Anyone consider replacing the aux battery with a capacitor?

[steven1955]

Anyone consider replacing the aux battery with a capacitor?

This would be like one of the capacitors used by some folks to stabilize power when running the super sized aftermarket audio equipment?

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[Thunderspud]

Havent heard of anyone doing it. Also, because I am curious, what benefit do you foresee this having?

 

[steven1955]

High quality capacitors should not degrade, at least over the typical life of a Jeep. It might be a good way to side step the need for auxillary battery replacement every few years.

 

[Mr._Bill]

Anyone consider replacing the aux battery with a capacitor?

This would be like one of the capacitors used by some folks to stabilize power when running the super sized aftermarket audio equipment?

The two batteries are separated during ESS restarts to keep low voltage from rebooting the computers. It would have to be a large enough capacitor to keep all the electronics stable during the restart.

 

[kb5zcr]

This guy made a start battery from capacitors and posted this youtube 12 yrs ago. The problem comes with a low power reserve. Caps can give a lot of currant for a short amount of time, but don't have much reserve capacity for low current needs.
Interesting concept.

 

[Flanders]

Anyone consider replacing the aux battery with a capacitor?

It would be a fun experiment and I've thought about it. AUX delete is probably the better option.

Ballpark, the capacitor bank would need to be capable of providing 20A for 1 second without dropping more than 1V.

That’s 20 farads minimum, EDLC “super capacitor” territory. These are typically rated for 2.7V max so we’d need 6 in series. Series capacitance follows the reciprocal-sum-reciprocal law, so 6 x 120F 2.7V super caps to get 20F at 16.2V max.

We also have to consider the voltage drop due to equivalent series resistance. DC ESR of a 100F super capacitor is typically around 50 milliohms, which means 300 millohms for a string of 6 and a voltage drop of 6V when drawing 20A.

That’s why the cheap capacitor banks marketed to car audio nerds don’t work. A car battery can provide hundreds of amps before dropping 6V.

We’d need DC ESR around 5 milliohms to get the voltage drop due to ESR down to 0.6V at 20A. And we’d have to double the capacitance to limit the voltage drop to 1.1V after 1 second when drawing 20A.

That’s 6 low DC ESR 250F (or greater) EDLCs on the BOM. These are not cheap and we’d want to protect them with an active balancing circuit. That’s a parasitic draw, typically several times that of the JT itself. Eliminating the AUX battery would not only cost us 15% of total battery capacity, we’d also be draining the main battery ~5x as fast while it sits.

Finally, the lifespan of an EDLC is severely reduced at high temperatures, often only 1000 hours at 65C (~150F). Some of the failure modes would result in the main battery being drained.

 

[Wildtoad]

Wouldn't it be a better idea just to keep the battries charged. From what I garnered those of us obliged to drive their Jeeps daily/weekly have had far less issues with the batteries. My '21 still going strong, but to be fair, have installed device to keep the ESS off.

 

[ShadowsPapa]

Anyone consider replacing the aux battery with a capacitor?

This would be like one of the capacitors used by some folks to stabilize power when running the super sized aftermarket audio equipment?

And how would this interact with the JT's smart charging system?
The charging system keeps both batteries topped off.
The IBS senses the temperature, amp hours out, amp hours in, battery state of charge and much more. When there is a start event, the two are separated for a brief moment at which time the PCM detects the voltage at the aux battery and compares to information on hand to help determine the battery health and state.
You'd have to account for system voltages ranging from 15.0 to 12.6 during normal driving situations, varying with weather/temperatures, load on the electric system and torque load on the engine and more.

I think people are missing the purpose of the aux battery and how the system acts during all possible events.

 

[UTRocky]

And how would this interact with the JT's smart charging system?
The charging system keeps both batteries topped off.
The IBS senses the temperature, amp hours out, amp hours in, battery state of charge and much more. When there is a start event, the two are separated for a brief moment at which time the PCM detects the voltage at the aux battery and compares to information on hand to help determine the battery health and state.
You'd have to account for system voltages ranging from 15.0 to 12.6 during normal driving situations, varying with weather/temperatures, load on the electric system and torque load on the engine and more.

I think people are missing the purpose of the aux battery and how the system acts during all possible events.

Only because I am brand new to Jeep Gladiator, can you refresh the point of AUX battery, or is there a thread I should search and read?

 

[ShadowsPapa]

Only because I am brand new to Jeep Gladiator, can you refresh the point of AUX battery, or is there a thread I should search and read?

Several companies have used this same setup - BMW, Chevrolet, Ford and some others, and it's been in Jeeps with the Pentastar Upgrade engine since 2015 for the 2016 model year (2016 Grand Cherokee is one example of Jeep products with this system)

When the vehicle is sitting - not running, both batteries are connected together in parallel. That means that you have the combined aH of the battery pair to handle the parasit load of the sitting vehicle, non-running.

