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Why not charge fridge battery on engine start alternator?

I understand you do not agree with charging fridge batteries from the engine start battery alternator via a split charge relay. Could you please explain your objection?

Asked by: david townsend  | 8.17pm, Monday 28 December

WW says:

I don't recall writing this but, thinking about it, I can't really see any reason why you should not do this if you really want to.
Provided the starter battery and the fridge batteries are well separated by a split charge relay or diode, the engine battery is isolated and should always be able to start the engine.
I suppose one possible problem is that the engine starter alternator is usually the smaller of the two on a twin alternator installation so it may struggle to charge the fridge batteries if they are heavily depleted.

Graham Booth  | 11.58AM, Wednesday 30 December

Readers say:

I am not a great supporter of the two alternator arrangement. The engine battery requires so little charging that it seems something of a waste to dedicate a whole alternator to it.
If you have a two alternator system then there are devices which will parallel the two so that you can add the extra capacity to the charging of the domestics.
I am a little confused as to the reason why you specify the fridge. Do you have a separate bank dedicated solely to the fridge or does the fridge bank also provide current for lighting etc.
With a little more detail I should be able to add to this answer.
Nick Cooke

Nicholas Cooke  | 11.59PM, Wednesday 30 December

Nick - Thank you for your comment. I also thought that the 'engine' alternator was underused so I fitted a split charge relay and a seperate bank of batteries for the fridge. This means that I can go to bed with low domestic bank without having to worry about the fridge cutting out overnight.
This has proved very successful for a number of years. I raised the question as I had been told that WW had writen an article about this setup and had some problems with this arrangement. Seams like I was missinformed.

dave townsend  | 11.54AM, Thursday 31 December

One possible reason for what you were told is that lead acid batteries do not charge and discharge exactly as the Amp hour rating would suggest. For example a 100Ah battery discharged at 50 amps would end up being More than half discharged after one hour (50A x 1 hour = 50Ah or half the battery capacity). However the same battery discharged at 1 amp for 50 hours (1A X 50h = 50Ah or half battery capacity)will end up being LESS than half discharged.
This means that to get the maximum Ah out of a battery combining the fridge and domestic bank and thus giving a larger bank will, in effect, increase the bank's capacity because you discharge them at a lower rate.
From what you say it looks to me as if you may have an undersized domestic bank or a charging problem but if the split works for you its fine but perhaps is not ideal for other boats and boaters.

A P Brooks  | 5.02PM, Saturday 2 January

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