Charge splitting/isolators

BruceB · 19 February 2008, 22:29

My old boat had one oversize battery through a Hella Isolator which was fine, the new console is huge and I am thinking of installing two batteries of the proper smaller size but linked for redundancy.
I want to run switched to one battery and have the second battery charged through a blocking diode so it is always charged and available. I would prefer one with the necessary diodes built in if possible and with the capability of switching to battery two or both as the needs may be. All the boat is to be run off the one battery with the second only for backup.
I don't really want to go for switching to keep batteries charged so Is there an isolator out there, preferably a good one from Blue Sea or Hella, that will do what I want?
Or is there a better way of doing this?
The c/b switch panel mentioned in the archives, all OK after some use? I am going down the same route and would appreciate reliability info before spalshing out the large amounts of cash from anybody using them?
Cheers

Cypman · 19 February 2008, 23:23

"BEP" VSR Voltage Sensitive Relay has provided the best solution for me. Not cheap, circa £50.

Cypman · 20 February 2008, 05:43

Further to my first reply I have looked on the BEP website (www.bep.co.nz) and they have at least 2 combined switch/vsr units, one of which would probably suit your needs. Either 714-100A or CC-801 are the simpler solutions, there is lots of other information under "Battery Management".

martini · 20 February 2008, 07:17

Your average twin battery isolator would suit your needs wouldnt it? Most can switch between batt 1 - batt 2 - both - off. Switch to both while underway to keep both charged and switch to either batt 1 or batt 2 if you are stopping for any length of time to keep one battery spare for engine starting etc.

Saves paying money for sensitive electronic charging gear which probably wont handle the rib environment.

v666 · 20 February 2008, 07:18

Merlin are very helpful, just give them a call for some good advice.

Their product web site is Power Store.

Tim

bedajim · 20 February 2008, 07:25

Hi

I use the Blue sea kit very nice

http://bluesea.com/category/1/productline/overview/329

Nasher · 20 February 2008, 12:44

Quote:

Originally Posted by martini

Your average twin battery isolator would suit your needs wouldnt it? Most can switch between batt 1 - batt 2 - both - off. Switch to both while underway to keep both charged and switch to either batt 1 or batt 2 if you are stopping for any length of time to keep one battery spare for engine starting etc.

Saves paying money for sensitive electronic charging gear which probably wont handle the rib environment.

Agreed
Keep it simple in 'our environment'.

I use 2 similar sized batteries, and a switch as above.
I use Batt 1 on the outward trip, and Batt 2 on the return.

Nasher.

alt · 20 February 2008, 13:12

A cheap and cheerful way to do what you're trying to do would be an "OFF-1-BOTH-2" switch

Make sure you get a 'Make before break' connection

OK so you're running your engine, switch to BOTH. That way both batteries will charge
Say, when you anchor up and have the stereo/vhf/whatever running, switch to battery 2
Then when you want to start your engine, switch to 1 and your engine will start
Then switch back to BOTH

You can install a diode to stop reverse charging, but since you're only going to have 1 battery running at a time, and when both are connected they're going to be charging, whats the point? My boat has a split-charge-diode in place but I could live without it - it was there when I bought the boat (I only ever really use 1 battery at one time)

Obviously I leave it at BOTH if im only going to be anchored up for a short while - I guess this is where the diode comes into play

-EDIT- I see that my above post has already been mentioned, d'oh

Rooster21 · 20 February 2008, 13:29

Answered my own question when i looked properly Duh

al40 · 20 February 2008, 15:16

Avoid blocking diodes at all costs as they will prevent the 2nd battery from getting fully charged. Average voltage drop over a diode is .7V which varies with current and temp.

Batts need to be charged to 14.4V to prevent sulphanation (usually in 3 steps - bulk, absorption then float). 2nd batt will never reach this with a diode.

I have always used switches in multi-batt installations and they have worked really well. It also allows you to easily compensate if the engine start battery dies for example - the only thing this setup does need is for you to remember to switch the switch at appropriate times!

Failing that, VSR's are great and relays are not that sensitive - fine for boats.

Ideal soln would be a proper intelligent split circuit charger but they are expensive!

