Dual Battery Systems with Temperature Differences due to Location

[fire_strom]

What do folks do about having a dual battery system with batteries remote from each other and therefore different temperatures? Specifically in my case I'll have the starting battery in the engine compartment stock location and a second battery on the frame rail (optional stock location in some configurations). The batteries will be connected when charging from the alternator but I think the alternator will be temperature compensated and thus under charge the cooler frame mounted battery. When charging off solar the frame battery will be similar temp to the charge controller so will be ok but the other battery will be possibly in the hotter engine compartment thus over charged. I do realize that most of the time on solar the engine bay should be pretty equal temp to the ambient but when it isn't that battery will be over charged right? What to do?
Thanks,
-G

 

[john61ct]

Get a DCDC charger with its own temp sensor for the House batt.

Or rewire the stock sensor NBD if the Starter isn't optimal

 

[DiploStrat]

Get a DCDC charger with its own temp sensor for the House batt. ...

That is a good idea, subject to all of the usual caveats, do you really need a B2B, etc.

In may former case, the camper batteries, 600Ah of AGM, were below the truck at the back, fully exposed to the weather. Starter batteries were under the hood. Chevrolet applied temperature compensation to the starter batteries.

The bottom line, however, was that the camper batteries were generally colder than the starter batteries and would thus expect a higher charge voltage. The warmer starter batteries probably pulled the voltage from the alternator down a bit, but the solar controller and the shore power, inverter/charger, both connected directly to the camper batteries, were both temperature compensated to the camper batteries.

So, in the end, it was a non-issue.

 

[john61ct]

Yes of course, I just meant, if it is a real issue, those are the two best solutions

 

[fire_strom]

What is NBD and B2B?

If I’m understanding, I could have gone with a dc to dc charger between the alternator and the house battery?

In that case I wouldn’t need the ACR?

All I’d lose is the potential of the house not being able to link to the starter battery to assist starting. Which I don’t think I care about since the whole point of this is to protect the starting battery and I could still use jump cables if I need.
And I’d lose solar directly charging both batteries unless I rewired, the solar input goes into the house system.
-G

 

[DiploStrat]

...
Results of our mouseclicker age of the keyboard. ...

Nah! We're just lazy and like to pretend we're cool by using arcane terms. Think of it as a secret handshake! ;-)

 

[DiploStrat]

...
In that case I wouldn’t need the ACR?

All I’d lose is the potential of the house not being able to link to the starter battery to assist starting. Which I don’t think I care about since the whole point of this is to protect the starting battery and I could still use jump cables if I need.
And I’d lose solar directly charging both batteries unless I rewired, the solar input goes into the house system.
-G

This goes to one of my pet peeves - too many oversimplified absolutes.

There are many ways to do this, including manual switches, relays, and chargers. The method that is best for your application will depend on many different factors, including:

-- The voltage of your vehicle's charging system, grossly, 14v+ or less than 14v.
-- The type of battery in your camper bank, lead acid, AGM or open, or lithium.
-- Presence or absence of solar/shore/generator power.
-- Your goals, including self-jump, winch support, etc.

If you look at the "Documents" section of my website you will find several presentations that you may find useful.

 

[john61ct]

What is NBD and B2B?

If I’m understanding, I could have gone with a dc to dc charger between the alternator and the house battery?

In that case I wouldn’t need the ACR?

All I’d lose is the potential of the house not being able to link to the starter battery to assist starting. Which I don’t think I care about since the whole point of this is to protect the starting battery and I could still use jump cables if I need.
And I’d lose solar directly charging both batteries unless I rewired, the solar input goes into the house system.
-G

Yes no big deal and battery-to-battery.

Yes one step up from plain ACR, for different chemistries or overcoming voltage drop.

One way, no self-jumping, but that's NBD.

Rarely should Starter get pulled down if system designed properly.

 

[fire_strom]

The charge controller I have says that it compensates for temperature at 18mV/degreeC (Samlex MSK-10a). If there is a 30c difference in temp between batteries. Is it right to calculate that the cooler battery will charge at 540 mV or .54 volts less than it would have gotten? I realize this is reversed, when I have the temperature differential it will be while the alternator (145a GM/ACDelco) is running and I don't know what the temp compensation parameters are for the alternator but I am just marinading on this to see if I understand.
-G

 

[dreadlocks]

or simpler, could just put a 2 battery switch in for house battery (off/1/2/combined).. normally its on the house battery since you have solar and starter battery has alternator, ideally they rarely need to operate in tandem and remain isolated, and if you want to jump start or charge house off alternator, combine em.. and if your house battery craps out or you think is starting to fail, you can switch everything over to the starter battery/alternator as an emergency backup, and it can act as a master disconnect switch for house battery all together in off season.. but yeah you have to act like a cave man and flip a switch, but its pretty flexible and bullet proof.. this is how I did the dual battery setup in my Westfallia and just got a dual output 120v charger for shore power/battery tending.