How to make a cheap isolated dual-battery setup for $50

[toyotech]

Sorry, was in a bit of a hurry on that last answer and totally muffed the other questions.

Sure you can add a switch. One of evldave's later graphics shows how. You're best bet is to wire it to ignition so it works automatically is you don't forget it, and then also add a switch that you can use to feed power to the solenoid when the ignition is off but you want to tie the batteries.

Yes you can use the frame for the ground. It's not optimal, but it will work. Might contribute to radio noise so if you're a radio guy, you might not like it.

Thank you. Good idea on the switch / ignition. I'll try the frame and if there is noise. I'll run another wire...


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[toyotech]

BTW, I noticed Samlex has gotten into the RV/solar game:

http://samlexsolar.com/


One rather nifty thing I saw on their site was a smart battery isolator (split-charge type):

http://samlexsolar.com/rv-power-products/ProductDetail.aspx?pid=482

That is pretty sweet. Will top of battery as needed


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[VanIsle_Greg]

That is pretty sweet. Will top of battery as needed


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Very nice...sweet unit.

 

[green]

Starting my setup this weekend. Is having a yellow top coupled with a red top a bad way to go? Looking at adding this to my setup! Awesome find.

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[TEC3]

I think I have ordered everything for this. Going to attempt it when it all arrives this weekend. Should go well. I have a deep cycle I was using for offshore power and just charging off a batt charger when I got home. I am going to use it and the starter battery to get it all wired in. then I will replace with 2 new batteries.

 

[dstn2bdoa]

First off, thanks for a great thread.

In the above picture where and how would I wire in a volt/amp meter to monitor the goings on of my system? I realize this is an incredibly basic question, sorry.

Thanks

 

[benjamin.bignell]

First off, thanks for a great thread.

In the above picture where and how would I wire in a volt/amp meter to monitor the goings on of my system? I realize this is an incredibly basic question, sorry.

Thanks

Someone may need to correct me on this, but...

The "running" voltage would be measured after the solenoid, between the hot wire (labelled On) and ground. Amperage would be taken from the hot wire after the solenoid.

You may also want individual voltage measurements on the batteries - in this case between the + and - terminals directly (or any equivalent location on the circuit that is convenient for measurement).

 

[dwh]

No, the wire labeled "on" is just the wire that supplies current to operate the electro-magnet in the solenoid.

If you want to measure the total amperage coming out of the alternator, you do that between the alternator and the first battery. If you want to measure the amperage going to the aux battery, you do that between the main battery and the aux battery.

If you wanted to use a Tri-Metric type amp counter, you'd put a shunt on each battery's negative.

If you want the voltage of the system while running, you can measure that anywhere - the entire "12v bus" will be at the same voltage as whichever battery is lower since that battery will drag down the bus voltage until it reaches a surface charge high enough for the voltage regulator to begin switching the alternator on and off to maintain bus voltage.

If you want to read the resting voltage of the two batteries individually when they are not tied, then you need one volt meter for each battery.

 

[DiploStrat]

Once More, with Feeling

What are you trying to measure?

-- Voltage? Where and when? (And why?) As dwh has noted again and again, when the system is on charge, the voltage will be whatever it is, everywhere. You will want to note the voltage to satisfy yourself that it is with the proper range for your battery type. You can probably do this from the voltmeter on your dash. A second voltmeter on the camper battery is interesting as a gross indication of state of charge, but its reading is only interesting when the camper battery is being discharged and accurate when it is not. The only time that a voltmeter can really give you an accurate state of charge is when the battery is at rest, neither under charge nor discharge. When under discharge, the voltage will read artificially low. Which is why I usually argue that a second volt meter should not be on the dash, but rather in your camper space.

-- Amperage? Specifically ampere hours? This is what you really want to know as this tells you how much charge you are putting into your camper battery (to a large degree, your starter battery is irrelevant.), how much charge you are taking out, and finally, whether your camper battery is fully charged or not. The only way to get a good approximation of all of this is with an hour counter like the Bogart TriMetric. (We will ignore, for the moment, the expensive but easy to install Smart Gauge, sold by Balmar in the US; too much argument about whether it really works.)

An hour counter requires a properly installed shunt on the negative leg of the battery you want to monitor, usually the camper battery, but a TriMetric will happily measure two different batteries.

Is all of this worth it? For a simply 100Ah deep cycle battery under the hood, as understood by this thread, probably not. (And if you install an hour counter you no longer have a cheap system.) For a 200+Ah battery bank, stuck back in the camper, I would consider it essential.

