Batteries (pre season test)

[cagey]

Are there any easy tests to check if my batteries are any good, are likely to hold a charge and how much approx life they have left ?
Many thanks
the Cornish Troll

 

[boatmike]

Put them on charge. Leave them a week and check voltage. Should maintain about 13 volts if not discharged. Best method however is check SG of electrolyte

 

[bluedragon]

Took mine to Halfords for them to check CCA (cold cranking amps) vs. the spec. It gives some idea about the "life" left in them.

 

[William_H]

You have multiple batteries so that there is redundancy at least for engine starting. So you don't need to know how much life is left. you will know when they don't do the job. There are much jmore worthwhile things to worry about.
olewill strikes again

 

[john_morris_uk]

The cold cranking test is ok - but it doesn't give the whole story and needs to be administered by someone who knows what they are doing.

For batteries that supply the domestics on your boat (which I hope are deep discharge or traction batteries or similar ones designed for deep discharge cycling) a better test is to charge the batteries up fully (check with a hydrometer) and then discharge them into a known load (a couple of car headlamp bulbs for instance) and measure how many hours it takes for the voltage to drop to 11.5 volts or thereabouts. (Someone can correct me if I have remembered the discharge voltage incorrectly)

You will know from the wattage of the bulbs (watts divided by 12) the current you have been drawing in the test. Multiply by the hours it took to discharge the batteries and you have the amp-hourage capacity of your batteries at that discharge rate. Chaeck this figure against the designed capacity of the battery which should be marked on it.

Don't leave the batteries discharged as you will damage them, but charge them back up fully straight away.

 

[VicMallows]

One other point to be aware of, which I have known to confuse some people:

Many batteries now have a 'magic-eye' indicator (which is really just a very crude hydrometer). However this only shows the approximate charge (spcific gravity) of one cell (out of six), and may well show "green, good" even though another cell has totally failed. If it possible, you need to check the SG of each cell individually,

Vic

 

[Krusty]

Timing the battery discharge through bulbs of known wattage, down to 11.5v, gives a good estimate of useful battery capacity. BUT -- the fall of voltage needs to be closely monitored, say every 2hrs down to 12.0, then every half hour. When the voltage drops below about 11.8, it just 'falls over the cliff', and the battery may not be recoverable!
I test mine (3 Delco 'Freedom's) this way every couple of years, but I once left one too long; voltage down to about 10v in minutes; and lost it!

 

[john_morris_uk]

Good point - perhaps I should have been more explicit on monitoring the voltage...

 

[halcyon]

Check your battery type and curve before doing the test, Delco Feedom run a differant curve to a standard lead acid, it's around 1 volt higher. Also differant loads will give a totally differant discharge curve.
If you apply a load of 1/10 the battery capacity to a lead acid battery, it will give approx 12.45volt for full charge. This will fall to 11.85 volt which is approx 50% capacity. If you double it you have your approx battery capacity, or more use it will equal your effective battery capacity.

Brian

 

[cagey]

Thanks everyone sound advice