[RE-wrenches] Marine battery bank switch

Thu Oct 11 17:16:19 PDT 2012

One thing to consider is that flooded batteries almost always have a 
slight short to ground.  Its in the milliamps, but its there.  It runs 
through the conductive acid mist that coats the outsides of the 
batteries.  So connecting the battery bank negatives together could 
cause some current flow between the banks even with the switch off. I 
found this out the hard way once with an ungrounded (vehicle) battery 
bank that shocked me when I touched the chassis and the battery at the 
same time.
I went to sealed batteries in that case and cured the problem.

Generally I'm not a fan of connecting new and old batteries in any 
way.   I have used a battery isolator on RVs to allow the alternator to 
charge both batteries, but it never allows them to be connected 
together.  The isolator is essentially a pair of large diodes wired to 
allow current flow to each battery, but no flow from either battery.  
Again I probably would never do that on anything but an RV.
Consider also that you could have some volt drop at high current across 
the switch, too.
If you like your new bank lose the switch, sell the old bank to a 
neighbor, or use it on a separate system.

Ray Walters

On 10/11/2012 5:51 PM, SunHarvest wrote:
> Thanks Allan,
> I always value your insight. There is a TriMetric on this system but 
> I'm looking at the FM80. I wrestled with the idea that the systems are 
> connected as the circuits between the two banks are not complete. 
> However, as soon as I made the connections the voltage on the new bank 
> dropped and the old bank "seemed" to be rejuvenated. Yes, it could 
> simply be an issue of  I didn't witness a "drop" but before and after 
> voltage was off by about 2 volts (down from 25 to 23) according to the 
> charge controller. Not a lot but enough to raise an eyebrow.
> Eric
>
>     ----- Original Message -----
>     *From:* Allan Sindelar <mailto:allan at positiveenergysolar.com>
>     *To:* RE-wrenches <mailto:re-wrenches at lists.re-wrenches.org>
>     *Sent:* Thursday, October 11, 2012 4:00 PM
>     *Subject:* Re: [RE-wrenches] Marine battery bank switch
>
>     Eric,
>     It still doesn't make sense to me. Unless you have a complete
>     circuit, the banks are not in parallel, and your switch on the
>     positives prevents a complete circuit of both at the same time.
>     Common negatives should not be an issue - after all, in a standard
>     off grid system, the DC - and the AC neutral are common to the
>     same ground rod and thus equipment bonding. A Trimetric TM2025 has
>     a second battery voltage sense function that requires the
>     negatives be common; mine tells me the voltage of my main 24V
>     system and my 12V generator starting battery, and all works well.
>
>     So I question your perception that one is stronger and the other
>     weaker. How do you perceive this new difference, and could it
>     simply be a error in perception? One issue with a switch on two
>     banks is that you can't use a single amp-hour meter, such as a
>     TriMetric. So I would tend to place less value on subjective
>     perception without the metrics to back it up.
>     Allan
>
>     *Allan Sindelar*
>     _Allan at positiveenergysolar.com_ <mailto:Allan at positiveenergysolar.com>
>     NABCEP Certified Photovoltaic Installer
>     NABCEP Certified Technical Sales Professional
>     New Mexico EE98J Journeyman Electrician
>     Founder and Chief Technology Officer
>     *Positive Energy, Inc.*
>     3209 Richards Lane (note new address)
>     Santa Fe, New Mexico 87507
>     *505 424-1112*
>     _www.positiveenergysolar.com_ <http://www.positiveenergysolar.com/>
>
>     *
>     *
>
>
>     On 10/11/2012 4:23 PM, SunHarvest wrote:
>>     I recently installed a Blue Sea 4-way marine battery switch to
>>     switch between two battery banks, one new bank and one old bank.
>>     I was hesitant to connect the negative home-runs from both banks
>>     onto one common bus (connected directly to a ground rod), which
>>     then connects back to the DC load center at the inverter. But
>>     that's how I was advised to wire it by an electrician friend.
>>     It appears now that the newer bank was performing quite a
>>     bit better prior to being connected to the marine switch (and neg
>>     lead to the neg bus) and the old bank is now performing better
>>     than before. I think my electrician buddy was incorrect and that
>>     even the positive leads being on a switch and neg lead on a neg
>>     bus, the banks are still, to some extent, effectively paralleled
>>     and that the new bank is being pulled down by being indirectly
>>     connected to the old bank.
>>     Thoughts and advice on how to correct the wiring if needed?
>>     Sorry to ask such a simple question, I'm just a little rusty on
>>     off-grid.
>>     Thanks guys.
>>     Eric Stikes
>>     SunHarvest Solar
>>     <www.sustainableenergygroup.com>
>>
>>
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>
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