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battery isolator

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newell
Navigator

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USA
113 Posts

Initially Posted - 12/29/2007 : 11:31:19

Happy New Year to all!

I would like to isolate my two batteries and am convinced i can do it with a simple diode arrangement and not pay $75 for a battery isolator. However, I am uncertain what value of diode to put into the circuit between the charger and the batteries. For reference sake, one of the batteries is a starting battery rated at 650 amp hours. I am unsure just how much of that would leak back through the charger under none-use conditions. Is there an electrical wizard out there who might have some idea of the size needed for the diodes involved??

newell

Newell
1980 C25 SR/SK
AVALON

Edited by - on

delliottg
Former Mainsheet C250 Tech Editor

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USA
4479 Posts

Response Posted - 12/29/2007 : 12:25:40

Newell,[url="http://www.amazon.com/Boatowners-Illustrated-Handbook-Wiring-Charlie/dp/0070710929"]
Boatowner's Illustrated Handbook of Wiring[/url] will tell you everything you need to know.

Edited by - on

Frank Hopper
Past Commodore

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Pitcairn Island
6776 Posts

Response Posted - 12/29/2007 : 14:08:13

Hi Newell,
You have stated that you want to do this. Just for argument's sake; don't. First thing I would bring up if we were sitting having a conversation on the dock is that the starting battery is inappropriate. Outboards do not need a starting battery. Remember that 99% of conventional knowledge has nothing to do with sailboats. You obviously have a dual battery setup, it should be two deep cycle batteries. A good charger has adequate circuitry to protect it from the charging voltage from your outboard, (if it has an alternator). I am a believer in quality onboard battery chargers as being money well spent. Good luck with your project.

Edited by - on

newell
Navigator

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USA
113 Posts

Response Posted - 12/29/2007 : 17:00:53

Thanks, Frank/ I agree with you regarding the batteries, However, when I got the boat last summer it had two starting batteries. One is now dead and I intend to replace it with a deep cycle and when the other one goes i will do the same with with it. My primary purpose in using the diodes would be to isolate the two batteries from each other and also to isolate the batteries from the solar panel. As you point out, I suspect that my Guest charger already has the necessary circuit protection.

newell

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sgoodhue
1st Mate

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USA
28 Posts

Response Posted - 12/29/2007 : 20:02:23

Best investment I made was a dual bank smart charger. Cost about $105. Never and I repeat, never have I not arrived at the dock to go out for a day and not had both batteries topped off and in the green. Mounted the charger underneath the bottom step of the companionway and ran the wires under the starbrd berth.

Edited by - on

Jefffriday
Navigator

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USA
198 Posts

Response Posted - 12/30/2007 : 11:08:37

Sure wish I had shore power, or for that matter, any power any where near the lake. I am 100% solar

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Dave Bristle
Master Marine Consultant

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Djibouti
10005 Posts

Response Posted - 12/30/2007 : 11:34:54

<blockquote id="quote">quote:<hr height="1" noshade id="quote">Originally posted by newell
 For reference sake, one of the batteries is a starting battery rated at 650 amp hours...<hr height="1" noshade id="quote"></blockquote id="quote">If so, you'd better have it mounted down by the keel--otherwise you'll be listing hard to starboard!

It's probably 650 "cranking amps"--the amount it can put out continuously over 30 seconds, and probably between 60 and 90 amp-hours if it's a Group 24. (A deep-cycle will have lower a CA and a higher AH.)

I'm curious--why do you want an isolator? Do you have a 1-ALL-2-OFF switch?

Edited by - Dave Bristle on 12/30/2007 11:36:15

newell
Navigator

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USA
113 Posts

Response Posted - 12/30/2007 : 14:58:53

Dave:

I do have a three way switch but my thinking was to isolate the batteries so that they would not equalize to each other and would remain separate unless I put the switch on both. I am aware the West Marine sells a battery switch with built in isolation but I just thought that it might be a fun winter project to see if I could build it myself.

newell

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delliottg
Former Mainsheet C250 Tech Editor

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USA
4479 Posts

Response Posted - 12/30/2007 : 15:16:24

Newell,
The book I recommended above will teach you how to build one (and a bunch of other cool stuff too).

