Battery Isolator

[Dale M.]

One does not even need to bridge rectifier to charge two batteries, two simple diodes will do the same thing.... Remember voltage/current only flows one way through a diode.... Think if it as a check valve to in a water system or a air system...

http://www.the12volt.com/diodes/diodes.asp

Dale

[bdosborn]

One does not even need to bridge rectifier to charge two batteries, two simple diodes will do the same thing.... Remember voltage/current only flows one way through a diode.... Think if it as a check valve to in a water system or a air system...

http://www.the12volt.com/diodes/diodes.asp

Dale

Don't forget that there are amps running through the diode 0.7V forward bias voltage drop. That means there is 7 watts of waste heat generated for every 10 amps that runs through the diode so you'll need to add heat sinks. That's why commercial versions are sinked so much:



I've got a #6 running from the truck battery to the RV plug and #10 wires back from there to the trailer batteries. I don't often get above 13.8 volts at my trailer battery so the isolator wouldn't work in my scenario; 13.1 volts doesn't hardly charge a battery.

Bruce

[PcHistorian]

well I know if you put that next to the tow vehicle battery that you get very little back at the spare, because the tow primary will regulate it to bare minimums. but it you have it right next to the alternator output, it depends if the voltage regulator is internal or external. and generally I wouldn't run any load of my trailer from the tow battery, so just heavy diodes to the trailer from the tow, right at the alternator. Otherwise if I have a charger from 120v in the trailer and two sets of batteries (litium ion to 12v for a separate led circuit to share with my deep charge, (AND the deep charge also gets the rest of the 12v loads, water pump, some non-led lights). then yes, both batteries next to the charger in the trailer. (see next photo. revised circuit, to show shared load and a "private" set of loads, right from the battery.

(sharing a charger by two batteries. if in the tow vehicle, put the diodes next to the alternator output. if the charger is a 120v in the trailer then put next to both batteries and decide what to share or keep private.) this is without the breakers, the fuses and the switches, just figure all that stuff goes as part of the "load."

[VermonTear]

Just found this thread.

I was planning on installing an isolator for my new build, but found out today the specs call for 4 gauge for my 20' run from the front of my auto to the back of my Tear. Not sure I want to do that. I think I can learn to live with unpluging from the vehicle when we park. So..... question is: What gauge do I need to run? And can that gauge run through my 7 pin plug?

Ken

[PcHistorian]

Ken, some people I have seen here replace the 12v +/- leads for the 7 pin connector with 10 gauge stranded wire. Are you planning a 120v charger as well? solar panel? I think it would depend on how low you consider going with your deep cycle's charge and so how much the wire is going to need to carry.

[VermonTear]

Thanks for quick response.

Are you planning a 120v charger as well? solar panel?

No solar. My small (1000 watt) inverter is an Inverter/charger, so yes, I will be using that as a charger when I am at home, or when I am at a campground with shore power. But, more often than not, I can imagine we'll be camping at campgrounds or remote spots without shore power, and maybe traveling cross country and won't want to have to depend on spots with shore power, so I'd like to be able to get charged back up from the vehicle as we travel.

I think it would depend on how low you consider going with your deep cycle's charge and so how much the wire is going to need to carry.

I don't really know enough to answer that yet, but I will try to live within my Lifeline's 100 AH specs and not draw it down below the recommended (I could go out in the shop and look it up but it's dark and late) percentage. I could live with 10 gauge easy I think -- maybe even 8.

Ken

[bdosborn]

Use a voltage drop converter to calculate what size wire to use. I like this one:
No Outage (but it was off line when I checked it).

This one isn't bad:
Southwire

Shoot for 3% voltage drop with a 10amp load and you should have an acceptable charge voltage at the trailer battery.

Bruce

[VermonTear]

Shoot for 3% voltage drop with a 10amp load and you should have an acceptable charge voltage at the trailer battery.
Bruce

Bruce...

Thanks for the links. Helpful, but.. like I indicated, I may not even know enough to be able to use the calculators intelligently. Hmmmm... what is my "Load current in Amps?" If I add up all (my few) DC stuff I want to be able to rely on when 'roughing it', it is only 4 Amps (3.93). Is that my "Load?" Or is it my worst case senerio Amp Hours (28.96)?

I have an AC coffee grinder that (with my inverter) I'd like to use even when roughing it. It is .91 amps. Would that mean I'd have to move my "Load" calc up to 4.84 Amps, even though I'm only going to be using it for 20 seconds a day? And if I wanted to use my toaster (which I'm totally okay with skipping in the rough) I'd have to add 7.73 amps to the Load, is that right?



'precciate the help.

