Second Battery Charging After Discharging

[LeeroyJ]

I have an aftermarket stereo installed from @SiMsound (see his thread here -- I am super happy with the improvement in sound). If I turn off the ignition the sound stops unless I turn on the Power and Int 1 overhead switches. I rarely need that feature though so have only used it once to test until last week when I was working on the Grenny and turned on those switches so I could listen to music without the car running. Of course I forgot to turn it off and when I came back five days later there was not enough charge to turn over the engine.

I jumped the Grenny (using the connections under the hood) and since I knew I had about two hours of driving ahead of me, I figured I'd just let the alternator top-up the batteries. From the screen on board, after 2 hours it read 14.2 V and 85% charged. Returning home another 2 hours and it read 14.2V and 99% charged. The head scratching thing was that during all of this driving, I couldn't get the amp to turn back on. So I did all that driving with no sound.

Confused this morning as to why the amp wouldn't power back on, I went back to my local installer who had installed the SiMsound unit for me and had him look at it to see what the problem might be. He first checked the fuse on the wire from the amp to the second battery (it was fine) and then checked the voltage on the second battery, turns out it was only at 3V. So apparently while doing all that driving, the CTEK 120 had not allowed the second battery to receive any of the charging.

When I got home I hooked a 6amp charger directly to the second battery and after only a few seconds I decided to check the voltage on the second battery again (with the charger disconnected) which suddenly was now at 11.8V. I tried the stereo and it worked as it should.

What I am guessing happened is that the CTEK 120 isolated the second battery from the starter battery when the second battery got too low (sadly not soon enough to keep that from drawing the starter battery too low). As a result, when the alternator was charging the starting battery, the second battery was not getting any of that charge. I guess that when I connected the 6amp charger directly to the second battery, that "tricked" the CTEK to think that the second battery was now ok to re-connect and as a result, when I read the second battery with a volt-meter only a few seconds after having the charger connected, the second battery read 11.8 because it was pulling power from the starting battery (through the CTEK). Hopefully the alternator will now charge both.

I don't trust my 6amp charger to leave it unattended (its literally from the 1970s), so I now have a 1.25amp "trickle charger" directly connected to the Second battery and I'll leave that connected for a day or two and see how things look.

As an unrelated aside (at least I guess its unrelated), to see if the amp now works, I turned the ignition to Aux and powered up the head unit first (I did not turn over the engine). I had the air conditioning fan in the off position before doing this (I always turn off the fan before I turn off the ignition). After verifying that the amp works, I turned the ignition to the off position and removed the key. When I did that, I noticed the vent fan had turned on low (i.e. I could hear the fan turn on and a small amount of air come out of the front vents, despite the vehicle being off and the fan switch being all the way off). When the computer finally went to sleep the vents stopped blowing. This air con system really does have a mind of its own.

 

[Dschubba]

What you suggest happened is usual in that all loads are taken from the starter battery. As the starter battery gets low, the aux battery will send power pulses to it, but when the aux battery gets to 12V it should isolate itself until a failed start attempt at which point it will assist to start the car.

So the stereo shouldn’t be running from it. Once the starter battery is above around 13V the auxiliary battery will then charge thru the ctek120.

The ROK guide on here has a full description.

I think your ctech unit Might need checking or perhaps you have an issue with one of your batteries being sulfates and self discharging
(As was case with mine)

Sean

 

[Logsplitter]

Exactly. The way the Grenadier is wired isn’t like a traditional starter and auxiliary load battery set up. The second battery is there to help the starter battery in the event of the starter battery being low on charge. Auxiliary loads are best connected to the main battery ,this will allow the Ctek unit to isolate the two batteries when the starter is flat and second battery still has enough charge to start the engine. I have all my extra auxiliary loads connected to the main /starter battery and never a problem.

 

[Tazzieman]

I don't trust my 6amp charger to leave it unattended (its literally from the 1970s),

That device is known as "toaster" . Best retired to a shelf , with the lead cut off!

 

[Tom109]

Get a new charger, charge from the post in the engine bay, and see how both batteries perform.

 

[Zimm]

Get a new charger, charge from the post in the engine bay, and see how both batteries perform.

I haven’t had luck with that. I took a shot charging the main, then the secondary, without unhooking and had them both at 100. Unfortunately that’s a state aid charge that angered Ineos Jesus so he just lets them loose 5% a night for the next week. Ineos Jesus moves in mysterious ways.

 

[Tom109]

I experienced the opposite. My SoC was hovering around 84% due to limited used. Threw it on a charger overnight, both batteries topped up, and SoC stays 97% even sitting for over a week.

I put one of those BLE monitors on the secondary battery, and I have one for the main battery but not yet installed due to the captured battery clamp bolts.

 

[Zimm]

I experienced the opposite. My SoC was hovering around 84% due to limited used. Threw it on a charger overnight, both batteries topped up, and SoC stays 97% even sitting for over a week.

I put one of those BLE monitors on the secondary battery, and I have one for the main battery but not yet installed due to the captured battery clamp bolts.

i have the monitors sitting in a box, but my truck has been closer to you than me for 3 weeks and counting.

 

[Tom109]

i have the monitors sitting in a box, but my truck has been closer to you than me for 3 weeks and counting.

Hope they sort everything out for you.

 

[hus-iq]

Exactly. The way the Grenadier is wired isn’t like a traditional starter and auxiliary load battery set up. The second battery is there to help the starter battery in the event of the starter battery being low on charge. Auxiliary loads are best connected to the main battery ,this will allow the Ctek unit to isolate the two batteries when the starter is flat and second battery still has enough charge to start the engine. I have all my extra auxiliary loads connected to the main /starter battery and never a problem.

