start and house batteries parallel

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start and house batteries parallel

Postby Wout on Fri May 20, 2016 1:34 am

Hi, I'm new to this forum :P and I have 2 basic question:

Question 1:
I have recently bought a 35ft sailboat on which a SAR V3 is installed. A single sense BEP Voltage sensitive relay switches start and house batteries parallel when the V-threshold for the start battery is reached.
From this point onwards, the SAR sees house and start batteries as one, right? Doesn't this cause over-charging of the start battery?
In the SAR documentation I read about the "parallel" connection, which becomes "high" when the SAR detects that the house battery is being charged. Is this at a certain V-level of the house bank? The doc doesn't elaborate on this.
Would it be better to use this SAR parallel output signal to switch a solenoid as in the SAR V3 diagram to switch house and start parallel? But in this case charging the house battery bank gets priority, correct? And still the risk that strat battery is over-charged.
Any recommendations?

2nd question:
The anchor winch is on the house battery bank at the moment. Wouldn't it be better to put it on the start battery, as the start battery is designed to deliver a high current for a short time?

Any recommendations?

Thanks in advance,

Wout in NZ
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Joined: Tue May 17, 2016 7:17 pm

Re: start and house batteries parallel

Postby Coulomb on Fri May 20, 2016 11:29 am

Sensing voltage on the house bank to trigger closing the solenoid works OK in most cases where the starter battery isn't deeply discharged. The SAR V3 uses a different method to determine when to close the solenoid.

Consider the case where both batteries are discharged. Depending on the alternator and the house battery capacity it could be hours before the voltage sensitive parallel switch closes to charge the starter battery. In the meantime, the starter battery may still be discharging if it supplies any loads like engine room fans or, more critically, bilge pumps.

The SAR V3 provides a smarter solution so that both batteries can be charged at the same time.

Regarding overcharge: Some amount of overcharging may be done to the starter battery depending on it's technology. If it's a conventional liquid electrolyte unit, the result will be water loss which requires more frequent maintenance.

The best starter battery is maintenance free with a lead-calcium plate. This type can withstand voltages applied to house banks without overcharge.

Why not use the starter battery for the anchor windlass?

Consider this: You've just started the engine to bail out of an anchorage due to a wind shift. The anchor is stuck on the bottom and sucks the starter battery down each attempt to get the anchor up. The voltage sensitive relay isn't going to close to replenish the starter battery, so you have to use the manual parallel switch.

If your boat is wired with the conventional 1-2-both switch, given the urgency of the situation, you may end up interrupting the alternator current in the process and blow up the alternator and regulator.

But, you're now using the house battery via the battery switch to operate the windlass. Why wasn't it just wired that way?
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