When it is running and you are driving it - same thing, batteries are in parallel at all times that it is running. The charging system sees it as "one battery". The IBS on the main battery knows the state of charge, state of health, temperature and so on of the main battery, and actually of the pair as the two are momentarily disconnected from each other as the starter is triggered - at which time the voltage reading is taken from the aux battery. It calculates from there.

During an ESS stop event, all of the electronics are "charged" and operational - radio, BCM, PCM and more. The power comes from both batteries still connected in parallel. You can tell this by having an external volt meter connected to ground and the positive post of the crank/main battery and another volt meter connected to the same ground point and N7 which is connected to the aux battery positive post. The voltage changes in lock-step as they share the load.
When an ESS restart is triggered, the starter takes power from the battery causing a sudden drop in voltage. To prevent the "electronics from seeing this", at that moment the two are disconnected from each other, the electronics don't see the sudden drop in voltage and thus there is no huge outrush from all of the electronic systems at that time. As quick as it's over, they are reconnected again.

It's not a problem on a cold, non-ESS start like first thing in the morning because the systems aren't fully operational and all circuits charged like during an ESS stop where everything is fully "charged" with power and operational.

The aux battery maintains a constant power to the electronics - all modules, radio head unit and more, during an ESS stop. To subject them to the sudden drop in voltage as the starter is triggered can cause issues.

Another fellow has demonstrated this effect with a scope, monitoring both batteries at the moment of a restart.

If you never ever use ESS, you can do away with the aux battery and upgrade the main battery to the same capacity as the two batteries combined. Otherwise to just disable the aux battery, you lose capacity.

 

[sharpsicle]

Anyone consider replacing the aux battery with a capacitor?

This would be like one of the capacitors used by some folks to stabilize power when running the super sized aftermarket audio equipment?

Not sure what the intended gain is here.

Capacitors are usually used where quick discharges are required. That's not the case in ESS. You want stability, which is what a battery is better at doing. Capacitors dump energy and lose voltage, the opposite of what you want. Not to mention, the battery is more efficient at storing energy.

So switching to a capacitor system for this is a lose-lose.

 

[Flanders]

Only because I am brand new to Jeep Gladiator, can you refresh the point of AUX battery, or is there a thread I should search and read?

The AUX battery exists mainly to keep the head unit from rebooting when the automatic start-stop system restarts the engine. Otherwise the current drawn during cranking can pull the battery voltage low enough that the head unit would drop out from undervoltage and then reboot when the engine starts.

As @ShadowsPapa said, the two batteries are always connected in parallel except for about one second during cranking from an ESS stop. They're also isolated for maybe 0.1 seconds before a cold start, for a quick self-test to ensure that the AUX is there and not completely dead.

I can't see any reason a capacitor would confuse the charging system. The IBS has no way of measuring AUX current anyway. The capacitor would just have to be large enough, and with low enough resistance, that its voltage doesn't sag too much when it's isolated. It would basically have to look like a healthy battery.

The problem is that the DC internal resistance of a healthy lead acid battery is an order of magnitude less than the ESR of a typical supercapacitor bank. The voltage drops instantly under load, proportional to resistance, and an inexpensive capacitor bank would likely fail the self-test.

 

[ShadowsPapa]

I can't see any reason a capacitor would confuse the charging system. The IBS has no way of measuring AUX current anyway.

But what about this -
anything coming out of the aux battery, since it's in parallel, is also a shared load on the main battery.
You can't take out of one partner of a pair without taking out of the other partner of the pair as they will try to stay in balance.

 

[Flanders]

But what about this -
anything coming out of the aux battery, since it's in parallel, is also a shared load on the main battery.
You can't take out of one partner of a pair without taking out of the other partner of the pair as they will try to stay in balance.

I'm not sure what you're saying. IBS can only measure current passing through the main battery. Anything in parallel with the main is completely invisible to it.

The instantaneous ratio of currents to a load, or from a souce, for two batteries in parallel and initially in equilibrium is the reciprocal ratio of their internal resistances. The same is true for a battery and a capacitor in parallel, or two parallel capacitors.

Point is, a large enough capacitor looks just like a battery for the time periods and currents in question.

 

[ShadowsPapa]

I'm not sure what you're saying. IBS can only measure current passing through the main battery. Anything in parallel with the main is completely invisible to it.

The instantaneous ratio of currents to a load, or from a souce, for two batteries in parallel and initially in equilibrium is the reciprocal ratio of their internal resistances. The same is true for a battery and a capacitor in parallel, or two parallel capacitors.

Point is, a large enough capacitor looks just like a battery for the time periods and currents in question.

I quoted the tech document from FCA, their training information.
They use the IBS historical data as well as the voltage history of the aux and so on to calculate............

but, whatever.

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