HOT IN THE ’HOLE

Canal Boat - - Back Cabin: -

QI have sound in­su­lated the en­gine bay on a semi-trad nar­row­boat, but on a hot day the tem­per­a­ture in the bay now rises to 45-48C. Will this dam­age the sealed lead acid bat­ter­ies? The al­ter­na­tors are okay up to 70C.

PETER CASWELL, via the CB web­site

ATONY REPLIES: If that re­ally was the am­bi­ent air tem­per­a­ture in the en­gine bay (rather than some­thing that has been heated by ra­di­a­tion or con­duc­tion) I would be con­cerned about the ef­fi­ciency of the com­bus­tion in the en­gine.

How well is the en­gine bay is vented? At 1,500rpm a 1.5-litre en­gine will con­sume about 900 litres of air a minute at 80% vol­u­met­ric ef­fi­ciency, which even on a hot day would help keep the area cool. I ex­pect the en­gine al­ter­na­tor will be okay be­cause af­ter half an hour or so it will have vir­tu­ally shut down. But the do­mes­tic al­ter­na­tor might be sub­ject to high loads at any time. The amount of heat trans­ferred de­pends upon the tem­per­a­ture dif­fer­ence be­tween the air and the al­ter­na­tor parts; you only have just over 20 de­grees dif­fer­ence, so cool­ing must be com­pro­mised.

I have found ref­er­ences stat­ing that ev­ery 8C above 25C halves the life of a lead acid bat­tery, but that prob­a­bly only ap­plies when the bat­tery is op­er­ated above 25C for the whole of its life. Un­less you are a very keen long-dis­tance cruiser your bat­ter­ies will only be above 25C for a small frac­tion of their life. Any re­duc­tion in life might not be no­tice­able if sul­pha­tion and ex­ces­sively deep dis­charge are also con­tribut­ing.

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