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Topic: Schottky based isolator vs combiner (Read 3379 times) previous topic - next topic

Schottky based isolator vs combiner

I've seen a number of posts talking about using battery combiners instead of battery isolators.  It is often stated that the diodes in isolators have a voltage drop of .7 volts, while the mechanical relay of a combiner does not.

Schottky diodes are pretty common now and they have a voltage drop of .3 volts, and they are used in many isolators.  So why continue to use .7 as the reference for voltage drop in isolators?

My understanding is that a combiner will physically tie two battery banks together, while an isolator will keep them separate but send power to both banks.

If a coach has three starting batteries of one group size and AH capacity, and three house batteries that are a differnt group size and significantly larger AH capacity, wouldn't a combiner cause issues with either short cycling, or improper voltage sensing, that could lead to either over charging or under charger of one of the banks?

I can understand using a combiner with battery banks where the batteries are all of the same group and AH capacity, but wouldn't an isolator be the better choice when the batteries and banks are different sizes?

Chris

Re: Schottky based isolator vs combiner

Reply #1 –
The battery combiners have a cut-in and cut-out voltage.  They are customizable based on the setting of your batteries.    Generally, you set them to make sure your house batteries are all full charge and don't get drawn upon unnecessarily and stay full, and so that your starting batteries are not dying.  To have the starting batteries peak efficiency packed at 100% isn't really necessary - you just want to give them a little something something every now and then so they don't go dead over long periods of non-use.

I have mine set up only to combine at voltage that the solar charge controller is sending over the last hour or so of absorb before going into float.  It disconnects automatically at bulk charge voltages because I don't want to overcharge, and disconnects automatically at float charge voltages because of the reasons you gave in your post.

I've seen others that you can combine according to time schedule instead of voltage, which would better for long periods of plugged-in storage.

It's not 100% ideal for the engine battery specs, but it'll keep them up, and it's better than letting them just sit!  If you want something that charges your starting batteries according to their exact specifications, go with a single solar panel and a small charge controller like John.

Re: Schottky based isolator vs combiner

Reply #2 –
Quote from: Blinded04 –
It's not 100% ideal for the engine battery specs, but it'll keep them up, and it's better than letting them just sit!  If you want something that charges your starting batteries according to their exact specifications, go with a single solar panel and a small charge controller like John.
After correcting our isolator problem we had when we bought the coach years ago, our batteries have lived a happy life and continue to do so. What I can't understand is why anyone would spend the money and time to go with a solar panel and charge controller to keep the engine batteries charged when a $20 trickle charger will do the same thing with only a 10 minute or so installation. The only time I could see any advantages would be if you stored the coach where there was no shore power but then you would need to charge all the batteries, not just the engine batteries. With our twin digital voltmeters up front, I can keep an eye on all voltages. The trickle charger keeps the engine batteries exactly to factory recommend specs of a little over 13 volts.

The parasitic drain on the engine batteries is very, very low and with our trickle charger running off of an inverter hot outlet, it more than charges the engine batteries when dry camping. So, it's not the most efficient method of using solar to generate 12V, invert it and then convert it back to battery charging voltage. But, SO WHAT! We have way more watts on the roof than we need and the amount of wattage needed to run the trickle charger is very low compared to our microwave, latte maker, TV, etc. The actual time needed for the trickle charger to keep the engine batteries up is only a couple minutes a day. I don't shut it off as I can't save the excess juice the solar panels put out. We only keep the inverter on part of the day anyway and turn it off at night when we don't need it. When we get home or stop at a campground with power, the moment we plug into shore power, the trickle charger starts maintaining the engine batteries.

This is as simple and cost effective as possible with zero drawbacks.

Pierce

Re: Schottky based isolator vs combiner

Reply #3 –
Thank you for the discussion.

It would be helpful to know what brand and model of products people are talking about when describing your system.  The battery combiners that I've looked at have fixed cut-in and cut-out voltages, and two terminals such as the Blue Sea ACR SI series.