Ken

[bdosborn]

I wouldn't worry about what the total load of all your devices in the trailer is (yet). Usually, everything is turned off when your towing so the only load on the wire from the tow vehicle battery to the trailer battery is the battery charging current. I've never seen the charge current seen go over 10 amps on my trailer so I use 10 amps as the load current for the charge wire. A #8 wire will carry 10 amps for 30' at a 3% voltage drop. A #10 will carry 10 amps for 30' with a 4.5% voltage drop. That still works but that little bit lower voltage will mean the battery will charge slower.

The total load of all your devices shown in your spreadsheet is what you use to size the wire from the battery to your distribution fuse block. That wire is usually so short you don't need to worry about voltage drop. I would use a #10 AWG wire, which is good for 30 amps. That's what I use on my trailer and my total load is way higher than 4 amps (but less than 30). You'll probably want to add a 12V receptacle or something in the future so I'd use the #10 to cover your future wants.

Are you going to use an inverter? They draw bucket loads of currents and usually have big, fat. dedicated wires and fuses. The #10 wont work for the inverter but I can size that wire if you need that info.
Bruce

[bdosborn]

P.S. Your spreadsheet is wrong. You can't add AC and DC amps together directly as they are at different voltages....

[VermonTear]

Bruce, thanks for chiming in to try to help me understand this stuff. I don't need to become a certified electrician, but I do need to learn enough to get it right before these wires get buried beneath glued down insulation and ply. I want to get it right before I get it wrong.

the only load on the wire from the tow vehicle battery to the trailer battery is the battery charging current. I've never seen the charge current seen go over 10 amps on my trailer so I use 10 amps as the load current for the charge wire.

Ahhh... so that's the load. I'll use 10 amps as the load.(How did you measure the battery charging current? Is that something you can do with a little hand held voltage meter?). I'll run a #8 if I can run it through my 7 pin plug, otherwise I'll run a # 10 cause I'm sure it will fit the plug okay.

to size the wire from the battery to your distribution fuse block. That wire is usually so short you don't need to worry about voltage drop. I would use a #10 AWG wire

Yeah, that is about 3' in my setup and I'm running #10.

Are you going to use an inverter? They draw bucket loads of currents and usually have big, fat. dedicated wires and fuses. The #10 wont work for the inverter but I can size that wire if you need that info.

Yes. I'm installing a Xantrex Freedom HF 1000. I've got #10 AC feed from Shore Power to the HF with a 30Amp AC inline fuse, then #10 to AC breaker box, #2 cables to battery with a 150 Hi-Amp DC CB. #8 Equipment Ground ties to DC Grounding Block. http://xantrex.com/documents/Inverter-Chargers/Freedom-HW/Freedom_HF_Install_Guide%28975-0395-01-01_Rev-C%29.pdf

Thanks again.

[VermonTear]

P.S. Your spreadsheet is wrong. You can't add AC and DC amps together directly as they are at different voltages....

Okay, but does the Amp Hours math look right? And can I add the Amp Hours of the AC and DC together? That's why I originally put this together -- to see how small an battery I could buy.

[PcHistorian]

Vermon, nope, you can't just add AC and DC amp hours together. because of voltage difference. (I tend to ignore the ac/dc form of electricty and just go by volt/amps) In your case look at it like this. To convert the DC to AC you multiply the voltage by 10 (12v DC to 120v AC). So to convert AC amps (hours) to DC amps (hours) multiply the AC amp hours times 10 to get the DC amp hours. 1 AC amp-hour = 10 DC amp hours (this is still a rough estimate as there is a tiny percentage of additional loss of amps due to the conversion process, so add maybe 3-5% after you do all the calculations for a safety factor/margin of error/margin of safety.) You'll find that a much more realistic take of what is going on.
That's my take anyway.

[bdosborn]

Ahhh... so that's the load. I'll use 10 amps as the load.(How did you measure the battery charging current? Is that something you can do with a little hand held voltage meter?). I'll run a #8 if I can run it through my 7 pin plug, otherwise I'll run a # 10 cause I'm sure it will fit the plug okay.

Most hand held meters are only good to 10 amps if they measure current at all. I used my Trimetric battery monitor to check my current. It can be a pain to measure with a meter as you need to do it when the battery is discharged a bit and the meter needs to be in the current path.

Bruce

[bdosborn]

So to convert AC amps (hours) to DC amps (hours) multiply the AC amp hours times 10 to get the DC amp hours.

That works. I would multiply by 11 to include the inverter efficiency and voltage drop. Most people are surprised by how much battery capacity they need after their first pass at a chart like yours. At the usage you've shown (around 70 amp-hrs), you would need 2 group 24 deep cycle batteries and you'd only get a single day of capacity.
Bruce