Join Date: Aug 2006
Location: Arlington, VA
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If you're worried about someone moving the battery switch while the engine is running, you've got more than battery problems aboard :-)
That's one...of many...reasons why an EchoCharge or DuoCharge device makes sense: THERE'S NOTHING TO SWITCH....EVER.
The Xantrex EchoCharge and the Balmar DuoCharge devices are intended to maintain a second battery by sensing whenever there is a charge on the main battery bank (the house bank). When the voltage rises above about 12.8VDC, these devices then bleed off a portion of the charging current and send it to the second (start) battery. The EchoCharge can handle a maximum of 15A. The DuoCharge is somewhat more sophisticated, programmable, and can handle up to 30A.
NOW, what most people fail to realize is that while starting your engine draws a lot of current, it does so for a VERY SHORT TIME, resulting in a VERY SMALL DRAW of amperage from your start battery. If you run your starter for 10 seconds (that's a lot...most diesels start much faster) and it draws 250amps while cranking, that's only less than ONE AMP HOUR total draw from the start battery. This is replaced in seconds by the alternator.
Please read the preceding paragraph again....for some reason, it's counter-intuitive and most folks think starting an engine draws down the start battery a lot. It doesn't. It's only a drop in the bucket. And, it's very easy and fast to replace that drop.
What's to choose between the EchoCharge and the DuoCharge? Some start circuits have other draws on them....exhaust fans, and any number of other things which are sometimes attached. If yours does, and you think the draw is going to be 10A or more while the engine is running, then go for the DuoCharge (or change your wiring). Otherwise, for the great majority of sailboats with small diesels -- like mine -- the EchoCharge is more than sufficient to maintain the start battery, and won't even be challenged. It'll just loaf along.
BTW, these are smart devices: they won't overcharge your batteries and they don't need any maintenance at all.
All the above is predicated on a situation where there is a start battery separate from the house bank. As I said in an earlier post, this is highly desirable because it allows you to combine all other batteries into a single large house bank, which has numerous benefits for exercising the batteries, recharging them quickly, and extending their usable life.
I'd certainly think about adding a small starting battery in the case of the original poster, maintained by an EchoCharge, and would combine the two 85AH batteries into a single large house bank.