Why isn't my aux. battery charging?

twblanset

Observer
I built a slide-in drawer box to go in my '99 Tacoma that included a deep-cycle, auxiliary battery to power an Edgestar frig., a water pump, and a couple of accessory outlets. On the first use I couldn't get the aux. battery to charge after a three hour drive.

I wired it from the starting battery, through an 80amp soleniod. I ran 10g wire from the engine compartment, back to the tailgate area, through a plug, and up to the front of the bed, where the battery lives in the drawer box. So there is probably about 25' of wire. I used a body ground instead of running a dedicated wire back to the negative pole.

After doing some reading (especially evldave's thread) I'm thinking there might be a couple of problems.
1. the 10 gauge wire is too small and too long.
2. the body ground isn't good enough (should have run a wire back).
3. both of the above.

Can anyone confirm my suspicions or give me some other ideas?
Thanks in advance.
Todd
 

Martyn

Supporting Sponsor, Overland Certified OC0018
Try running 4 gauge wire back to the battery. That should solve your problem.

Make sure you have a fuse or resettable breaker as close to the starter battery as possible. Use Anderson connectors if you need to plug and unplug the system.
 

TeCKis300

Observer
I assume you checked continuity between the batteries positive node through the solenoid when the motor is on?

10 gauge is slightly on the smaller end (rated at ~30amps on 12V) but should suffice.

I'd suspect it's your body ground. Make sure to really clean the grounding post of any paint and that it has a solid connection. Though some body parts just don't make a good ground. No need to run a separate ground wire back, though that would be a good test. Or run a second grounding wire temporarily to a known good ground.
 

surfbark

New member
Definitely run a larger gauge wire.... You always fuse (or breaker) for the wire size. In this case A number 2 awg would be a good idea with an 80 amp "service"
 

Martyn

Supporting Sponsor, Overland Certified OC0018
I assume you checked continuity between the batteries positive node through the solenoid when the motor is on?

10 gauge is slightly on the smaller end (rated at ~30amps on 12V) but should suffice.

I'd suspect it's your body ground. Make sure to really clean the grounding post of any paint and that it has a solid connection. Though some body parts just don't make a good ground. No need to run a separate ground wire back, though that would be a good test. Or run a second grounding wire temporarily to a known good ground.

There may well be something going on with this system other than the gauge of the wire, and everything should be checked out. The 10 gauge wire run over 25 ft will only produce a trick charge to the aux battery. If the battery was fully charged when it was put in then it may run the system for a couple of days, depending on how it was used and the battery size, but the recharge from the vehicle wouldn't be able to keep pace with the deficit.

It's something we see all the time with the 10 gauge wiring to the towing package on vehicles. It produces a slow recharge rate for a trailer auxiliary battery and doesn't keep pace with the discharge caused by running a battery.

Increasing the gauge of the wire up to 4 gauge would take care of the recharge rate.
 

McZippie

Walmart Adventure Camper
There may well be something going on with this system other than the gauge of the wire, and everything should be checked out. The 10 gauge wire run over 25 ft will only produce a trick charge to the aux battery. If the battery was fully charged when it was put in then it may run the system for a couple of days, depending on how it was used and the battery size, but the recharge from the vehicle wouldn't be able to keep pace with the deficit.

It's something we see all the time with the 10 gauge wiring to the towing package on vehicles. It produces a slow recharge rate for a trailer auxiliary battery and doesn't keep pace with the discharge caused by running a battery.

Increasing the gauge of the wire up to 4 gauge would take care of the recharge rate.

Exactly... assuming everything is wired correctly and solenoid is working. The negative also needs to be 4 guage, a short run to frame is OK with a clean ground.
With a 10 gauge wire, it can actually be worse than "recharge from the vehicle wouldn't be able to keep pace with the deficit" ... depending on the alternator, voltage regulator and state of charge in the starting battery, a fully charged aux battery can discharge to the lower state of charge starting battery without any load on the aux battery.


Good: to have both aux and starting batteries fully charge before beginning trip.
Better: have same type, size and age of batteries for starting and aux.
Best: upgrade aux and starting to AGMs.

