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Old 11-24-2019, 10:50 PM   #81
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So where do you put the controller? Does it tell you the voltage so you know when to switch?
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Old 11-24-2019, 10:53 PM   #82
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Originally Posted by alanmalk View Post
https://www.etrailer.com/Battery-Cha...a/DW08770.html - diodes sold for this purpose.

But not very common in trailers - more for boats where having two independently charged batteries could be the difference between making it home, or not.

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thats not a 'diode', its a battery isolator, which is typically put on an vehicle to isolate the camper/house battery from the engine/starting battery. its a solid state relay, controlled by the ignition switch, when the ignition is on, the vehicle alternator is connected to the camper battery in addition to the engine battery, and charges both.
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Old 11-24-2019, 11:12 PM   #83
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Looking at this article:Battery choices again



Quote:
o join batteries in parallel, use a jumper wire to connect both the positive terminals, and another jumper wire to connect both the negative terminals of both batteries to each other. Negative to negative and positive to positive. You CAN connect your load to ONE of the batteries, and it will drain both equally. However, the preferred method for keeping the batteries equalized is to connect to the positive at one end of the battery pack, and the negative at the other end of the pack.
Why is the second method preferred, and how much difference would it make, and why? It solves my problem if I use the first method (connect one battery to the load. Keeping in mind that the largest battery drain at any time is probably a few lights, fridge, and propane heater running.
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Old 11-24-2019, 11:50 PM   #84
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Originally Posted by John in Santa Cruz View Post
thats not a 'diode', its a battery isolator, which is typically put on an vehicle to isolate the camper/house battery from the engine/starting battery. its a solid state relay, controlled by the ignition switch, when the ignition is on, the vehicle alternator is connected to the camper battery in addition to the engine battery, and charges both.

Please see the screen shot.
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Old 11-25-2019, 12:21 AM   #85
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problem with diodes is that 0.7 V forward drop. so now the bulk phase of 13.8V or so will be reduced to 13.1V which isn't enough to put a bulk charge on the battery. And, the absorption phase of 14.2 V is reduced to 13.5V which will not fully charge the battery, not even close.
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Old 11-25-2019, 12:54 AM   #86
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Originally Posted by John in Santa Cruz View Post
problem with diodes is that 0.7 V forward drop. so now the bulk phase of 13.8V or so will be reduced to 13.1V which isn't enough to put a bulk charge on the battery. And, the absorption phase of 14.2 V is reduced to 13.5V which will not fully charge the battery, not even close.
Just like using wire that is too thin - your precious power is going up into the air as heat.

But the advantages of a diode-based isolater are: cheap, simple, mechanically and electrically rugged, almost foolproof installation.

I designed and built a processor controlled isolater which would automatically put batteries in parallel when their voltage was within certain tolerances. Used so-called "ideal" diodes (MOSFET based) that could handle many hundreds of amps but could be switched on/off with a tiny voltage. Had a nice display to show which batteries were being charged and which was being discharged. (Could totally isolate batteries so charge and discharge could be going on at the same time.) Integrated with the solar panel and WFCO converter to use both as input. Used very small heat sinks since the voltage drop was on the order of 0.05 Volts at my typical amperage. Amazingly efficient...

Tossed the whole thing out the window when I realized that AGM batteries didn't need any special treatment to be paralleled and that I was overthinking the problem. I also realized (but slowly) that I wanted to camp and ignore my batteries - which I have done so now for 5 years. I look at them in the spring and say "good to go for another year".

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Old 11-25-2019, 01:27 AM   #87
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yeah. I just use a pair of gc2 220AH 6V golf cart batts in series. wet cells. I check the juice once or twice a year, haven't had to add any yet. I was a bit tight on cash when I was buying them, so I got the Costco Interstates for $100 each. if they die in another year or two, oh well, I'll get a couple more cheap batts. At 2 years, they are working great.
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Old 11-25-2019, 01:21 PM   #88
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Quote:
Originally Posted by Bobbie54 View Post
Looking at this article:Battery choices again

Why is the second method preferred, and how much difference would it make, and why? It solves my problem if I use the first method (connect one battery to the load. Keeping in mind that the largest battery drain at any time is probably a few lights, fridge, and propane heater running.
it is a way to make both batteries 'see' the same wiring resistance.
the top battery has the resistance of A and C and D
the bottom battery has A and B and D.
A is the same for both batteries
D is the same for both batteries
So if you use the same size and length cables for B and C each battery will have the same resistance in-line to the charger or to the dis-charger (the load).
Please accept my crappy labeling of the picture.

I think the resistance in 18 inches of number 2 wire is so small that I personally would prefer perfection if it didn't cost, but I wouldn't swap out cases or spend a lot of time worrying about the level of perfection. I would just run the two different lengths of B and C and move on.

Maybe if you can upload a picture of what you are up against it might help
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Old 11-25-2019, 02:59 PM   #89
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Originally Posted by alanmalk View Post
https://www.etrailer.com/Battery-Cha...a/DW08770.html - diodes sold for this purpose.

But not very common in trailers - more for boats where having two independently charged batteries could be the difference between making it home, or not.

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In RVs on land, these are normally mounted in the tow vehicle (to isolate the trailer battery from the tow vehicle battery, to avoid running the tow vehicle battery down), or in a motorhome (to isolate the coach and chassis batteries).

Despite the eTrailer description, this is not a regulating device.