The way I am talking about using them is as a charging method for the banks when the coach is running down the road and being charged by the engine alternator.  Just a drop in replacement for an isolator.  Maybe I'm missing something about how our coaches are wired, but that's also why I am asking the questions.

I've looked at combiners over the years, but I've stayed away from them because it's been instilled that banks of dissimilar size/capacity and construction should not be physically joined together.  On a combiner, when the relay closes it connects two dissimilar sized battery banks, and possibly two battery banks of different construction (lead acid, gel, AGM) that may need different voltage levels for proper charging.

Say you are dry camping and leave early in the morning.  Your house battery bank is a set of 3 8D batteries and this morning they need to have 100 AH replaced.

What I understand will happen is that after the coach is started the combiner relay will close causing the starting battery bank and the house battery bank to be physically connected.

Since they are connected, won't the two battery banks try to equalize - one bank giving up amps to the other bank until they are even?

Once the engine is running the alternator sense wire sees that voltage is needed, and the alternator ramps up.  Now your batteries are being charged, but the two banks are also constantly trying to equalize.

Can't the smaller bank be stressed under these conditions - having higher voltage state applied to it for a longer period of time that is good for it?

Chris

Re: Schottky based isolator vs combiner

Reply #4 –
James mentions my name and what I use for solar and the engine batteries. I do not plug in even here at home so solar does all my charging. I did use the trickle charger for a coupe of years but it was not keeping it up to snuff so decided on the panel. The coleman panel was on sale for $79 with the controller so was a no brainer at that cost and it keeps the engine batteries in top shape since I did this. Just did the same for Brother Geoff and his GV as he had seperate small panels for both sets as they store their coach with no power and no "array" of solar for coach on roof. He had to keep moving them on ground etc etc so now they catch the sun and both banks he reports are doing well. He does not have to worry any more about them.
JohnH

Re: Schottky based isolator vs combiner

Reply #5 –
The diode based isolators were a recommendation to me five years ago. 

As the stock .7 drop isolator unit still worked ok we deferred that and went with a 30 amp auto connect trickle charger.

Turns out by accident I left the headlights on till the engine batteries were dead.

Turned on the gen and the auto connect worked fine.  Except the draw exceeded the units internal fuses limits.

Blown fuse.  No imdication.  Using the stock boost switch would not have overloaded the unit. 

We have a 14amp solar set on the roof which worked well with the trickle charger unit unitil it blew.

Our usage kept the engine batteries up fairly well so we had not noticed the units failure.

When upgrading the inverter the failure was noted and the limitations of its 30 amp fusing.

My buddy said all the newer coaches use combiners.

So we installed a sea land 500 amp auto combiner.

Replaces the three amp power consumption when onstock boost switch with a magnetic unit.  No power draw.

The combiners noted programming seems to be ideal for our Rv use.

Much more even voltages shown both on solar backup and alternator and the magnum sw2812.

Fixed as far as we can see.

Yes the AGM red tops and the east penn gels are different chemically.

Yes the batteries I assume are trying to equalize a bit.

Best workaround was the set everything at 13.6 volts at the batteries.

Solar, alternator and float.  Depending on the battery monitoring systems temp sensor for the house charging.

If memory serves me most alternators charge in cars at 14. 2? Volts.

So if the battery temp management system allows 14.15 volts charging at cold temps the AGM redtops would not be receiving more voltage from the sw2812 charging circuit than they were normally rated for.

The east penn gels had a tag on them new that allowed a max charge of 14.15 also.

My thoughts and my buddies experience was that long term this worked great.

Solar and alternator are set lower at 13.6 as neither are multistage yet.

Float on the sw2812 set for gels is 13.6. 

So the solar matches the float on the charger pretty close.

Xantrex pro link learns the system over time and the gauge shows "full" after long sw charging and long term storage from the solar and both engine and house show 13.4-13.5 at the dash after the long wire runs to the dash.

The engine battery dash gauge shows roughly 13 volts and as it is after the ignition switch my guru says that's correct.