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Gloss
Master Marine Consultant

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USA
1916 Posts

Response Posted - 01/02/2008 : 08:08:26

I checked out that book, but decided not to get it for my library as it was published 14 years ago. Gotta wonder if it covers the new smart chargers, AGM batteries, etc.

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Leon Sisson
Master Marine Consultant

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USA
1893 Posts

Response Posted - 01/02/2008 : 09:09:14

<blockquote id="quote">quote:<hr height="1" noshade id="quote">Originally posted by newell
 ...I would like to isolate my two batteries and am convinced i can do it with a simple diode arrangement and not pay $75 for a battery isolator...<hr height="1" noshade id="quote"></blockquote id="quote">Newell, I highly recommend using a [url="http://www.defender.com/product.jsp?path=-1|328|51495|606044&id=605576"]Battery Combiner[/url]. All diodes impose a voltage drop, typically 0.6V or so. As I recall 12V lead acid batteries have a voltage range of only about a volt between fully charged and 50% discharged. The [url="http://www.yandina.com/combInfo.htm"]Yandina Battery Combiners[/url] use a relay to connect the batteries, which introduces almost no voltage drop. If the retail price presents an obstacle, you might try contacting the manufacturer to see of they have any reconditioned units currently available. I've been using a factory refurbished pair of the smallest model to combine three batteries in a C-25 for about 10 years, and am very pleased with the performance, reliability, and convenience.

-- Leon S.

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Gloss
Master Marine Consultant

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USA
1916 Posts

Response Posted - 01/02/2008 : 18:36:44

55 bucks seems cheap for what a combiner does for you Leon.

So please check me out if I'm getting the procedure right to get the benefits. On my Ericson, which has a diesel, I have a starting battery, on switch position 1, and a deep cycle for house loads on switch position 2. I start the engine with switch position 1 (starting battery) and then once I start I switch to position 2 so when I anchor out I'm using the deep cycle battery. But the combiner makes sure that both batteries are getting charged when the engine is running, but I won't accidently discharge my starting battery? Right?

Of course if I want to start my engine, I switch back to position #1.
And if it is cold outside like this afternoon's 20 degree temp and I want to start my engine I can combine them both using the combiner, or putting my battery switch on "both" (I had to do this today when I went to check out the boat and wanted to stir up the engine some.)

Am I getting this right Leon? I guesse I should read up about this in Calder's book too.
Thanks

Edited by - on

Stu Jackson C34
Admiral

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844 Posts

Response Posted - 01/02/2008 : 18:47:21

If you didn't want the book because it was 14 years old, then forget about the isolators, too, because that technology is even older!

You have a 1-2-B switch. You'll end up having two separate batteries, both deep cycle (because you do not need a start technology battery for your outboard). You want to charge both batteries (banks) from a single source without one bank discharging into the other. You also want to do this with your switch in the off position, since you use solar and leave the boat unattended during the week.

Then use a combiner, which is a simple relay that engages whenever there is a charging source and opens when it disappears (the sun goes down).

Isolators eat up almost a volt of electricity, and since the difference between a full battery and a discharged one is only a volt, why use something that eats up your charging voltage?

You run the solar panel and ALL other charging sources to one battery, then wire the combiner between the banks. The output of the banks go to the 1-2-B switch. Simple.

In your case, you do NOT want an echo charger, which is like a combiner, a relay, but it limits current to the second battery. It is used with large house banks and smaller start batteries to avoid overcharging the start bank. In your case, both banks are the same size, so use a combiner.

Frank, the combiner ONLY works when there is charging occurring. No charge (voltage over 12.8 V), it opens and no current flows. That's why it replaces the isolators, because it keeps the banks separate when no charging is occurring.