Edit: While driving the reason a fully charged aux battery can discharge to a lower state of charge (SOC) starting battery or vise versa:
The voltage regulator controls the alternator amp output by monitoring the voltage.
The voltage regulator senses the higher (SOC) voltage in the aux battery and reduces amp output before the lower (SOC) starting battery is charged.
With the higher (SOC) in the aux battery the amps can flow back to the lower (SOC) starting battery.
Eventually both the starting and aux will reach the same (SOC) and the voltage regulator will control alternator amp output correctly.
 
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rryd

Observer
One problem I discovered with my Toyota van, the alternator have a sensor cable who "feel" on my start battery. So when my leisure battery was empty my alternator still feel that my start battery was fully charged.
Result? Only a few ampere goes to my leisure battery, had to drive forever to charge.

So I mounted a extra alternator to charge my leisure batteries.
Fast charge and a extra electrical system is some of the positive.

Sent from my GT-I9300 using Tapatalk 2
 

twblanset

Observer
Thanks for all the great info. The solenoid is continuous duty rated and it's a generic Camping World model. The wiring is "correct" and the solenoid is working. I probed around with a voltmeter and there is 12+ volts everywhere when the engine is running but when the key is off the voltage on the aux. side of the solenoid drops down around 11.5 and the starting battery is around 12.5.
 

McZippie

Walmart Adventure Camper
Thanks for all the great info. The solenoid is continuous duty rated and it's a generic Camping World model. The wiring is "correct" and the solenoid is working. I probed around with a voltmeter and there is 12+ volts everywhere when the engine is running but when the key is off the voltage on the aux. side of the solenoid drops down around 11.5 and the starting battery is around 12.5.

To make sure the aux is holding a charge; fully charge the aux with a 120 volt charger, remove charger, put one to two amp load on aux for 1/2 hour to remove surface charge, then wait a day for voltage to settle (battery at rest) check the voltage, it should be in the 12.5+ range.
 
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twblanset

Observer
I did this prior to installing it and it held a charge just fine. It's sounding like I should run a 4ga wire with a ground wire.
 

McZippie

Walmart Adventure Camper
I did this prior to installing it and it held a charge just fine. It's sounding like I should run a 4ga wire with a ground wire.

Great! if you didn't do so already, charge the aux battery, shouldn't let it sit at 11.5 volts.
 

brian90744

American Trekker
charging duel batteries

Maybe change your set-up to an isolator. disconnect the main wire at the alt, and run it to the center post of the isolator. run from the side post of the isolator wire back to the alt. from the other post run a wire to your aux battery.
 

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wrcsixeight

Adventurer
Make sure whatever circuit you are activating the solenoid with is hot all the time the engine is running.

25 feet of 10 awg will not pass many amps. The body ground cannot be trusted, at least attach the body ground to the frame if you don't want to run 4 awg ground back to the engine battery.

Even if you do get it working, you will still be disappointed with the amount of charged returned to the aux battery. The Original charge circuit is not beefy enough to really handle the extra amps a distant aux battery can ask for. If the (+) post on the alternator is hot with the key not in the ignition, then add a 4 awg cable from alternator (+) to the engine battery side of the solenoid, properly fused of course.

Otherwise recharging that distant battery is Like breathing through a cocktail straw after running a marathon.

Fully charge that aux battery with a grid powered charger right after and right before every outing, do not rely on the alternator, they do very poorly squeezing in the last 20%. The last 20% will take many many hours whatever the charging source.
 

4RunAmok

Explorer
Maybe change your set-up to an isolator. disconnect the main wire at the alt, and run it to the center post of the isolator. run from the side post of the isolator wire back to the alt. from the other post run a wire to your aux battery.

Diode isolators reduce the amount of voltage delivered to BOTH batteries and neither will fully charge...ever...., while constant duty solenoids do not have any such circuitry restricting flow.

You already have your solution at larger wires and running both positive and negative wires to the main battery, the only other thing you might consider upgrading is the solenoid. Consider a Trombetta 250amp constant duty, or a Blue Sea 7622 500amp constant duty. The beauty of the Blue Sea is that it is an Automatic Charging Relay, no need to run control wires, it pairs automatically with the OPTION of running an override switch to pair or disconnect when you wish, it also has a switch on the unit itself.
 
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