Picked up my Gren yesterday and out on a trip already. Have the aux battery and one of those coolers connected to power through the cigarette charger in the boot.

How long could I realistically leave the aux power on without potential issues?

 

[Logsplitter]

Picked up my Gren yesterday and out on a trip already. Have the aux battery and one of those coolers connected to power through the cigarette charger in the boot.

How long could I realistically leave the aux power on without potential issues?

That’s a difficult one without knowing the power draw of the cooler. As an example I can easily run my 47L ARB fridge set at 0c for 12 hrs in hot weather with no problem. When camped up for two or three days at a time , I use 2 x 100w suitcase style solar panels with inbuilt mppt controllers to keep the batteries topped up and have never had a problem.

 

[LeeroyJ]

Picked up my Gren yesterday and out on a trip already. Have the aux battery and one of those coolers connected to power through the cigarette charger in the boot.

How long could I realistically leave the aux power on without potential issues?

The second battery on the Grenadier (if you have that option) is rated as a 105 amp hour battery. I'm not sure how many amp hours the starter battery has, but let's guess 80, so a combined total of 185 amp hours.

Amp draw from fridges vary quite a lot between fridges and dependent on how cold you have it set, how hot it is outside, and how many times you open it. But for the sake of argument, let's say you average 1.5amps per hour.

In theory that would give you 185/1.5 = 123 hours of run time. But that's not a realistic number because you need to keep the batteries from dropping below a certain voltage to ensure you can start the car and for battery health. Also, you probably are drawing other loads.

Here is how Google's AI answers the problem (it says 61.67 hours):

AI Overview

What you're solving for


You are determining the maximum run time of a lead-acid battery under a specific load while maintaining a minimum voltage.


What's given in the problem

• The battery capacity is
  
  
  
  185 Ah185 Ah
  .
• The load current is
  
  
  
  1.5 A1.5 A
  .
• The minimum desired battery voltage is
  
  
  
  12 V12 V
  .
  
  

Helpful information

• Lead-acid batteries should not be discharged below
  
  
  
  50%50%
  of their capacity to prolong lifespan.
  
  

How to solve


Calculate the usable amp-hours and then divide by the load current to find the run time.

1. 
   Step 1 . Calculate the usable capacity.
   
   • Usable capacity is
     
     
     
     50%50%
     of the total capacity.
   • Usable capacity
     
     
     
     =185 Ah×50100=92.5 Ahequals 185 Ah cross 50 over 100 end-fraction equals 92.5 Ah
     .
   
   Step 2 . Calculate the run time.
   
   • Run time is the usable capacity divided by the load current.
   • Run time
     
     
     
     =92.5 Ah1.5 A=61.67 hours equals the fraction with numerator 92.5 Ah and denominator 1.5 A end-fraction equals 61.67 hours
     .

Solution


The battery can run the load for approximately

61.67 hours61.67 hours
while staying above

12 V12 V
.

 

[hus-iq]

That’s a difficult one without knowing the power draw of the cooler. As an example I can easily run my 47L ARB fridge set at 0c for 12 hrs in hot weather with no problem. When camped up for two or three days at a time , I use 2 x 100w suitcase style solar panels with inbuilt mppt controllers to keep the batteries topped up and have never had a problem.

Many thanks. Sorry I looked on the cooler but no info. Will look at solar panels once settled onto the car.

 

[hus-iq]

The second battery on the Grenadier (if you have that option) is rated as a 105 amp hour battery. I'm not sure how many amp hours the starter battery has, but let's guess 80, so a combined total of 185 amp hours.

Amp draw from fridges vary quite a lot between fridges and dependent on how cold you have it set, how hot it is outside, and how many times you open it. But for the sake of argument, let's say you average 1.5amps per hour.

In theory that would give you 185/1.5 = 123 hours of run time. But that's not a realistic number because you need to keep the batteries from dropping below a certain voltage to ensure you can start the car and for battery health. Also, you probably are drawing other loads.

Here is how Google's AI answers the problem (it says 61.67 hours):

AI Overview

What you're solving for


You are determining the maximum run time of a lead-acid battery under a specific load while maintaining a minimum voltage.


What's given in the problem

• The battery capacity is
  View attachment 7902311
  185 Ah185 Ah
  .
• The load current is
  View attachment 7902308
  1.5 A1.5 A
  .
• The minimum desired battery voltage is
  View attachment 7902313
  12 V12 V
  .
  

Helpful information

• Lead-acid batteries should not be discharged below
  View attachment 7902309
  50%50%
  of their capacity to prolong lifespan.
  

How to solve


Calculate the usable amp-hours and then divide by the load current to find the run time.

1. 
   Step 1 . Calculate the usable capacity.
   • Usable capacity is
     View attachment 7902312
     50%50%
     of the total capacity.
   • Usable capacity
     View attachment 7902314
     =185 Ah×50100=92.5 Ahequals 185 Ah cross 50 over 100 end-fraction equals 92.5 Ah
     .
   
   Step 2 . Calculate the run time.
   • Run time is the usable capacity divided by the load current.
   • Run time
     View attachment 7902307
     =92.5 Ah1.5 A=61.67 hours equals the fraction with numerator 92.5 Ah and denominator 1.5 A end-fraction equals 61.67 hours
     .

Solution


The battery can run the load for approximately
View attachment 7902310
61.67 hours61.67 hours
while staying above
View attachment 7902306
12 V12 V
.

Cheers Leyroy, that's very helpful!!! Will leave it overnight and see what the display shows and look at ways to increase consumption

 

[Ovrland Bill]

That device is known as "toaster" . Best retired to a shelf , with the lead cut off!

I have the same charger from the same decade and yes, it works and no, I don’t trust it either!