Batteries - how can I tell??

[tim3057]

I have three batteries, two from the boat and one from an old Nissan Sunny. All look superficially similar, although there are obvious differences in amperage rating and one is labelled easy-start or something. My daughter wants to sell the Nissan Sunny (who wouldn't!) and I need to be sure I've got the worst battery in it... The one in the car is fairly new and frankly is probably worth more than the car, but I don't know how old the ones in the boat are.

So, question: How can I test each battery to see how much life it's got left in it?

PS

Nissan Sunny jokes only welcome if you're prepared to give battery advice too!








<hr width=100% size=1>Wayfarer Woodie done, now I need a new fix..

 

[john_morris_uk]

May I suggest that its not simply a matter of putting the worst battery in the car, but ensuring that you keep the batteries thats best fit for the purpose of boating. I don't know how you've got your boat wired or how many batteries you have in it but if you want an 'engine start only' battery for the boat, it can be argued that a car type battery would be appropriate. However if you want a battery for running the domestics (echo sounder, lights etc etc) off whilst you are sailing or at anchor, a car battery will be destroyed very quickly by the charge/recharge cycle and you need to make sure you keep the 'deep discharge/traction/leasure batter(ies).

If all the batteries are the same internal design then the only way to see how they stand is to charge them up (check the specific gravities of each cell to check they are all ok) then discharge them into a known load until the voltage drops to 11 volts or so and time the process. (A car head lamp bulb is a good load for this) Number of Amps into load times hours to discharge gives Amp-hour rating of the battery (at that discharge rate)

If you are trying this on a car type battery don't leave the battery discharged for any time at all - recharge straight away!

Sorry, but this isn't an straightforward answer.

If they are all car batteries, an auto electritian will be able to do a cranking amps type discharge test which might help. Its quick and easy and if they think they are about to sell you a battery they will probably do it for free.

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[Leisure27]

If you have a digital voltmeter then you can tell the approximate state of charge of a battery.

12.65v is 100% charged.
12.24v is 60% charged.
11.89v is discharged.

These are values at 80 degrees Farenheight (warm room temperature).

Charge them all up, leave them a few hours to settle then test them. The one that retains the highest charge is in the best state.

But take note of John's previous comment about car and leisure type batteries.

Brendan.

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[VicS]

The figures are not quite what I would have quoted but near enough.

The batteries should be charged for long enough to ensure that they are as fully charged as possible then left standing for 12 hours before checking the voltages. The exact time is not as important as the treating each one the same in this case.
Then monitor the rate at which reading falls over a peroid of many days preferably several weeks. I consider this to be as important as the initial reading as you don't wnt to keep a battery which, although apparently charging well does not hold its charge.
I personally prefer the digital voltmeter approach to the hydrometer method, it seems to give more precise figures than a cheap hydrometer and the identification of a duff cell is not of any use even if interesting.

<hr width=100% size=1><font color=purple>Ne te confundant illegitimi.</font color=purple>

 

[Joe_Cole]

You could do a capacity test. Charge the battery then run it down on a known load. Time how long it takes to run down, then do the calculations. You would need to do it on all the batteries; and that's assuming, as others have said, that they are all starter batteries.

But is it worth it? A new battery for a Nissan will cost what, £20?

Incidentally, remind me never to buy a used car from you. /forums/images/icons/smile.gif

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[VicMallows]

If you think all three might have some life in them, keep them all. Go to the local marina and pick up another of suitable size from the waste area. Mind you, .... that one will quite likely be the best of the lot! ... amazing what some people throw out.

Vic

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[FullCircle]

I have always acquired my batteries from the local car breakers for about a fiver a go. A lottery maybe, but bear in mind cars are broken much earlier than in previous years due to their low value after 10 years.
I havent had a lot of problems, and I keep a stock of 2-3 batteries at home ready to go when one of the boats batteries go pop.

<hr width=100% size=1>Jeanneau 35 - only 13,272 lbs displacement, not even 6 tons,what a lightweight!!!