 20. Thus, if your battery is rated at 100Ah you should be able to discharge it at 5A for 20h. Charge up the battery you want to test, apply the calculated load [turn on the appropriate number of lights] and time how long it takes before the 'big dip'.

The testing devices they use in garages apply a massive load for a few seconds. This is not as good a test of how the battery will perform over slow discahrge conditions as a run-down test but it will give a rough guide. Just measuring the voltage after charging and briefly discharging will tell you absoltely nothing at all. All but a very dead battery will read the same.


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[William_H]

I agree with MedMan however if you have 2 batteries in paralell as a bank then check them individually. This is good because you can compare the results. Firstly try starting your engine on only one battery then the other. The enthusiasm of the starter will indicate how good the batteries are. especially if you inhibit the engine from starting so giving the stater at least 5 seconds of crancking. Then after ensuring the batteries are fully charged, discharge them fully over a period of time as described at a know discharge current. You will be able to decide from the results if you want to stick with the old batteries or get new ones. Regards Will

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