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Old 05-04-2009, 04:36 PM   #1
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Battery separators: uni-directional or bi-directional?

Hello!

(Funnily enough, searching for 'uni-directional' tells me that the term searched for is too common, and is thus ignored! So apologies if this has been gone over before.)

Wondering whether a battery separator should be uni- or bi-directional. A quick overview, as we would use them:

- uni-directional will charge the house battery from the alternator, once the starting battery is charged
- bi-directional will do the above, and also charge the vehicle battery from any other charging source (ie. solar) once the house battery is charged

My concern about bi-directional - when the house battery is charged, the separator will parallelize it with the starting battery. Is there any chance of damaging the house battery when doing this (such as sudden discharge of house battery into starting battery, if the starting battery is low)? Will having these two batteries in parallel confuse a charger, so that it sends too much current into the house battery? I don't have a perfect understanding of how chargers read battery state when there are 2+ charging sources and/or 2+ batteries!! I do know that any authority on batteries recommends against hooking up 2 different batteries in parallel, but this is usually relating to permanent battery bank connections.

SMB seems to use a bi-directional, btw. The Sure Power 1315, I believe.

The advantage of bi-directional, for my purposes, is that I wouldn't have to rewire the stereo and accessory sockets to run off of the house battery.

Cheers!
Mike.

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Old 05-04-2009, 07:53 PM   #2
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Re: Battery separators: uni-directional or bi-directional?

Yes, our battery separators are bi-directional. But, don't worry about it. The losses in the wiring and separator will prevent battery damage, even if you ran the starting battery down trying to start the engine. Or, even if you ran down the house batteries with the microwave and then started the engine to recharge.

Charging batteries in parallel is no big deal. All the 6.0 diesel vans have two (parallel) starting batteries and many have two (parallel) house batteries. The only time parallel batteries are an issue is when one is much better than the other. Then the bad battery will pull down the good one, never the other way around.

Mike
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Old 05-05-2009, 12:05 PM   #3
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Re: Battery separators: uni-directional or bi-directional?

Quote:
Originally Posted by Ford_6L_E350
Charging batteries in parallel is no big deal. All the 6.0 diesel vans have two (parallel) starting batteries and many have two (parallel) house batteries. The only time parallel batteries are an issue is when one is much better than the other. Then the bad battery will pull down the good one, never the other way around.
But starting batteries and house batteries are always the same type and size and age of battery - they're never mixed. I'm concerned about the starting battery being older, and weaker, and pulling down the (good) house battery when the separator hooks them up in parallel. With a one-way separator, they'll only be connected when the house battery is the weaker one (I'm not so concerned about my starting battery - it costs much less ).

During my searches, I pulled up the Optima site, who had this to say about separators:

"If you are using an Optima and a conventional battery, your system must charge each battery independently."

Maybe some sort of relay? Seems too complicated for me!

Cheers,
Mike.
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Old 05-05-2009, 08:46 PM   #4
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Re: Battery separators: uni-directional or bi-directional?

The separator only hooks them up when the voltage level reaches a certain point in the charged battery (or battery set) at which they're nearly or fully charged. You can force the separator to hook them up (manual override) for things like jump-starting the van off the house batteries for dead starters or (I can't imagine doing this) powering your microwave off the starter batteries.

The way SMB wires the separator it manual overrides whenever the key is in the Start position, that's both Diesels and Gassers, so you can effectively completely hide trashed starter batteries because they're jumped with the house each and every time... which is what happened to me.

I know my starter batteries charge when the house batteries are topped off, and the house charge while driving (you can watch the blueSky voltmeter jump when the starters are topped off), and I'm pretty sure that was true even when the starter batts were toast (of course I didn't know that until I pulled them and I wasn't putting them back after they tested bad!)

Anyway, I guess what I'm saying is I don't see where the concern is. Changing the start wire over to a manual switch solves the "always-jump-on-start" problem. Unless you're saying putting good, fully charged batteries with bad ones to be charged, during charging, ruins the good ones... is that it?

And I'd be much more concerned when the starter is the weak one... popcorn won't get you out of Death Valley.
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Old 05-05-2009, 09:15 PM   #5
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Re: Battery separators: uni-directional or bi-directional?

Mike this has come up before. I'll send you a PM on this issue. Not that I know what's correct but it's my view. Check your messages. There are several links that might be of value. If you feel it helps go ahead and post it.

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Old 05-06-2009, 09:03 AM   #6
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Re: Battery separators: uni-directional or bi-directional?

My SMB doesn't have the 'Connect on Start' hooked up. I've never needed it and don't plan to hook it up.

Before one set of batteries could pull down the other set, the seperator would disconnect. It only connects when one set is charge fully, and disconnects when the voltage comes down. Unless it fails in the 'connected' position, no damage to the good batteries could occur.

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Old 05-06-2009, 10:52 AM   #7
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Re: Battery separators: uni-directional or bi-directional?

Quote:
Originally Posted by Ford_6L_E350
Before one set of batteries could pull down the other set, the seperator would disconnect. It only connects when one set is charge fully, and disconnects when the voltage comes down. Unless it fails in the 'connected' position, no damage to the good batteries could occur.
That seems to be the part I overlooked - like you say, it would prevent what I'm worried about, which is the pulling down of the good battery by the bad. Thank you!

Dave's message to me had a lot of good information, which is also contained in his blog post, here:

http://autoramblings.com/blog/?p=386

Thanks Dave for passing it along!

Cheers,
Mike.
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