Edited by - on

newell
Navigator

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USA
113 Posts

Response Posted - 01/02/2008 : 20:11:24

Stu:

You had me confused with another person. Actually, I did purchase the book. I really like the idea of the combiner and will, no doubt, go that direction.

newell

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Leon Sisson
Master Marine Consultant

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USA
1893 Posts

Response Posted - 01/06/2008 : 23:52:59

<blockquote id="quote">quote:<hr height="1" noshade id="quote">Originally posted by Gloss
 55 bucks seems cheap for what a combiner does for you Leon.

So please check me out if I'm getting the procedure right to get the benefits. On my Ericson, which has a diesel, I have a starting battery, on switch position 1, and a deep cycle for house loads on switch position 2. I start the engine with switch position 1 (starting battery) and then once I start I switch to position 2 so when I anchor out I'm using the deep cycle battery. But the combiner makes sure that both batteries are getting charged when the engine is running, but I won't accidently discharge my starting battery? Right?

Of course if I want to start my engine, I switch back to position #1.
And if it is cold outside like this afternoon's 20 degree temp and I want to start my engine I can combine them both using the combiner, or putting my battery switch on "both" (I had to do this today when I went to check out the boat and wanted to stir up the engine some.)

Am I getting this right Leon? I guesse I should read up about this in Calder's book too.
Thanks
<hr height="1" noshade id="quote"></blockquote id="quote">Here goes one of those thousand word explanations which should really be a schematic diagram!

At least for the moment, remove the [Off/1/Both/2] high current battery selector switch from the picture. In its place, imagine two high current SPST cutoffs, one in series with each battery. Wire the alternator output to the cranking battery through its cuttoff (with some sort of over current protection so a shorted alternator doesn't start a fire). Wire solar panels, wind charger, etc. to house bank through its cuttoff w/appropriate current protection. Wire the battery combiner between the two battery's positive terminals. Install the combiner's remote control switch and LED indicators on electric control panel. Set combiner in automatic mode and forget it's there.

With all batteries slightly discharged and combiner initially open circuit:

While motoring, the alternator will first recharge cranking battery, then parallel it with house bank to charge house bank from alternator.

Or while anchored, the solar panels will first charge the house bank, then route any excess charge to top up the cranking battery.

If you need to parallel partially charged batteries, flip the combiner's remote control switch to [On]. (For example, to crank engine using house bank, or to run essential house equipment from cranking battery with house bank flat, such as VHF, bilge pumps, or blender.)

To isolate the batteries, flip the combiner's remote control switch to [Off]. (For example, to work on cranking circuit without the house bank suddenly getting switched in parallel when the solar panels have charge to spare.)

The original voltage regulators on the alternator, solar panels, shore pwer charger, etc. prevent over charging any of the batteries. Note this all assumes all batteries are of same technology (all flooded cell, or gelcell, or AGM, etc.).

-- Leon S, Chief Electrical Obfuscator

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glivs
Admiral

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USA
836 Posts

Response Posted - 01/07/2008 : 06:31:46

Are combiners and ACRs (Automatic charge relays) the same technology?

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ilnadi
Captain

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452 Posts

Response Posted - 01/07/2008 : 11:51:33

<blockquote id="quote">quote:<hr height="1" noshade id="quote">Originally posted by newell
 Happy New Year to all!
I would like to isolate my two batteries and am convinced i can do it with a simple diode arrangement and not pay $75 for a battery isolator. <hr height="1" noshade id="quote"></blockquote id="quote"> Happy new year back.

A battery isolator is actually a pair of diodes with circuitry around it to improve protection and performance (like transient protectors, sometimes relays to bypass the diodes when you do not need them, etc). I cannot do it off the top of my head, but if I read up enough I could design one and I would bet you it would cost more than $75, take up more space and perform about the same (if not worse). Just for startes, 200A rectifiers are $25 or so.

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