 

[dstn2bdoa]

Thank you for the quick and detailed replies.

I left my question vague because I'm unsure what I'm looking for. Your responses helped narrow it down a bit.

I had several vague goals in mind. When I put a house battery in my truck and I'm out camping, I would like to know 1) How much "life" do I have left in my house battery? And 2) How much is being drawn when various things are running. Or alternatively, when everything is shut off is there leakage? 3) And when I'm driving or charging, is it full?

In my very limited understanding, I though a volt/amp meter would do this and I would just wire in the system somewhere.

 

[dwh]

If the total load you are trying to measure is less than 50 amps, then you could use a Watts Up meter:

http://www.amazon.com/Watts-Meter-A..._sim_hi_3?ie=UTF8&refRID=189GWZA3CJ8DXEFWMZ14

You would put that in between your battery and your aux loads. To know how much life is left in the battery, you'd have to take an educated guess


To do any more than that, you'd need to drop the bucks on a real battery monitor, such as the Tri-Metric, Xantrex LinkPro or Victron BMV - any of which is smart enough to do the math and take the educated guess for you. :

http://www.victronenergy.com/battery-monitors/bmv-600s and bmv-602s

 

[benjamin.bignell]

No, the wire labeled "on" is just the wire that supplies current to operate the electro-magnet in the solenoid.

If you want to measure the total amperage coming out of the alternator, you do that between the alternator and the first battery. If you want to measure the amperage going to the aux battery, you do that between the main battery and the aux battery.

If you wanted to use a Tri-Metric type amp counter, you'd put a shunt on each battery's negative.

If you want the voltage of the system while running, you can measure that anywhere - the entire "12v bus" will be at the same voltage as whichever battery is lower since that battery will drag down the bus voltage until it reaches a surface charge high enough for the voltage regulator to begin switching the alternator on and off to maintain bus voltage.

If you want to read the resting voltage of the two batteries individually when they are not tied, then you need one volt meter for each battery.

Thank you for correcting me! I have been working my way through this interesting thread but misunderstood the diagram.

Hopefully no-one was "shocked" by my wrong answer!

 

[green]

BTW, I noticed Samlex has gotten into the RV/solar game:

http://samlexsolar.com/


One rather nifty thing I saw on their site was a smart battery isolator (split-charge type):

http://samlexsolar.com/rv-power-products/ProductDetail.aspx?pid=482

Given this Automatic charge isolator, and the accompanied drawing with it (see below), is the drawing saying that you charge the starter battery by connecting a home charger to the Aux battery. Does this work both ways from the alternator that connected to the starter battery?

 

[DaveNay]

Given this Automatic charge isolator, and the accompanied drawing with it (see below), is the drawing saying that you charge the starter battery by connecting a home charger to the Aux battery. Does this work both ways from the alternator that connected to the starter battery?

View attachment 274881

From their PDF:

If a battery charger is connected to the auxiliary battery
and this battery reaches a voltage of 13.2V (26.4V)
during at least 7 seconds, the relay closes and charges
at the same time the starter battery. This is an advantage
if the vehicle remains stationary over a long
period of time. If the battery charger is disconnected
and the battery voltage reaches 12.8V (25.6V) during
at least 60 seconds, then the ACR-160 separates again
the two batteries.

 

[DiploStrat]

Given this Automatic charge isolator, and the accompanied drawing with it (see below), is the drawing saying that you charge the starter battery by connecting a home charger to the Aux battery. Does this work both ways from the alternator that connected to the starter battery?

You have hit on one of the best arguments in favor of an intelligent, bidirectional relay. Basically, with variations of capacity, times and voltages, depending on the preferences of the designer, once the controller detects a real charge, on either side, it closes the relay. Once either battery drops below a given voltage, it opens the relay. The differences are time delays, high and low voltage settings, ability of the user to customize (or not), remote and manual overrides, tell tale lights, etc.

Assuming appropriately large wiring (think 1/0 AWG or larger), once the relay closes, a charger attached to EITHER battery simply sees one large battery and charges it. In my case, with two large alternators attached to the starter batteries and 500w of solar attached to the camper batteries, my relay is rarely open.

Diode based systems and most B2B's do not offer bidirectional charging. (The CTEK D250S, when coupled with a SmartPass, is an exception.)

Bidirectional charging can be really important for folks whose vehicles sit for long periods, especially in cold weather.

N.B. You do need to be sure that your solar kit and shore power will drop to a true float.