Quote:
Originally Posted by John in Santa Cruz View Post
thats not a 'diode', its a battery isolator, which is typically put on an vehicle to isolate the camper/house battery from the engine/starting battery. its a solid state relay, controlled by the ignition switch, when the ignition is on, the vehicle alternator is connected to the camper battery in addition to the engine battery, and charges both.
That one was just a couple of diodes. My motorhome has a relay-based isolator, which is certainly the better-performing and more functional choice. Good relay-based devices can include functions such as
  • not connecting the coach/house/trailer/auxiliary battery until the supply voltage is high enough (engine running and alternator working), and
  • temporarily connecting the batteries when a button is pushed so the coach battery can be used to boost the chassis battery in case the chassis battery runs down or fails (not so helpful with a trailer).

As John explained, the problem with diode-based isolators is that there is always a voltage drop across the diodes, so there is a bit of energy wasted and more importantly the battery doesn't see all of the charging voltage.
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Old 11-25-2019, 03:06 PM   #90
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Originally Posted by jxoco View Post
it is a way to make both batteries 'see' the same wiring resistance.
the top battery has the resistance of A and C and D
the bottom battery has A and B and D.
A is the same for both batteries
D is the same for both batteries
So if you use the same size and length cables for B and C each battery will have the same resistance in-line to the charger or to the dis-charger (the load).
Please accept my crappy labeling of the picture.

I think the resistance in 18 inches of number 2 wire is so small that I personally would prefer perfection if it didn't cost, but I wouldn't swap out cases or spend a lot of time worrying about the level of perfection. I would just run the two different lengths of B and C and move on.

Maybe if you can upload a picture of what you are up against it might help
The only difference from your picture is that the batteries are end to end and the existing wiring won't reach to D. (Which is one reason I'm pretty sure it was done with A and D to the same battery in the first place- in fact I have a photo of that, just no second battery in the photo.

Seems like it isn't going to make a lot of difference anyway- I can see it might for a battery in continuous use but they should equalize when there is no load regardless of resistance when current is flowing.
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Old 11-25-2019, 03:43 PM   #91
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... but they should equalize when there is no load regardless of resistance when current is flowing.
And they will equalize! Someday I will tell the horror story of sailing across the Atlantic in a used boat with a burned out alternator, while carefully keeping the house battery and the starting battery isolated with the A-B switch. Not knowing that some backyard mechanic had constructed a convoluted path between the two with some 18ga hookup wire. Then reaching the Chesapeake Bay and finding that the starter was as weak as the house battery...

That's why I am so opinionated about keeping things simple: Plain parallel connection, no A-B-Both switch, no isolator, not worrying about a few inches of missing/extra wire, etc. And I don't even worry about disconnecting the tow vehicle for a day or three and let it's (rather large) battery add a few of its own watt-hours to the grand total.

Remember Murphy's Law!
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Old 11-25-2019, 04:11 PM   #92
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And they will equalize! Someday I will tell the horror story of sailing across the Atlantic in a used boat with a burned out alternator, while carefully keeping the house battery and the starting battery isolated with the A-B switch. Not knowing that some backyard mechanic had constructed a convoluted path between the two with some 18ga hookup wire. Then reaching the Chesapeake Bay and finding that the starter was as weak as the house battery...

That's why I am so opinionated about keeping things simple: Plain parallel connection, no A-B-Both switch, no isolator, not worrying about a few inches of missing/extra wire, etc. And I don't even worry about disconnecting the tow vehicle for a day or three and let it's (rather large) battery add a few of its own watt-hours to the grand total.

Remember Murphy's Law!
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So I just added a plain off and on marine switch for better quality and most important moved it out from under the table . Improving what was but keeping it simple ! Pat
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Old 11-25-2019, 05:37 PM   #93
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Like this;
( B and C would be the same length for the ideal )
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Old 11-25-2019, 07:29 PM   #94
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I don't think the A wire is long enough but I'll check it out as the D wire would be, it is only an inch too short the other way.
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Old 11-26-2019, 10:25 AM   #95
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switch batteries in the case and that would move A and D, maybe that would work?
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Old 11-26-2019, 01:33 PM   #96
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switch batteries in the case and that would move A and D, maybe that would work?
Great, made it work! (Not sure which diagram but one I hadn't thought of, so thanks very much!)

Connected up and voltage is the same on both batteries so they quickly equalized if they weren't already. Lights work. I guess I'm safe to recharge the batteries when needed!
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Old 11-26-2019, 02:10 PM   #97
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...
Connected up and voltage is the same on both batteries so they quickly equalized if they weren't already.

...
At the risk of overthinking...
Voltage on each battery - when connected in parallel - actually tells you very little. Much more interesting would be amperage going between both batteries with nothing running.

If you are so inclined: Using John's diagram above, disconnect the A+ and B wire from the right hand battery. (With A+ disconnected then nothing is acting on the batteries.) Then use your multimeter on the Amp setting (move the probe to the Amp connection if your meter has one). Touch one lead to the + post on the right hand battery, touch the other lead to the disconnected B wire. If you are lucky then on the Amp setting the meter will read near zero. If true, then switch the meter to the milliAmp setting. That reading will not be zero for sure. Tell us what you get!
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Old 11-26-2019, 03:40 PM   #98
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Reads 2-3 milliAmps. (On the 20 mA setting.)
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Old 11-26-2019, 06:48 PM   #99
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Reads 2-3 milliAmps. (On the 20 mA setting.)
Can't get much better than that. At my best - when the batts were brand new - I saw 5 milliAmps. And that was after they had been connected for a week.
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