Tuesday, December 15, 2009

One for battery anoraks

The remaining Herbie Awards follow in a day or so, but meanwhile . .

Today we have tiny whisps of snow, and the thermometer is showing an inexorable slide into frosty weather. Poor Herbie will be feeling the chill I thought, so yesterday I popped out to Iver to make sure she was OK.

Apart from making sure the plumbing was drained to prevent freeze ups I checked on the batteries. The results shown by our Smartgauge show that we have lost 18% of our capacity (from 100% to 82%) in the last 29 days with the boat lying unused and nothing switched on (except the Smartgauge itself). These are "leisure" batteries two years old, so is that good or bad or normal, I ask myself. I'm getting to know a fair amount about batteries but there always seems to be something new to learn so another half hour googling revealed the following interesting (confusing??) facts about lead acid battery self discharge.

Apparently lead acid batteries should self discharge about 5% per month when in fit condition, except if they have antimony in the plates in which case they lose 15% or more. Reading around it looks as though most leisure batteries use some antimony in the plates, so that might explain my higher rate of discharge. This might also explain why I was surprised by the amount of top up water I needed last time I topped them up, because antimony makes batteries boil off more water when being charged. (Note to self - must top up more often).

Temperature has a significant effect. Batteries exposed to high temperatures, especially above 25 degrees C will have their life reduced by as much as 50% for every 15 degrees over. Now I'm not sure how hot it gets under Herbie's engine cover, but I'm sure its hotter than 25 when cruising in warm weather. Not so good then, but in cold weather the reverse is true, so at the moment they should be doing fine.

Capacity is the reverse. Higher in hot weather, lower in cold (as we all know from car batteries being poorer in the cold). So could some of my 18% loss be due to the cold decreasing capacity rather than normal discharge?

That's the trouble with real life. The number of variables pulling in opposite directions makes it hard to know where you stand.

Anyhow I charged them up to 100% yesterday and I'll keep an eye on them . I suspect that by this time next year I might have new batteries.


Anonymous said...

I'll alert Richard to your musings - he'll be interested in your findings.
Our batteries had gone down alarmingly (and inexpicably) over the last 2 weeks. But they hadn't lost that much power in the previous weeks.
The smartguauge is definitely paying its way but, like you, now that we know, should we be worried?!
Sue, Indigo Dream

saltysplash said...

Steve at Highline infmd me that they lose a considerable amount of charge if stood on a concrete base, I wonder if the same is true if stood on a metal base? My 5 new bats on Amy are sitting on a wooden base. your thoughts?

prashant said...

now that we know, should we be worried?!

Work from home India

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