You could always have a batt switch plus a VSR :-)

Nos4r2 · 20 February 2008, 15:36

Like this?(link) Expensive though!

jwalker · 20 February 2008, 16:15

Quote:

Originally Posted by al40

Avoid blocking diodes at all costs...

Nonsense. We've had this conversation before.
Your notion of a battery never reaching full charge is not correct and it presumes the battery with the diode is initially fitted in a partially discharged state. Fitting a fully charged battery and then using a diode to preserve the charge works fine. Even if you accept the voltage drop as 0.7v (which it usually isn't) you'll still have a charging voltage of approx 13.7v which is just fine for a float charge. If you're wise, you'll choose to use a schottky diode which has a voltage drop of approx 0.4v.
If you end up with a really difficult situation and you've partially discharged your backup battery too, all you need to do is ensure your switch is in the correct position to recharge it fully.

All these suggestions of switching....what a hassle.

I use a diode with complete success. Both my batteries are now over 4 years old and still work fine and they are starting a big diesel engine not a little outboard motor so I guess they're still delivering satisfactory current.

I have a 1-both-2 switch but I never switch it, it's always on both but I also have an isolating switch for the battery with the diode so it's the equivalent of always being on 1. During my boat build I felt it may be useful to have the switching facility but I've never used it. Some folk say it gives safety but I feel it's very little - after all, do you disconnect your car battery each time you park?

Quote:

Originally Posted by BruceB

All the boat is to be run off the one battery with the second only for backup.

I think that is the way to go. Personally, I'd use two simple single switches; one as a main switch and one in series with the negative connection of the backup battery with the diode wired across it. Dead simple, no switching necessary in normal use and both batteries remain charged.

BruceB · 20 February 2008, 17:01

Tis funny how an innocent question can open up so much controversy :-)..............
I had a Hella Contour switch as an isolator on my last boat which looks identical to the BEP ones mentioned.
Is this what you mean about the isolator wiring and if so would it not be easier to wire it in parallel with the switch #2 so charging takes place all the time?
Or could the same wiring be used with a single dual battery switch as a neater way of setting up switches?

jwalker · 20 February 2008, 17:28

Much like the second one but I used a switch in the negative line. I did this because negative isolators are available which are in the form of a negative battery terminal which is 'jumped' by tightening a thumb screw. This made it very easy and the diode can have an 8mm terminal on each end which is simply put behind the clamp bolt nuts.

Here's one.

The end with the bolt through it is a taper which your standard battery cable fits onto. I'm sure you could adapt that fuse holder to house the diode to make it a neat solution.

BruceB · 20 February 2008, 17:34

So you don't have two isolators as such. You have one main battery isolator and this discarnect unit connected to your neg terminal on the second battery with the fuse holder removed and a diode across it. When required on a flat battery you screw in the black plug to bridge the diode for use? as shown in first drawing.
Does that make any sense and is it correct?

Or how about the scond drawing with one switch for dual batteries but with a diode joining both? The disadvantage I can see with this is that when switched to battery two the first battery doesn't get charged and if the second battery is flat the first will try and equalise the charge between them, more thinking required I think

jwalker · 20 February 2008, 17:35

Quote:

Originally Posted by BruceB

So you don't have two isolators as such. You have one main battery isolator and this discarnect unit connected to your neg terminal on the second battery with the fuse holder removed and a diode across it. When required on a flat battery you screw in the black plug to bridge the diode for use?
Does that make any sense and is it correct?

Spot on.

BruceB · 20 February 2008, 18:12

I think I have found the answer an BEP 714-100A this has one switch which can be set to battery 1 or 2. Whatever battery is selected as the running battery the other is charged through a VSR. It isn't even particularly dear as £69!

Cypman · 20 February 2008, 18:25

Buce
You won't go wrong with that, its a very good quality unit and they also sell a very neat mounting plate for it.

jwalker · 20 February 2008, 18:52

Quote:

Originally Posted by BruceB

It isn't even particularly dear as £69!

Bruce, you live in a different world to me...

BruceB · 20 February 2008, 19:01

It isn't dear compared to a lot of other bits needed and when you have gone through several "cheaper" isolators at £15-20 each it makes sense to buy one good quality one that will last.
The hella one I had was still going strong after several years use, the cheaper ones at about half the £35 cost of this lasted one season only before developing high resistance and having to be changed.